Power System Protection lectures22_6.ppt
- 1. Transformer Protection A number of fault conditions can arise within a power transformer. These include: Earth fault on a transformer winding. Core faults due to insulation breakdown which sufficient eddy current to flow causing over heating. Inter-turn faults occur due to winding flashover caused by line surges.
- 2. Phase to phase faults. This are rare in occurrence but will result in a substantial currents of magnitude similar to earth faults. Tank faults due to loss of oil which produces abnormal temperature rises. External abnormal conditions such as overloading, overvoltages due to transient core losses and corresponding temperature rise.
- 3. Transformer Size Fuses usually protect transformer with capacity less than 500 KVA in industry and 2500 KVA in residential areas. With ratings up to 5000 KVA in residential areas, instantaneous and time delay over current relays may be more desirable. For industrial loads greater than 1500 KVA and for transformers that are part of the bulk power system it is recommended to use differential protection on harmonic restraint percentage differential relays. Also, higher voltage leads to more sophisticated and costly the protective device.
- 4. 1. Differential Protection The protection of transformers is usually performed by differential protection. The differential protection responds to the vector difference between two similar quantities. The C.T. connected on the transformer windings should be arranged so that the same current is flowing between the two sides. A General Rule Is To Connect the CTs on Any Star Windings in Delta and In Any Delta Windings Connect CTs in Star Two basic requirements that the relay connection must satisfy are: The relay must not operate for loads or external faults. The relay must operate for internal faults.
- 5. Fig. 1 represents differential protection of Delta-Star transformer
- 6. Fig. 2 shows a star-star transformer to which circulating current protection has been applied. Here it will be noted that the current transformers on both sides are connected in delta.
- 7. Next development was the use of kick of fuses to shunt the relay coils as shown in fig. 2. These fuses are of the time limit type that do not operate in the time of switching under sustained fault conditions, the fuses operate and the current then passes through the relay coil and trip the C.B. This also is a slow protection and may cause some problems. It also depends on the fuse. The next development was to desensitizing the relay for a short period of 0.1 to1 sec during switching. After this time the shunt across the relay coil is removed. This method can lead to switching on a transformer for long period during faults. The latest method adopted is harmonic current restraint.
- 8. The overload fuses shown in fig.2 provide a form of back up protection. In the event of sustained through fault, damage may be caused to the transformer. One or more of the overload fuses will operate; leaving the relay to be fed from one of CTs and thus causing relay operation. Fig. 3 (a) is included to show how had the current transformers been connected in star, operation of the protective relay would occur on a fault outside the protected zone which we wish to avoid while Fig 3 (b) shows how this can be avoided by connecting the current transformer secondaries in delta.
- 11. 2. Frame leakage protection
- 12. 3. Restricted earth fault protection (differential protection )
- 13. Earth faults on secondary side are not reflected on primary side when the primary winding is delta connected or has unearthed star point. In such cases, an earth fault relay connected in residual circuit of 3 CTs on primary side operates on internal faults in primary windings only. Because earth faults on secondary side do not produce zero sequence currents on primary side, restricted earth fault protection may then be used for high speed tripping for faults on star connected earthed secondary winding of power transformers.
- 14. Figure 5 shows the connections of the earth fault relays connected in the residual circuit of the line CTs. Figure 6 shows the connection of the restricted earth fault protection relay in the secondary side and earth fault protection in the primary side. If the fault F1 is beyond the transformer windings, I1 and I2 will flow so that the current in the earth fault relay is negligible. For earth fault within the transformer star connected windings, I2 flows and I1 is negligible. Hence I2 causes the relay to trip the circuit .
- 15. When fault occurs very near the neutral point of the transformer, the voltage available for driving the earth fault current is small and the fault current would be low. If the relay is adjusted to sense such small currents, it may operate under normal unbalance conditions. It is common to set the relay to pick up at about 15% of the rated current. Such setting leaves a portion of the windings unprotected. Therefore it is called unrestricted.
- 17. Busbar Protection Busbars are important and need special attention due to: Fault level at busbars is very high. The faults at busbars cause disconnection of power to large portions of the system. Busbars faults should be cleared in very short time ( 50 ms ) to avoid damage to insulation. Stability of the system is affected by busbars faults.
- 18. It’s desirable to include the following in busbar protection: High speed. Stability for external faults. Freedom from unwanted operation. No operation due to CT saturation. Interlock with generators for over current. Use main and check protection to assure disconnection only when desirable.
- 19. Methods of busbar protection Overcurrent relays of connected circuits. Directional interlock. Differential protection. Frame leakage earth fault.
- 20. 1. Overcurrent Relays The fault in bus A can be sensed by R5 and R4. In this type of protection, all the circuit breakers of the busbar zone are disconnected. These are C1, C2, C3, C4 & C5. R6 & R7 will act as backup protection if C4 and C5 do not clear the fault. For fault in bus B, R8 will sense the fault and C6, C7 and C8 will open. This type of protection is slow and evolves complicated control system to discriminate faults within the zone. Also, the zone of the busbar is not clearly identified.
- 21. 2. Directional Interlock It uses directional relays in source circuits and over current relays in load circuits. It makes the discrimination between internal and external faults possible. The contacts of the relays are interlocked in such a way that if power flows the busbar is sufficiently low, all the circuit breakers on the source side and the load side are tripped.
- 23. 3. Differential Protection For normal conditions, the vector sum of currents entering the bus zone is equal to the vector sum of currents leaving the bus zone. That is, I i = zero During internal faults, the vector sum of currents in the circuits connected to bus bar is equal to fault current, that is, I i = If
- 25. Disadvantages Large number of circuits having different current levels. Saturation of CT cores due to d.c. component in s.c. current. Sectionalizing of bus makes the circuit complicated. 4. Frame leakage Earth Fault
- 26. One of the famous connections for busbars: 2 out of 3 Only 2 of the 3 circuit breakers can operate at the same time. The circuit breakers on the transformers are normally closed but the circuit breaker in the centre of the busbar is normally open.