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Protection Systems & Devices (Relays) 3rd Year

CHAPTER 3 Types of Relays according to their function 1. Directional Relays Directional relays are used where it is desirable to trip the circuit breaker for current flow in one direction. That is, the direction is made responsive to the directional flow of power. This is achieved by making the relay distinguish certain differences in phase angle between current and reference voltage or current. The directional relay has a current winding and directional winding. The current winding is connected to the current transformer, whereas the directional winding is connected to the potential transformers to provide the circuit voltage for polarizing the unit. Therefore, the pick-up of the relay is dependent on the magnitude of current and voltage and the phase relationship between them. The directional relay thus establishes one boundary of the protected zone; that is, it protects the circuit only in one direction. Directional relaying is often used where coordination becomes a problem, such as in tie lines between two supply substations or to provide protection against the motoring of a generator.

Directional relay responds to the flow of power in a definite direction or the flow of current in a particular direction. They can be directional power relays, directional over-current relays or directional earth relays. In these relays, the induction type Whatthour construction can be modified to sense the direction. This can be done by using two actuating coils called current coil and voltage coil as shown in figure. 19

Protection Systems & Devices (Relays) 3rd Year

When fault takes place, the fault current flows in the current coil and produces a flux in the upper magnet (ΦI) and the voltage coil produces a flux in the lower Φv . The two fluxes produce a torque = KVI sin (θ + α) Where θ is the angle between V & I ( leading ) α is the angle between V & Φv ( lagging ) o when α = α1 then ( θ + α ) = 90 and the torque becomes maximum . When θ + α = 0 or 180o , the torque = zero. When θ + α between 180o and 360o, the relay will restrain .

2. Over Current Protection Over current protection is that protection which the relay picks up when the magnitude of the current exceeds the pickup level. Over current protection includes the protection from overloads, which means that equipment takes more current than its rated current. This is usually protected by thermal relays. Overload and maximum permissible temperature rise have limits based on insulation class. When excessive current flows in a circuit, it is necessary to trip the circuit breaker protecting that circuit. This type of protection is usually provided by either time-delay or instantaneous overcurrent relays. The instantaneous relay, although inherently fast, requires a short time to operate, whereas time-delay relays have intentional time delay built into them to provide coordination with other overcurrent relays for selectivity. The selectivity is obtained by adjustment of current setting (sensitivity) and time, using the most applicable of several time-characteristics. The relay time characteristics differ by the rate at which the time of operation of the relay decreases as the current increases. The time characteristics for each family of overcurrent relay consist of inverse, very inverse, extremely inverse, definite time, short time, and long time. The application of overcurrent relay is generally more difficult and less permanent than that of other types of relaying. This is because the operation of overcurrent relays is affected by variations of short-circuit current magnitudes. These magnitude variations in short-circuit current are caused by changes in system operation and system configuration. Short circuit protection is provided using fuses or circuit breakers fitted with or tripped by over current relays or series connected trip coils operating switching devices. Overcurrent protection is the most basic form of short circuit protection, but in many parts of the system it is not adequate to provide the level of protection (dependability, security, selectivity, etc.) that is demanded of the modern electric power system protective relaying system. Nevertheless, in some areas it still finds application, especially on radial feeders that serve only loads, on small and medium sizes of motors, and on certain transformer primaries. Each of these, and other, applications will be covered in detail later, but we will find it helpful to review the fundamental basis for overcurrent relays now. Besides introducing an important topic, it will allow some more concrete examples of the philosophical issues previously discussed. Instantaneous overcurrent relays

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Protection Systems & Devices (Relays) 3rd Year

