# Basics of Protective Relaying in Power System

- Link: https://studyelectrical.com/2016/07/basics-of-protective-relaying-in-power-system.html
- Published: 2016-07-16T10:35:00+00:00
- Author: 

⌊Protective%2BRelaying%2BIntro⌉⌊Protective%2BRelaying%2BIntro⌉
 Protective relaying
is necessary with almost every electrical plant, and no part of the power system
is left unprotected. The choice of protection depends upon several aspects such 
as type and rating of the protected equipment, its importance, location, probable
abnormal conditions, cost, etc. 

Between generators and the final load points, there are several electrical equipment
and machines of various ratings. Each needs certain adequate protection.

The protective relaying senses the abnormal conditions in a part of the power system
and gives an alarm or isolates that part from the health system.

## How Protective Relaying Works?

The [relays ](http://www.studyelectrical.com/2015/08/how-protective-electrical-relays-work.html)
are compact, self-contained devices that respond to an abnormal condition. The relays
distinguish between normal and abnormal conditions. Whenever an abnormal condition
develops, the relays close its contacts. Thereby the trip circuit of the circuit
breaker is closed. Current from the battery supply flows in the trip-coil of the 
circuit-breaker and the circuit breaker opens and the faulty part is disconnected
from the supply. 

The entire process, _‘occurrence of a fault, operation of the relay, the opening
of the circuit breaker, removal of the faulty part from the system’_, is automatic
and fast. Circuit breakers are switching devices that can interrupt normal currents
and fault currents.

Besides relays and [circuit-breakers](https://studyelectrical.com/2018/12/circuit-breaker-operating-principle-and.html),
there are several other important components in the protective, relaying scheme,
these include _protective [current transformers](https://studyelectrical.com/2018/09/current-transformer-ct-construction-working-operating-principle-advantages-uses.html)
and voltage transformers, protective relays, time-delay relays, auxiliary relays,
secondary circuits, trip circuits, auxiliaries, and accessories, etc._ Each component
is important. Protective relaying is teamwork of those components.

## Function of Protective Relaying

The [functions of protective relaying](http://www.studyelectrical.com/2015/12/function-of-protective-relays-in-power-system.html)
include the following :

 * To sound an alarm or to close the trip circuit of circuit-breaker so as to disconnect
   a component during an abnormal condition in the component, which includes overload,
   under-voltage, temperature rise, unbalanced load, reverse power, under-frequency,
   short circuits, etc.

 * To disconnect the abnormally operating part so as to prevent the subsequent faults,
   e.g.over-load protection of a machine protects the machine and prevents insulation
   failure.

 * To disconnect the faulty part quickly so as to minimize the damage to the faulty
   part, e.g. If a machine is disconnected immediately after a winding fault, only
   a few coils may need replacement. If the fault is sustained, the entire winding
   may get damaged and the machine may be beyond repairs.

 * To localize the effect of fault by disconnecting the faulty part, from the healthy
   part, causing the least disturbance to the health system.

 * To disconnect the faulty part quickly so as to improve system stability, service
   continuity, and system performance. Transient stability can be improved by means
   of improved protective relaying.

Faults cannot be avoided completely. They can be minimized. Protective relaying 
plays an important role in minimizing the faults, and also in minimizing the damage
in the event of faults.

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