• Author / Uploaded
• Sureshraja9977

Citation preview

What is hybrid substation? Answer: Hybrid substation is a combination of both conventional air insulated substation (AIS) and Gas Insulated Substation (GIS). Some of the bays in the substation are gas insulated (enclosed in modules filled with SF6 gas) and some of the bays are air insulated type (open to atmosphere). The design of hybrid substation is based on the convenience, local conditions, availability of the area and cost

Why Earthing Grid is Required in Substation? Answer: In electrical substation in order to limit the step potential and touch potential to sufficiently safe values and to control the influence of the short circuit currents on telecommunication installations. A Strong earthing network is provided. This is provided to effectively earth the systems with high earth fault currents

What are the different current transformers used in substation? Answer: Current transformers in substations are generally used for reducing or stepping down a.c currents from higher value to lower value for the purpose of measuring, protection and control purposes. There are two classes of current transformers. MEASURING CURRENT TRANSFORMERS:

They are used for measuring purposes.They are used with ammeters, watt meters, KVA meters and kWh meters for reducing line current to 1 ampere or 5 amperes PROTECTIVE CURRENT TRANSFORMERS:

They are used with over-current protection, earth fault protection, differential protection, impedance protection

What is the power frequency spark-over voltage? Answer: Power frequency spark-over voltage is the r.m.s value of the power frequency (50Hz or 60Hz) voltage applied between the line and earth terminals of the arrester and earth which causes spark-over of the series gap

What is Follow Current with respect to Surge Arrester? Answer: Follow current is the current which flows from the connected power source through lightning arrester following the passage of discharge currents

Which is the basic busbar scheme employed in 220kV and 400kV substations? Answer: In case of 220kV and 400kV substations double main busbar scheme or one-and-half breaker schemes are generally employed

Function of Isolators in Substation? Isolators are disconnecting switches which are used for disconnecting of the circuit under no load conditions. Isolators are placed in the substation such a way that any part of the circuit can be isolated from the live parts for maintenance purposes. Isolators plays an important role during maintenance of the substation. Isolators should be always operated under no load condition. Isolators should be open after opening the circuit breaker and closed before closing the circuit breaker contacts.

Function of earth switches in substation? Earth Switches in substation are employed to connect the conductors to the earth thereby discharging any charging currents due to capacitance. Earth switches are employed for the protection of operating personnel. Earth switches are operated along with isolators. When isolators opens, earth switches immediately connects the conducting parts to ground

     

What are the different substation components Answer: Circuit Breakers, Disconnecting switches, Grounding switches, Current Transformers, Potential Transformers or Capacitor voltage Transformers, Line Traps, Lightning Arrestors, Power Transformers, Shunt Reactors, Current limiting reactors, Station Buses and Insulators, Grounding system, Series capacitors and shunt capacitors. What are different types of busbar arrangement Schemes in Substations? Answer: Different switching schemes or busbar arrangement scheme employed in switch-yard are: Single Bus Scheme Double bus single breaker scheme Main and Transfer busbar scheme Double bus single breaker scheme Breaker and half scheme Ring main bus scheme Which type of bus arrangement is costlier and more reliable? Answer: One and half breaker scheme is most reliable and costlier scheme. In one and half breaker scheme 3 circuit breakers are provided for 2 bus-bars. Hence the name is coined as One and half breaker scheme For distribution network which type of Switching scheme is employed? Answer: For distribution network Ring Main busbar scheme is employed. It has the provision that if power interruption occurs to one bus section due to maintenance or fault, power can be fed the bus from other side of the power system.

  

Factors on which material of station busbar depends? Answer: Some of the factors on which station busbar depends are: Current Carrying capacity Short Circuit Stresses Establishing minimum electrical clearances What are the different electrical clearances in Substation? Answer: Ground Clearance: This is the minimum distance between any live conductor and the earth or ground. Phase Clearance: It is the clearance between the different phases in a circuit or same phases in different circuits Sectional Clearance: This is the minimum clearance between the live conductor and the limits of work, where spacial clearance is related to the safety of the operating personnel. The limits of work section may be ground or the platform on which the person is working.

Why EHV busbars are hallowed and circular? Answer: To avoid corona effect What are different Bus bar materials used in Substation? Answer: Busbars are generally made of copper and Aluminum. Aluminim has the advantage of one third the weight of the copper and also Aluminum requires less

maintenance and proper use of alloys provide necessary rigidity required for bus material. Aluminum is used widely in EHV and HV stations What is the first equipment of the substation connected to the incoming transmission line? Answer: Lightning arrestor is the first equipment of the substation connecting the incoming transmission lines. It protects other substation equipment from lightning and switching surges by suppressing them to ground. Lightning arrestors also placed in the substation between both the ends of major equipment such as power transformers for protection. Reasons for using Stones/Gravel in Switch yard: 1. During Short circuit current Step and Touch potential increases. Step potential : It is the potential developed between the two feet on the ground of a man or animal when short circuit occurs. This results in flow of current in the body leads to electrical shock Touch potential: It is the potential that is developed between the ground and the body of the equipment when a person touches the body during fault condition. When operating personnel touch an electrical equipment during short circuit condition, fault current flows through the human body. This is defined as touch potential.

Gravel/Stones in the substation is provided to reduce the step potential and touch potential when operators work on switch yard. 2. Power Transformers installed in the substations will have oil as cooling and insulating medium. Oil leakage takes place during operation or when changing the oil in the transformer. This oil spillage which can catch fire is dangerous to the switchyard operation. So Stones/Gravel is provided to protect from fire when oil spillage takes place. 3. Stones eliminate the growth of small weeds and plants inside the switch yard 4. Improves yard working condition 5. Provides protection from wild life and snakes.

Methods to Reduce Step Potential and Touch Potential in Substation Step Potential and Touch Potentials are very important in substations because during ground faults all the ground current returns to the substation transformer (as the substation transformer is grounded). The current that returns through the earth can create a significant voltage gradient along the ground and between ground and conducting objects.A step potential in substation creates a path through the legs from one foot to the other. A touch potential in substation is normally considered a hand-to-foot or hand-to-hand contact. Step potential and touch potentials are of concern during normal conditions and during ground fault.Under normal conditions, unbalanced currents can rise the neutral to earth voltage. This is not normally dangerous but it can cause shocks.

Step potential and Touch potentials during faults are more dangerous. Therefore it is important to reduce the step potential and touch potential to be within limits during the substation design.

Methods to Limit Step Potential and Touch Potential: Step potentials and touch potentials can be reduced by employing one of the following methods: By proving low resistance path to ground 2. By providing insulation layer between operating personnel and earth 3. By proper placing of ground conductors Some of the methods employed to reduce step potential and touch potential in substation are: Multi-grounded neutral helps to reduce the dangerous step potential and touch potential during line-to-ground faults. By creating low impedance path back to the source, faults are quickly cleared by fault interrupters. Multiple grounding electrodes tied together helps to reduce the touch potentials at the fault point. With multiple neutrals, step potentials are usually not dangerous since fault currents spreads between several grounding electrodes Using a reactor on the substation transformer neutral helps to limit the step and touch potentials. While utilities normally use the neutral reactors to limit the fault currents. the reduction of ground fault currents also reduce the step and touch potentials and reduces current in grounding and bonding connectors. By wearing electric hazard shoes. These shoes when dry can have offer millions of ohms of resistance which can save the operating personnel against these dangerous potentials. By using insulating materials such as rubber gloves can protect the personnel. By providing resistive surface layers in and around the substation. It is often provided with the surface of crushed rock or pebbles which acts as insulation medium between the operating personnel and earth. 1.









Factors affecting Soil Resistivity Soil resistivity is the resistance of certain volume of the soil between opposite faces of the cube of soil with a volume of 1m 3. Soil resistivity vary widely. Soil resistivity varies significantly with increase in the depth. Some of the important significant factors that affect the soil resistivity are listed below:

Soil Resistivity is affected by: Moisture content: Moisture content in the soil is one of the major factors that determine the soil resistivity. The dryer the soil the more is the resistivity of the soil, wet soil will have low soil resistivity. Change in the moisture level of the soil through out the year during different seasons is the biggest reason for change in the ground electrode resistance. With increase in the moisture content in the soil, ground resistance or soil resistance decreases and about after 22% of moisture content there will be very little change in the soil resistivity

Temperature: Above the freezing point of the water, temperature does not impact the soil resistivity significantly but the temperature below the freezing point of the water soil resistivity rises and will have significant impact

Salts in Soil: The presence of soluble salts in the soil will significantly impact the resistivity of the soil. One of the option to reduce the electrode resistivity is by chemical treatment of soil near the electrode. Bentonite backfilles or salt treatment such as sodium chloride or calcium chloride can significantly reduce the soil resistivity. The only disadvantage is some chemicals enhance the metal corrosion rate which can damage the grounding rod to corrode. If the grounding rod sustains significant current, the current flowing through the grounding rod may dry out the surrounding soil and that can increase the resistance of the electrode. When high currents flowing through the grounding rod or earthing electrode, this may be due to lightning or from faults the soil surrounding the electrode breaks resulting in the increase in the resistance

Creepage Distance for Insulators in Substation Creepage Distance for Insulators: The creepage distance for insulators is the shortest distance along the insulator surface between the metal parts at each end of the insulator. Creepage distance can also be refer as leakage distance for insulators. Insulators in substation are provided to avoid any leakage current from live electrical conductors to flow to the earth through supports. The atmospheric dust sticks to the insulator surface forming a conducting layer. The leakage current flows from the live conductor to the earth through such surface layers. The leakage properties (creepage properties) of an insulator s in substation are characterized by the length of the leakage path. While designing the insulator sheds, the leakage distance for insulators requirement should be satisfied. The requirement of the leakage distance or creepage distance for insulators depends on the  

Rated phase to ground voltage Degree of atmospheric pollution If the surface of the insulator is clean, smooth and well gazed the dust particles tend to fall down. When an a.c voltage is applied to the apparatus, the dust particles have a tendency to align to the direction of electrical field lines on the surface of the insulator. If the surface of the insulator is rough and moist this alignment will be faster. The continuous application of voltage causes slow alignment of these particles. To prevent a continuous conducting track, the insulator should have sufficient ceepage distance or leakage distance. During power frequency withstand test of an unclean insulator, the flash-over occurs along the dirty surface of the insulator. In case of internal gas filled or oil filled apparatus, the internal surface should also be free from moisture and dust. Otherwise internal flash over can occur

along the surface by tracking. Some of the typical creepage distance for insulators in substation provided based on the level of pollution are tabulated below

S Degree of No. Pollution

Recommended Creepage Distance for Insulators

1

Clean areas

16 mm/kV

2

Moderately polluted areas

20 mm/kV

3

Industrial areas

22 mm/kV

4

Heavily polluted areas

25 mm/kV

Shunt Vs Series Capacitors Advantages Capacitors aid in minimizing losses and helps in reducing operating expenses. Capacitors should be placed where the reactive power demand is more as capacitors generate reactive power and helps to maintain the voltage. Basically reactive power demand will be more at load side of the power system, hence capacitors should be placed as close as possible to load. In specific, capacitor banks should be placed where the low voltage problem occurs.Shunt capacitors are series capacitors are employed in power system for different purposes 





 

Shunt capacitors and series capacitors in the power system generate the reactive power to improve the power factor and voltage, thereby enhancing the power system capacity and reducing the losses. In series capacitors reactive power generation is proportional to the square of the load current (I2Xc), whereas in shunt capacitors reactive power generation is proportional to the square of the voltage (V2/Xc). The cost of installation of series capacitors is higher than that of the shunt capacitors. This is because the protective equipment for the series capacitors is often complicated. In addition to that, series capacitors are generally designed for higher power to cope up with the future increase in the load For the same voltage improvement, the rating of the shunt capacitors will be higher than that of series capacitors. Series capacitors compensation may create certain disturbances: ferro-resonance in transformers, sub-synchronous resonance during motor starting, shunting of motors during normal operation and difficulty in protection of capacitors from system faults. Series capacitors are more effective on distribution circuits with higher X/R ratio and for load variations involving a higher reactive content. Series capacitors are generally employed to improve the stability of the system and shunt capacitors are generally employed to improve the power factor of the system

Some of the factors which influence the choice between the shunt and series capacitors are tabulated below:

Objective

Series Capacitors

Shunt Capacitors

Improving Power Factor

Secondary

Primary

Improving Voltage level in overhead line system with a Primary normal and low Power Factor

Secondary

Improving Voltage level in overhead line system with a Not Used high Power Factor

Primary

Reduce Line Losses

Secondary

Primary

Reduce Voltage Fluctuations

Primary

Not used

Substation Safety Clearances Majority of electrical substations will be air insulated type. All the electrical equipment live parts will be exposed .All the electrical equipment in the substation shall be arranged in such a manner to ensure the clearance space between the live parts and other equipment of the substation (grounded or ungrounded). Adequate clearances should be provided for safety of the operating and maintenance personnel and also to prevent any conducting path between the phases. Safety clearances associated with different voltage levels are given in the table below. 

 



Phase to phase clearance is the minimum clearance required to be provided between the live parts of different phases or is the minimum clearance between the same phases Phase to earth clearance is the minimum clearance required to be provided between the live parts of the system and earth structures Sectional clearance is the distance between the live parts of the phases and the terminals of the work section. This limits of work section or maintenance section may be a platform or ground on which operation personnel can carryout his task safely Ground clearance is the vertical clearance between the live parts of the equipment and the earth surface Safety Clearances in Substation:

Minimum Clearances (mm)

765 kV

400 kV

220 kV

132k V

Phase to Phase

9400

4200 2100 1300

320

Phase to earth

6400

3500 2100 1300

320

Sectional Clearance

10300 6500 5000 3800

3000

Ground Clearance 14000 8000 5500 4800

3700

33kV

Corona Ring on Surge Arresters and other EHV equipment Have you ever seen a ring around the Surge Arrester or other EHV equipment? What is the purpose of the Ring around the Extra High Voltage (EHV) equipment? What is the use of big rings at EHV testing centers?

