DET 308 POWER SYSTEM II CHAPTER 3 b

DET 308: POWER SYSTEM II CHAPTER 3 b : PROTECTION POWER SYSTEM

Types of Protective Relays Overcurrent relay Directional relay Differential relay Distance relay Pilot relay

Overcurrent Protection Protection against excess current. Connected to a current transformer and calibrated to operate at or above a specific current level. When the relay operate and energize a trip coil in a circuit breaker and trip (open) the circuit breaker. Protection against current was naturally the earliest protective system to evolve. The relay coordination can be achieved by using either - Time - Current - Combination of both time and current

Overcurrent Protection (cont. ) 1. Discrimination by Time An appropriate time interval is given to ensure that the breaker nearest to the fault opens first. Fig 1: Radial System with Time Discrimination Each protection unit comprises a definite time delay in which the operation of the current sensitive element simply initiates the time delay element.

Overcurrent Protection (cont. ) The relay at H is a set the shortest time delay permissible to allow a fuse to blow for a fault on the secondary side of transformer G. A time delay of 0. 25 is adequate. The disadvantage is that the longest fault clearance time occurs for faults in the section closest to the power source where the fault level (MVA) is highest.

Overcurrent Protection (cont. ) 2. Discrimination by Currents The fault current varies with the position of the fault because of the difference in impedance values between the source and the fault. The relays controlling the various circuit breakers are set to operate at suitable tapered values such that only the relay nearest to the fault trips its breaker.

Overcurrent Protection (cont. ) Fig 2: Radial System with Current Discrimination

Overcurrent Protection (cont. ) Relay setting of 8800 A at J would protect the whole of the cable section between J and H. However, the disadvantages: - It is not practical to distinguish between a fault at F 1 and a fault at F 2, which is a short distance. - Variations in the source fault level would not protect the section concerned. Therefore, the method is not practical proposition for correct grading between circuit breakers J and H. However, the problem changes when there is significant impedance between the two circuit breakers concerned.

Overcurrent Protection (cont. ) 3. Discrimination by Time and Current The relay operation time is inversely proportional to the fault current level. The actual characteristic is a function of both ‘time’ and ‘current’ settings. Fig 3: Radial System with Discrimination by Time and Current

Overcurrent Protection (cont. ) Fig 4: Example of Protection Coordination

Inverse Definite Minimum Time (IDMT) Overcurrent Relay IDMT overcurrent relay is a device which senses the current flow in one phase of an electrical power component, such as transmission line and generates a trip signal after a delay. This delay is short for heavy fault current, long for lighter fault current and infinite for load current. There are two basic type of IDMT relay: solid state and electromagnetic induction. The most widely used is the induction-disc type relay.

Inverse Definite Minimum Time (IDMT) Overcurrent Relay (cont. ) 1. Basic Principle and Construction of IDMT Relay The relay works same as induction motors. They need only one electrical quantity for operation: alternating primary current. The relay comprises of an electromagnetic system which operates on a movable conductor, usually in form of a metal disc. This disc, made from copper or aluminium is free to rotate between the poles of the magnetic system. The movement depends upon the torque produced on it. These two fluxes is divided into two components

Basic Principle and Construction of IDMT Relay (cont. ) The torque is produced by the interaction of 2 electromagnetic fluxes which induce eddy current in the disc. These fluxes must be mutually displaced both in space and phase to cause a torque. Due to only a single energizing quantity involved in this relay, these two fluxes are divided into two components as shown in Figure 5(a). Displacement is then achieved by putting a copper band called shaded ring around one portion of the poles.

Basic Principle and Construction of IDMT Relay (Cont. ) Fig 5: Inverse Time-Overcurrent Relay with C-type Magnet Core

Basic Principle and Construction of IDMT Relay (Cont. ) The flux, φ1 induced a voltage, E 1 in the disc as shown in Figure 5(b). The eddy current, I 1 is produced by E 1 lagging with small angle. the flux, φ2 lags by an angle α behind φ1. The torque of the relay is proportional to the sine product of the two magnetic fluxes cutting the disc. So the torque, T is given by: Т∞φ1φ2 sinα The fluxes are produced by the same coil current and the geometry is fixed, so the torque is proportional to the square of current, I: Т = ΚΙ 2 Thus the bigger current will produced bigger torque and therefore the faster operating time to trip.

