8/4/2019 Prot 123 Reviews
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UNIVERSITY OF MOARATUWA
POWER SYSTEME PROTECTION
EE 4
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 andremote backup protection?
8. Define unit protection and non unit protection.
9. Give examples for unit protection and non unit protection
schemes10. Describe in general the protection schemes employed to
protect Sri Lankas
power system from LV level upto the generators.
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UNIVERSITY OF MOARATUWA
POWER SYSTEME PROTECTION
EE 4
Review 2 - Relays
1. Draw a diagram showing the basic structure of a protection
system showing theflow 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 delayedcontacts?
5. What is meant by the auxiliary supply for protection systems
and identify the
standard values6. 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 overcurrent relay.
8. Describe the general structure of a numeric relay.
9. Compare the advantages/disadvantages of numeric relays and
electro-mechanicalrelays
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 theadvantages and disadvantages.
8/4/2019 Prot 123 Reviews
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UNIVERSITY OF MOARATUWA
POWER SYSTEME PROTECTION
EE 4
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 circuitedsecondary. 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
CT7. 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 of the
Rathmalana GSS, 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 Rathmalana GSS
13. Explain how the knee point voltages change with different
tappings for the same winding.
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 undertransient 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 Pockels effect in
optical voltage transformers.29. How is Rogowsky coil different
from a current transformer?
30. Why couldnt Rogowsky coils or resistive voltage dividers be
used as instrumenttransformers as the input devices for
electro-mechanical relays?
