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Saturday, 7 February 2015

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.