EE448-Electrical Power Engineering-lecture 14.pptx

electric power protection

1. EE448-Electrical Power Engineering Dr. Mona Ibrahim

2. Lecture ( 14) –transmission line protection using distance relays

3. contents

4. What are distance relays? As the length of electrical transmission line is generally long enough and it runs through open atmosphere , the probability of occurring fault in electrical power transmission line is much higher than that of transformers and alternators . That is why a transmission line requires much more protective schemes than a transformer and an alternator .

5. Features of protection of transmission line During fault, the only circuit breaker closest to the fault point should be tripped . If the circuit breaker closest the faulty point, fails to trip the circuit breaker just next to this breaker will trip as back up. The operating time of relay associated with protection of line should be as minimum as possible in order to prevent unnecessary tripping of circuit breakers associated with other healthy parts of power system. What are distance relays?

6. A distance protection element measures the quotient V/I (impedance), considering the phase angle between the voltage V and the current I. In the event of a fault, sudden changes occur in measured voltage and current, causing a variation in the measured impedance. The measured impedance is then compared against the set value. Distance element will trip the relay (a trip command will be issued to the CB associated with the relay) if the measured value of the impedance is less then the value set . What are distance relays?

7. What are distance relays? The voltage element of the relay is excited through a potential transformer (P.T.) from the line to be protected. The current element of the relay is excited from a current transformer (C.T.) in series with the line. The portion AB of the line is the protected zone. Under normal operating conditions , the impedance of the protected zone is Z . Under fault condition , the relay is so designed that it closes its contacts whenever impedance of the protected section falls below the pre determined value i.e. Z in this case.

8. Protection Zones of Distance Relays The need for zones arises from the need of selective protection; i.e. the distance element should only trip faulty section. We can set the distance element to only trigger a trip signal for faults within a certain distance from the relay, which is called the distance element reach . The setting impedance is represented by Zr = hs . Z L , where Z L is the line impedance. The distance element will only trip when the measured impedance Z is less than or equal to the setting impedance hs . Z L

9. Protection Zones of Distance Relays

10. Protection Zones of Distance Relays

11. Protection Zones of Distance Relays

12. Protection Zones of Distance Relays

13. Protection Zones of Distance Relays

14. Protection Zones of Distance Relays

15. Example 1

16. Example 2

17. Example 3

18. Example 4

19. Example 5