Corona Ring:

This Metal Ring what we see in the substations and other transmission line network is "Corona Ring". At higher voltages such as in EHV transmission networks corona discharge effect is prominent. Corona Ring distributes the electrical potential gradient evenly and maintains the maximum value below the threshold limits of the corona effect. Corona Rings are employed at high altitude electrical equipment where density of the air is less.

Gas Insulated Substations Advantages Disadvantages Gas Insulated Substations (GIS): Gas Insulated Substations (GIS) are differ from Air Insulated Substations (substations we usually see) such that all the substation equipment such as busbars, circuit breakers, current transformer, potential transformers and other substation equipment are placed inside SF6 modules filled with SF6 gas. SF6 gas which has high dielectric property acts as insulating medium to the High Voltage or Extra High Voltage substation equipment. Generally Gas Insulted Substations (GIS) are indoor type as it requires 1/10th of the space required

for conventional substations. Some of the advantages and disadvantages of Gas Insulated Substations (GIS) are:

   

   

Advantages of GIS Substation: It occupies very less space (1/10th) compared to ordinary substations. Hence these Gas Insulated Substations (GIS) are most preferred where area for substation is small (eg: Cities) Most reliable compared to Air Insulated Substations, number of outages due to the fault is less Maintenance Free Can be assembled at the shop and modules can be commissioned in the plant easily

Disadvantages of GIS Substation: Cost is higher compared to Ordinary Conventional Substations Care should be taken that no dust particles enter into the live compartments which results in flash overs When fault occurs internally,diagnosis of the fault and rectifying this takes very long time (outage time is high) SF6 gas pressure must be monitored in each compartment, reduction in the pressure of the SF6 gas in any module results in flash overs and faults

Locations where Gas Insulated Substation is preferred Gas Insulated Substations are preferred in the places where land requirement for the substation is difficult such as in populated areas and highly polluted areas where outdoor switchyard is not preferred. In Gas Insulated Substations all the switching electrical equipment such as circuit breakers, isolators, earth switches, and busbars are completely enclosed inside modules which are filled with SF6 gas. These modules are factory made and site assembled. Hence takes very little time for commissioning compared to air insulated substations.

    

The main advantage of this Gas Insulated Substation is because of its compact size due to high dielectric strength of the SF6 gas. And the availability and reliability of the GIS substations are more compared to air insulated substations (Faults occurring chances of Air insulated substation or conventional substations are more. Hence GIS substations are provided where high reliability of electric power is required such as nuclear plants and and other important facilities where un interruption of power is more required. Some of the places where Gas insulated substations are preferred are: Large towns where space available is limited Industrial complexes where un interruption of power is necessarily Mountain regions and valleys Underground substations Off-shore (On sea or lake) substations



HVDC transmission system terminal substations In large cities area for commissioning an ordinary Air insulated substation is difficult because of the lack of space available. In such conditions GIS substations out rule the ordinary substation. However it is to be considered that GIS substation requires high capital cost compared to ordinary substation. Off shore and highly polluted areas where equipment present in the outdoor will be corroded due to the pollution and saline environment. Under such case indoor GIS substation is very much helpful.

Properties of Sulfur Hexafluoride (SF6) gas Sulfur hexafluoride gas (SF6) gas properties:        

  

Heavy, chemically inert, non toxic No poisonous effect on the human body but decomposition products are poisonous Color less and odor less It is gaseous at normal room temperature and pressure Density is about 6.6g/l at 20oC (5 times denser than air) Critical temperature is at 45.6oC and can be liquefied by compression Very good insulant with high dielectric strength SF6 gas is electo-negitive ( tends to attract the free electrons and has the arc quenching property). Because of this main reason SF6 gas is used for arc quenching and insulation medium in circuit breakers. The gas is highly stable Unlike solid insulation materials, electrical breakdown of sf6 gas does not result in permanent deterioration of its properties Decomposition occurs on the exposure to the electric arc. (Disassociation products will be SF2 and SF4 lower order fluorides)

Substation Interview Questions Answers What are different insulators employed in substations? Answer: Some of the insulators employed in substations are:       

Pin insulators Post insulators Strain insulators Suspension insulators Hallow apparatus insulators Dead-end insulators Solid core insulators What is creepage distance?

Answer: Creepage distance is the shortest distance between two conducting points along the surface of the insulating material.

What is Basic Impulse Level (BIL)? Answer: Basic Impulse Level (BIL) refer to the peak value of lightning impuse voltage withstand capability of the equipment

What are the disadvantages of corona? Answer: Some of the disadvantages of corona are:   

   

 

Corona causes power loss Corona causes unacceptable noise Corona cause radio interference

What is the significance of Corona rings in substations? Answer: It is observed that corona discharge occurs mostly at sharp corners, edges near the conductor fixing points. Corona rings also called grading rings are hallow metallic rings having large diameter and will have a smooth surface. These corona rings are placed suitably to metal clamps, joints to encircle the conductors surface. This have the advantage of shifting of high stress point shifted from conductor surface to corona ring surface. As the diameter and surface area of the ring is higher than that of the conductor. This helps in: Field distribution is made uniform Surface stress is reduced to much lower value Corona discharge is eliminated Dust deposition is also minimized What is the neutral point earthing and its advantages? Answer: In substation all the neutrals of rotating machines, transformers, busbars and other equipmentare connected to ground. Some of the advantages are: Earth fault protection is based on the method of neutral grounding The system voltage during earth faults depends on the neutral grounding. During line to ground faults, grounding of neutral helps to reduce the over-voltages which occur on the onehealthy phases Advantages of Outdoor Substation (AIS):



This type of substation arrangement is best suited for low voltage rating substations (step down substations) and for those substations where there is ample amount of space available for commissioning the equipment of the substation

    

The construction work required is comparatively less to indoor switch yard and the cost of switchgear installation is also low In future the extension of the substation installation is easier The time required for the erection of air insulated substation is less compared to indoor substation All the equipment in AIS switch yard is within view and therefore the fault location is easier and related repairing work is also easy There is practically no danger of the fault which appears at one point being propagated to another point for the substation installation because the equipment of the adjoining connections can be spaced liberally without any appreciable increase in the cost

Disadvantages of Air Insulated Substation (AIS):  



More space is required for outdoor substation when compared to indoor gas insulated substation (GIS) Outdoor switch yards are more vulnerable to faults as it is located in outside atmosphere which has some influence from pollution, saline environment and other environmental factors. Deposition of saline particles on insulators can cause insulator failures. They are also vulnerable to direct lightning strikes and other external events such as heavy winds, rains and cyclones. Therefore reliability wise air insulated substation or outdoor substations are relatively low compared to indoor substation Regular maintenance is required compared to indoor substations (Maintenance for Gas Insulated Substation is very minimal and reliability is very high) as they are exposed to outside environment

Power System Earthing Updated on January 2, 2014

It is necessary to earth a power system at a suitable point by a suitable method as it offers many advantages as 1.

It provide Safety to the electrical equipments against over-current

2.

It provides better safety

3.

It reduces the maintenance expenditure

4.

It improves the service reliabilty

5.

It provides improved lightning protection

Solid Earthing When the neutral of the power transformer and generator is directly connected to the earth, then the system is said to be solidly earthed. The solidly earthing does not make a zero impedance circuit as generator or transformer would have its own reactance in series with the neutral circuit. The direct earthing of a generator without external impedance causes earth fault current from the generator to exceed the maximum 3-phase fault current if the impedance of the generator is too low. This results in Stator winding damage as the short circuit current during fault will exceed the short circuit rating of the winding for which it was designed. For this system of earthing, it is necessary that the earth fault current shall be in the range of 25% to 100% of the 3-phase fault current to prevent the development of high transit over voltages.

Resistance Earthing In resistance earthing the neutral of the generator or transformer is connected to the earth trough a resistance in series. Advantage Of Resistance Earthing are: 1.

It reduces the line voltage drop caused when earth fault occurs

2.

It reduces electric shock hazards to the persons, caused by stray earth fault currents in the return path

3.

It reduces the mechanical stresses in the circuit carrying fault current

4.

It reduces the effect of burning of faulted electrical equipment

The magnitude of the resistance to be used should be such that it should limit the earth fault current to a value which will reduce minimum damage at the point of the fault.

Type Of Earth Resistance Methods

Reactance Earthing: In reactance earthing a reactor is connected in between the neutral of the machine and earth. A low reactance is connected in series with the neutral of the machine to limit the earth fault current through the generator. This current should not be greater than the 3-phase fault current of the generator. The earth fault current of the earthed system should not be less than 25% of the 3-phase fault current in order to minimize the transient voltages

The Non linear lightning arrester basically consists of set of spark gaps in series with the silicon carbide non linear resistor elements. Lightning arresters are connected between the phase conductors and ground. During normal system operating voltage conditions, the spark gaps are non conducting and isolate the high tension (HT) conductors from the ground. However whenever an overvoltge of magnitude dangerous to the insulation of the apparatus protected occurs ( these over voltages or over surges may be caused due to lightning strikes on the conductors or due to Extra High Voltage (EHV) switching) the spark gap breaks down and allows the high voltage surge current to flow through the ground. Working Principle of Silicon Carbide (SIC) Lightning Arresters: The volt-ampere characteristics of the non linear resistor in the lignting arrester can be approximately described by expression V = KI β. Where K and β are dependent on the composition and manufacturing process of the Non linear Resistor (NLR). The value of β lies generally in the range of 0.3 and 0.45 for modern silicon carbide (SIC) lightning arresters. If the voltage across the Non Linear Resistor (NLR) doubles, the current would increase approximately by 10 times. Therefore, with multiple spark gaps arresters can withstand high Rate of Recovery Voltage (RRRV). The non-uniform voltage distribution between the gaps (which are in series in lightning arresters) presents a problem. To overcome this, capacitors and nonlinear resistors are connected in parallel across each gap. In case of lightning arresters employed for high voltage applications, capacitors and nonlinear resistors are connected across the stock of gaps and NLRs. With the steep voltage wave surge the voltage is mainly controlled by the capacitor and at the power frequency by the nonlinear resistors. It is obvious that when the over voltages cause the break down of the series gaps, the current would be very high so as to make the voltage to subside very fast. The highest voltage that appear across the lightning arrester would be either the spark over voltage of the arrester or the voltage developed across the non-linear resistor during the flow of surge current. The lowest spark over voltage of the arrester is called the hundred percent impulse spark over voltage of the arrester. The voltage developed across the non-linear resistor during the flow of surge current is called residual voltage. The lower the value of the voltage developed the better the protection of the lightning arrester. Disadvantages of Silicon Carbide (SIC) Arresters:

  



Some of the disadvantages of silicon carbide arresters compared to gapless arresters are given below: Silicon Carbide (SIC) arresters have inferior V-I Characteristics compared to Zno arresters (Metal oxide arresters). Decrease in energy absorption (surge wave) capability compared to Zno arresters. Probability of sparking between the gaps. Advantages of Silicon Carbide (SIC) Arrester : Due to the presence of gaps the normal power frequency voltage during normal operation isnegligibly less compared to gap less arresters. Hence no leakage current flow between theline and earth in SIC arresters

Inrush Currents in Transformers - Causes When a transformer is energized after a short interruption, the transformer may draw high inrush currents from the system due to core magnetization being out of synch with the voltage. The inrush currents will be as high as short circuit currents in the transformer (almost 20 to 40 times the rated normal full load current of transformer). Inrush currents may cause fuse, relays or reclosers to falsely operate. It may also falsely operate the faulted circuit indicators or cause sectionalizers to mis-operate When the transformer is switched in, if the system voltage and transformer core magnetization are not in synch, a magnetic transient may occurs. This transient may drive the core into saturation and drives a large amount current into the transformer causing transformer core to damage

Factors Significantly Impact Inrush Currents in Transformer: 

  

A transformer that is designed to operate lower on the saturation curve draws less inrush currents as there is more margin between the saturation point and the normal operating. The extra flux during switching is less likely to push the core into saturation Large transformers draw more inrush current. Large transformers will have smaller saturation impedance Higher source impedance relative to the transformer size limits the currents that the transformer can pull from the system

the point where the circuit breaker close (position of flux wave in sine wave). The worst case will be when the flux is at maximum (peak) and voltage is minimum (in transformer the applied voltage lag behinds the flux by 90 deg). Other factors have little significance. The load on the transformer does not significantly change the inrush currents. While switching transients cause high inrush, other voltage transients especially voltage transients with dc components can saturate the core of the transformer and cause inrush currents

 



When the nearby fault was cleared and transformer voltage is recovering from the voltage sag, the sudden rise in voltage can drive the transformer to saturation Energizing a transformer can cause the nearby transformer to also draw inrush currents. The inrush currents into the switched transformer has a significant dc component that can cause the voltage drop. The dc component can push the other transformer into saturation and draws inrush A lightning flash near the transformer can drive the transformer core to saturation THREE WINDING TRANSFORMER: Generally in power system mostly two winding transformers are employed. But three winding transformers are employed because of some advantages:

  



The most common reason for having a three winding transformer is to provide a delta connection tertiary winding To limit the fault level on the low voltage system of the transformer by dividing the LV infeed (in order to provide double secondary windings) Providing tertiary winding helps to interconnect different power system operating at different voltages (Three winding transformer helps provide power supply at two different secondary voltages, 220kV/11kV/6.6kV transformer can able to provide power at two different voltage levels (11kV and 6.6kV) To regulate the voltage and reactive power of the system by providing synchronous capacitor connected to one of the terminals of the transformer

Why Delta winding prefer: It is always desirable to have one delta connection winding in the three phase transformer as delta connected three phase winding will offer low impedance path for the three phase currents. Also the presence of delta connected three phase winding allows to circulate the current around the delta winding in the event of unbalance loading condition Although power system designers aims to avoid use of star/star transformer in power system but cases will arise when the phase shift between the star/delta and delta/star is not applicable such as in the power station supplying power to auxiliary system. Therefore it is common practice to have a star/star with delta tertiary three winding transformer supplying power to the plant auxiliary system B/H Curve of the magnetic material (core of the transformer) is not linear. Is a sinusoidal voltage (flux) is applied across the primary winding, the magnetizing current obtained will not be sinusoidal in nature and consists of fundamental component and several harmonics. Third harmonic components predominate with several other higher harmonic components. If there is no delta connected winding, or if the star connections of the transformers are not grounded, the line to earth capacitance currents supply system lines supply the harmonic components. If the harmonic components cannot flow in any one of these paths then, secondary voltage will be distorted. 

