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CHAPTER-1 TRANSMISSION LINE 1.1 INTRODUCTION Electrical power is the basic need for the economic development of any country. The energy consumption is the main index for the overall development and growth of a country. The process of modernization, increase in productivity in industry and agriculture and the improvement in the standard of living of the people basically depend on the adequate supply of electrical energy. The electrical energy is generated by hydroelectric power plants, thermal power plants and nuclear power plants. The electrical power is transmitted from these power plants to the consumer’s premises by using transmission and distribution systems. The power from the generating station is transmitted at high voltage (such as 132, 220, 440 kV) over long distances to the major load centres. The line should have sufficient current carrying capacity so as to transmit the required power over a given distance without excessive voltage drop and overheating. The line losses should be small and insulation length should be adequate to cope with the system voltage. The transmission system of an area is known as ‘GRID’. The different grids are interconnected through the ‘TIE’ lines to form a ‘regional grid’ and the different regional grids are further interconnected to form a ‘national grid’. Each grid operates independently. Power can be transmitted from one grid to another, over the tie lines under the condition of sudden loss of generation or increase in load. A single phase AC circuit requires 2 conductors. A 2-phase AC circuit using same size conductor as a 1-phase circuit can carry 3 times the power which can be carried by a single phase circuit and uses 3 conductors of 3-phase and 1-conductor of neutral. Thus a 3-phase circuit is more economical then a 1-phase circuit in terms of initial cost as well as the losses. All transmission and distribution systems are, therefore, 3-phase systems. In fact, a balanced 3-phase circuit does not require the neutral conductor as the instantaneous sum of the 3 line currents is zero.

1

Therefore, the transmission lines and feeders are 3-phase, 3-wire circuit. The distributors are 3-phase, 4-wire circuit because a neutral wire is necessary to supply the 1-phase load for domestic and commercial consumers. The standard frequency in India and many other countries is 50 Hz. The overhead line conductors are bare and not covered with any insulating covering coating. The line conductors are, therefore, secured to the supportive structures by means of insulating fixtures, called the insulators, in order that there is no current leakage to the earth through the supports. The material most commonly used for overhead line is ‘Porcelain’. But toughened glass, steatite and special composition materials are used to limited extent. Insulators are required to withstand both electrical and mechanical stresses. In the present work, we have designed a 3-phase transmission system to transmit a given power through a given distance. Subjected to the constraints such as efficiency and regulation for a given power factor of the load. We have also attempted mechanical design of a transmission line. The mechanical design comprises of selection and number of insulators, proper sag and minimum distance of the line from the ground and based on this, we have selected a suitable tower.

1.2 HISTORY OF TRANSMISSION LINE Before we dig deep into the principles of Transmission Line Losses, let us first review a brief history of the power transmission line particularly with Overhead Transmission Line.

Fig no. 1.1: View of a transmission line. 2

The first transmission of electrical impulses over an extended distance was demonstrated on July 14, 1729 by the physicist Stephen Gray, in order to show that one can transfer electricity by that method. The demonstration used damp hemp cords suspended by silk threads (the low resistance of metallic conductors not being appreciated at the time). However the first practical use of overhead lines was in the context of telegraphy. By 1837 experimental commercial telegraph systems ran as far as 13 miles (20 km). Electric power transmission was accomplished in 1882 with the first high voltage transmission between Munich and Miesbach. 1891 saw the construction of the first three-phase alternating current overhead line on the occasion of the International Electricity Exhibition in Frankfurt, between Lauffen and Frankfurt. In 1912 the first 110 kV-overhead power line entered service followed by the first 220 kV-overhead power line in 1923. In the 1920s RWE AG built the first overhead line for this voltage and in 1926 built a Rhine crossing with the pylons of Voerde, two masts 138 meters high. In Germany in 1957 the first 380 kV overhead power line was commissioned (between the transformer station and Rommerskirchen). In the same year the overhead line traversing of the Strait of Messina went into service in Italy, whose pylons served the Elbe crossing 1. This was used as the model for the building of the Elbe crossing 2 in the second half of the 1970s which saw the construction of the highest overhead line pylons of the world. Starting from 1967 in Russia, and also in the USA and Canada, overhead lines for voltage of 765 kV were built. In 1982 overhead power lines were built in Russia between Elektrostal and the power station at Ekibastusz, this was a three-phase alternating current line at 1150 kV (Power line Ekibastuz-Kokshetau). In 1999, in Japan the first powerline designed for 1000 kV with 2 circuits were built, the Kita-Iwaki Powerline. In 2003 the building of the highest overhead line commenced in China, the Yangtze River Crossing.

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CHAPTER-2 COMPONENTS OF A TRANSMISSION LINE 2.1 INTRODUCTION The transmission lines are like the arteries of the power system. Transmission lines act as medium for carrying bulk energy from one substation to other. The electric energy transmission is carried out at High and Extra High Voltages (EHV). Voltage above 220 kV is usually referred as Extra High Voltage. The transmission lines can be constructed over head or underground. The overhead lines are bare conductors with proper clearances from earthed structures and between the phase conductors.

2.2 TRANSMISSION SYSTEM REQUIREMENTS Listed below are the typical points to be considered before starting or even operating an Electrical Power System. These factors can be best categorized into three main points; Electrical Design, Mechanical Design & Structural Design. Electrical Design of AC system involves; •

power flow requirements

•

system stability and dynamic performance

•

selection of voltage level

•

voltage and reactive power flow control

•

conductor selection

•

losses

•

corona-related performance(radio, audible, and television noise)

•

electromagnetic field effects

•

insulation and over voltage design

•

switching arrangements 4

•

circuit-breaker duties

•

protective relaying.

Mechanical Design includes •

Sag and tension calculations

•

conductor composition

•

conductor spacing (minimum spacing to be determined under electrical design)

•

types of insulators

•

selection of conductor hardware

Structural Design •

selection of the type of structures to be used

•

mechanical loading calculations

•

foundations

•

guys and anchors.

Miscellaneous features •

line location

•

acquisition of right-of-way

•

profiling

•

locating structures

•

inductive coordination (considers line location and electrical calculations)

•

means of communication

2.3 HARDWARE COMPONENTS OF TRANSMISSION LINE

5

Fig no. 2.1: Picture common components a transmission line. The following are the mostshowing commonmost overhead transmission lineofcomponents: •

Structures for Support (Poles & Towers)

•

Wires and Cables (phase conductors & OHGW)

•

Insulators (ceramics & polymer)

•

Connectors

•

Guying for support

•

Line Arresters

•

Others (vibration damper, corona ring, spacers, etc.

2.4 CONDUCTORS IN TRANSMISSION LINE In the past, electric power was transmitted through the use mostly of copper conductors. Copper is rank among the most ideal metals for transmitting electricity due to its low resistivity also, of which it is second to silver. However, in the modern days, aluminum replaced copper as a main material for transmitting electricity simply because of the much lower cost and lighter weight of an aluminum conductor in contrast to a copper conductor 6

with the same resistance. Another advantage of an aluminum is when compared to a copper with the same resistance, aluminum tends to have a larger diameter. It is an advantage because with a conductor with a relatively larger diameter the lines of electric flux originating on the conductor will be farther apart at the conductor surface for the same voltage. Electrical conductor

Fig no. 2.2 View of overhead conductors carry electric power.