Relay Adjustments Three types of relay adjustments: Current plug setting (PS) Time setting multiple (TSM) Grading margin time

Current Plug Setting (PS) Taps on the coil of the relay are used to adjust the current setting which are call the current plug setting. This adjusts the setting by means of a plug bridge, which varies the effective turns on the upper electromagnet. The plug setting (PS) can be given either directly in amperes or indirectly as a percentage of the rated current. They are usually in the range from 50% to 200%, in 25% intervals, of the rated relay current. Effectively, the relay characteristic is moved horizontally by altering the plug setting, PS.

Time Setting Multiple (TSM) The distance traveled by the moving contacts can be controlled by means of the time setting multiple (TSM). This adjustment alters the position of the backstop of the disc with reference to the moving contact. This effectively varies the operating time of the relay. On most designs of relay, dial positions are available with increments of 0. 005 to the highest setting of 1. 0. The effect of altering the TSM is to move the characteristic vertically. Changing both PS and TSM results in moving the

Grading Margin Time 1. 2. 3. 4. The operating time of IDMT relay must be long enough to ensure that the circuit breaker nearest to the fault trips first but short enough to trip its circuit breaker. This grading margin (or time interval) is depend upon several factors: Circuit breaker interrupting time: the circuit breaker must interrupt the fault current completely before the back up relay ceases to be energized. Overshoot time: when the driving torque is removed, the relay disc continues to rotate, and the time of rotation after the relay is de-energised is call the overshoot time. Errors: all measuring devices, including the relay, are subject to manufacturer’s error and a tolerance must be allowed. Final margin: some extra allowance is required to ensure that a satisfactory operation of the relay is obtained. This also called safety margin.

Earth Fault Protection More sensitive protection against earth faults can be obtained by using a relay which respond only to the residual current of the system, since the residual component only exists when fault currents flows to earth. Thus the earth fault relay is unaffected by load current. It can be setting which is limited by the design of the equipment only. However, setting of few percentage tolerances is considered due to maybe unbalanced leakage or capacitance currents to earth may produce residue current that can damage the equipments.

Fig 6: Residual connection of current transformers to earth fault relays

Determination of IDMT Relay Setting Two methods: Graphical method Mathematical analysis

Calculation of Relay Settings

Where, CT = current transformer primary rating in A PS = plug setting TSM = time setting multiplier RSI = relay setting current in A FC = fault current in A PSM = relay plug setting multiple RCOT = relay characteristic operating time in s TSM = relay time setting multiple ROT = real relay operating time in s DT = discrimination time in s Rn. OT = operating time of relay nearest the fault in s Rj. OT = operating time of the next adjacent relay in s

Example: Setting Radial System Fig 7: An example of Setting Radial System Table 1: Data for Radial System

Example: Solution

Fig 8: IDMT Relay Characteristic

Example

Example

Example Table 2: Relay Time Setting Calculation Fault Location Relay at D C B A PSM RCOT TSM ROT D 13. 95 2. 6 0. 05 0. 13 C 6. 975 3. 6 0. 175 0. 63 13. 45 2. 6 0. 175 0. 455 B 5. 38 4. 1 0. 233 0. 955 9 3. 15 0. 233 0. 735 A 7. 5 3. 45 0. 358 1. 235 13. 08 2. 65 0. 358 0. 95

(b) Ring Circuits A ring system is usually applied to a situation where the continuity of supply is the priority, such as hospital and mines. Each substation in the ring has either one or two infeeds supplied from the same sources. Directional relays are commonly applied to ring systems.

Ring Circuits Fig 9: Grading of a ring circuit

Ring Circuits (cont. ) The usual procedure for grading relays in interconnected system is to open the ring at the supply point and to grade the relay first clockwise and then anti-clockwise. Thus, they are treated as two radial circuit superimposed on each other as Figure 9. The relays looking in a clockwise: sequence 12 -3 -4 -5 -6 and anti-clockwise: sequence 5 -4 -32 -1 -6. The arrow in Figure 9 indicates the direction of current flow that will cause the relay to operate.

Ring Circuits (cont. ) Double arrow is used to indicate a non-directional relay. The relay at R(1, 6) and R(5, 6) are the starting point for the setting. Can be set independently of load currents. Furthermore, these relay should operate as fast as possible, so that their setting can be set the minimum. Usual values assigned to this situation are 50% PS and 0. 1 TSM. The procedure to determine the remaining relay setting is same with the radial system that has been discussed before.