Noise In Transformer: The basic cause for the noise in the transformer is due to magnetostriction of the sheets in the magnetic circuit (core of the transformer). Variations in the magnetic







 



 

 



induction subjected to the sheets to periodic variations in the length, the amplitude which is in the order of microns per meter length. The fundamental frequency with which these vibrations occur is double that of the system frequency, (for 60Hz frequency vibration frequency will be of the order of 120Hz) and also constitute numerous harmonics. Also various parts of the transformer, starting with the magnetic circuit (core) are liable to vibrate due to magnetostricition effect. The noise generated due to magnetostricition effect transmitted from the magnetic circuit to the tank of the transformer either through direct conduction to supporting points or through the oil and insulating material used in transformer. The transformer tank and the radiator radiate the acoustic noise or energy in to the ambient atmosphere Another source is due to the vibration of magnetic sheets perpendicular to the surface either at the edge or at the core packets, or at the joints between the leg and the yoke The current carrying windings is also a source of noise, however the amplitude of the noise is very less and is not detectable. Cooling fans and pumps employed for cooling the transformer is also acts as source of noise Methods to reduce Transformer noise: The main source of noise in the transformer is due to magnetostriction effect of the magnetic circuit or core. In order to reduce the noise cold rolled grain-oriented plant, with low magnetostriction and improved flatness is employed Ensuring uniform flux distribution and reduction in the cross flux also reduces the noise Elimination of the clamp bolt holes, use of resin impregnated glass-fibre bands instead of core bolts, gluing of core packets can reduce the noise These above specified remedial methods not only reduces the noise level by 5 to 10 dB, but also reduces the losses and no-load current Faults in Transformers: Some of the faults in the transformers are likely to be over-voltages which resulting from the atmospheric phenomenon (lightning) transmitted by overhead lines. Switching in the power system (especially high voltage switching more than 400kV) can produce over-voltages of less steep but longer duration surges stressing both liquid and solid dielectrics (insulation). These over-voltages should be restricted in amplitude to a value below the transformer insulation breakdown withstanding level. Short circuits in the power system subject the transformer to currents of 10 to 20 times the rated currents (short circuit current level will be severe when fault occur close to transformer). Power transformer is generally designed to withstand tens of short circuits, lasting not more than 2 sec duration in its life time. If there are more short circuits than the designed limit special construction is required. Short circuits should

eliminated (by isolating faulty power system by opening circuit breakers) as quickly as possible to limit the short circuit intensity on transformer. 

  

 

Overloads can arise in transformers from planned or fortuitous (unexpected) circumstances. In the first case, temperature increase in transformer insulating material should not exceed the standard value. In the second case, certain time limit can be tolerated but this will have certain cost in reduction in the life of the transformer. Internal Faults in Transformer: Electrodynamic faults: which occurs between insulation and current carrying conductors, HV and LV winding due to external and internal short circuits Electromagnetic faults: Which occur due to eddy currents induced in the magnetic circuits or the clamping structure. Electrical faults which occurs due to bad contacts in the leads or bad contacts in the tap changer Dielectric faults: Which occur due to shorting between windings or between live parts and earth, partial discharges Thermal faults: Which occurs due to abnormal temperature rise, hot spot, thermal ageing or pollution in transformer oil Mechanical faults: Which occur due to vibrations, leakages or defective operation of the tap changers Different types of defects originated in transformers will have degree of gravity depending on the amount of damage it can do on transformers and their consequences. Some of the defects (vibrations, partial discharges) will not immediately endanger the equipment but care must be taken before causing major damage. On the other hand, defects such as (over-voltages and short circuits, and initial breakdown) requires immediate attention Ferroresonance: Ferroresonance phenomenon can be triggered off through interaction of the system capacitance with a non-linear inductance, when transformer is at no-load. This oscillatory phenomenon or resonance phenomenon result in over-voltages which can damage the cable and the power transformer. Series ferroresonance is dangerous to power transformer

Ferroresonance can occur when: 

 

The power transformer is on no-load (even an active load which can be less than one tenth of the rated power can be sufficient to prevent the phenomenon of ferroresonance to occur by absorbing the energy). A large capacitance exists in line or the cable connected to at least one terminal of the transformer while the other terminals remain at normal potential An unbalanced situation is created by a single phase trip out, a break in the conductor or blowing of one or two fuses

Methods to Prevent Ferroresoance:  

Ferroresonance can be reduced by reducing the length between the transformer terminals and the section circuit-breaker or fuses Avoid non simultaneous switching of power transformer



Earth the neutral point directly or through a resistor limiting the single phase fault current

Auto-transformer Advantages:  



 

Auto-transformers in comparison with the double winding transformers are generally advantageous of the voltage ratio is favorable from the point of view of the equivalent size The reduction in the equivalent power in relation to the throughput, reduction in the weight and size, reduction in the no load (iron losses) and on load losses (cooper losses), the reduction in the no load current and the short circuit impedance If P is the power of the transformer, linear dimensions of the auto transformer vary as P0.25and the weight and volume vary as P0.75 thus reducing the losses and weight of the transformer The short circuit impedance of the auto-transformer goes down with equivalent rating which is advantageous because of the lower voltage drop and better regulation. For connections between the two systems at very high voltage, where the power is to be transmitted in the order of GVA (Giga Volt-amperes), the use of auto transformer make it possible for high power equipment and can be easily transportable. This will not be possible with normal transformers

Auto-transformer Disadvantages: 





An electrical connection between the primary and secondary is not always desirable, particularly when the voltages levels on both the sides of transformer are quite different. If the neutral point is not solidly earthed, the lower voltage side can be subjected to high potential in the event of earth fault on high voltage side. In general practice, auto transformers are only used on distribution systems where the neutral is connected to the earth Auto-transformers are particularly sensitive to the atmospheric over-voltages. Therefore the auto-transformer requires much highs standard of insulation compared to normal transformer. Surge Arrester protection is required As the short circuit impendace values of the auto-transformer are low, short circuit currents can reach higher critical values.

Sources of Harmonics: Several sources of harmonic currents that may be found on electrical power distribution networks are listed below:        

Variable speed motor drives Rectifiers Arc furnaces Wielding equipment Uninterrupted power supplies Switching mode power supplies Compact fluorescent lamps Electronic ballasts

Disadvantages of Harmonics: Harmonics degrades the performance of power system. Some of the disadvantages of harmonics in the power distributed network are listed below:

       

The harmonics flowing in the distribution network downgrade the quality of the electrical power supply. There can have several negative effects on the operation of the power system Increased losses on the distribution system due to increase in the effective rms current Over-load in neutral conductors due to cumulative increase in the third harmonics created by the single phase loads Overloads, vibration and premature ageing of the generators, transformers and motors as well as increase in the noise level Overloads and premature ageing of the power factor correction capacitors Distortion of the supply voltage that can disturb the operation of the sensitive loads Disturbances in the communication networks and telephone lines Resonance between the supply inductance and capacitance of the power factor correction capacitors DRY TYPE TRANSFORMER: Dry type transformers are most suitable in locations where conventional oil filled transformers present a safety hazard. Some of the locations where dry type transformers are best suited are: Oil refineries, Chemical plants, Marine applications, Metro railways where there is significant safety is required against fire.

Advantages of Dry Type Transformers: 

Dry type transformers have no risk of fire



Dry type transformers are lighter in weight compared to oil filled transformers



It requires less floor area to accommodate



No toxicity unlike synthetic liquid filled transformers



Dry type transformers are have efficient noiseless operation



Reduced installation cost



They are Maintenance free

Type of dry type transformers: Different type of dry type transformers are: 

Cast resin type



Resin Impregnated type Cast resin type transformers are Class F glass fibre reinforced epoxy resin. It can withstand 155 oC continuously. The disadvantages of cast resin type transformers is tat in the event of failure, the complete unit including HV and LV winding is to be replaced, whereas, with resin impregnated transformer, only the affected winding is required to be replaced.

What is meant by Current Chopping? Answer: Current Chopping is a phenomenon of current interruption before the natural current zero is reached. Why Isolation switches are provided along with the circuit breakers?

Answer: Isolation switches are provided for separating the disconnected element from the live portion of the system for the purpose of safety. Why asymmetrical breaking current is higher than the symmetrical breaking current in a circuit breaker? Answer: Asymmetrical breaking current is the rms value of the combined sum of the dc and ac components of the short circuit current at the instant of separation of the breaker contacts while symmetrical breaking current is the rms value of the ac component alone. Thus asymmetrical breaking current is higher than the symmetrical breaking current. What is recovery voltage in circuit breaker? Answer: Recovery voltage in circuit breakers is defined as the power frequency rms voltage which appears across the breaker contacts after the transient oscillations die out and the final extinction of the arc has resulted in all the poles of the circuit breaker. It is the voltage developed after the arc extinction is competed When does an arc extincts? Answer: If the rate of rise of re-striking voltage (RRRV) is less than rate of rise of dielectric strength between the breaker contacts arc will not sustain and extincts What is re-striking voltage? Answer: The Re-striking voltage may be defined as the resultant transient voltage which appears across the breaker contacts at the instant of arc extinction For an Resistive Load and Inductive load considered for switching operation of circuit breaker, Switching of which load is difficult and Why? Answer: In case of switching resistive load, current and voltage will be in phase i.e,when during current zero voltage also will be zero in case of resistive load. Hence the voltage at the breaker terminals during arc extinction is zero or minimal. On the other hand, for switching inductive load during current zero voltage across the breaker will be maximum. This results in re-striking of arc once again. Therefore switching inductive load will be difficult Why Earth Wire is provided in the overhead transmisssion lines? Answer: Earth wire is provided above the overhead transmission lines for protection against lightning strokes. The ground wire or earth wire protects the phase conductors or line conductors by attracting itself the lightning strokes which in its absence would strike the phase conductors. Also the ground wire reduces the voltage electrostatically or electromagnetically induced in the conductors by the discharge of a neighboring cloud. It also provides the additional protective effect by causing the attenuation of traveling waves set in the lines by acting a short circuit secondary of the line current What is meant by protective angle? Give its value for reliable operation?

Answer: Protective angle is the vertical angle through the ground wire axis and the line passing from the ground wire axis to the outermost phase conductor. Its value for reliable protection is taken equal to 20-30 degrees What is the difference between the lightning arrester and surge absorber? Answer: Lightning arrester limits the duration and amplitude of the follow current while a surge absorber reduces the steepness of the wave front for a particular surge What is meant by Lightning arrester or surge diverter? Answer: A Surge Diverter is a device that is connected between line and earth, i.e, in parallel with the equipment under protection at the substation. It limits the duration and amplitude of the follow current.

What is meant by impulse ratio of any lightning arrester? Answer: Impulse ratio of any lightning arrester is the ratio of breakdown voltage of the wave of special duration to breakdown voltage of a 50Hz wave What is meant by voltage surge? Answer: Voltage surge is sudden excessive rise in voltage that may be dangerous to the electrical equipment of an installation. Voltage surge cause damage to insulation of the system. Voltage surges are caused because of lightning, switching on and off the load at high voltages How standard lightning impulse wave is represented? Answer: Standard Lighting impulse wave is represented by 1.2/50 micro sec wave

Transmission Lines Protection Questions Answers What is meant by Tine Graded Protection? Answer: Time Graded protection is a scheme of over current protection, in which the discrimination is incorporated i.e, the time setting of the relays is so graded that in the event of the fault, the smallest part of the system is disconnected

What are the main elements of current carrier protection? Answer: The main elements of the carrier current protection are: Transmitter, Receiver, Coupling equipment and Line trap

Why Merz-Price Protection is not suitable beyond 33kV? Answer: Difficulties are experienced in balancing the secondaries of the CTs and this is why Merz-Price protection is not employed beyond 33kV

What types of relays are suitable for the protection of radial feeders? Answer: Induction type IDMT relays are more suitable for protection of the radial feeders because the time current characteristics are similar in shape and in no case they cross each other at any time

What is the purpose of Line trap and Coupling capacitors in carrier channels? Answer: Line trap is an LC network inserted between the busbar and connection of coupling capacitors to the line and tuned to resonance at the high frequency and are used to confine the carrier currents to the protected section so as to avoid interference with or from adjacent carrier current channel. Coupling capacitors is used to connect the high frequency (carrier) equipment to one of the line conductors and simultaneously serves to isolate the carrier equipment from high power line voltage What is Unit Protection? Answer: Unit system of protection is one in which the protection responds to the faults in the protected zone alone and it does not respond to through faults (faults beyond the protected zone). None unit systems does not have zone boundry What is the minimum time interval set for two adjacent circuit breakers to operate in radial feeders? Answer: It will be usually around 0.4 sec For what type of faults does differential protection is provided? Answer: Differential protection responds to the phasor difference between two or more electrical quantities. It operates for the internal faults occur in generators or transformers. For external faults differential protection does not operate What is the disadvantage of ordinary differential protection? Answer: When differential relaying is used for protection, the CTs at both sides of the generator winding must be of equal accuracy. Otherwise if the CT errors are excessive it will cause the mal operation of the relay. To safeguard against such disadvantages percentage differential protection is employed.