In physics and electrical engineering, a conductor is a material which contains movable electric charges. In metallic conductors such as copper or aluminum, the movable charged particles are electrons. Positive charges may also be mobile in the form of atoms bound in a crystal lattice which are missing electrons (known as holes), or in the form of mobile ions, such as in the electrolyte of a battery, or as mobile protons in proton conductors employed in fuel cells. In general use, the term "conductor" is interchangeable with "wire." Physics All conductors contain electric charges, which will move when an electric potential difference (measured in volts) is applied across separate points on the material. This flow of charge (measured in amperes) is what is meant by electric current. In most materials, the direct current is proportional to the voltage (as determined by Ohm's law), provided the temperature remains constant and the material remains in the same shape and state.

7

2.4.1 CONDUCTOR MATERIALS Copper has a high conductivity. Silver is more conductive, but due to cost it is not practical in most cases. Because of its ease of connection by soldering or clamping, copper is still the most common choice for most light-gauge wires. Aluminum has been used as a conductor in housing applications for cost reasons. It is actually more conductive than copper when compared by unit weight, but it has technical problems that have led to problems when used for household and similar wiring, sometimes having led to structural fires: •

A tendency to form an electrically resistive surface oxide within connections, leading to heat cycling of the connection (unless protected by a well-maintained protective paste);

•

A tendency to "creep" during thermal cycling, causing connections to become loose due to a low mechanical yield point of the aluminum; and

•

A coefficient of thermal expansion sufficiently different from the materials used for connections, accelerating the creep problem and addressed by using only plugs, switches, and splices rated specifically for aluminum.

These problems do not affect other uses, and aluminum is commonly used for the low voltage "drop" between a power pole and the household meter. It is also the most common metal used in high-voltage transmission lines, in combination with steel as structural reinforcement. Listed below are some of the known types of aluminium conductors that are used by many transmission and distribution utility worldwide; •

AAC All-Aluminium Conductors

•

AAAC All-Aluminium-Alloy Conductors

•

ACSR Aluminium Conductor, Steel Reinforced

•

ACAR Aluminium Conductor, Alloy Reinforced

Due to the low tensile strength of aluminium, experts created a way to fill this void. They were able to create a higher tensile strength conductor by incorporating aluminium with 8

other types of metal. ACSR which consists of a central core of steel strands surrounded by layers of aluminium strands is now the type of configuration that are popularly used as conductors for transmission lines.

Fig. no. 2.3 Aluminium with Steel

Fig. no. 2.4 Different kind of ACSR cables according to composition The most common conductor materials are hard drawn copper and aluminium. Their properties are given in table 2.1. Table 2.1: Properties of Copper and Aluminium conductors

Electrical conductivity (silver = 1.0) Resistivity (μ Ω-cm) Specific gravity

Copper 0.975 1.777 8.89

Tensile strength(

3.84

)

Aluminium 0.585 2.826 2.70 to

180

430 Coefficient of linear expansion per

17

9

23

to

234

Temperature coefficient of resistance 0.00393 per

0.004

at 20 Ratio of conductivities for equal area 1 Ratio of diameters for equal 1

0.6 1.29

resistance Ratio of weights for equal resistance

1

2

2.4.2 TYPES OF CONDUCTOR 1. Stranded Hard Drawn Copper. Hard drawn copper has the advantages of very high conductivity (i.e., very low resistivity), good tensile strength and weather resisting properties. Many years back it was widely used for construction of overhead lines. Due to non-availability and high cost involvement, it is generally not use in India. In other countries, too, it is very rarely used.

2. Aluminium. Aluminium has the advantages of much lower cost and lesser weight as compared to copper. The fact that an aluminium conductor has a larger diameter than a copper conductor of the same resistance is also an extra advantage. A large diameter. For the same voltage, leads to a lower voltage gradient at the conductor surface with a tendency of reduced ionisation level of air and corona.

3. Aluminium Conductor Steel Reinforced (ACSR). ACSR (Aluminium Conductor Steel Reinforced) conductor comprises hard drawn aluminium wires stranded around a core of single or multiple strand galvanised steel wire. Fig. 2.1(b) shows an ACSR conductor having 7 strands of steel and 30 strands of aluminium. Aluminium provides the necessary conductivity while steel provides the necessary mechanical strength. During manufacture, a layer of grease is put between aluminium and steel to reduce electrolytic action (corrosion) between zinc and aluminium (The steel strands are galvanised with zinc). All transmission lines and most of the distribution lines use ACSR conductor. These conductors are

10

manufactured in a wide variety of sizes from 5 mm to over 40 mm overall diameter. Aluminum conductor steel reinforced (or ACSR) cable is a specific type of highcapacity, high-strength stranded cable typically used in overhead power lines. The outer strands are aluminum, chosen for its excellent conductivity, low weight and low cost. The center strand is of steel for the strength required to support the weight without stretching the aluminum due to its ductility. This gives the cable an overall high tensile strength. 4. Galvanised Steel. Galvanised steel conductors have been used to advantage for extremely long spans, or for short line selections exposed to normally high stresses due to climatic conditions. These conductors are found most suitable for lines supplying rural areas and operating at voltages of about 11 kV, where cheapness is the main consideration. Iron or steel wire use is most advantageous for transmission of small power over a short distance, where the size of copper conductor desirable from economical consideration comes out to be smaller than SWG, which cannot be used because of poor mechanical strength. This conductor is not suitable for EHT lines for the purpose of transmitting large amounts of power over a long distance due to its following properties: (i)

Poor conductivity, 13% that of copper.

(ii)

High internal reactance.

(iii)

It is subjected to eddy current and hysteresis.

5. Cadmium Copper. The conductor being used in certain cases is copper alloyed with cadmium. Addition of 1 or 2 % of cadmium in copper increase the tensile strength by about 40% and reduces the conductivity only by 17% below that of pure copper. However, cadmium copper is costlier than the pure copper. Use of cadmium copper will be economical for a line with long spans and small crosssection i.e. where the cost of conductor material is comparatively small in comparison to that of supports etc. Cadmium-copper conductors are also employed for telephone and telegraph lines where currents involved are quite 11

small. However, owing to scarcity of copper, cadmium-copper conductors on communication lines are being replaced by ACSR conductors.

6. Copper-clad Steel. A composite wire, known as copper-clad or copper-weld steel wire, is obtained by welding a copper coating on a steel wire core. Line conductors made of copper-clad steel are preferable stranded, and have a considerably large tensile strength than the equivalent all-copper conductors. The proportion of copper and steel is so chosen that the conductivity of composite wire is 30% to 40% of that of copper conductor of equal diameter. Such material appears to be very suitable for river-crossings or other places where an extremely long span is involved.

7. Phosphor Bronze. When harmful gases such as ammonia are present in atmosphere and the spans are extremely long, phosphor bronze is most suitable material for an overhead line conductor. In this conductor some strands of phosphor bronze are added to the cadmium copper.

(a)

(b)

Fig no. 2.5 Stranded Conductors.

