Classification of Protective Relay and Busbar Arrangement

 Classification of Protective Relay and Busbar Arrangement

Calculation of Relay Operation Time 

• For calculating actual relay operating time, we need to know these following operation

• Current setting. 

• Fault current level. 

• Ratio of current transformer. 

• Time / PSM curve. (A plotted graph showing the relation between time of operation and plug setting multiplier is plotted on a graph paper) 

• Time setting. Calculation of Relay Operation Time 

• Step-1: From CT ratio, we first see the rated secondary current of CT. Say the CT ratio is 100 / 1 A, i.e. secondary current of CT is 1 A. 

• Step-2 From current setting we calculate the pick up current of the relay. Say current setting of the relay is 150 % therefore pick up current of the relay is 1 × 150% = 1.5 A. 

• Step-3 Now we have to calculate PSM for the specified faulty current level. For that, we have to first divide primary faulty current by CT ratio to get relay faulty current. Say the faulty current level is 1500 A, in the CT primary, hence secondary equivalent of faulty current is (1500x1)/100 = 15 A 

 • Step-4 :Now, after calculating PSM, we have to find out the total time of operation of the relay from Time / PSM curve. From the curve, say we found the time of operation of relay is 3 second for PSM = 10. 

• Step-5 Finally that operating time of relay would be multiplied with time setting multiplier, in order to get actual time of operation of relay. Hence say time setting of the relay is 0.1. Therefore actual time of operation of the relay for PSM 10, is 3 × 0.1 = 0.3 sec or 300 ms.

Relay 

A Relay is an electromechanical device that can be used to make or break an electrical connection. It consists of a flexible moving mechanical part which can be controlled electronically through an electromagnet, basically, a relay is just like a mechanical switch but you can control it with an electronic signal instead of manually turning it on or off. Again this working principle of relay fits only for the electromechanical relay.