  

What is advantage of using percentage differential relay protection? Answer: Advantages: It does not require CTs with air gaps or special balancing features It permits a low fault setting to be used and this ensures maximum protection of the windings It ensures complete stability under the most severe through fault conditions Explain Differential Protection? Answer: Differential protection is generally provided for the equipment or group of equipments which are to be protected against internal faults. They are the primary

protection systems (operates faster) for any internal faults occurring within the protection zone. Under healthy conditions the currents at both ends of the windings will be equal. EMFs induced in the secondaries of the CTs will be equal and so no current flows through the operating coil of the relay. When an earth fault or phase to phase fault occurs the condition no longer holds good and the differential current flows through the relay operating coils makes the relay to operate. Relay operates for the faults occurring within the zone of protection Where does Negative phase sequence relay is employed? Answer: Negative sequence relay is employed for the protection of generators and motors against unbalanced loading that may arise due to phase to phase faults What is the operation principle of differential relay? Answer: A differential relay operates when the phasor difference of two or more similar electrical quantities exceeds a pre-determined amount Why distance protection is preferred as primary protection compared to over current protection for transmission lines? Answer: Distance relay is superior to over current protection for the protection of transmission lines. Some the reasons are faster protection, simpler coordination, simpler application, permanent settings without need for readjustment, less effect of the amount of generation and fault levels, fault current magnitude, permits the high line loading Why biased differential protection is preferred over simple differential protection? Answer: Biased differential relay is preferred because its operation is not affected by the trouble arising out of the difference in the CTs ratios for high values of external short circuit currents Where Impedance relay, Reactance relay and Mho relays are employed? Answer: The Impedance relay is suitable for the phase faults relaying for the lines of moderate lengths Reactance type relays are employed for the ground faults while Mho type of relays are best suited for the long transmission lines and particularly where synchronizing power surge may occur What is percentage differential relay? Answer: It is a differential relay where the operating current required to trip can be expressed as a percentage of load current

   

What are the main functions of Differential Relays? Answer: Differential Relays must have the following features: High speed operation High sensitivity Adequate short circuit thermal rating Ability to operate operate with low values of voltage

 

Burden must not be excessive There should be no voltage and current creep What is meant by "Relay Settings"? Answer: Relay settings means actual value of the energizing or characteristic quantity at which the relay is designed to operate under given conditions Define Plug Setting Multiplier? Answer: Plug Setting Multiplier and is defined as the ratio of fault current in the relay coil to the pick up value Where is directional relay used? Answer: Directional relay are used when graded time overload protection is applied to ring mains and interconnected networks For what type of fault does Buchholz relay is employed? Answer: Buchholz relay provides protection only against transformer internal fault How definite time lag is achieved in attraction armature relays? Answer: The instantaneous type attraction armature can be made a definite time lag or inverse time lag by using a oil dash pot, an air escapement chamber a clock work mechanism or by placing a fuse in parallel wit it. ADVANTAGES OF STAR AND DELTA CONNECTED SYSTEM: In a 3-phase system the alternators or generators may be star connected or delta connected. Likewise 3-phase loads may be star connected or delta connected. Some of the advantages of star and delta cconnected systems are listed below

Star Connection: 







In a star connection, phase voltage Vph = VL/ (3)1/2. Since the induced emf in the primary winding of an alternator is directly proportional to the number of turns, a star connected alternator will require less number of turns than a delta connected alternator for the same voltage. For the same line voltage, a star connected alternator requires less insulation than a delta connected alternator. Due to the above reasons three phase alternators are generally star connected. In star connection, we get 3-phase and 4-wire system. This permits the use of two voltages (phase voltages as well as line voltages). Single phase loads can be connected between any one lie and neutral wire while the 3-phase loads can be put across the three lines. Such a flexibility is not available in delta connection In star connection, the neutral point can be earthed. Such a measure offers many advantages. For example, in case of line to earth fault (L-G fault), the insulators have to bear 1/31/2 (57.7%) times the line voltage. Earthing of neutral also permits the use of protective devices (relays) to protect the system in the case of ground faults

Delta Connection:  



This type of connection is most suitable for rotatory conveyers Most of the three phase loads are delta connected than star connected. One reason for this, atleast for the case of unbalanced load, is the flexibility with which loads may be added or removed on a single phase. This is difficult to do with star connected 3-wire load Most of the 3-phase induction motors are delta connected

Why Sine Waveform Chosen for Alternating Voltage and Current Commercial alternators produce sinusoidal alternating voltage. A sinusoidal alternating voltage is produced by rotating a coil with a constant angular velocity in a uniform magnetic field. Sinusoidal voltage always produce sinusoidal current unless the circuit is non linear. Why Sine Wave is chosen rather than a simple curve such as a square or triangular wave. Some of the reasons are given below: 

In alternating current (a.c) machines such as induction machines, synchronous machines, transformers.., sinusoidal voltages and currents respectively produce the least iron and copper losses for a given output. The efficiency of the machine therefore is better

 



Sinusoidal voltages and currents produce less interference (noise) on telephone lines The sine waveform produces the least disturbance in the electrical circuit and is smoothest and efficient waveform Due to above advantages, electrical supply is generate sinusoidal alternating voltage and currents.

Hysteresis Loss: When a magnetic material is subjected to a cycle of magnetization (magnetized first in one direction and later magnetized in opposite direction in a cyclic manner), an energy loss takes place. This energy loss is due to molecular friction in the material. That is, the domains (or molecular magnets) of the material being turned first in one direction and then the other. Energy is thus expended in the material in overcoming this opposition. This loss is in the form of heat and is called hysteresis loss. Hysteresis loss is present in all electrical machines whose iron parts are subjected to cycle of magnetization like transformers, induction motors and other machines operated on ac supply. When an alternating supply is provided to the electrical machines the flux in the iron of these machines change in both direction and value alternatively. During this process energy is lost and this loss constitutes the core loss of the machine. The obvious effect of hysteresis loss is the rise of temperature of the machine Transformers and most electric machines operate on alternating current. In such devices, the flux in the iron changes continuously, both in magnitude and in direction. Hence hysteresis loss occurs in these machines



Hysteresis loss also occurs when iron parts rotates in constant magnetic field egdc machines Hysteresis Loop or B-H Loop area: Hysteresis loop is obtained by plotting B-H (B: flux density, H:magnetizing force) of iron for one cycle of magnetization.

When the core of the machine such as transformer, induction motors is magnetized with the magnetizing force (H) by applying voltage, magnetic flux density (B) increases as shown in the figure (dotted lines) and saturates (first quadrant). Saturation region in one in which increase in the magnetization force (H) will not further appreciably increase the magnetic flux density (B) in the material. When the applied voltage reaches back to the zero position from the peak (first half cycle), magnetizing force will also becomes zero. However the magnetic flux density in the core will not reach zero but have some finite value. This property of magnetic material to withhold some flux (B) when magnetizing strength becomes zero is called Retentivity. During the next half cycle all the domains in the magnetic core tries to align in opposite direction as the applied voltage polarity changes and hence the magnetizing force applied. With increase in the magnetizing force (H) a point is reached where the magnetic flux density(B) becomes zero. This point is called coercivity. Beyond this point, magnetic flux will be in opposite direction with increase in field strength and reaches saturation as shown in figure (third quadrant). Further applied magnetic field will starts decreasing from peak and reaches zero. This decrease in the magnetic flux with decrease in field strength can be seen in figure (third quadrant) and reaches back when the voltage polarity changes. This forms the B-H loop of hysteresis loop of the magnetic material of the machines



Importance or Significance of B-H Loop: The shape and size of the hysteresis loop largely depends on the nature of the magnetic material. The choice of a magnetic material required for a particular application often depends on the shape and size of the hysteresis loop. The smaller the hysteresis loop area of a magnetic material, the less is the hysteresis loss. For example, the hysteresis loop area for silicon steel has very small,



for this reason silicon steel is widely used for manufacturing of transformer cores and rotating machines which are subjected to rapid reversals of magnetism The hysteresis loop for Hard Steel (large hysteresis loop area) indicates that the material has high retentivity and coercivity. Therefore hard steel is quite useful in making permanent magnets. But due to large area hysteresis loss is quite high. This is the reason hard steel is not used for construction of electrical machines Methods to reduce Hysteresis Loop or B-H Loop: Hysteresis loop can by using soft magnetic materials (like CRGO core magnetic material in transformers) having smaller loop to reduce the hysteresis loss. DIFFERENCE BETWEEN LAN, WAN AND MAN NETWORKS:

A Computer network that spans relatively small area is called Local Area Network (LAN). Most LANs are confined to small building or group of buildings. However one LAN can be connected to other LANs over any distance via telephone lines and radio waves. A system of LANs can be cconnected in this way is called Wide Area Network (WAN) Most LANs can connect workstations and personal computers. Each node (individual computer) in a LAN has its own CPU with which it executes programs, but it is also can able to access data and devices anywhere in the LAN. This means that many users can able to share expensive devices, such as laser printers, as well as data. Users can also use the LAN to communicate with each other, by sending e-mails or engaging in chat sessions or can able to play games together Metropolitan Area Network or MANs are large computer networks usually spanning a campus or a city. They usually use wireless infrastructure or optical fiber communications to their sites For instance a university or college may have MAN that joins together many of the local area networks (LANs) situated around site of a fraction of square kilometer. Then from their MAN they could have several Wide Area Network (WAN) links to other universities or the internet. Specially this type of MAN is known as campus area networks 1. What is the main reason for providing metallic sheath in underground cables? Answer: The metallic sheath is provided around the insulation to protect it against the ingress of moisture, gas and other damaging liquids (acid or alkalies) from the soil and atmosphere 2. What are the main requirements of the insulation materials used for underground cables? Answer: Some the properties that insulation material require to possess are high insulation resistivity, high dielectric strength, good mechanical properties, nonhygroscopic, non inflammable, immune to attacks by acids and alkalies 3. What are the advantages and disadvantages of oil filled cables? Answer: Some of the advantages and disadvantages of oil filled cables are listed below: Advantages:

       

    

        

Smaller overall size and smaller weight for given voltage and kVA rating due to reduction in the thickness of the dielectric required No ionization, oxidation and formation of voids More perfect impregnation Smaller thermal resistance due to decrease in the dielectric thickness, so higher current rating More maximum permissible stresses Fault identification is easy Disadvantages: Greater cost Complicated laying of cables and maintenance 4. Advantages of XLPE cables compared to PVC cables? Answer: XLPE cable can withstand higher temperature (90 0C) compared to PVC (700C) XLPE cables can have higher over-load capacity XLPE cables are lighter in weight and smaller bend radius hence lower installation cost XLPE cable has high short circuit rating It has lower dielectric and constant power factor which can result in saving of power 5. What are different insulation materials used in cables for high voltage and medium voltage? Answer: Medium Voltage: Polyethylene (PE) Cross linked polyethylene (XLPE) Tree retardant cross linked polyethylene (TR-XLPE) Ethylene-Propylene Elastomers (EPR) PILC High Voltage: Cross linked polyethylene (XLPE) Paper/Oil Paper/Polypropylene SF6 gas NEUTRAL GROUNDING:



  



Generally on neutral grounding is provided at each voltage. There will be several voltage levels between the generation of the power and distribution of the power in the power system. Only one ground is provided for each voltage level of the power system Grounding of the power system is provided at the source and not at the load end Each of the major bus section in the system are grounded For generator grounding, neutral of the generator is grounding through a resistance which limits the stator fault current. The value of the resistor employed for the grounding the generator decides the percentage of the generator windings left unprotected Synchronous motors and synchronous capacitors are provided with reactance type of grounding. This reactance grounding provides additional reactance which provides additional lagging currents which nullifies the capacitive grounding currents  When several generators are connected to a common neutral bus, the bus is connected to the ground through a single grounding device. Disconnect switches are used to ground the desired generators to the neutral bus  When several generators are operating in parallel, only one generator neutral is earthed. This is to avoid the interference between the zero sequence currents  In generating stations there is a provision to ground neutral of at least two generators, though one at a time. The other generator neutral is grounded when the first generator is out of service  When there are one of the two supply sources, no switching equipment is used in the grounding circuit.  For the protection purpose, the neutral point of the star side of the power transformer is usually grounded  The star connected secondary sides of the protective CTs and PTs are grounded at one point. This ensures stable neutral, proper measurement of the voltages and currents, kWh and kVA on the secondary side measuring instruments and controls  For the circuits between 3 kV and 33 kV resistance or reactance grounding is used. But for low voltages less than 600V and high voltages above 33 kV solid or effective grounding is used. Effective grounding limits the voltages of healthy phases to line-to-neutral values in the events of ground faults and also eliminates the arcing grounds. The effective grounding causes the ground fault currents of very high magnitudes flow through the machine. But modern day protection systems are very sensitive and fast operating so that faults are cleared in very short time What is a Circuit Breaker? Answer: Circuit Breaker is a mechanical device designed to close or open contact members, thus closing or opening an electrical circuit under the normal or abnormal conditions How does a Circuit Breaker different from Switch?