2.5 INSULATORS The overhead line conductors are bare and not covered with any insulating covering/coating. The line conductors are, therefore, secured to the supporting structures by means of insulating fixtures, called the insulators, in order that there is no current 12

leakage to the earth through the supports. Insulators are mounted on the cross-arms and the line conductors are attached to the insulators so as to provide the conductors proper insulation and also provide necessary clearances between conductors and metal work. The important properties that an overhead line insulator must possess are: 1. High mechanical strength so as to bear the load due to the weight of line conductors, wind force and ice loading if any. 2. High relative permittivity so as to provide high dielectric strength. 3. High insulation resistance in order to prevent leakage of currents to earth. 4. High ratio of rupture strength to flash over voltage. 5. Ability to withstand large temperature variations i.e., it should not crack when subjected to high temperatures during summer and low temperature during winter. The dielectric strength should remain unaffected under different conditions of temperature and pressure.

2.5.1

INSULATOR MATERIALS

A true insulator is a material that does not respond to an electric field and completely resists the flow of electric charge. In practice, however, perfect insulators do not exist. Therefore, dielectric materials with high dielectric constants are considered insulators. In insulating materials valence electrons are tightly bonded to their atoms. These materials are used in electrical equipment as insulators or insulation. Their function is to support or separate electrical conductors without allowing current through themselves. The term also refers to insulating supports that attach electric power transmission wires to utility poles or pylons. The material most commonly used for overhead line insulators is porcelain but toughened glass, steatite and special composition materials are also used to a limited extent. 1. Porcelain. Porcelain is produced by firing at a controlled temperature a mixture of kaolin, feldspar and quartz. It is mechanically stronger than glass. It gives less 13

trouble from leakage, and is less susceptible to temperature variations and its surface is not affected by dirt deposits. On the other hand, it is not so homogeneous as glass, owing to the fact that each component shell of a porcelain insulator is glazed during manufacturing process and its satisfactory performance in service depends to a considerable extent on the preservation of this glaze which is only of the order of 25 microns in thickness. Also fault cannot detect easily as it is not transparent. In tension his material is usually weak and does not withstand tensile stresses exceeding

. The

dielectric strength and compressive strength of a mechanically sound porcelain insulator are about 6.5 kV/mm of its thickness and

respectively.

2. Glass. Glass is cheaper than porcelain in the simpler shape and if properly toughened and annealed gives high resistivity and dielectric strength (14 kV per mm of thickness of the material). Owing to high dielectric strength, the glass insulators have simpler design and even one piece design can be used. Glass is quite homogeneous material and can withstand higher compressive stresses as compared to porcelain. It has also a lower coefficient of thermal expansion which minimises the strain due to temperature changes and owing to its transparent nature flaws in the material can be readily detected by visual examination. The main disadvantage of the glass is that moisture more readily condenses on its surface and facilitates the accumulation of dirt deposits, thus giving a high surface leakage. Also in large sizes the great mass of material combined with the irregular shape, may result in internal strains after cooling. Glass insulator however, can be used upto 25 kV under ordinary atmospheric conditions as well upto 50 kV in dry atmosphere.

3. Steatite. Steatite is a naturally occurring magnesium silicate, usually found combined with oxides in varying proportions. It has a much higher tensile and bending stress than porcelain and can advantageously be used at tension towers or when a transmission line takes a sharp turn.

14

2.5.2

TYPES OF INSULATORS

Various types of insulators used for overhead transmission and distribution lines are: 1. Pin Type Insulator. A pin insulator is small, simple in construction and cheap. It is used on lines upto and including 33 kV lines. The conductor is bound into a groove on the top of the insulator which is cemented on to a galvanised steel pin attached to the cross arm on the pole or tower. To avoid a direct contact between the porcelain and the metal pin, a soft metal (generally lead) thimble is used. An adequate length of leakage path is obtained by providing the insulator with two or three petticoats or rain sheds. These are so designed that even when the outer surface of these insulator is wet due to rain, sufficient leakage resistance is still given by the inner dry surface. In its electrical behaviour, a pin type insulator may be compared to a complicated series of conductors with resistances in series and shunt. The petticoats with the inverting air spaces from the condenser system and the leakage paths over the surface and through the body of the material are represented by the resistances.

Fig. no. 2.6 Pin Insulators (a) 11 kV (b) 33 kV

15

Pin type insulators are used only up to about 33 kV because for higher voltages they tend to be very heavy and more costly than suspension type insulators.

2. Suspension Type Insulators. The cost of a pin insulators increases very rapidly with increase in line voltages. Therefore, suspension insulators are used for line above 33 kV. They are also known as disc insulators or string insulators.

Fig. no. 2.7 Picture of a Suspension Insulator A suspension insulator consists of porcelain disc units mounted above the other. Each disc consists of a single shed of porcelain grooved on the under surface to increase the creep age distance. The upper surface of each disc is inclined at a suitable angle to the horizontal in order to ensure free drainage of water. Each disc is provided with a metal cap at the top and a metal pin underneath. The cap is recessed so as to take the pin of another unit and thus a string of any required number of units can be built up. The most commonly used disc is the cemented cap type. 3. Post Insulators. These are used for supporting the bus bars, and disconnecting switches in sub-stations. A post insulators is similar to a pin type insulator but has a metal base and frequently a metal cap so that more than one unit can be mounted in series. In extra high voltage sub-stations (400 kV and above) polycon post insulators are used. In this insulator the porcelain elements are in the form of cones smugly fitting one inside the other and bounded by special cement. The puncture path is through many layers of porcelain cones and the voltage required to puncture this path is many times the external flash over voltages so that insulator is almost puncture proof.

16

Fig. no. 2.8 Picture of a Post Insulator 4. Strain Insulators. These are special mechanically strong suspension insulators and are used to take the tension of the conductors at the line terminations and at positions where there is a change in the direction of line. The discs of a strain insulator are in a vertical plane as compared to the discs of suspension insulator which are in a horizontal plane. On extra long spans, viz, at river crossings, two or three strings of strain insulators, arranged in parallel, are often used.

Fig. no. 2.9 Picture of a Strain Insulator The electrical breakdown of an insulator due to excessive voltage can occur in one of two ways: •

Puncture voltage is the voltage across the insulator (when installed in its normal manner) which causes a breakdown and conduction through the interior of the insulator. The heat resulting from the puncture arc usually damages the insulator irreparably.

•

Flashover voltage is the voltage which causes the air around or along the surface of the insulator to break down and conduct, causing a 'flashover' arc along the outside of the insulator. They are usually designed to withstand this without damage.

