PROTECTIVE RELAYING                          CHAPTER - 3

INTRODUCTION

In order to generate electric power and transmit it to customer’s millions of rupees must be spent on power system equipment. The equipment is designed to work in normal conditions. But faults may occur due to failure of insulation in the following cases:

1.    Over voltages due to switching

2.    Over voltage due to direct and indirect lightening strokes

3.    Bridging of conductors by birds

4.    Breakdown of insulation due to decrease of its dielectric strength

5.    Mechanical damage to the equipment.

     This causes short circuit currents to flow. These short circuit currents may cause a heavy damage to equipment. To protect the system under abnormal conditions protective relays are used.

“Protective relays are the devices that detect abnormal conditions in electrical circuits by constantly measuring the electrical quantities which are different under normal and fault conditions.”

Under abnormal condition current, voltage, phase angle, frequency may change. The relay operates when it detects the fault which completes the trip circuit and thus opening the circuit breaker which disconnects the fault circuit.

PROTECTIVE RELAY

It is a device that detects the fault and indicates the operation of C.B.  to isolate the defective element from the rest of the system as explained below.

                                               

A typical relay circuit is shown when S.C. occurs at point F on transmission line. C.T. current increases current in relay coil which closes trip coil of C.B. and C.B. contacts get open. Thus faulty section is isolated.

Protective Zones

The protective relaying of a power system is planned along with system design. The C.Bs. are located at appropriate points. Depending upon the rating of the machine, its location, importance, probability of faults and abnormal conditions etc. each power system component of transmission line, e.g. bus bar, cables, capacitors, loads etc, is covered by protective zone.

A part of the system protected by a certain protective scheme is called as protective zone or zone of protection.

The entire part of a power system is covered by several protective zones and no part of the system is left unprotected.

Overlapping of protected zones is done to ensure safety of each and every element in the system. If there is a fault in the region where overlapping is done, more breakers will open than min. necessary to trip or disconnects the faulty part. But if there were no overlaps between the zones then the region between the zones would not lie in either region. Therefore, no C.B. will be opened and if C.B. is not opened then this may cause damage to the rest of the healthy system. For this reason certain amount of overlaps are provided between adjacent zones.

Probability of failures in overlapping regions is very low hence tripping of too many C.Bs. will be quite infrequent.

Primary and Backup Protection

Primary protection (main protection) is the first to act i,e. first line of defence and is responsible to protect power system element from all types of fault. If primary protection fails then backup protection comes into action and removes the faulty part from the healthy system.

Backup protection is provided for the following reasons:

i)        If due to some reason (e.g. failure of relay, auxiliary relay failure, fault in C.T., P.T., trip circuit, C.B. etc.) the main protection fails then there should be an additional protection otherwise the fault will not be cleared and it will damage the equipments.

ii)       If maintenance, testing work is to be carried out on protective equipments, the main protection. protection is made inoperative and backup protection acts like main

The arrangement of backup relay should be such that anything that might cause failure in primary relaying should not cause failure in backup relaying. This can be satisfied only if backup and primary relays have nothing common. Therefore, generally backup relays are at different stations from primary relays.

Another important thing is that backup relaying must operate with sufficient time delay so that primary relaying is given a chance to operate.

Generally, backup protection is given against short circuit protection only for economy.

The Methods of backup Protection

1.    Relay backup: Same breaker is used for primary and backup protection, only separate trip coils are provided.

2.    Breaker backup: Different breakers are provided fro primary and backup protection and both are at the same station.

3.    Remote backup: Primary and backup protection is provided at different stations and are completely independent.

4.    Centrally co-ordinate backup: Main protection is at various stations and backup protection is at central control centre.

Backup by Time Grading Principle

                                   

This is widely used. The current at various points in a path is measured e.g. at source, intermediate location like substations etc., consumer’s end etc.

Consider above figure. If fault occurs beyond ‘C’ region C.B. at ‘C’ region operates i.e. primary protection. In the meanwhile relays at B and A may also start operating. They have given enough time lag, so that if breaker at ‘C’ does not operate, breaker at ‘B’ will operate providing backup protection.