Answer: Switch is just a device when can be able to open and close the circuit during normal operation. Whereas on the other hand circuit breaker has the ability to open and close the contacts during abnormal or fault conditions. Thus circuit breaker has the potential to break and make heavy short circuit currents. Auto-reclosures in the circuit beaker has the ability to re-close after certain designed duration to verify whether the short circuit was cleared What is meant by making capacity of the circuit breaker? Answer: The making capacity of the circuit breaker when closed on a short circuit is the peak value of the maximum current wave (including dc component) in the first cycle of the current after the circuit is closed by the circuit breaker Why current chopping is not common in oil circuit breakers? Answer: Current chopping is not common in oil circuit breakers because in most of the oil circuit breakers the arc extinguishing power is proportional to the magnitude of current to be interrupted Name the materials used for the contacts of vacuum circuit breakers? Answer: Copper-Bismuth, Copper-lead, Copper-tellurium, Silver-bismuth, Silver-lead and Silver-tellurium are some of the alloys employed as contact materials in the vacuum circuit breakers Why current chopping considered as serious drawback in a circuit breaker? Answer: The current chopping is considered as serious drawback because it sets up high voltage transient across the breaker contacts. For EHV applications which type of circuit breaker is mostly preferred? Answer: For Exta High Voltage (EHV) applications SF6 gas insulated switchgear is preferred

     

What are the duties of Circuit Breakers? Answer: Some of the duties of the circuit breakers are listed below: Interruption of small inductive currents Switching of unloaded transmission lines and unloaded cables Switching of capacitor banks and reactors Interruption of terminal faults Interruption of short line faults Asynchronous switching

Circuit Breaker Making Capacity During short circuit, circuit breaker contacts opens and automatically closes after few cycles to determine whether the fault is cleared. 90% of the faults occur in power system are transient in nature and lasts for few cycles and clears on its own eg: touching of transmission lines with each other beacause of wind, touching of tree branches to transmission line momentarily, etc. Such faults clears on its own and after the transient power

system restores to normal operation. It is not advised to open the breaker contacts and isolate the line for such small transients. Therefore auto re-closers are employed which when relay senses the fault trips the circuit breaker contacts. After some time breaker contacts again try to close, if there is no fault in that instant power system restores to normal operation. However if fault further exists circuit breaker contacts open up permanently until it is reset. The cycles of closing and opening is designed for the circuit breaker depends on the application. Therefore circuit breaker should close under short circuit condition. The MAKING CAPACITY of the circuit breaker depends on the ability to withstand the effects of electromagnetic forces which which are proportional to the square of the peak value of the making current. Making current of circuit breaker when closed on short circuit is the peak value of maximum current wave (including dc component) in the first cycle of the current after the circuit is closed by the breaker For determing the making current of the circuit breaker we must multiply symmetrical breaking current by (2)1/2 to convert to peak value from rms value. and then by 1.8 to take into account the doubling effect of maximum asymmetry. Rated Making Current = 1.8 x (2)1/2 rated short circuit breaking current = 2.55 x rates short circuit breaking current = 2.55 x symmetrical breaking capacity

Auto-Recloser Circuit Breaker in Power System Most of the faults on over head lines are transient in nature. About 85% to 90% of faults are momentary and caused by tree branches, lightning, birds etc. These conditions results in arcing faults which lasts for very small duration and clears after that moment. The arc generated can be extinguished and the line can be re energized. For this momentary faults which recovers on its own normal circuit breaker operation of opening the faulty part is not advisable. Some provision should be permitted in circuit breakers to close the breaker contacts if the fault is cleared momentarily. This fact is employed as a basis for auto- reclosures. In this scheme after the relays of both ends have picked up, the circuit breakers are tripped as for as possible at the same time and reclosed after time has be allowed for deionization. The fault disappears if it is transient and line is restored to normal service after the reclosure. If the fault is not cleared after the first reclosure a double or triple attempt of separation and reclosure is made. If the fault still persists, the breaker may permanently open till it is manually reset. Auto reclosures may be single or three phase type. Mostly single phase auto reclosing breakers are preferred as most of the transmission faults are single phase to ground faults. Auto reclosures of single pole type improves the stability of the system as power remains transmitted through the remaining two healthy phases when fault on one phase occurs. The breakers may be rapid auto reclosing type (about 20 cycles or 0.4 sec) or delayed auto reclosing (5 to 30s) type. For rapid reclosing type it is not required to check synchronism while reclosing however for delay reclosing synchronism should be checked before reclosing. For this purpose synchronous relays are employed

The current limiting reactors are used to perform the following functions: Functions of Reactors: 

Protective reactors are used to reduce the flow of short circuit so as to protect the apparatus from excessive mechanical stresses and from the overheating and thus protect the system as whole



Protective reactors are used to reduce the magnitude of the voltage disturbances caused by the short circuits



Reactors also localize the fault by limiting the current that flows into the fault from other healthy feeders or parts of the system, thereby avoiding the fault from spreading. This increases the chances of continuity of the supply



Reactors reduce the duty imposed on the switching equipment during the short circuits to be within economical ratings. So they are used (1) In the systems where extensions have been made and the circuit breaker rupturing capacities have become inadequate (2) In large systems, so as to limit the short circuit MVA to mach with the rupturing capacity of the circuit breakers In general reactors should be placed at the points in the network where they can be most effective. Very few occasions arise where it is necessary or desirable to introduce reactance in the generator circuits as modern alternators have sufficient inherent reactance to enable them to withstand the forces of the short circuit. However when older machines operate in parallel with the older machines, a case may arise where the added reactance in the circuits of the older machines will provide protection and give them a roughly the same characteristics as the new machines Reactors installed in the individual feeder circuits are not an economical proposition as often a considerable number of feeders are involved. Generally reactors are employed so that a group of feeders where the insertion of additional reactance is necessary to protect the group of circuit breakers of rupturing capacity. Similarly interconnection between the new and the old sections of the installation may profitably include a reactor and thus eliminate the need of replacing old circuit breakers BATTERY:

Valve Regulated Lead Acid (VRLA) Batteries are popularly termed as maintenance free lead acid batteries. Each cell in VRLA battery will have a value (automating operating) to allow the gas to escape if developed by the over charging the cells and to hinder the entrance of oxygen from the air into the cell These VRLA batteries are also called sealed lead acid batteries and also recombinant batteries. Oxygen which evolve during discharging at the positive plate of the battery recombine with the hydrogen which evolve at the negative plate of the cell to from water. This results in prevention of the water loss and unlike flooded batteries water is not needed to be added periodically. The valve present in VRLA cell acts as a safety valve which opens when the concentration of hydrogen gas inside the battery increases dangerously high. Some of the advantages of VRLA cell is listed below:

Advantages: 

VRLA battery is maintenance free and no periodic water refilling is not required



Reduced demand on ventilation



The capacity range of VRLA battery is high (12000 Ah)



VRLA batteries will have longer life. Life span of this batteries will be around 10 years

 

Feasibility for float charge operation with the same voltage of the cells, to continue in using existing charging equipment VRLA batteries are economical

Battery Interview Question and Answers What is primary cell and secondary cell? Answer: Battery which can be discharged only once is called primary cell ex: Manganese Zinc and alkaline Secondary cells are the battery which can be charged and discharged for a number of times ex: Lead acid and Nickel cadmium Do temperature have effect on battery? Answer: Temperature is one of the major factors that have impact on battery performance, shelf life, charging and battery voltage control. At higher temperature batteries will have more chemical activity compared to batteries which operate at the designed temperature



What factors that decide the VRLA battery to completely charged from fully discharged condition? Answer: Some of the important factors that determine the duration of the discharged battery to charge are: Duration of the discharge occurred



Temperature at which battery is operating



Size and efficiency of the charger Can VRLA batteries be enclosed in sealed containers? Answer VRLA batteries should not be enclosed in the sealed containers. Oxygen and hydrogen gases are produced in the battery, however they once again convert back to water. But during over charging these gases escape out of the cell. If the batteries are enclosed inside a sealed container, the pressure inside the container due to hydrogen and oxygen increases and may explode What is stratification in the battery? Answer: During charging of the battery acid is produced at the plate surface and during discharging of the battery acid will be consumed, due to this nonuniform concentration heavy parts of the electrolyte settles at the bottom and lighter parts of the electrolyte floats on the top. This phenomenon is call stratification. Stratification affects the life of the battery.



What is end of life of the battery? Answer: If a battery cannot able to hold proper charge or if the battery capacity is falls below 80% of its rated capacity then it is considered as the end of life of the battery. Many factors affect the life of the battery Charging and discharging cycles



Temperature



Leakage of the gases or electrolyte



corrosion of the plates



over discharge rates How positive and negative terminals of a battery are identified? Answer: Positive terminal is identified painted with red color and negative terminal is identified painted with blue color on the top of the battery

CURRENT & VOLTAGE TRANSFORMER:

Current Transformers (CT) and Potential Transformers (PT) are used to measure the current and voltage in a circuit of the order of hundreds of amperes and volts respectively. A CT has large number of turns on its secondary winding, but very few turns on its primary winding. The primary winding is connected in series with the load so that it carries full load current. A low voltage range ammeter (0-5A) is connected across the secondary winding terminals. Secondary of the CT is practically short circuited since the ammeter resistance is very low. It should be remembered that secondary of the CT should not be made open as it draws heavy current and damages the primary winding of the CT A PT has large number of turns in the primary and fewer turns in the secondary and hence it steps down the voltage. The primary winding is connected across the supply voltage and low range voltmeter (0-110V) is connected across the secondary winding terminals Some of the main difference between current transformers (CT) and potential transformers (PT) are given below:



The secondary of the CT is almost short circuit, whereas the secondary of the PT is practically a open circuit



The primary winding of the CT is connected in series with the load so that it carries the full line current, but there is only a small voltage across it. However the primary winding of the PT has the full supply voltage applied across it



In CT the excitation current I 0 and flux density vary over a wide range whereas in PT, they vary over a limited range only,

Advantages of three phase system:



Three phase system is widely used in generation, transmission and distribution. Single phase system is only used to operate small electrical appliances at consumer ends where the power rating is very less. Some of the advantages of three phase system or poly phase systems compared to single phase system is explained below Advantages of three phase system: The output of 3 phase machine is always greater than single phase machine of same size. The output will be approximately 1.5 times than single phase machine. So for given size and voltage 3 phase alternator or electrical machines occupy less space and less cost compared to single phase machine having same rating



For transmission of electrical power three phase supply requires less copper or less conducting material than that of single phase system for given volt-amperes and voltage ratings. Hence 3 phase system is more economical compared to single phase system



Single phase machines are not self starting machines. On the other hand three phase machines are self starting due to rotating magnetic field. Therefore in order to start a single phase machine an auxiliary device is required which not in the case of 3 phase machine.

 

Power factor of single phase machines is poor compared to three phase machines. In single phase system the instantaneous power is function of time. Hence fluctuates with respect to time. The fluctuating power will cause significant vibrations in the single phase machines. Hence performance of single phase machines is poor. While instantaneous symmetrical three phase system is always constant



Three phase system gives steady output



Single phase system can be obtained from three phase supply system, vice-versa is not possible



For converting systems like rectifiers, the dc voltage waveform becomes more smoother with the increase in the number of phases of the system. Hence three phase system is advantageous compared to single phase system

 

3 phase motors will have uniform torque whereas single phase motors will have pulsating torque Parallel operation of three phase generators will be simple compared to single phase generators because of pulsating reaction in single phase generator

Why rating of Synchronous Generators and Alternators in MVA or KVA Normally the rating of synchronous generators or alternators will be in KVA or MVA instead of kW rating. Electrical apparatus or machines are usually rated the load which it can carry without overheating and damaging to insulation. i.e, rating of the electrical machines are governed by the temperature rise caused by the internal loss of the machine. The copper loss in the armature depends on the strength of the armature current and core loss depends on voltage and these losses are independent of the power factor. The reason for which the transformers and synchronous generators are rated in voltamperes instead of watts is that manufacturer does not know at what power factor does these equipments are going to operate. On the other hand while manufacturing the motors manufacturer specifies the power factor at which the motor should be operate. This is the reason why the motor loads (including synchronous motors are rated in kw) are rated in wattage power. In the case of alternator and transformers manufacturer does not know the operating power factor. Hence they are rated in KVA or MVA Consider a generator whose output power is 1000kVA, for different power factors 0.2, 0.5 and 0.8 and unity respectively the output power in kW will be 200, 500, 800 and 1000kW.

Insulation Materials used in Transformers and Induction Motors Insulation materials used in transformers and motors are classified based on the temperature withstand capability. Some of the insulation classes used in transformers and induction motors are: Class Y, Class A, Class E, Class B, Class F, Class H and Class C. 