17

Most high voltage insulators are designed with a lower flashover voltage than puncture voltage, so they will flash over before they puncture, to avoid damage. Dirt, pollution, salt, and particularly water on the surface of a high voltage insulator can create a conductive path across it, causing leakage currents and flashovers. The flashover voltage can be more than 50% lower when the insulator is wet. High voltage insulators for outdoor use are shaped to maximize the length of the leakage path along the surface from one end to the other, called the creepage length, to minimize these leakage currents. To accomplish this surface is molded into a series of corrugations or concentric disk shapes. These usually include one or more sheds; downward facing cup-shaped surfaces that act as umbrellas to ensure that the part of the surface leakage path under the 'cup' stays dry in wet weather. Minimum creep age distances are 20–25 mm/kV, but must be increased in high pollution or airborne sea-salt areas. 2.6 LINE SUPPORTS The function of line support is obviously to support the conductors. Line support must be capable of carrying the load due to insulator and conductors including the ice and wind loads on the conductor along with the wind load on the support itself. The main requirements of the line supports are: 1. High mechanical strength to withstand the weight of conductors and wind loads etc. 2. Light in weight without the loss of mechanical strength. 3. Cheaper in cost. 4. Low maintenance cost. 5. Longer life. The choice of line supports for a particular situation depends upon the line span, crosssectional area, line voltage, cost and local conditions

18

Fig. no. 2.10 Picture showing different parameters of a transmission line. 2.6.1 TYPES OF LINE SUPPORTS The line supports are of various types including wood, steel and reinforced concrete poles and steel towers either of the rigid or flexible type. 1. Wooden Poles. These supports are cheapest, easily available, provide insulating properties and therefore, are extensively used for the distribution purposes specially in rural electrification keeping the cost low. Their use is usually limited to low pressures (upto 22kV) and for short spans (upto 60 meters). The wooden poles well impregnated with creosite oil or any preservative have life from 25 to 30 years. Wooden poles are very elastic and lines employing wooden supports are often designed throughout for the transverse load. Longitudinal strength at terminals and for anchor support is provided by means of guys. Double pole structures of A or H types are often employed for obtaining a higher transverse strength than that could be economically provided by means of single poles.

19

Fig. no. 2.11 Picture of a Wooden Pole.

2. RCC Poles. Poles made of reinforced cement concrete (RCC), usually called the concrete poles, are extensively used for low voltage distribution lines upto 33 kV. Their construction should conform to the standard specification for RCC work, but in low case the dimension shall be less 25 cm

25 cm at the bottom and 13cm

13cm at the top. These poles are of two types in shape. One type is square crosssection from bottom to top. The other type has rectangular bottom and square top with rectangular holes in it to facilitate the climbing of poles and reduce the weight of poles. These give good appearance, require no maintenance, have got insulating properties and resistance against chemical action, very strong, have longer life and can be used for longer spans (80-200 m). Such poles are most suitable for water logged situations where other types will not be at all suitable, as due to standing water wooden poles will decay very rapidly, and steel construction will be having deposit of rust. Since these poles are very bulky and heavy, therefore, transportation cost is heavy and need care in handling and erection.

20

Fig. no. 2.12 Picture of a RCC Pole.

3. Steel Poles. The steel poles are of three types (i) tubular poles (ii) rail poles and (iii) rolled steel joists. The tubular poles are of round cross-sections, the rail poles are of the shape of track used for railways and rolled steel joists are of I crosssections. Such poles possess greater mechanical strength and permit use of longer spans (50-80 m) but cost is higher. Their life is longer than that of wooden poles and life is increased by regular painting. These poles are set in concrete muffs at the foundation in order to protect them from chemical action. The average life of steel poles is more than 40 years.

21

Fig. no. 2.13 Picture of a Steel Pole 4. Lattice Steel Towers: The steel tubular poles and concrete poles are usually used for distribution in urban area to give good appearance and steel rails or narrowbase, lattice-steel towers are used for transmission at 11 kV and 33 kV and broadbase lattice-steel towers are used for transmission purposes at 66 kV and above. The broad-base, lattice-steel towers are mechanically stronger and have got longer life. Due to their robust construction long spans (300 m or above) can be used and are much useful for crossing fields, valleys, railways lines, river etc.

Fig. no. 2.14 Picture of a Lattice Steel Tower

CHAPTER-3 22

DESIGN OF TRANSMISSION LINE

3.1 INTRODUCTION The design of a transmission line involves a number of technical and economical aspects. The power capacity and distance of transmission are specified. The voltage regulation and efficiency are also specified. The design details include line voltage, size of phase conductors, span, spacing and configuration of conductors, number and size of earth wires, number of insulators, clearances, sag under operating and erection conditions, etc. Once these design features are available, the voltage regulation and efficiency can be calculated.

3.2 CHOICE OF VOLTAGE The cost and performance of the line depend, to a great extent, on the line voltage. An empirical formula for optimum voltage is

V

(3.1)

Where V = line voltage in kV L = distance in km P = power in kW A standard voltage nearest to this value should be adopted. The above formula gives only a preliminary estimate. The choice of the most economical voltage requires a detailed study of many technical and economical aspects. One a preliminary estimate is available a detailed analysis is necessary. This becomes all the more necessary when the final choice is likely to fall in EHV/UHV range.

System Voltages in Transmission Lines 23

Table shown is the standard system voltages from ANSI standards C84 and C92.2 According to ANSI standards C84 and C92.2, system voltages are recommend to be within the table shown below. 345kV, 500kV and 765kV are considered to be in the Extra High Voltage (EHV) level. The choice of system voltage is in the decision of the utility. However, some points needs to be considered in choosing such, like voltage economics, conductors, distances, equipments, etc. Table no. 3.1 Standard voltages listed in ANSI standards C84 and C92.2

3.3 SELECTION OF CONDUCTOR SIZE The cost of conductor size is about 30 to 45 percent of the total cost of the line. Moreover the cost of towers, foundation and line losses also depend on the conductor size. A proper selection of the size of phase conductors is, therefore, very important. Overhead transmission lines invariably use ACSR conductors. These conductors are manufactured in a variety of sizes (Appendix A). The size of the conductors should be such that it can carry the rated current continuously without excessive rise in temperature. The temperature affects the sag and the loss of the tensile strength (due to annealing) of the conductor. For copper and aluminium, annealing starts at about

and the operating temperature should be well below this value. The

standard practice is to design the line for a conductor temperature of

24

.

The temperature rise of the conductor depends on the conductor heating due to

loss

and heat dissipation. In overhead lines heat is dissipated by convection and radiation. A steady temperature will be reached when (3.2)

Where

= rms value of conductor current, amperes.

= ac resistance of conductors, ohms/meter length.

= heat loss due to convection, watts per

= heat loss due to radiation, watts per

surface area.

surface area.

= conductor surface area per meter length. The heat loss due to convection is given by the equation.

(3.3)

Where p = pressure in atmosphere, Ta is the temperature of air in air in m/sec, d is the diameter of the conductor in m and

, v is the velocity of

is the difference between the

temperatures of conductor and air. The above formula is valid is v 0.15 m/sec and d m. The heat loss due to radiation is proportional to the difference of the fourth power of the temperature of the conductor and the surroundings. This loss can be found from the equation

25

(3.4)

The T1 is the conductor temperature in

, T2 is the temperature of surroundings

and e

is the relative emissivity of the surface (e = 1.0 for black body and about 0.5 for oxidised copper).