To protect against very severe short circuits, such as a bolted three-phase short circuit close to the source end of a line, the overcurrent relay may be used to instantaneously trip the circuit breaker. Here instantaneous means with no intentional time delay. Obviously, this is an example of a pure magnitude type relay, which lacks flexibility to protect against low-level short-circuit currents, such as a line-to-line short circuit at the far end of a long line. However, the most damaging faults may be detected much more quickly by instantaneous overcurrent than any other type of relay. Consequently, this relay is often used with relatively high settings to detect very severe faults, in conjunction with other types of relays to detect the other faults. Time-overcurrent current relays

A more or less obvious improvement is to introduce time as a variable, producing the timeovercurrent relay. This device will trip its breaker if a short-circuit current exists for a certain time, but not if the same current exists for a shorter time. This allows much more flexibility in coordination between adjacent relays on a line, or between line and transformer protection. The over current protection should not operate for starting current, permissible over current or current surges. To achieve this, the time delay is provided or high-set instantaneous relay is used. Over current protection should be coordinated with neighboring over current protections for discrimination. Over current protection is used for motors and transformers. For small motors, thermal relays and HRC fuses are employed. Thermal relays are used for overload and HRC fuses for short circuits. For large motors (larger than 1200 HP) relays are used. Also, for transformers less than 500 KVA, fuses are usually used. For large transformers, differential relays or O.C. relays are used.

3. Earth Fault Protection (ground fault protection) When the fault current flows through earth return path, the fault is called earth fault. The following are some methods of earth fault protection. a- Residually connected earth fault relay

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Protection Systems & Devices (Relays) 3rd Year

In the absence of earth fault Ia + Ib + Ic = 0 = IR In case of fault IR ≠ 0 then the relay operates b- Neutral to earth protection

c- Core balance current transformers

d- Frame leakage protection

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Protection Systems & Devices (Relays) 3rd Year

4. Reverse Power Protection In directional over current relay, the directional element does not measure the magnitude of power. It senses only direction of power flow. The over current protection responds to over current for a particular direction flow. If power flow is in the opposite direction, the directional over current protection does not operate. Reverse power protection operates when the power direction is reversed in relation to the normal working direction. It senses both the magnitude and direction of power flow.

5. Differential Protection Differential protection responds to vector difference between two or more similar electric quantities. It is used for protection of large transformers, generators, motors, feeders and bus bars. a- Circulating current differential (Merz - Prize) This relay may operate even for external faults or may loose its stability for through faults due to C.T. Ratio errors during short circuit or saturation of C.T. magnetic circuits during short circuit conditions. Also, magnetizing current ( in rush current ) during switching or changing the tap changer may cause the relay to trip at normal conditions. To overcome these problems we may use the percentage differential relay or biased differential relays.

b- Biased or percent differential relay In this relay, the operating coil is connected to the mid point of the restraining coil. The total number of ampere turns in the restraining coil = I1N 2

+

I2N 2

=

( I1 + I2 ) N 2

For external faults, I1 & I2 increases and there by the restraining torque increases which prevents the mal- operation. c- Balanced voltage differential protection

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Protection Systems & Devices (Relays) 3rd Year

6. Distance Relays Distance relays differ in principle from other forms of protection in that their performance is not governed by magnitude of the current or voltage but rather on the ratio of these two quantities. In impedance relays, there is a balance between voltage and current the ratio of which can be expressed in terms of impedance which represents a measure of distance along a transmissions line. a- Impedance relay In impedance relays, the torque produced by a current element is balanced against the torque of a voltage element. The current element produces positive ( pickup ) torque proportional to I2 whereas voltage element produces negative ( reset ) torque proportional to V2. The torque equation is T = K1I2 – K2V2 – K3 where K3 is the control spring effect At the balance point, T = 0 then K1I2 = K2V2 + K3 or K2V2 = K1I2 – K3 V2 = K1 I2 K2

K3 K2I2

or

V = Z = √ K1 - K3 ≈ √ K1 I K2 K2I2 K2

Then

Z = √ K1 = constant K2 The operating characteristic in terms of voltage and current is shown in fig.1, where the effect of the control spring is shown as causing a noticeable bend in the characteristic only at the low-current band. For all practical purposes, the dished line, which represents a constant value of Z, may be considered the operating characteristic.

