UNIVERSITY OF MORATUWAREVIEW QUESTIONS – OC, EF, DOC and DEF PROTECTION

1. What is the purpose of Over current protection2. How is the discrimination achieved when applying the principle of over current

protection to protect a power system.3. Discuss the advantages and disadvantages of the methods identified in 2.4. What are the main features of inverse time over current protection?5. Name the commonly available inverse time over current characteristics and

identify where each characteristic could be best used.6. What is meant by “achieving discriminative over current grading”?7. What is discriminative margin and how it is derived for grading between relays.8. Describe each of the factors identified in 7 9. Briefly explain plug setting, time multiplier setting and Plug setting multiplier?10. Fill the blanks.

CT ratio

Primary Fault current (A)

Secondary Current (A)

Plug Setting

Relay Type OC, EF, SEF

PSM TMS Operating time (Secs)

NI VT EI

200/5 2000 100% 1.0-do- 20 125% 0.2-do- 75% 20 0.1-do- 1000 10 0.125-do- 200 20% 0.1-do- 60 5% Definite time - ( ) Secs

11. At what fault level should two relays be graded?12. What is the main issue that arises when grading LV and HV over current relays

protecting a Delta star transformer?13. Draw the CT connection diagram for RB Over current and EF relay for a 11kV

network.14. Draw the CT connection diagram for RYB OC and EF for a LV network.15. What is a ZSCT ? State another term for the same unit. What are these used for?16. Explain the use of ZSCT.17. What is polarization?18. Explain the terms current polarization, voltage polarization and dual polarization.19. What is the difference between the maximum torque and and angle of maximum

sensitivity?20. Why are directional over current directional earth fault relays are required?21. What are the factors that need to be considered when selecting inputs for

directional over current and directional earth fault relays?22. What is “open delta connection”? Explain the operation of it by drawing the

connection diagram and the vector diagram.JK-2009 DEC