3.4 CHOICE OF SPAN A longer span means a smaller number of towers but the towers are taller and more costly. The higher the operating voltages, the greater should be the span to reduce the high cost of insulators. Moreover the insulators constitute the weakest part of a transmission line and a reduction in the number of towers per km (by using longer span) increases the reliability of the line. For every proposed line there is a definite length of span which will give the minimum cost of the line. From mechanical consideration there is a maximum value of span for each conductor size. Many a time it happens that the conductor size, as determined from electrical calculations is very small and it is possible to reduce the cost of line by using thicker and stronger conductor so that a longer span may be employed. Sometimes it is not feasible to determine the tower height and span length on the basis of the line cost alone because lighting hazards increase greatly as the height of conductors above ground is increased. Modern high voltage lines have spans between 200 to 400 m. For river and ravine crossings exceptionally long spans up to 800 m or so have been satisfactory employed. Specifications Long span overhead transmission line •

Minimum wear

•

Anti-loose

•

Well corrosion resistance 26

•

Easy installation

Characteristics 1. For a single wire material, whether it is damaged or continue, preformed line splicing section 100% recoverable mechanical strength, and the length of the connecting wire inside can greatly improve conductivity. 2. For ACSR for repair were not damaged, steel core, wire aluminium wire connecting section can be restored to 100% strength and 10% of the steel core strength, and the installation of wires within the article follow, lead performance greatly improved . 3. If the steel core damage, please select the article follow the whole tension. Table 3.2: The usual spans With wooden poles With steel tubular poles With RCC poles With steel towers

40-50 m 50-80 m 80-200 m 200-400 m and above

3.5 CHOICE OF CONDUCTORS Many conductor configurations are used in practice. There is no special advantage in using symmetrical configuration and in most cases flat horizontal or vertical configuration are used from mechanical consideration. A flat horizontal configuration means a lesser tower height but a wider right of way. A vertical configuration means a taller tower and increased lighting hazards. In spite of these facts, flat horizontal and vertical configuration is used in many cases. For single circuit lines an L type configuration is quite popular. A transmission line may be a single circuit line or double circuit line. A double circuit line has a higher power transfer capability and greater reliability than a single circuit line. Each circuit of a double circuit line is usually designed for 75% of the line capacity. In India, both single circuit and double circuit lines exist in the EHV and high voltage class (66 kV, 132 kV, 220 kV and 400 kV). In foreign countries also both single and double circuit line exist. The number of circuits for a proposed line can be determined from the surge impedance loading (SIL).

27

3.6 SPACINGS AND CLEARANCES There must be adequate spacing between conductors so that they do not come within sparking distance of each other even while swinging due to wind. An empirical formula commonly used for determining the spacing of aluminium conductor lines is Spacing =

meters

(3.5)

Where S = Sag in meters V = Line voltage in kV Table 3.3: Some typical values of spacing Line voltage(kV)

0.4

11

33

66

132

220

400

Spacing (m)

0.2

1.2

2.0

2.5

3.5

6.0

11.5

The Indian Electricity Rules specify the minimum clearance between the ground and the conductor. These values are: Table 3.4: Minimum clearance between the ground and the conductor kV Clearance to ground (a) Across Street (m) (b) Along Street (m) (c) Other Areas (m)

0.4

11

33

66

132

220

5.8 5.5 4.6

5.8 5.5 4.6

6.1 5.8 5.2

6.1 6.1 5.5

6.1 6.1 6.1

7.0 7.0 7.0

These rules also specify the minimum clearance for power lines from buildings, railway tracks and telecommunication lines, etc.

3.7 INSULATION DESIGN

28

The insulation design affects the performance of the line to a great extent. Line insulation should be sufficient to take care of switching over voltages, temporary over voltages and atmospheric over voltages. The insulation level of the transmission lines is based on the switching surge expectancy on the system. The maximum switching surge over voltage to the ground is taken as 2.5 p.u and the insulation is designed for this voltage. In addition adequate protection against atmospheric over voltages (direct lighting strokes) is provided. In EHV and UHV lines over voltages due to switching surge assume a greater importance than atmospheric over voltages. Determination of line insulation: The insulation of line has to be based upon the consideration or lightning and switching surges and power frequency over voltages. With the present day knowledge of lightning behaviour it is possible to build lines to a certain predetermined level of performance. In case of high voltage lines of 132 kV and above, these can be made particularly lightning proof by (i) efficient sliding, (ii) low tower footing impedances. Good shielding is obtained when the shielding angel is about 300 and similarly optimum conditions are generally obtained when the tower-footing impedance is reduced to about 10 ohms. The line insulation must be sufficient to prevent a flashover from the power frequency over-voltage and the switching surges, taking into account all the local unfavourable circumstances which decrease the flash-over voltage (rain, dust, insulator pollution, etc.). it is usual to adopt the following over-voltage factors:

Table no. 3.5: Over voltage factors

For 220 kV For 400 kV

Switching surge flash-over

Power frequency flash-

voltage 6.5 Vpn 5.0 Vpn

over (wet) 0.3 Vpn 3.3 Vpn

Where Vpn is the phase to neutral voltage (rms.) 29

It is a good practice to make an allowance for one or more insulator discs to take care of the possibility of an insulator unit in the string becoming defective, and also for hot line maintenance, over and above those required to withstand the above flash-over values. Accordingly, for lines upto 220kV, one extra disc, and 400 kV lines two extra discs may be used.

Table no. 3.6 F.O.V. of standard Discs (254×146 mm) Impulse No. Of Discs

Dry FOV kV rms.

Wet FOV kV rms.

FOV(standard full waves) kV crest

1

80

50

150

2

155

90

255

3

215

130

53

4

270

170

440

5

325

215

525

6

380

255

610

7

435

295

695

8

485

335

780

9

540

375

860

10

590

415

945

11

640

455

1025

12

690

490

1105

13

735

525

1185

14

785

565

1265

15

830

600

1345

16

875

630

1425

17

920

660

1505

18

965

690

1585

19

1010

720

1665

20

1055

750

1745

25

1280

900

2145

30

30

1505

1050

2550

In the light of the above discussion, the number of isolator discs of 254×146 mm size required to withstand switching surge and the power-frequency over- voltage for 132 kV, 220kV, and 400 kV lines is given below:

Table no. 3.7 Recommended Insulation Level for Lines Normal

Vpn

Switching

No.

Power freq.

No.

system

kV

over-voltage

Of

Over-

Of

kV crest

Disc

voltage

Disc

)

s

(wet) kV

s

132

76

76×6.5=495

reqd. 5

76×3=228

reqd. 6

7

9/10

220

12

127×6.5=82

9

127×3=381

10

11

15/16

400

7

5

13

231×3.3=76

20

22

24

23

231×5=115

1

5

voltage(kV

No. Of Discs Recommende

Employe

d

d at present

2

It can be worked out to see that lines working at voltages 132 kV and above are immune to lightning provided, of course, if proper shielding and low tower footing resistance are provided. For example, assuming a value of 50 kA (rms.) for the severest lightning discharge and a tower footing resistance of about 10 ohm, the required impulse strength of the insulation should be √2×50×103×10 i.e. 700 kV for a line to be immune from lightning affects. 7discs as recommended in table above for a 132 kV line, would provide impulse strength of almost (695 kV) the same value (700 kV), still better results in this case can be obtained by reducing the tower footing resistance. For 132 kV lines the maximum tower footing resistance kept is 7 ohms.

31

3.8 SELECTION OF GROUND WIRE The primary function of ground wires is to shield the phase conductors from the lightning strokes. They are placed above the phase conductors and are grounded at every/alternate towers. Thus they help in dissipating the lightning currents to the ground. The selection of the number and configuration of the ground wires is of great importance in the protection of transmission line against direct strokes. The number of ground wires may be one or two. A shielding angle of about 30 is considered to be adequate for high voltage lines. However, for high voltages lines in areas with low lightning hazards, shielding angle up to 45 have been used. EHV lines are usually provided with two ground wires and the shielding angle for such lines is kept at about 20 . To prevent back flashover from the earthed metal to the phase conductors, the tower footing resistance should not exceed 10 ohms. The vertical separation between the ground wires and phase conductors should be greater at mid span than at the supports, i.e., the ground wire should have lesser sage as compared to the phase conductors. The material most commonly used for ground wires is galvanised steel. A ground wire should be able to carry the maximum expected lightning current, without undue heating. It should also have sufficient mechanical strength. Experience has shown that if a ground wire is mechanically strong, it can carry the maximum, it can carry the maximum lightning current without excessive heating. Therefore, the size of ground wire is generally decided on the basis of mechanical strength.

