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(a) Buchholz relay
(b) Static Relay
2. Which of the following conditions actuate a negative sequence relay?
3. A relay is said to have an overreach if
(a) it can extend protection over a greater line length
(b) the measured fault impedance is greater than the actual one
4. A zero sequence relay responds to
(a) L-G fault
(b) L-L fault
(c) 3-phase fault
5. A typical zone 1 fault clearing time in a 3-zone distance protection scheme is about
6. A typical zone 2 reach of a 3-zone protection scheme is
It is required to generate sufficient electrical power at the most suitable locality, transmit it to the load centres at the most economic and ecological price giving due regard to security, continuity and reliability. Interconnected operation of a power system leads to economic power supplies. However, it presents stability problems and threatens the reliability & continuity of supply. The timely isolation of faulty sections of the system in the interest of stable operation cannot be overemphasized.
Identify the following components in the simple scheme shown in Fig.10
1. sensors for monitoring the operating conditions
2. relay system to process and decide if isolation is desired and if so to send trip signals to the circuit breaker
3. circuit breakers to effect actual isolation
The efficiency of a protection scheme depends upon its ability to
· correctly identify the faulty section
· isolate it in minimum time
· avoid false tripping
Discuss the basic difference between unit & non-unit type of protection. Give examples.
Distinguish between primary & back-up protection
Stress the need for duplication of protection using 3-zone protection scheme.
1. principle of operation
2. speed of operation
3. derivation of relaying signals
4. functions performed
5. nature of actuating quantity (current, voltage, power, reverse power etc.}
6. number of quantities measured and compared
7. method by which a relay acts upon the circuits protected- Direct/Indirect acting
· Distinguish between time overcurrent, directional and differential relays
1. Back-up protection
2. Directional feature of protection & its application
3. Negative sequence relay
· Explain the principle of operation of the induction relay. How are time & current settings provided?
4. Speed of operation
What are internal & through faults?
1. Discrimination to fault location-
· By time
· By current magnitude
· By time and direction
· By distance measurement
· By time and current magnitude
· By current & voltage balance
· By direction of power
· By phase comparison
2. Discrimination to type of fault
· By sequence networks
3. Discrimination to location & type of fault
The blocks of the scheme shown in FIG.11 should be explained.
· Quantities measured by relays
· Measuring element: means of measurement, CTs, PTS, and their characteristics
· Comparing and control elements
State the functional requirements of a relay.
Explain the requirement of discrimination, and methods employed for achieving proper discrimination.
Explain the following terms:
· Type of contacts
· Definite, inverse, and very inverse characteristics of a relay
Power system protection
Principles of differential protection
· Circulating current systems
Role of stabilizing resistor and biasing
· Balanced voltage systems
Examples of differential relays
Principles of distance protection
3-zone protection, requirements of zonetime settings, schemes of 3-zone protection
Relay characteristics in the impedance plane
Protection of system components
A. Generator protection for
· External faults, thermal overloading, unbalanced loading
· Stator protection for L-L,L-G, and inter-turn faults
· Rotor protection
· Reverse power protection
B. Transformer protection
· Differential protection
· Winding protection
· Buchholz relay
· Over-current protection
C. Busbar protection
1. Explain the methods & type of earth fault protection
2. Explain the different polar characteristics of distance protection indicating positive & negative torque regions
3. Explain the method of 3-zone protection and method of providing it.
Wiring, electrical operation
To inspect the ASSEMBLY including wiring & function
To check protective measures by application of control voltage and current injection at secondary
Dielectric test at 2.5 kV for 1 min.
To verify withstanding high voltage test
· Insulation resistance
· Verification of items as per approved drawing/ Bills of materials
· Verification of routine test reports of major bought out items, like relays , meters etc.
· To verify insulation provided on conductive parts
· To verify the quantities of required material in the panel as per approved drawing/B.O.M.
· To verify routine test certificates of relays , meters, etc.
NOTES: 1. Routine tests are to be considered as Acceptance Tests
2. The routine test Certificates of manufacturer of all bought out items are to be verified.