Backup by Duplication Principle

Very popular in U.S.A. Both primary and backup protections are provided at same station. The protective devices (C.T., P.T., relays etc.) are duplicated. Primary and backup protections operate at same speed. This protection scheme is costly and used in EHV lines after considering economy.

REQUIREMENTS AND QUALITIES OF PROTECTIVE RELAYING

1. Selectivity and Discrimination: Selectivity is the ability of the protective system to select correctly the faulty part of the system and disconnect the faulty part without disturbing rest of the system.

And in order too provide selectivity to the system entire system is divided into several protection zones.

Discrimination means ‘distinguishing the difference between’. This property of protective relaying enables it to distinguish between the normal condition and abnormal condition. The protective scheme should operate during abnormal condition and not during normal condition.

                                   

Refer above figure. If fault occurs at F₁ the CB₁ only should trip and other system continues to work. Similarly for F₂ both CB₃ and CB₂ should open. Thus, the protective relaying should isolate only the faulty part of line without disturbing neighboring line.

2. Speed: The relay system should disconnect the faulty section as fast as possible.

Relay time; It is the time from the instant of actuating element is energized to the instant when relay contacts are closed.

This is important for the following reasons:

a)    Electric equipment may be damaged if it carries fault current for a long time.

b)    Chances of developing other fault due to previous fault are reduced.

c)    Power system stability is improved if speed is high.

3. Sensitivity: It is the ability of the system to operate for lower values of actuating quantity.

Sensitivity of a relay is a function of volt ampere input to the coil of relay necessary for its operation. The smaller the volt ampere the more is the sensitivity of relay. Thus, 1VA relay is more sensitive than 3 VA relay.

4. Reliability: It is the ability of the relay system to operate under predetermined conditions.

The protective relaying should not fail to operate in the events of fault, there should not be any fault in protective relaying components and the protective relaying should not operate unnecessarily.

5. Simplicity: The relaying system should be simple so that it can easily be maintained.

Reliability is closely related to simplicity, the simpler the protective scheme, greater is its reliability.

6. Adequateness: The relay should provided sufficient protection to the system under abnormal conditions.

7. Economy: The most important factor in choice of protective relaying is economic factor.

As a rule the protective gear should not cost more than 5% of total cost. However, the equipment to be protected is of utmost important e.g. generator, main transmission line etc. then more expenses are allowable.

In this course, we are expected to learn in brief some of the important types of relays, their circuit operation and how the circuit is protected in abnormal conditions/faults by the relay operation.

CLASSIFICATION OF RELAYS

a)    As per construction and operation

b)    On the basis of applications.

c)    According to time required for operation.

d)    D) Microprocessor based relays.

A) As Per Construction and Operation:

Following are the different types:

(a)  Electromagnetic attraction type:

1. Attraction armature type:

i)      Plunger type:

ii)     Hi8nged armature

iii)    Balanced beam relay

iv)   Polarized relay.

2. Moving iron type

i) Rotating vane type

3. PMMC relays

i) Coil rotating between poles

ii) Rotating vane type.

b) Electromagnetic induction type

1. Induction disc type

i)  Plunger type

ii) Watt-hour meter type

2. Induction cup type

(C)  Thermal relays

i)        Thermo couple

ii)       Bimetallic strip

(d) Static relays

(e) Gas actuated relays

i) Buchholz relay.

ii) Pressure release valve type

(f) Rectifier relay

(g) Digital relay

(B) On the basis of Application:

Following are the different types

1.    Under voltage, under current, under power relays

2.    Over voltage, over current, over power and reverse power relays

3.    Distance relays.

4.    Differential relays

5.    Directional relays

(C) According to the Required for Operation:

1. Instantaneous relays

2. Definite time-lag relay

3. Inverse time relay

4. Inverse Definite Minimum Time relay (I.D.M.T)   

(D) Microprocessor Based Relays:

1. Micro process (μp) based relay

2. μp based over current relay

3. μp based static relay.