3.9 EVALUATION OF LINE PERFORMANCE The line parameters are used to evaluate the efficiency and regulation. It is sufficiently accurate to represent the line by a nominal

or

circuit for the efficiency and regulation

calculations. However, if the line is very long, the calculations should be based on ABCD constants. If the efficiency and regulations are not within the prescribed values, it may be necessary to revise the design by selecting a thick conductor cross-section and changing 32

the conductor configuration. In some cases it may be necessary to use a higher transmission voltage in the revised design.

3.10 HEIGHT OF TOWER Number of insulation strings = x Height of one string = h1 Total height of insulation strings = x × h1 Minimum clearance between the ground and the conductor = h2 Height of tower above insulation strings up to ground wire = h3 Total tower height = (x×h1) + h2 + h3

(3.6)

3.11 LOSSES IN TRANSMISSION LINES Total transmission line losses can be broken down into three relevant parts namely; conductor losses, dielectric heating & radiation losses, and coupling & corona losses. Conductor Losses: Conductor losses is also popularly known as line heating losses since electric current that passes through a conductor releases heat. It is known that any metallic materials possess inherent resistive nature that is why it is inevitable that during electrical flow through these materials unavoidable power loss occurs. Typical transmission line conductors consist of resistance that is uniformly distributed throughout the system; as a result it is safe to say that the total power loss in the line is directly proportional to the square of the current that passes and the total resistance of the wire. In addition to that, resistance of the wire is inversely proportional to the diameter of the conductor thus, the bigger the wire diameter, the lower resistance it can give. The discussion of transmission lines so far has not directly addressed LINE LOSSES; actually some line losses occur in all lines. Line losses may be any of three types COPPER, DIELECTRIC, and RADIATION or INDUCTION LOSSES. NOTE: Transmission lines are sometimes referred to as rf lines. In this text the terms are used interchangeably. Copper Losses 33

One type of copper loss is I 2R LOSS. In rf lines the resistance of the conductors is never equal to zero. Whenever current flows through one of these conductors, some energy is dissipated in the form of heat. This heat loss is a POWER LOSS. With copper braid, which has a resistance higher than solid tubing, this power loss is higher. Another type of copper loss is due to SKIN EFFECT. When dc flows through a conductor, the movement of electrons through the conductor's cross section is uniform. The situation is somewhat different when ac is applied. The expanding and collapsing fields about each electron encircle other electrons. This phenomenon, called SELF INDUCTION, retards the movement of the encircled electrons. The flux density at the center is so great that electron movement at this point is reduced. As frequency is increased, the opposition to the flow of current in the center of the wire increases. Current in the center of the wire becomes smaller and most of the electron flow is on the wire surface. When the frequency applied is 100 megahertz or higher, the electron movement in the center is so small that the center of the wire could be removed without any noticeable effect on current. You should be able to see that the effective cross-sectional area decreases as the frequency increases. Since resistance is inversely proportional to the cross-sectional area, the resistance will increase as the frequency is increased. Also, since power loss increases as resistance increases, power losses increase with an increase in frequency because of skin effect. Dielectric Losses DIELECTRIC LOSSES result from the heating effect on the dielectric material between the conductors. Power from the source is used in heating the dielectric. The heat produced is dissipated into the surrounding medium. When there is no potential difference between two conductors, the atoms in the dielectric material between them are normal and the orbits of the electrons are circular. When there is a potential difference between two conductors, the orbits of the electrons change. The excessive negative charge on one conductor repels electrons on the dielectric toward the positive conductor and thus distorts the orbits of the electrons. A change in the path of electrons requires more energy, introducing a power loss. The atomic structure of rubber is more difficult to distort than the structure of some other dielectric materials. The atoms of materials, such as polyethylene, distort easily. 34

Therefore, polyethylene is often used as a dielectric because less power is consumed when its electron orbits are distorted. Radiation and Induction Losses RADIATION and INDUCTION LOSSES are similar in that both are caused by the fields surrounding the conductors. Induction losses occur when the electromagnetic field about a conductor cuts through any nearby metallic object and a current is induced in that object. As a result, power is dissipated in the object and is lost. Radiation losses occur because some magnetic lines of force about a conductor do not return to the conductor when the cycle alternates. These lines of force are projected into space as radiation and this results in power losses. That is, power is supplied by the source, but is not available to the load. Corona loss Corona as defined by IEEE standard 539-1990 Power lost due to corona process. On overhead power lines, this loss is expressed in watts per meter (W/m) or kilowatts per kilometre (kW/km). A luminous discharge due to ionization of the air surrounding an electrode caused by a voltage gradient exceeding a certain critical value is called corona. What is Corona Effect? One of the phenomena associated with all energized electrical devices, including highvoltage transmission lines, is corona. The localized electric field near a conductor can be sufficiently concentrated to ionize air close to the conductors. This can result in a partial discharge of electrical energy called a corona discharge, or corona. What is Corona? •

Electric transmission lines can generate a small amount of sound energy as a result of corona.

•

Corona is a phenomenon associated with all transmission lines. Under certain conditions, the localized electric field near energized components and conductors 35

can produce a tiny electric discharge or corona that causes the surrounding air molecules to ionize, or undergo a slight localized change of electric charge. •

Utility companies try to reduce the amount of corona because in addition to the low levels of noise that result, corona is a power loss, and in extreme cases, it can damage system components over time.

•

Corona occurs on all types of transmission lines, but it becomes more noticeable at higher voltages (345 kV and higher). Under fair weather conditions, the audible noise from corona is minor and rarely noticed.

•

During wet and humid conditions, water drops collect on the conductors and increase corona activity. Under these conditions, a crackling or humming sound may be heard in the immediate vicinity of the line.

•

Corona results in a power loss. Power losses like corona result in operating inefficiencies and increase the cost of service for all ratepayers; a major concern in transmission line design is the reduction of losses.

Source of Corona: •

The amount of corona produced by a transmission line is a function of the voltage of the line, the diameter of the conductors, the locations of the conductors in relation to each other, the elevation of the line above sea level, the condition of the conductors and hardware, and the local weather conditions

•

The electric field gradient is greatest at the surface of the conductor. Largediameter conductors have lower electric field gradients at the conductor surface and, hence, lower corona than smaller conductors, everything else being equal.

•

Irregularities (such as nicks and scrapes on the conductor surface or sharp edges on suspension hardware) concentrate the electric field at these locations and thus increase the electric field gradient and the resulting corona at these spots.

•

Corona also increases at higher elevations where the density of the atmosphere is less than at sea level. Audible noise will vary with elevation.

•

Raindrops, snow, fog, hoarfrost, and condensation accumulated on the conductor surface are also sources of surface irregularities that can increase corona. 36

•

However, during wet weather, the number of these sources increases (for instance due to rain drops standing on the conductor) and corona effects are therefore greater.

•

Corona produced on a transmission line can be reduced by the design of the transmission line and the selection of hardware and conductors used for the construction of the line.