Class Y: Maximum hot spot temperature for this type of insulator is 90 oC

    

   

   

Some of the materials used are cotton, silk, paper, wood without impregnated Class A: Maximum hot spot temperature for this type of insulator is 105 oC Cotton, Silk, and Paper when suitably impregnated are the type of materials used Class E: Hot spot temperature is 120oC Materials possessing degree of thermal stability allowing them to be operated at a temperature 15oC above higher than Class A. Ex: wire, enamel etc Class B: Maximum hotspot temperature is 130oC Mica, Glass fibre, asbestos, etc with suitable impregnation or coating substance Class F: Hot spot temperature is 155oC Mica, glass, asbestos etc with suitable binding impregnation or coating substances capable of withstanding 25oC higher than Class B Class H: Maximum hotspot temperature of 180oC Combination of materials such as mica, glass fibre, asbestos suitably bonded Class C: Hotspot temperature above 180oC Mica, porcelain, glass, quartz and asbestos with and without inorganic binder

Parallel Operation of Alternators Requirements of parallel operation of alternators: The alternators which are to be operated in parallel should meet the following requirements so as to make the operation successful. Requirements are: 

Two alternators or synchronous generators which are to be operated in parallel should have same output voltage rating



The rated speed of the machines which are operating in parallel should be such as to give the same frequency (f is equal to (PxN)/120)



The alternators should be of the same type so as to generate voltages of the same waveform. Generators operating can have different power rating (KVA rating)



The prime movers of the alternators should have same speed load characteristics, the nature of characteristics should be drooping so that load on the generator is proportional to output rating of the alternators



Without reactancce the alternators will not develop generating power and parallel operation would not be possible. Hence the alternators should have reactances in armature circuits, otherwise they will not operate in parallel

Why Reactive Power is Generated at Load 

Reactive Power if possible should be generated close the load centers or close to the point of its consumption. Some of the reasons why reactive power cannot be transmitted to long distances are: It is inefficient during the high real power transfer and also requires higher voltage magnitude gradients.



It causes high real and reactive power losses if reactive power is transmitted through long distances. Real power losses should be kept minimum for economic point of view and reactive power should be reduced to minimize the investment in reactive power devices such as shunt capacitors.



It can lead to damaging temporary over voltages following load rejections. When the receiving end circuit breaker opens with transmission line still energized from sending end, then over voltages occurs as the voltage magnitude depends upon the reactive power in the system. Over voltage will damage the insulation of the conductor and wingdings.



If reactive power is to be transmitted through long distances in the power system It requires larger size of the equipment (transformers and cables) in the power system.

Harmonics effect on Transformer Harmonics distortion effect on Power System and their impact on Induction Motors are alreadydiscussed. Some of the disadvantages of harmonics and their impact on the transformers are: 

RMS current increase



Eddy Current loss increases



DC offset current saturation

RMS current Increase: Harmonics currents in the system will increase the RMS current in the transformers. This increase in the RMS current will increase the losses (copper losses) in the transformer which results in the reduction of the overall efficiency of the transformer

Eddy Current Loss Increase: Eddy currents loss is proportional to the square of the applied frequency. As the harmonic components will have the frequencies of the order of multiples of the fundamental frequency eddy current losses will increase with the presence of the harmonic components in the current and voltage waveforms. Therefore because of the increase in the eddy current loss overall efficiency of the transformer comes down. Harmonic component also increase the temperature of the windings.

DC offset current saturation: Due to the presence of dc component in the harmonics tend to saturate the core of the transformer.

Why Symmetrical Components Used for Unbalanced faults in System Symmetrical Components: Any unbalanced three phase system of currents, voltages or other sinusoidal quantities can be resolved in to three balanced of phasors which are known as symmetrical components of the original unbalanced system. This three balanced systems constitutes three sequence networks which are solved separately on single phase basis. The symmetrical components differ in phase sequence, i.e, the sequence in which the phasor quantities attains the maximum value. Any unbalanced system quantities (current, voltage, other sinusoidal quantities) can be represented in set of symmetrical quantities which constitutes positive sequence components, negative sequence components and zero sequence components.

Why Sequence Component: In symmetrical power system (it is the system in which the three phases are identical), phasor quantities such as current, voltage of different sequences does not react to each other. Current in the one sequence will induce the voltage drop in respective sequence only. Since in symmetrical system as there is no interaction between the quantities of different sequences, currents of any sequence is considered to flow in an independent network associated with that sequence only. In simple words we will get different sequence networks for positive, negative and zero sequence components, for example positive sequence current will flow in positive sequence network only. The single phase equivalent circuit of any component consists of impedances associated with that circuit and it is called sequence network for particular sequence component. This helps in analyzing the system performance when there is unbalanced fault, interaction between the quantities (voltage, current) of different sequences can be determined by interconnecting the sequence networks (positive, negative and zero) at the fault location. Main advantage of sequence components are isolating the quantities (voltage, current) in to components (positive, negative, zero) which helps in better criteria for controlling factors for different applications 

In Stability analysis investigations the synchronizing force between the machines is affected by the positive sequence components.



Heating effect due to unbalanced currents depend on the negative sequence components. (when heating is observed due to unbalanced currents that determines presence of negative sequence component currents flowing the system. By reducing the unbalanced currents heating effect and negative sequence components can be reduced.



When earth fault occurs zero sequence currents flows through the system. Hence all ground or earth fault relay operates on determining the zero sequence components.

Where Star (wye) Delta transformer is used in Power system Star-Delta Transformer (Y-Δ ): Primary Y (High Voltage side) and Secondary ( Low Voltage side) Δ is generally used at the generating side for stepping up the voltage levels. Power is generated at 11kV to 33kV having current ratings in thousands of kilo amps. This generation voltage has to be stepped up to higher voltage for transmission as stepping up the voltage results in the reduction of losses while transmission Generation side of the transformer (LV side) voltage applied is low and current flowing through the windings of the transformer is high. At secondary side (LV side), delta connection (Δ) is employed because in delta connection Iphase=ILine/√ 3 and Vphase=VLine. Current carrying capacity of windings depends on the thickness of the conductor. Large current flowing through the conductor means more copper utilised for the windings. Therefore in order to reduce the flow of huge currents by above equation delta winding is employed in the LV side of the transformer. On the other hand star type (Y) of connection is employed on the HV side of the transformer. In star connection Vphase=VLine/√ 3 and Iphase=ILine. Voltage withstand ability of conductor depends on the insulation strength of the conductor. Hence to reduce the phase voltage Star (Y) connection is employed in the high voltage (HV side) of the transformer.

Why Transformer makes "hum" noise



Transformer makes "hum" noise because of Magnetostriction effect. Magnetostriction effect is the property of ferromagnetic magnetic such that change in the dimension of material when the varying magnetic field is applied. In electrical transformer alternating voltage is applied to the core. This results in the varying magnetic filed direction in the core. Grain directions will be varied in every half cycle. Due to this varying magnetic field, dimensions of the material will be changed causing the vibrations. Some of the methods employed to reduce the noise are: Preventing vibration of core-plate, which use the lower flux density



Tightly bolting and clamping the laminations and structural parts of the transformer



Sound insulating the transformer from the tank by cushions, padding and oil barriers



Preventing transformer tank wall vibration by suitable design of tank and stiffeners



Sound insulating the tank from the ground or surrounding air

Why Silica gel is used in transformer When load on transformer increases current flowing the transformer windings increases. Heat generated by the rise in the current will be dissipated in to the transformer oil. Heated transformer oil becomes less denser

compared to the normal transformer oil and moves up in to the conservator. This heat will be dissipated in to the atmosphere from hot transformer oil. When load on transformer increases, transformer breather breathes air in. Air contains moisture when come in contact with the oil will affect the purity of the transformer oil. Moisture and impurity level in the transformer oil increases results in the damage of insulation property of the transformer oil. To avoid entry of moisture air in to the transformer silica gel crystals are used. Silica gel will absorb the moisture in the air and allows dry air in to the transformer. Silica gel will be blue when they are installed or dry. When the crystals absorb the moisture they turn to pink color. Thus silica gel protects the insulation strength of the transformer oil by absorbing the moist air entering in to the transformer.

Low Voltage winding near core of the Transformer For explaining the operation of the transformer Low voltage winding (LV winding) and High voltage winding (HV winding) are wound on different limbs of the transformer for easy explanation. However in practical each limbs of the core will have both LV winding and HV winding wound together. In Transformer design LV winding will be wound close to the core of the transformer because the insulation provided between the LV winding and core of the transformer is quite less compared to the insulation provided between the HV winding and the core of the transformer which results in less cost for insulation and reduction in the size of the transformer for same MVA rating. Hence due to this reason LV winding is placed near to the core of the transformer. On the core of the transformer insulation is placed and LV winding is wound. Once again insulation is placed between the LV winding and HV winding.

Current flowing in 2 adjacent conductors in same direction and opposite direction When current flows in the conductor it produces magnetic field. This is explained by Flemming's Right Hand Thumb Rule. When current flows in the direction of right hand thumb. Magnetic field will be produced around the conductor in the direction of other figures as shown in fugure

Consider two conductors carrying current in the same direction "+" sign represents current flowing into the plane and "." sign represents current flowing out of the plane

When current flowing in the same direction in the two adjacent conductors magnetic filed will be produced in such a manner that they attract each other On the other hand, when current flowing in opposite direction in two adjacent conductors magnetic field will establish in such a manner such that both the conductors repel each other

How Transformer Impedance is designed Certain considerations were determined while designing the impedance value of the power transformer. Impedance for the transformer is due to the inductance and resistance present in the transformer. During short circuit in the power systems short circuit current in the power system is limited by the presence of impedance drop due to transformers, transmission lines and cables. These impedance drops in the power system reduces the short circuit level. This the advantage of having high impedance values for the transformer.

However large transformer impedance cause large drop in the power system during the normal operation. This will cause a significant impact on the starting of the large motors (when large motors are started during low voltage condition other loads connected to the bus will trip due to huge drop in voltage at the bus) and voltage regulation of the power system Hence transformer impedance is designed for a optimum value considering the above conditions.

Difference between Solid Conductor and Stranded Conductor Solid Conductor: It consists of single piece of metal wire. It is cheap for manufacturing.Skin effect is higher in solid conductors as at higher frequencies current flow on the surface of the conductors results in the increase in the effective resistance. The main disadvantage of the solid wire is its more rigid property. It cannot be bent easily

Stranded Conductors: Stranded wire consists of sub conductors touch each other. It is costlier to manufacture compared to solid wire. For the given current carrying capacity the size of the stranded conductor is large compared to solid wire. Different elements of strands can be wound together to get the transmission line of desired property (eg: ACSR conductor contains Aluminum and Steel stands wound together). Proximity and skin effect is reduced using stranded conductors.

Bundled Conductors and Composite Conductors Difference Composite Conductors: In Composite conductors sub conductors touch each other. Composite conductors are typically stranded conductors. In Composite conductors different elements are used (In ACSR conductors aluminum has the properties of light weight, good conductivity and rutlessness and steel has the property of high tensile strength). Composite conductors are employed as they are flexible compared to solid conductor. Composite conductors reduce proximity effect and also reduces skin effect up to certain extent.

Bundled Conductors: Bundled conductors are employed in Extra High Voltage (EHV) transmission as at higher voltages corona effect is significant. In bundled conductors sub conductors are placed as certain distance throughout the transmission lines. This reduces the corona discharge loss and interference with the communication lines nearby.

Advantages of Bundled Conductors in Transmission Lines For transmission of more power for long distances to load centers Extra High Voltage (EHV) transmission is employed. Implementing Extra High Voltage has advantage of reduction in the copper losses and improves efficiency. However transmission of voltage beyond 300kV will poses some problems such as Corona effect which causes significant power loss and interference with communication circuits if round single conductor per phase is used. In order to reduce corona effect hollow round conductors are used. Keeping economical constraints other option is instead of using hallow round conductor it is preferable to use more than one conductor per phase which is called Bundled Conductors. For transmission of power beyond 400kV bundled conductors are employed.

Advantages of Bundled Conductors: Bundled conductors are primarily employed to reduce the corona loss and radio interference. However they have several advantages: 



Bundled conductors per phase reduces the voltage gradient in the vicinity of the line. Thus reduces the possibility of the corona discharge. (Corona effect will be observed when the air medium present between the phases charged up and start to ionize and acts as a conducting medium. This is avoided by employing bundled conductors) Improvement in the transmission efficiency as loss due to corona effect is countered.



Bundled conductor lines will have higher capacitance to neutral in comparison with single lines. Thus they will have higher charging currents which helps in improving the power factor.



Bundled conductor lines will have higher capacitance and lower inductance than ordinary lines they will have higher Surge Impedance Loading (Z=(L/C)1/2). Higher Surge Impedance Loading (SIL) will have higher maximum power transfer ability.



With increase in self GMD or GMR inductance per phase will be reduced compared to single conductor line. This results in lesser reactance per phase compared to ordinary single line. Hence lesser loss due to reactance drop.

Mobile Substations Advantages and Applications Mobile Substations are emergency power supply substations employed temporarily to provide power supply in the areas affected by the natural disasters such as storms, cyclones and during wars. These substations will have maximum rating of about 220kV. Mobile Substation typically contains Trailer for unit, Distribution Transformer, Cooling system for Transformers, Switchgear equipment (relays, circuit breakers, isolators etc..)

Advantages of Mobile Substation: 

Mobility in service



Quick access for service



Less Maintenance cost



Minimum Clearance for utilization



Increase in Reliability



Maximum KVA with minimum height.

Application of Mobile Substations:   

Mobile Substations are employed to provide emergency power supply during natural disasters and during wars It also used to supply power when ordinary conventional substation is under maintenance or fault It also employed for providing temporary load supply to industrial plants, remote locations.

2011 Interview Questions: These are some of the interview questions asked in BHEL 2011 Electrical Interviews. Questions are collected from the Net and some of my friend's interviews. Be prepare to answer some basic questions such as 

Tell me about yourself



What do you know about BHEL



Why do you want to join BHEL



Final Year Project



Some basic questions from Bio-data.

