Let’s see how Protective Relays are classified

Protective relays and relaying systems detect abnormal conditions like faults in an electrical circuit and operate automatic switchgear to isolate faulty equipment from the system as quickly as possible. 

There are various types of protective relays used in a power system for protection. Normally the actuating quantity is an electrical quantity but sometimes the actuating quantity may be pressure or temperature also. Relays must have certain functional qualities such as reliability, selectivity, speed and sensitivity.

Classification of Relays:

One can classify electrical relays in a number of ways as given below:

1.       According to the function: Relays may be classified as main, auxiliary and signal relays according to their function in the protective scheme. 

Relays which respond to any change in the actuating quantity are called as main relays. The auxiliary or supplementary relays are those relays which are controlled by other relays to perform some auxiliary function. The auxiliary function may be the introduction of a time delay, increase in number of contacts, increase in making or breaking capacity.

Signal relay's function is to indicate the operation of some relay with the help of flag or target. Simultaneously, these relays may also actuate an alarm circuit.

2.       According to the nature of the actuating quantity: Relays may also be classified according to the nature of the actuating quantity i.e. as current, voltage, impedance, frequency relays etc.

Such relays are also differentiated as over relays and under relays. Relays which respond to the actuating quantity when they exceed a predetermined value are called “over-relays”, e.g. over-current relay. Relays which operate when the value of the actuating quantity drops below a predetermined value are called “under relays”, e.g. under-voltage relay, under-frequency relay etc.

3.       According to the connection of the sensing element: According to the connection of the sensing elements, relays may be classified as primary and secondary relays. Primary relays are those relays whose sensing elements are directly connected in the circuit or element they are supposed to protect. The sensing elements of secondary relays on the other hand are connected through a CT and/or PT. Relays normally used in the power system protection are the secondary relays because of the involvement of heavy currents and high voltages.

4.       According to the action upon the circuit breaker: Relays are divided as direct acting relays and indirect acting relays according to the method by which these relays act upon the circuit breaker. Direct acting relays are those relays whose control element act mechanically to operate a circuit breaker whereas in an indirect relay, the control element switches in an auxiliary power source to operate the circuit breaker.

5.       According to the principle of operation and construction: The protective relays used in an electrical system can be broadly classified as electro-magnetic relays and static relays. According to the principle of operation and construction, they may be further classified as electromagnetic attracted armature type, electromagnetic induction type, moving coil type etc.

6.       According to the time of operation: The relays can also be classified according to the timing characteristics i.e. as instantaneous relays, definite time-lag relays, inverse time-lag relay and inverse definite minimum time relays.

Instantaneous relays are those relays in which operation takes place after a negligible small interval of time after the incidence of the operating quantity. 
In definite time-lag relays the time of operation is quite independent of the magnitude of the actuating quantity. Similarly, in inverse time-lag relays, the time of operation is approximately inversely proportional to the magnitude of the quantity causing the operation of the relay. 
For inverse definite minimum time relays, the time of operation is inversely proportional to the smaller values of actuating quantities and tends to a definite minimum time as the value increases.        

Protective Relays and their functional characteristics

An electric power system should ensure the availability of electrical energy without interruption to every load connected to the system. The most severe electrical failures in a power system are the shunt faults which are characterized by an increase in system current, reduction in voltage, power factor and frequency. 

"Protective relays and relaying systems detect abnormal conditions like faults in the electrical circuits and operate automatic switchgear to isolate faulty equipment/equipments from the system as quickly as possible."

Thus a protective relay is an automatic device which senses an abnormal condition in an electrical circuit and closes its contacts. These contacts in turn close the circuit breaker trip coil circuit, thereby it opens the circuit breaker and the faulty part of the line or equipment is disconnected from the rest of the system. 

The protective relays should operate only the concerned circuit breaker so as to disconnect the faulty equipment from the system as quickly as possible without affecting the healthy section. Relays are tested thoroughly to ensure that they will operate correctly and will assist the related circuit breaker to clear a fault, only within their specified zone. Figure 1 shows the wiring connection of Relay and Circuit Breaker.  

Fig.1: Wiring connection of Relay and Circuit Breaker.

The protective relays do not eliminate the possibility of fault occurrence. It can take action only after the fault has occurred. Situation would be ideal if the protection system could anticipate and prevent fault occurrence but this is next to impossible except where the original cause of fault creates some effect which can operate a protective relay. However Buchholz relay is one of such devices which can anticipate and prevent major faults.

Functional Characteristics of Protective Relays:

A protective relay should have certain qualities. These essential qualities are:

1.      Reliability – A protective should be reliable and must operate when it is required. Every component and circuit which is involved in the operation of the relay is vital and should be considered as a potential source of failure. Failure can be reduced by reliable design well supported by regular and thorough maintenance. Some features of design and manufacture which make relays inherently reliable are good contact material, high contact pressures, dust free enclosures, well braced joints and robust construction.

2.      Selectivity – The protective relay must be capable of selecting the part of the system which is faulty and should isolate it from the healthy one i.e. a relay should differentiate between the faulty part and the healthy part.

3.      Speed – A protective relay must be quick acting and fast otherwise may result in damage to the equipment and the system. The operating time of a relay is of the order of 30 to 100 ms, depending upon the fault level. Also the relays should not be extremely fast (less than 10 ms), otherwise it may result in undesired operation during transient faults such as lightning surges.

4.      Sensitivity – Sensitivity of a protective relay refers to the smallest value of actuating quantity at which the protective relay starts operating in relation with the minimum value of fault current in the protected zone.   

Faults in Power System

"A fault in an electrical circuit is defined as the defect in the electrical circuit because of which the current in the circuit is diverted from the intended path."

For example suppose a circuit has two parallel paths, opening up of a path will divert the current to the other path and in the process it may damage the path or the conductor. Thus, faults can damage or disrupt the power system in many ways.

Causes:

In a power system, the faults occur because of insulation failure which may be because of a system over-voltage such as switching surges or lightning stroke. Faults may also be due to a broken insulator or a conductor. Various other reasons such as improper operating habits may also lead to a fault; for example, loading a distribution transformer beyond its normal rated capacity. 

Nearly one half of the faults occur on power lines which are widely branched, have greater length, operate under variable weather conditions and are more exposed to atmospheric disturbances.  

Also read:

• Classification of  Electrical Faults

Effects of fault

Faults give rise to abnormal operating conditions. When a fault occurs at any point in the power system large currents, large forces and or abnormal voltages are developed. The excessive current because of the fault is determined by the internal e.m.f.s of the machines in the network, their impedances, and the impedance in the network between the machines and the fault.

Faults currents, also called short circuit currents, are many times greater than the normal currents. Large voltage stresses the insulation of the various equipments, which are on the way, beyond their breakdown value causing the failure. 

Similarly large currents overheat the equipment or the element of the power system. Sometimes faults lower the system voltage below the permissible voltage limit causing unwanted and teasing interruption of various equipments and components. Faults can also cause a three-phase system to become unbalance.

Action to be performed during a fault

It is necessary that the faults or the faulty section should be removed immediately so that the normal operation of the rest of the system is maintained. The protective relays employed in the power system or network should immediately detect the faults or the faulty section without fail and send trip signal for the operation of circuit breakers.

To obtain proper setting of the protective relays and the interrupting capacities of circuit breakers, the values of these fault currents and voltages should be known with great accuracy. Short circuit studies and calculations provide currents and voltages on a power system during fault conditions.