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 Components of protection schemes

 Unit & non-unit type of protection

 Classification of relay

 Functional requirements of a relay

Methods of discrimination

Power system protection

Basic structure of a relaying scheme








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  1. Which of the following relays gives a fore-warning of a possible major fault

(a) Buchholz relay

(b) Static Relay

  1. Thermal relay

Ans: (a)

2.        Which of the following conditions actuate a negative sequence relay?

  1. L-L fault
  2. Overvoltages in the system
  3. Reverse power flow

Ans: (a)

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

  1. the measured fault impedance is less than the actual one

Ans: (a)

4.        A zero sequence relay responds to

(a) L-G fault

(b) L-L fault

(c) 3-phase fault

Ans: (a)

5.        A typical zone 1 fault clearing time in a 3-zone distance protection scheme is about

  1. 20 ms
  2. 80 ms
  3. 200ms

Ans. (b)

6.        A typical zone 2 reach of a 3-zone protection scheme is

  1. 50%
  2. 80%
  3. 150%

Ans: (c)






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.


Components of protection schemes


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



Unit & non-unit type of protection


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.


Classification of relay


Based upon

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?


Functional requirements of a relay


1.        Reliability

2.        Sensitivity

3.        Discrimination

4.        Speed of operation


What are internal & through faults?


Methods of discrimination


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


Basic structure of a relaying scheme


The blocks of the scheme shown in FIG.11 should be explained.


Explain the

         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.




[IS: 8623-1993]




S. NO.

Routine test



Wiring, electrical operation

To inspect the ASSEMBLY including wiring & function


Protective measures

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.