Electrical Questions: 1. What are the different conditions for parallel operation of Transformers? 2. What are V and inverted V curves in Synchronous motor and their significance? 3. Explain electrical corona? and methods to reduce the electrical corona? 4. Explain about P, PI and PID controller? Their advantages and disadvantages? 5. What are the different braking methods used in dc motors 6. What is water hammering phenomenon? 7. Advantages of DC motors over AC motors? 8. What is Load factor, Demand factor, Installed capacity, spinning reserve? 9. Which power plants can be used as a peak load plants. Reason?

10.What is String efficiency in insulators? 11. What is Transient stability and methods to improve transient stability? 12. What is single line diagram and the assumptions required? 13. What is Step Potential and Touch potential in substations? 14. What are different switching schemes employed in substations? 15. How Speed of DC shunt motor is controlled? 16. What is the difference between Microprocessor and Microcontroller? 17. What is SIL and its significance 18. Realize Ex-OR using NAND gates 19. What is Race around condition 20. Why single phase induction motor is not self starting and methods to start single phase induction motor?

What is a Signal Anything which carries information is a signal. A signal is a real (or complex) valued function of one or more real variable(s).When the function depends on a single variable, the signal is said to be one dimensional. A speech signal, daily maximum temperature, annual rainfall at a place, is all examples of a one dimensional signal. When the function depends on two or more variables, the signal is said to be multidimensional. An image is representing the two dimensional signal, vertical and horizontal coordinates representing the two dimensions A signal is a description of how one parameter is related to another parameter.

Types of Signal

For example, the most common type of signal in analog electronics is avoltage that varies with time. Since both parameters can assume a continuous range of values, we will call this a continuous signal. If this signal is passed through an analog-to-digital converter force each of the two parameters to be quantized. Signals formed from parameters that are quantized in this manner are said to be discrete signals. Discrete signals are quantized in one dimension ( as discrete signal is obtained by sampling of a continuous signal the signal obtained will be discrete with respect to time axis and amplitude of the signal is continuous . Digital signals are those which obtained by taking finite number of distinct values. Digital signals are discrete with respect to both time and amplitude. Digital signals are quantized in both dimension ( X-axis and Y-axis particular value exist; signal represented as just like a set of points in the plot depends on the sampling rate)

Why Transformer rating is in KVA not kW Rating of Electrical Transformer is in VA (Volt Amperes ) because: Maximum current carrying capacity of the transformer is determined by the thickness of the conductor and Maximum voltage supplied to the winding depends on the insulation strength of the conductor

Manufacturers of Transformers and alternators does not know at what power factor the consumer uses the machine and the nature of the load (capacitive, resistive and inductive) connected to the machine. Hence they rate the transformer depends on the maximum current carry and the voltage to be applied. This rating of the transformer corresponds to apparent power of the machine (VA). Also iron losses in transformer depends on the the voltage applied and copper losses depends on the current flowing through the winding. These losses are independent of the power factor at which the transformer operates. On the other hand induction motors are rated with real power (Watts) as the machine operating at defined power factor at full load is pre determined.

Eg: Consider a transformer can carry maximum current of 50A and max voltage applied is 200V. Then Power rating of the at full load (Unity power factor) = 50x200

= 10kW

Power rating of the at full load (0.5 power factor) = 50x200x0.5 = 5kW (This means load having 0.5 power factor can connect maximum of 5kW to the transformer. 5kW is the full load of the transformer)

Reactive Power and Active Power flow in Power System Active Power: The power that is generated in the power system which is utilised in useful work (running the motor, lighting and heating energy)

Reactive Power: Reactive power is the power which will not do any useful work in the power system but helps the active power to flow. Without Reactive Power active power will not able to flow in the system.

Different Power Flow In Power System: Active power in the power system depends on the power angle, means active power will flow in power system from bus having leading voltage power angle to lagging voltage power angle busbar. On the other hand, reactive power flow from the high voltage bus to low voltage bus in the power system, under this condition reactive power generated in the line (normally generated by alternator or shunt capacitance) is more compared to the reactive power absorbed (normally sink for reactive power will be series inductance) at the receiving end.



P (active power ) and delta (power angle) are closely coupled



Q (Reactive Power) and V (magnitude of the voltage) are closely coupled

Hence it is understood that for the flow of reactive power from one point to other in power system difference in the voltage should exist. Active power will flow in power system from leading power angle to lagging power angle even the voltage at receiving end is higher compared to sending end (still active power flows).

SCADA Systems Interview Questions and Answers SCADA Interview Questions: Different Levels of SCADA? Answer: Different levels are SCADA systems are:

1. 2. 3. 4. 5.

Field level devices and instruments eg: Sensors Remote Terminal Units (RTUs) Communication system present between the RTUs and Main Station Main Control Station or Master Station Computer systems pertained to processing the data for displaying to operator

Why SCADA systems are Implemented? Answer: Some of the reasons or advantages why SCADA systems are implemented are:

1. SCADA systems improves the performance of the operation of the plant 2. SCADA systems provides better protection to the equipment of the plant 3. SCADA systems improves productivity of the personnel 4. Information receives very fast, process the information and display it to opertor in graphs and plots. Hence helps the operator to take the decisions fast. 5. Provides better energy savings and saves economy.

What is the capacity of CPU of Remote Terminal Unit (RTU)? Answer: Remote Terminal Units (RTUs) contain generally microprocessor having 16 bit or 32 bit (eg: 68302 or 80386)

What is the Power Supply provided to the RTU? Answer: Remote Terminal Units (RTUs) are operated from 110V/240V + or - 10% and 50Hz or 12/24/48V + or - 10% DC Voltage

What are advantages of PLCs over RTUs? Answer: Advantages of Programmable Logic Controllers (PLCs) are they can be used for general purpose can easily set for verity of functions. Some of the advantages of PLCs over RTUs are: PLCs are reliable, cheap, flexible compared to RTUs, sophisticated control and troubleshooting in PLC is easier.

what is Progrmmable Logic Controllers (PLCs) Programmable Logic Controllers (PLCs): Programmable Logic Controllers (PLCs) are the micro processor controller devices situated at the remote sites of the plant or at the field of the process systems. PLCs purpose is to monitor the process parameters located at the field and adjust the outputs based on the inputs received by the PLCs. PLCs will operate on any systems which will have input field devices that are On-Off type (discrete or digital ) or analog input devices. Similarly it will operate on the field devices which will be discrete or analog output type. In simple words, Hence PLC acts as interface between the input and output devices at the process side of the industries. It monitors the inputs receive from the input devices and takes necessary output control functions by executing the programs stored in its memory. The term Logic is used in Programmable Logic Controllers because all the program inside the PLC will be of Logic programming (eg: Ladder diagrams ). In industrial applications hard wired relay logics were replaced by Programmable Logic Controllers due to its reliablity, simplicity, low cost, easier programming and many other functions

Transposition of the power lines Transposition: Transposition of the conductors in a transmission system means exchange in the position of the transmission line conductors at regular intervals in a transmission system along the line in a manner that each transmission line conductor occupies the position of every other transmission conductors over an same distance. In a power transmission system if the power lines are not equally spaced ( asymmetrically spaced ) even though the system operates under balanced condition, voltage drops of different magnitudes exist in the three conductors. This difference in voltage drops is because the unequal inductance of the three phases (mutual inductance of conductors are different in unsymmetrical spaced lines). Also due to asymmetrical spacing the magnetic fields external to the conductors is not zero. This causes the induced voltages in the adjacent electrical circuits particularly telephone lines that may result in Interference in the telephone circuits.

To reduce this effect to minimum the conductors are transposed. Transposition can be made either to the power conductors or telephone (communication ) lines.In a transposed line each of the three conductors occupies all the three positions relative to the other conductors for one third of the total length of the transmission line. The purpose of transposition is due to balance the capacitance of the line so that electrostatically induced voltages balances out in the transposed lines. Under balanced condition the conductors magnetic field linking an telephone line is shifted by 120 o out of phase. As the power lines are transposed, over the length of one complete transposition cycle ( three different

positions of the power conductor) of power line, the net voltage induced in the telephone line is zero. This is because the sum of three induced voltages which are displaced by 120 o in phase combines zero.

Transformer Questions and Answers 3 Why Low Voltage (LV) winding is placed near to the transformer core? Anwer: For a given conducting material insulation required depends on the voltage. Hence if High Voltage (HV) winding is placed near to the transformer core, more insulation is required to insulate between the transformer core and the High Voltage (HV) winding. This results in increase in the cost of the insulation material and also size of the transformer increases significantly. Thus Low Voltage (LV) winding is placed near the core which requires less insulation between the core and LV winding

What are the different insulation materials used in power transformers? Answer: In power transformers the primary insulation medium and cooling medium is transformer oil. It serves the purpose of both insulation and cooling. Apart from transformer oil different insulation materials employed are oil impregnated paper, press board, wood, mica, and asbestos. What is Transformer Breathing? Answer: When transformer under full load, power transformer oil present in the transformer heats up and gets expand. During this process gas at the top of the oil gets expel out in to the conservator present at the top of the power transformer along with hot oil and cool oil from conservator comes down. This process is called breathing out of the transformer. When the load on the power transformer is removed or during no load condition, transformer oil cools and air is drawn in to the transformer. This is called breathing in of the transformer. The incoming air may consists of moisture which should be removed else results in the deterioration of the dielectric strength of the transformer oil. Hence the air entering the power transformer is made to pass through the breather where moisture is removed from air through silica gel.

Transformer Testing methods Transformer Tests: Tests on the transformers are carried for verifying the capabilities of transformer to withstand Thermal stresses, Dielectric stresses, short circuit electro-dynamic stresses and environmental stresses.

Routine tests: These are carried out on every transformers before dispatch to ensure that it is in accordance with the specifications.Some of the tests come under routine test include:   

Measurement of the winding resistance Verifying the polarity of the windings Measurement of load losses and the impedance voltages

 

Measurement of no load losses and no load no load current. Electrical tests at the power frequency

Type Tests: Type tests are performed on the first transformer of one type and are intended to check the design characteristics. It is presumed that every transformer would also comply with the type test, since its design is identical. 

Temperature rise test



Tests of ability to withstand full wave impulse



Tests for switching impulse withstand



Noise level test

Special Test: Special tests are conducted in the presence of the purchaser or his representatives as specified in the tender 

Partial discharge test



Checking the level of Radio interference voltage



Vibration test



Test on ability to withstand Short -circuit current



Measurement of Noise level

Transformer parallel operation conditions Parallel operation of transformers is required in cases such as the power to be delivered is more than the individual transformer rating. In such cases operating two or more transformers to facilitate the power flow is possible, but certain conditions to be followed while operating transformers in parallel condition

Conditions for parallel operation of Transformers: 1. Polarities of the transformers must be same (wrong polarity leads to dead short) 2. The voltage rating of both primary and secondaries are identical. This means the that the transformation or turns ratio must be same for the transformers which are operated to be parallel. (Voltage ratio is to be maintained to avoid circulation current) 3. Percent (or per unit ) impedance of the transformers are to be same in magnitude and should have the same phase angle (X/R ratio should be same for transformers operating in parallel else division of load will not be proportional to the kVA ratings of transformers) 4. Phase displacement between the primary and secondary line voltages of the transformers should be same (transformers of star/star and delta/star cannot be paralleled because of the phase difference of -30 o making paralleling impossible (cannot be compensated))

Phase sequence of the transformers should be same. (Phase sequence is the order in which the terminal voltage attains their maximum value. Therefore in paralleling the two three phase transformers those terminals whose voltage attains maximum values simultaneously must be paired up. Conditions 1,4 and 5 are absolutely essential and must be fulfilled, condition 2 must be satisfied to a close degree and condition 3 (X/R ratio) must be satisfied in order to have equal loading on the transformers. 5.

When DC supply given to Transformer what happens ? DC supply to Transformer: A Transformer cannot be operated on the DC source or never connected to DC supply. If a rated dc voltage is applied to the primary of the transformer, the flux produced in the transformer core will not vary but remain constant in magnitude. So therefore no emf is induced in the secondary winding except during the moment of switching on the dc supply. As no induced emf is produced current cannot be delivered from the secondary side to the load. Also the flux flowing through the iron core from primary winding to secondary winding not only links the secondary winding but also primary winding. Due to this flux linkage self induced emf is produced in the primary winding. This self induced emf in the primary winding will oppose the applied voltage and hence it acts as back emf. This back emf limits the primary current flowing through the primary winding in normal operating condition (similar like dc machine armature current). When a dc supply is provided to the transformer primary no self induced emf will be generated (no back emf). Therefore heavy current will flow in the transformer primary winding which may result in burning down the transformer primary winding. In many interviews this question is most common:

When dc supply is given to transformer what happens? and which winding will burn (primary or secondary)?

Electrical Seminar Topic: Smart Grid Seminar Topic: Seminar topics during under graduation course enhances the student to look beyond the textbooks and make them innovative. Seminar topics should be chosen in such a manner that the topic should be simple and explains about the present technology which can be find very little in text books and should be useful to audience. I am introducing a topic called SMART GRID so that you can add the required resources and present in a seminar or can do a mini projects which helps during the placement.

Electric Grid: Electrical Grid is an interconnection of power system constitutes generating plants, transmission network and distribution system ( to consumer ends). In normal grid, energy losses takes place due to the poor transmission and distribution networks including power thefts which results in loss in economy.No communication exist between the consumers and grid authorities. Power consumed is charged at same rate during all the day. So many flaws in today power grid makes it weak to sustain small disturbances.

