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

· Explain

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:

· Reset

· Pick-up

· Type of contacts

· Over-reach

· 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.

TOP

CONTROL & RELAY PANELS

[IS: 8623-1993]

ROUTINE TESTS

S. NO.	Routine test	Purpose
1	Wiring, electrical operation	To inspect the ASSEMBLY including wiring & function
2	Protective measures	To check protective measures by application of control voltage and current injection at secondary
3	Dielectric test at 2.5 kV for 1 min.	To verify withstanding high voltage test
4	· 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.