Physical Parameters of Corona: •

Corona is caused by the ionization of the media (air) surrounding the electrode (conductor)

•

Corona onset is a function of voltage

•

Corona onset is a function of relative air density

•

Corona onset is a function of relative humidity

Methods to reduce Corona Discharge Effect: 1.

By minimizing the voltage stress and electric field gradient.: This is accomplished by using utilizing good high voltage design practices, i.e., maximizing the distance between conductors that have large voltage differentials, using conductors with large radii, and avoiding parts that have sharp points or sharp edges.

2. Surface Treatments: Corona inception voltage can sometimes be increased by using a surface treatment, such as a semiconductor layer, high voltage putty or corona dope. 3. Homogenous Insulators: Use a good, homogeneous insulator. Void free solids, such as properly prepared silicone and epoxy potting materials work well. 4. If you are limited to using air as your insulator, then you are left with geometry as the critical parameter. Finally, ensure that steps are taken to reduce or eliminate unwanted voltage transients, which can cause corona to start. 5. Using Bundled Conductors: on our 345 kV lines, we have installed multiple conductors per phase. This is a common way of increasing the effective diameter 37

of the conductor, which in turn results in less resistance, which in turn reduces losses. 6. Elimination of sharp points: electric charges tend to form on sharp points; therefore when practicable we strive to eliminate sharp points on transmission line components. 7. Using Corona rings: On certain new 345 kV structures, we are now installing corona rings. These rings have smooth round surfaces which are designed to distribute charge across a wider area, thereby reducing the electric field and the resulting corona discharges. 8. Weather: Corona phenomena much worse in foul weather, high altitude 9. New Conductor: New conductors can lead to poor corona performance for a while. 10. By increasing the spacing between the conductors: Corona Discharge Effect can be reduced by increasing the clearance spacing between the phases of the transmission lines. However increase in the phase’s results in heavier metal supports. Cost and Space requirement increases. Corona Detection •

Light Ultraviolet radiation: Corona can be visible in the form of light, typically a purple glow, as corona generally consists of micro arcs. Darkening the environment can help to visualize the corona.

•

Sound (hissing, or cracking as caused by explosive gas expansions): You can often hear corona hissing or cracking Sound.

•

In addition, you can sometimes smell the presence of ozone that was produced by the corona.

•

Salts, sometimes seen as white powder deposits on Conductor.

•

Mechanical erosion of surfaces by ion bombardment

•

Heat (although generally very little, and primarily in the insulator)

38

•

Carbon deposits, thereby creating a path for severe arcing

•

The corona discharges in insulation systems result in voltage transients. These pulses are superimposed on the applied voltage and may be detected, which is precisely what corona detection equipment looks for.

Power factor Power Factor is defined in the fundamentals of electrical engineering as the cosine of the phase angle between the voltage and the current. An inductive circuit is said to have a lagging power factor, and a capacitive circuit is said to have a leading power factor indicate, respectively, whether the current is lagging or leading the applied voltage. (Stevenson Jr.)

CHAPTER-4 SAMPLE EXAMPLE

Example: It is proposed to transmit 80 MW at 0.9 power factor lagging over a distance of 150 km. The line efficiency and regulation at full load should be better than 95% and 10% respectively. Work out the following details of the transmission line. Make suitable

39

assumptions. (a) Select line voltage and number of circuits. (b) Choose proper conductor and span for this line. (c) Select a suitable value of inter-phase spacing and a suitable configuration of conductors. (d) Calculate line parameters. Estimate the line efficiency regulation for full load condition. (e) Estimate corona loss. (f) Find the capacity of shunt compensation equipment to improve the receiving end power factor to 0.95 lagging. (g) Estimate line efficiency and regulation for full load at 0.95 power factor lagging. (h) The line will be erected a temperature of 30°C in still air condition. It is desired that a factor of safety of 2.5 should be maintained under bad weather condition when the temperature is 5°C and wind load is 378 N/m 2 of projected area. Find the sag and tension under erection condition. Also find the sag under the bad weather conditions. (i) Select a suitable number and size of ground wires for this time.

Solution:(a) Using Eq. (3.1) the optimum line voltage is, P   L V = 5.5 +  1 . 6 100  

Where,

1

2

= Line voltage in kV.

= Distance in km. 40

= Power in kW.

= kV = 164.43 kV The nearest standard line voltage is 220 kV. Therefore it should be a 220 kV line. The surge impedance of a single circuit line is about 400 ohms. Surge impedance loading (SIL) = = = 121 MW Since the required power transfer is less than SIL, a single circuit is sufficient. (b)

= = A = 233.27 A

Let the ambient temperature be

. Therefore, temperature rise of

can be allowed.

Referring to Appendix A, a suitable conductor for this current is ACSR 6/1/3.66 mm conductor . It is necessary to calculate the line losses and the line efficiency to check the suitability of this conductor. Line losses are approximately equal to resistance per phase

.

MINK (ACSR 6/1/3.66 mm) :-

41

where

is the total line

For the ACSR 6/1/3.66 mm conductor the resistance at calculate the resistance at

is 0.4565 Ω/km. To

we use Eq.,

= =

303 248

= = = .56 Ω/km = r75 × L

=0.56 = 84 ohms

Line efficiency = = .85 or 85% The efficiency is very poor. Hence this conductor size is not suitable.

TIGER (30/7/2.36 mm):If we choose the ACSR conductor 30/7/2.36 mm conductor. The resistance of this conductor at

is 0.2220 ohms/km.

r75 = r20 ×

303 248 42

= = 0.27 ohms/km R= = 40.68 ohms outputpower

Line efficiency = outputpower + losses = = 0.9233 = 92.33% The efficiency is still poor, that shows Tiger is still not a correct selection.

PANTHER (30/7/3.0mm):For the ACSR 30/7/3.0 mm conductor the resistance at

is 0.140 ohms/km.

= = 0.171 ohms/km R= = 25.65 ohms 80 ×10 6 Line efficiency = 2 80 ×10 6 + 3 × ( 233.27 ) × 25.65

(

) [

]

= 0.95 or 95% The ACSR conductor 30/7/3.0 mm (PANTHER) has much higher current rating than the rated current of the purpose line. The line efficiency for this conductor will be higher than 95%

43

Hence the characteristics of this conductor are: •

Number of aluminium strands

=

30

•

Diameter of each Al strand

=

3.0 mm.

•

Number of steel strands

=

7

•

Diameter of each steel strand

=

3.0 mm

•

overall diameter

=

21 mm

•

Weight of conductor

=

974 kg/km

•

Ultimate strength

=

89.67 kN

•

Cross section area of Al

=

212.1 sq mm

The conditions governing the selection of span has been discussed in section 3.4. Hence experience has shown that a Span of 300 m is suitable for a 220 kV line. Minimum clearance between the ground and the conductor is estimated as 7 m using Table 3.4 Number of insulation strings is calculated as 16 using Table 3.7 Now using Table 3.6 total insulation string length = 0.254 × 16 = 4 m Hence, total tower height using eq. 3.6 is calculated as 4 + 7 + 6 = 17 m

(c) As per values given in Table 3.3, an inter-phase spacing of 6 meters is suitable for a 220 kV line. The conductor configuration can be horizontal or L-type. Choose horizontal configuration and 6 meters spacing between adjacent phases (12 meters between the two phases).