Smart Grid: Smart Grid is an an intelligent, future electricity system that connects all the generating systems (supply), grid and distribution systems ( consumer terminals) through an intelligent communication system.Smart Grid is a family of network control systems and asset management tools, empowered by sensors, communication pathways and information tools. Simply smart grid is an secure integration of electrical and information infrastructures. Smart Grid at Consumer's side: Major focus on the smart grid is on the distribution level or consumers. Demand is what drives the supply, so better management at consumer level helps the overall system. Important component of smart grid is 'smart metering'. Older meters connected to consumers just indicated the units consumed by the consumer. Smart meters evolved in to advanced metering system where the meter not only stores but also communicates loads and other power statistics in real time and even advice the consumers for better and optimal load control for reduction in prices. Smart grid explains and gives choice to the consumers to decide the timing and amount of power consumption based on the price per unit at that instant. Thus smart grid provides many wide features to the consumers for reducing the electricity bills and participating the consumers in improving the performance of the grid. Smart Grid at Transmission side: Smart grid helps the Generation and Transmission companies in dealing effectively with the power thefts and system losses through smart metering and advanced communication features. This also reduce in the reduction in the CO2 emissions in to atmosphere. Thus smart grid helps in reducing the losses, electricity bills and global warming and makes the electrical grid stronger to disturbances. Please find some more sources related to this and i hope this can become a good seminar topic as it is the emerging technology and everybody talking about. Add some surveys, reports and case studies while presenting the seminar topic which makes the subject more informative and useful.

SCADA systems SCADA systems: SCADA systems consists of SCADA Master which is generally referred as SCADA Master Station which includes SCADA hardware system and SCADA software system and Remote Terminal Unit (RTU). SCADA master station job is to communicates with Remote Terminal Unit (RTU) for carrying out monitoring, control, data storage and Human Machine Interface (HMI) operations. SCADA hardware consists of hardware components such as very reliable PC ( work station ) and its peripherals present in the master station and able to control the RTUs or PLCs (programmable Logic Controllers) through SCADA software. SCADA software system was developed to communicate with the PLCs or RTUs for data acquisition and control operations.

SCADA Master System: SCADA Master Station has the centralized control to monitor and control the remote stations. SCADA master station should perform tasks such as: 

Display the real time data received from PLCs, RTUs, and IEDs



Stores the received historical data received



Activates alarms when necessary



Displaces sequences of events during disturbance



Provide an Human or Operator Interface for supervisory control



Perform the communication control over the network

SCADA Hardware system: Master station is a part of the WAN or LAN network. The hardware for the master station includes high end and more reliable PC (workstation), its peripherals and its communication ports. Reliability of the hardware in master station is important else entire SCADA system will tamper. Redundant workstations are provided to improve the reliability of the system.

SCADA software System: SCADA software systems consists of

1.The operating system software: program on which SCADA software programs runs

2.SCADA system software: SCADA system software includes 4 modules Data acquisition: Refers to receiving and analyzing and processing the data from the field. Control: Controlling the remote units (RTUs) by commands initiated either by operator or automatic according to predefined parameters. Data Storage: Storing the historical data obtained from the field.

Human machine Interface: Includes input and output devices for interaction between the operator and software, the way the data is displayed in graphs and the commands given to the system

3.SCADA application software: SCADA application software is optional software to enhance the performance of the SCADA main software eg: software tools for analyzing the system disturbance recordings

Remote Terminal Units (RTU): Remote Terminal Units employed at remote units o network and it acts as interface between the SCADA master system and field instruments. It receives and stores all the information from field such as values, alarms, meter readings in the memory. When master station asks the data it transmits the data to master station. Based on the commands received from master station it performs certain tasks such as opening value, tripping the relay

Why Generation Voltage in Power Plant is Low (11kV to 33kV) ? Generally electrical power is generated at low voltages (generation voltage will be around 11 to 33kV) depending on the power output rating of the generator and transmitted to the load centers by stepping up the generating power through step-up transformer. In Electrical Generation plants, the generation voltage will be in the range between 11kV and 33kV. Generator designers wishes to generate the electric power at high voltages as possible to limit the stator current (for large alternators stator current will be in the order of thousands of amperes) required to achieve the desired output. But certain technical and economical problems will arise which results in designing the generator for particular voltage and current ratings. 



Low voltage generation for particular power level (say 500MW) requires large size of the stator conductors as huge currents flow through the stator because of low voltage. ( Any current carrying conductor size is decided by the amount of current flowing through the conductors). With larger the stator current flowing through the higher the copper loss and the heat dissipated. Therefore requires better and larger heat removing medium. Thereby increase in the cost and size of the machine. Low voltage power generation also results in increase in the size of the generator and occupies larger space. On the other hand, increase of the machine voltage significantly beyond the minimum necessary value results in requirement of more insulation for the generator stator windings, thus increasing its size and cost. This increase in voltage for power generation also increases the size of the machine. Therefore an optimum value of Voltage and Current values are attained for designing the generator considering the cost and size factors of the generator. Example:

660 MW electrical generation plant usually operate at 23.5 kV with line currents of about 19,000 A at 0.85power factor. Electrical Objective Questions With Answers: 1) In a transmission line the distributed constants are: a) Resistance and Capacitance b) Capacitance and Inductance c) Resistance, Inductance, and Capacitance only d) Resistance, Inductance, Capacitance and Shunt Capacitance 2) If the frequency of the transmission line increases then: a) Line resistance increases b) Line resistance decreases c) Shunt rectance increases d) Shunt reactance decreases 3) The phenomenon in which the conductor surface carries more current compared to core when alternative voltage is applied is a) Corona b) Skin Effect c) Ferranti Effect d) Lenz's Law 4) On what factors does Skin Effect depends: a) Cross section of the conductors b) Supply Frequency c) Permeability of the conductor material d) all the above 5) The Skin Effect of a conductor reduces with increase in the: a) Cross section of the conductor b) Supply frequency c) permeability of the conductor d) Resistivity of the conductor material 6) Skin effect of the conductor results in the: a) decrease in the dc resistance b) Increase in the dc resistance c) decrease in the ac resistance d) Increase in the ac resistance

7) Which of the following is correct: a) with increase in the frequency skin effect decreases b) skin effect decrease with decrease in the conductor diameter c) with increase in the conductor permeabilty skin effect decreases d) with increase in resistivity of the material skin effect increases 8) With the presence of earth in case of overhead lines: a) Capacitance of the line decreases and Inductance increases b) Capacitance of the line increases and Inductance increases c) Capacitance and Inductance of the line increases d) Only Capacitance of the line Increases Answers: (1) d (2) d (3) a (4) d (5) d (6) d (7) b (8) d Explanation for Objective Questions: 1. Answer (d). For simulating a transmission lines, equivalent circuit can be represented as resistance and inductance in series and capacitance and shunt conductance parallel to the circuit. Hence a transmission line can be represented as equivalent circuit of these parameters. 2. Answer (d). If the frequency of the line increases, then shunt reactance = 1/(2 x pi x f x c). So when frequency (f) inceases shunt reactance decreases. 3. Answer (a). This phenomeon is called Skin Effect 4. Answer (d). Skin Effect depends on all the above said factors 5. Answer (d). The Skin effect reduces with increase in the resistivity of the conductor 6. Answer (d). When an alternating current is passed through the conductor, then due to the high inductance of the inner part of the conductor compared to external all the current will tend to flow on the outer surface of the conductor, this phenomenon is called skin effect. As there is resistance for the ac current flow, therefore the effective conductor resistance for ac is higher than dc 7. Answer (b). Skin effect increases with the increase in the conductor diameter (size), increase in the frequency and increase in the permeability and skin effect decrease with the increase in the resistivity of the material. 8. Answer (d). With the presence of the earth, charging currents phenomenon will come in to picture which increases the shunt capacitance of the transmission line. However Inductance do not have any effect on the presence of earth

Electrical Objective Questions: DC Machines (Set 1) Electrical Objective Questions With Answers:

1.The nature of the current flowing in the armature of the DC machine is: a) ac b) dc c) pulsating d) dc superimposed on ac

2.The Commutator in a dc machine can be convert a) ac to dc b) dc to ac c) both a and b d) None of the above

3.The emf produced in the dc generator is ____ induced emf a) Statically b) Dynamically c) magnetically d) Electrostatically

4.DC generator operates on the principle of a) electro magnetic induction b) Lenz's law c) Biov Savart's law d) none of these

5.Reluctance torque in rotating machines is present when: a) airgap is not uniform b) Reluctance seen by stator mmf varies

c) Reluctance seen by rotor mmf varies d) Reluctance seen by working mmf varies

6.A Conductor is rotating within a magnetic field. At which position do the peak voltage occur: a) At the right angles to the axis of magnetic field b) Along the axis of the magnetic field c) At 45 degree angles to the axis of magnetic field d) Anywhere

7.Fleming's left hand rule is applicable for: a) dc generator b) dc motor c) alternator d) Transformer

8.EMF induced in a coil rotating in a uniform magnetic field will be maximum when the: a) flux linking with the coil is maximum b) rate of change of flux linkage is minimum c) rate of change of flux linkage is maximum d) rate of cutting flux by the coil sides is minimum

Answers: (1) a (2) c (3) b (4) a (5) d (6) b (7) b (8) c Explanations for the Objective Questions: 1. Answer (a).In the armature of the DC machine the current flow direction will be alternating manner. This alternating current is made to uni directional with the help of Commutator. This is the main use of the commutator in the DC machine.

2. Answer (c). Commutator is used to convert the ac current or voltage to dc and dc variables in to ac. This is interesting, in DC generator the current flow direction in armature winding is ac in nature, so commutator helps in rectifying ac to dc. In DC motor the current we supply to the motor is dc, however for armature of dc motor ac current is required to be supplied to produce alternative flux for generating rotating torque. Hence commutator in DC motor converts dc to ac. 3. Answer (b). The emf produced in the dc generator is dynamically induced emf. This can be explained with the help of faradays law electro magnetic induction. 4. Answer (a). DC generator works on the principle of faraday's law of electro magnetic induction 7. Answer (b). Fleming's left hand rule is applicable for DC Motors.

Methods to reduce corona Loss in Power System Critical breakdown voltage of the corona can be increased by following ways: 



By increasing the spacing between the transmission line conductors. However the spacing between the lines cannot be increased very much as this effects the size of the tower, weight of the supporting structures and the land occupied by the tower.Hence the distance between the conductors should be chosen optimum so that the critical breakdown voltage increases. Other factor on which the critical breakdown voltage is diameter of the conductor. By increasing the diameter of the conductor breakdown voltage of the corona can be increased. The increase in the conductor diameter results in reduction of the electric field intensity which reduces the corona. Hence hollow conductors are employed for reducing the corona in EHV lines.

By using the bundled conductors the diameter of the conductor increases compared to diameter of the normal conductor for same power rating. Increase in the conductor diameter reduces the electric field intensity. Hence corona loss can be reduced. In simple corona can be reduced by increasing the distance between the current carrying conductors, increasing the diameter of the conductors (employing Hollow conductors) and by using Bundled conductors corona loss can be reduced or minimized. 

Factor affecting corona Loss in Power System Atmosphere: The voltage gradient for breakdown of the air is proportional to its density. In stormy weather (bad climate) the number of ions may be more than normal. As more number of ions present between the conductors means reduction of insulating property of the medium. So corona can occur at less voltage than critical breakdown voltage during bad climatic conditions than fair weather conditions.

Conductor:

The corona is conserved to be affected by the Size(diameter), shape(stranded or smooth) and surface condition (dirty or clean) of the conductor. Corona decreases with the increase in the conductor diameter. This is due to the fact that increase in the conductor diameter reduces the electric field intensity. For Stranded conductor corona is more compared to solid conductor. As the potential gradient of the stranded surface is more compared to the normal solid conductor for same power rating.

For dirty surfaced conductor the break down voltage required for corona formation is less than clean surfaced conductor. Spacing between Conductors: With increase in the spacing between the conductors the electrostatic stresses reduces results in reduced corona effect Line Voltage: At low line voltage there will be no corona. When the voltage applied increases breakdown voltages then corona formation takes place

What is Travelling Waves When a transmission line is connected to a voltage source, the whole of the line is not energized in one instant. This is because of presence of distributed constants (Inductance (L) and Capacitance(C)). When switch S is closed, the inductance L1 acts as open circuit and C1 acts as short circuit instantaneously. In this instant the next section (i.e, L2 and C2) cannot be charged as voltage across C1 is zero. So in order to charge the next section (i.e, L2 and C2) it will take finite time. The same argument applies to third and fourth sections and so on. Thus voltage build up in the sections will be in the form of a voltage wave traveling from source to load through the transmission lines and gradual charging of capacitance is due to associated current wave. Thus in a long transmission lines the voltage and current travels from source to load in the form of waves.

What is electrical substation? Electrical Substation: Electrical substations in electrical power system can be find in power plants (for stepping up the voltage for transmission of power to long distances), switching stations where power can be transmitted in desired manner during transmission and distribution centers where the power is stepped down and provide power to the house hold and commercial terminals. Hence Electrical substation plays an important role and can be find in different locations of the power system ( generation centers to distribution centers). Substations can be assumed as a nodal point in the power system where different conductors coming from different places connects together to a point and on the other hand different conducting lines travel out to different locations in the power system. Electrical substations are different types:

Air Insulated Substations: In this subtatation where all the substation equipment are exposed to environment or the all the substation equipment stands outside in the open air. This type of substation has advantage of cheap in cost and

disadvantage of all the equipment ( bus bars, insulators ) expose to the external atmosphere susceptible to fault and life of equipment decreases.

Gas Insulated Substations: This type of substations are completely enclosed in sF6 gas modules. This substation takes very little place (1/8 th of Air Insulated Substation). This has the advantage of more reliable and disadvantage of high cost and difficult to operate if fault occurs.