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Power System Protection




Review 1 - Introduction 1. Why are protection systems required for power systems? 2. How do the short circuits or abnormal operations affect power systems? 3. What is the first automatic protection device and explain where these are used now. 4. What are the main functions of protective relays and those of circuit breakers? 5. Explain the general characteristics/features that define the quality of protective relaying. 6. Identify the basic information necessary to design a protection system for a power system. 7. What is primary protection, backup protection, local backup protection and remote backup protection? 8. Define unit protection and non unit protection. 9. Give examples for unit protection and non unit protection schemes 10. Describe in general the protection schemes employed to protect Sri Lanka’s power system from LV level upto the generators.


Review 2 - Relays 1. Draw a diagram showing the basic structure of a protection system showing the flow o f information from one unit to another. 2. What is relay rated current, rated voltage and setting? 3. Compare the functions of an auxiliary relay with a protection relay. 4. What are normally open contacts, normally closed contacts, time delayed contacts? 5. What is meant by the auxiliary supply for protection systems and identify the standard values 6. Identify the different relay types 7. Sate the principle of operation of an induction type relay and explain how this principle is used in the construction of an over current relay and a directional over current relay. 8. Describe the general structure of a numeric relay. 9. Compare the advantages/disadvantages of numeric relays and electro-mechanical relays 10. What are IEDs? 11. Explain the functions of a compact bay control unit? 12. Compare the present day protection schemes with the conventional and name the advantages and disadvantages.

POWER SYSTEME PROTECTION Review 3 – Instrument Transformers 1. What are the main functions of instrument transformers in general and also of CTs for measuring, protection and of voltage transformers? 2. What are the two basic modes of operation for transformers and explain the difference between the two. 3. In one mode, instrument transformers can not be operated with an open circuited secondary. Explain why. 4. Explain the operation of CTs and PTs with the help of vector diagrams. 5. What is the main cause of errors in Cts and PTs, explain with the help of vector diagrams 6. Write down equation for the secondary induced voltage for a CT 7. How can the CT polarity, ratio and knee point can be determined? 8. What is knee point voltage and name the factors on which it depends? 9. Discuss the importance of knee point voltage when CTs are used with protective relays? 10. From a CT name plate, identify the cores/windings used for protection and metering 11. Work out the knee point voltages for the different ratios of the above CT and try to establish a relationship. 12. Comment on the CTs, VTs you saw during the visit to a GSS 13. Explain how the windings with different knee point voltages available in the same CT. 14. What are the classes applicable for the protection and metering CTs. 15. Explain the basis used for defining the classes. 16. Why can not metering CTs be used for protection? 17. Explain the difference between 15VA5P10 CT and a 30VA10P20 CT 18. What is meant by the burden of a CT. 19. Calculate the impedance offered by a relay of rated current of 5Amps when used at 20% setting. Assume the relay burden to be 3VA at setting. 20. At 20 times the Plug setting calculate the voltage developed across the relay. 21. What should be the knee point requirements of the CT 22. How do CT requirements under steady state conditions differ from transient conditions? 23. What are the protection schemes that need CTs capable of operating properly under transient conditions? 24. What are the standard secondary currents for CTs. 25. Explain the reasons for choosing a particular secondary current rating for a scheme. 26. What is CVT? Where can those be used? 27. What is the voltage factor and on what basis the voltage transformer classes are decided. 28. Explain the use of Faraday effect and Pockel’s effect in optical voltage transformers. 29. How is Rogowsky coil different from a current transformer? 30. Why couldn’t Rogowsky coils or resistive voltage dividers be used as instrument transformers as the input devices for electro-mechanical relays?

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