Distance Protection Faults on a power system circuit should be cleared quickly; otherwise could result in the disconnection of customers, loss of stability in the system and damage to equipment. Distance protection meets the requirements of reliability and speed needed to protect these circuits, and for these reasons is extensively used on power system networks. Distance protection is non unit type of protection It has the ability to discriminate between faults occurring the different parts of the system depending on the impedance measured. A relay that operates on the basis of voltage-to-current ratio is called an impedance relay (distance relay/ratio relay)

Impedance Relay Block And Trip Region R = Re. Z Fig. 10 Impedance relay block and trip region

Impedance Relay Block And Trip Region (cont. ) Impedance Z is define as the voltage-to-current ratio at the relay location. The relay trips for |Z|<|Zr|, where Zr is an adjustable relay setting. The impedance circle that define the border between the block and trip region passes through Zr. A straight line is called the line impedance locus. This locus is a plot of positive sequence line impedances, predominantly reactive, as viewed between the relay location and various point along the line. The relay setting Zr is a point in the R-X plane through which the impedance circle that defines the trip-block boundary must pass. This relay is not directional; fault to the left or right of the relay can cause a trip

Impedance Relay With Directional Capability Fig. 11 Impedance relays with directional capability

Impedance Relay With Directional Capability (cont. ) The ways to include directional capability with an impedance relay are shown in Figure 11. In Figure 11(a), an impedance relay with directional restraint is obtain by including a directional relay in series with an impedance relay. In Figure 11(b), a modified impedance relay is obtained by offsetting the center of the impedance circle from the origin. This modification is sometime called as an mho relay. The radius of the impedance circle for the modification impedance relay is half of the corresponding radius for the impedance relay with directional restraint. The modified impedance relay has the advantage of better selectivity for high power factor loads

§ Figure 12 shows the typical block and trip region for both type of three -zone, directional impedance relays Figure 12 Three-zone, directional impedance relay

Figure 13 shows the relay connection for a three-zone impedance relay with directional restraint. Figure 13

Distance Protection: Zones of Protection In general, distance protection includes three steps of protection, with each step reaching a fixed preset distance and operating in a preset time. Zone 1 reaches 80% - 90% of the protected line. The tripping is instantaneous. Zone 2 extend beyond the protected line up to about 50% of the adjacent line. The tripping ha a time delay, usually set to a value between 0. 3 s to 0. 5 s. Zone 3 covers the protected line, the adjacent line and up to 25% of the line next to the adjacent line. Tripping is delayed between 0. 6 s to 1. 0 s.

Example: Three-zone impedance relay setting Table 3 shows the positive-sequence line impedance as well as CT and VT ratios at B 12 for the 345 k. V system as shown in Figure 14. (a) Determine the settings Zr 1, Zr 2 and Zr 3 for the B 12 threezone, directional impedance relays connected as shown in Figure 13. Consider only solid , three-phase faults. (b) Maximum current for line 1 -2 during emergency loading condition is 1500 A at a power factor of 0. 95 lagging. Verify that B 12 does not trip during normal and emergency loadings. Table 3: Data for example

Figure 14: 354 k. V Transmission loop

Example: Solution a) Denoting VLN as the line-to-neutral voltage at bus 1 and IL as the line current through B 12, the primary impedance Z viewed at B 12 is: Using the CT and VT ratios given in Table 5, the secondary impedance viewed by the B 12 impedance relay is

Example: Solution(cont. ) we set the B 12 zone 1 relay for 80% reach, that is, 80% of the line 1 – 2 (secondary) impedance: Setting the B 12 zone 2 relay for 120% reach: From Table 5, line 2 – 4 has a larger impedance than line 2 – 3. Therefore, we set the B 12 zone 3 relay for 100% reach of line 1 – 2 plus 120% reach of line 2 – 4.

Example: Solution(cont. ) b) The secondary impedance viewed by B 12 during emergency loading, using; is:

Example: Solution(cont. ) Since this impedance exceed the zone 3 setting of 9. 07∠ 80. 9 o Ω, the impedance during emergency loading lies outside the trip regions of the three-zone, directional impedance relay. Also, lower line loadings during normal operation will result in even larger impedances farther away from the trip region. B 12 will trip during faults but not during normal and emergency loadings.

Differential Protection Differential relay are commonly used to protect generators, buses and transformer. Figure 15 illustrates the basic method of differential relaying for generator protection. The protection only shown for one phase. The same method shown in Figure 15 is repeated for the other two phases. The current on both sides of the equipment are compared. Under normal conditions, or for a fault outside of the protection zone, current I 1 is equal to current I 2. Therefore, the currents in the current transformers secondary are also equal, i. e. I’ 1– I’ 2 and no current flows through the current relay. If a fault develops inside of the protected zone, current I 1 and I 2 are no longer equal, therefore I’ 1 and I’ 2 are not equal and there is a current flowing through the current relay.

Differential Protection (cont. ) Figure 15: Differential Protection Of A Generator

END OF CHAPTER 3