(d)

=

44

= 7.5595 m D = 21 mm so,

r=

21 mm 2

= 10.5 mm = 10.5 ×10 −3 m Geometric mean radius (GMR), = 0.7788 × r = 0.7788 ×10.5 ×10 −3 m = m Line inductance = mH/km = mH/km = 1.36 mH/km Impedence

Z = R+jωL = R+j2πf L

(where l = length of transmission line)

= ohms/phase = 25.65+ j64.1 ohms/phase = 69.04∠68.2° ohms/phase 0.02412 Line capacitance = log Deq  µF/km  r'   

= µF/km = μH/km Y = ωC 45

= = siemens/phase Since the length of the line is 150 km, a sufficiently accurate results can be obtained by the nominal π or T representation. Let we use nominal π representation. 1+

ZY 1 = 1 + ( 69.04∠68.2°)(398.66 ×10 −6 ∠90°) 2 2

=

(

) 1 + 14 ( 69.04∠68.2°)(938.66 ×10

ZY   −6 Y 1 +  = 398.66 ×10 ∠90° 4  



−6

)

 ∠90°  

= =

1 V 3

(V = voltage of transmission line in volts)

= = 127017

V

= = 233.27∠− 25.84° A Now, using equation

 ZY  1+ Z Vs  2  Vr  I  =   ZY ZY  I   s   Y  1+  1+   r    4  2  = = 46

=137745.97 ∠48° = = = 213.31 ∠−13.27°A Sending end power factor = = 0.951 lagging Sending power factor

= = 3 ×137745.97 × 213.31 × 0.951 ×10 −6 = 83.83 MW

Line efficiency

= = 95.43% Vs

at no load

=

1+

ZY 2

=

137745.97 0.987

= 139560.2533 V Regulation

= =

139560.2533 −127017 ×100 127017

=

(e) Taking

θ=

And usin

δ=

=

, Pressure(p) = 74 cm of mercury 3.86 p 273 + θ 3.86 ×74 273 + 50 47

= 0.884 Let,

= 0.84

Using equation, Vd =

Vd =

3 ×10 6 2

3 ×10 6 2

 Deq   r δ m° ln   r 

 7.5595  ×10.5 ×10 −3 × 0.884 × 0.84 ln  3   10.5 ×10 

Vd =108.5 Kv

Vr 127017 = ×10 −3 Vd 108.5

= 1.17 ≈1.2 Using table given in section 6.4 F = 0.08 Corona loss = = = 0.166 The corona loss ( Pc ) of less than 0.2 kW/phase/km is considered to be tolerable. Hence the corona loss for this line is within limits. Total corona loss = 0.166 × 3 ×150 = 74.7 kW (f) When

=

lagging

= 0.4843 Receiving end reactive power = 48

= 38.744 MVar lagging When

=

lagging

= Receiving end reactive power = 80 × 0.3287 = 26.296 MVar lagging Capacity of shunt capacitors to improve the receiving end power factor from 0.9 lagging to 0.95 lagging = 38.744 − 26.296 = 12.448 MVar leading (g) When the receiving end power factor has been improved to 0.95 lagging. =

= = 220.99 = A = 221 ∠−18.20° A = = (127017∠0°) ( 0.987∠0.3°) + ( 221∠−18.20°) ( 69.04∠68.2°) = V ZY  ZY    I s = Vr Y 1 +  + I r 1 +  4  2   

=

(127017∠0°) (396.11 ×10 −6 ∠90.14°) +( 221∠−18.20°) ( 0.987∠0.3°) 49

= 208.12 ∠− 4.6° A Sending end power factor = = 0.985 lagging Sending end power = = 83.47

≈

83.5 MW

Line efficiency = = 95.8 %

at no load =

Vs

= 137521.9 Regulation =

= 135734.13

ZY 1+ 2

0.987

≈

137522 V

137522 −127017 ×100 127017

= 8.27 % (h) d(overall diameter of conductor) = 21 m d = 2.1 ×10 −2 m Area A = number of strands ×π r 2 Where r =

diameter of each strand 2 2

∴

1  3 A = 37 ×π ×  ×  2 1000  

m2

= 2.6 ×10 −4 sq. M Now, Young’s Modulus of elasticity E = 91.4 × 10 9 And co-efficient of linear expansion α = 18.44 ×10 −6 (Where E and α are constants) 50

N / m2 per °C

Weight w = 974 kg/km

w = w ×g ×10 −3 = 974 ×9.8 = 9.54 N/m For bad weather conditions (subscript) Fw = wind load × d =378 ×2.1 ×10 −2

= 7.938 N/m

(w

Ft1 =

2

+ Fw2 )

(9.54 ) 2

=

+ ( 7.938)

2

= 12.41 N/m T1 =

=

ultimate strenght safety factor

89.67 2 .5

kN

= 35868 N

, θ1 = 5 °C

For erection condition (subscript 2) , θ2 = 30°C

T2 = ?

Using equation,   A E Ft1 l 2  T  T2 −  T1 − α A E ( θ 2 − θ1 ) −  = 24 T12    2 2

Where T1 and T2 are tension in N, Α is area in sq m α is co-efficient of linear expansion, 51

A E Ft 22 l 2 24

E is Young’s Modulus of elasticity in N/m

θ1 and θ 2 are temperatures in °C Ft1 and Ft 2 are forces in N/m

l is the length of span in m

now,

α A E (θ 2 − θ1 ) −

A E Ft12 l 2 24 T12

24

24 T12

A E Ft22 l 2

=

24

2.6 × 10 −4 × 91.4 × (12.41) × ( 300 ) 2

=

=

A E Ft22 l 2

2

A E Ft1 l 2

24 × ( 35868)

2

10645.1613

2.6 × 10 −4 × 91.4 × 10 9 × ( 9.54 ) × ( 300 ) 24 2

= =

[

]

T23 − 14267.6347 T22 − 8110.5 × 10 9

= 8110.5 × 10 9 = 0

Using hit and trial technique, = 26140 N

Sag under erection condition =

w l2 8 T2

m

Where l = length of span S

=

9.54 × 300 × 300 8 × 26140

= .41057

2

8110.5 ×10 9

T22 T 2 − 35868 + 10955.204 + 10645.161

T2

2

≈

4.11 m

Sag under bad weather condition =

Ft1 × l 2 8 T1 52

m

S

=

12.41 × 300 × 300 8 × 35868

=

3.89 m

Vertical sag under bad weather condition Vertical sag

= S cos γ

tan γ = where

Fw = wind load or wind force in N Fw

Where

Fw w + wi

=

pD

p = wind pressure D =

Since tan γ =

diameter of conductor + diameter of ice coating Fw w + wi

 Fw = tan -1   w + wi

  

hence

γ

Where

w = weight of conductor wi =

weight of ice

Since there’s no ice hence γ

=

F  tan −1  w   w 

vertical sag

=

S cos γ

=

 −1  Fw S cos tan   w 

=

 −1  7.938   3.89 × cos tan   9.54  

m   

53

=

2.99

≈

3

m

CHAPTER-5 CONCLUSION In this project we have designed transmission line which comes to be single circuit line since the required power transfer through a given length is less than SIL (Surge impedance loading). As per our design requirement the efficiency and regulation of the line comes within the stipulated limits.

54