Z=

1 Slope of the line

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Protection Systems & Devices (Relays) 3rd Year

A much more useful way of showing the operating characteristic of distance relay is by means of “Impedance diagram” or the “ R – X diagram “. The numerical value of the ratio V to I is shown as the length of a radius vector Z and the phase angle between V and I determines the position of the vector. Any value of Z less than the radius of the circle will result in the production of a positive torque and any value of Z greater than Z will produce negative torque regardless of the phase angle V & I . OY represents the feeder on the RX diagram. If fault occurs within distance OZ, the relay operates. For faults beyond Z, the relay does not operate.

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Protection Systems & Devices (Relays) 3rd Year

b- Directional Impedance Relay Directional feature senses the direction in which the fault power flows with respect to the location of CT and VT. This means that the directional unit will permit tripping only in its positive torque region. The net result is that the tripping will occur only for points that arc both within the circle (characteristic impedance) and above the directional unit characteristic. This means that the relay will operate only when faults occur on one side of the relay.

c- Modified ( shifted ) characteristic The modified impedance relay is like the impedance type except that the impedance characteristic is shifted by a “ current bias “ which merely consists of introducing into the voltage supply an additional voltage proportional to the current ( I 2 ), then T = K1I2 – K2 ( V + cI )2

d- Reactance type distance relay The reactance type is an over current relay with directional restraint. The directional element is arranged to develop maximum negative torque when its current lags the voltage by 90 o.

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Protection Systems & Devices (Relays) 3rd Year

T = K1I2 – K2VI sinθ - K3 Or

V sinθ = Z sinθ = X = constant I which is a straight line on the R-X diagram. In other words the relay operates only on the reactance.

e- Mho type distance relay It is also called the admittance relay. This relay is similar to the impedance relay but is made inherently directional by the addition of a voltage winding known as the polarizing winding. The characteristic equation of this relay is Z = K cos (θ - α) K1 This equation represents a circle of diameter K = ZR K1 This is the ohmic setting of relay which passes through the origin. This means that the characteristic equation of a Mho relay is a circle passing through the origin.

C

A

B

We consider the two lines AB and AC, with mho relay located at (A), it will only operate for faults occurring in the line AB but not for faults in the line AC. This relay does not need a directional relay (separate) because it is inherently directional. Notice: Any fixed setting such as 30 o, 45o, 60o or 75o can be given to ( α ) which is called the characteristic angle of the relay.

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Protection Systems & Devices (Relays) 3rd Year

Example: A line section has an impedance of 2.8 +J5 ohms. Show this on R-X diagram as impedance vector. If the relay is adjusted to just operate for a zero impedance short circuit at the end of the line section. Show the operating characteristic of: 1. an impedance relay. 2. a reactance relay. 3. a mho relay ( assume that the centre of the mho relay operating characteristic lies on the line impedance vector. If the arcing short circuit occurs having an impedance of 1.5 + j 0 ohms anywhere along the line. Find for each type the maximum protected portion of the line. Solution : OA = 2.8 + J 5 The circle with O as centre & OA as radius represents the characteristic of the impedance relay. The circle with OA is the diameter and passing by the origin is the mho relay characteristic. The line parallel to OB and passing by A is the characteristic of the reactance relay. The impedance of the S.C. = 1.5 + J 0 represented by the line OD. The total impedance of the line & the S.C. is the line from D and parallel to OA. i.e. DFE. This line cuts the mho circle in F and the impedance circle in E. Drawing FN and EM parallel to OD, then the protected part of the line in the mho relay id ON and for the impedance relay is OM. The percentage protected zones are ON / OA = 82 % for the mho relay and OM / OA = 85 % for the impedance relay. The reactance relay is unaffected and the percentage protected zone = 100 %.

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