OVER CURRENT RELAY The relay has two settings. These are the time setting and the plug setting. The time setting decides the operating time of the relay while the plug setting decides the current required for the relay to pick up.

The plug-setting multiplier, PSM, is defined as follows: where I relay is the current through the relay operating coil and PS is the plug-setting of the relay = pickup current. The value of PSM tells us about the severity of the current as seen by the relay A PSM less than 1 means that normal load current is flowing. At PSM > 1, the relay is supposed to pick up. Higher values of PSM indicate how serious the fault is. For example, let us consider a 1.0 A relay (i.e. a relay with current coil designed to carry 1.0 A on a continuous basis) whose plug has been set at 0.5 A, i.e. at 50%. Assume that, for a certain fault, the relay current is 5.0 A. The relay, therefore, is said to be operating at a PSM of (5.0/0.5) = 10.

Inverse Time Over-current Relay Inverse time characteristic fits in very well, with the requirement that the more severe a fault is, the faster it should be cleared to avoid damage to the apparatus

Inverse definite minimum time (IDMT) over-current relay This is possibly the most widely used characteristic. The characteristic is inverse in the initial part, which tends to a definite minimum operating time as the current becomes very high. The operating time is Very inverse time over-current relay The inverseness of this characteristic is higher than that of the IDMT characteristic. The operating time is Extremely inverse time over-current relay The inverseness of this characteristic is higher than that of the very inverse characteristic. The operating time is

Application of IDMT Relays for Protection of a Distribution Feeder The purpose of relay R B is to provide primary protection to line BC. The purpose of relay R A is to provide primary protection to line AB and back-up to line BC. We start the setting process from the tail end of the system. (a) Deciding the CT ratios and plug settings (pickup current): (i) At relay B, the maximum load current, assuming 25% overload is: 80 A + (0.25 x 80 A) = 100 A Assuming 1 A relay to be used, the CT ratio can be selected to be 100 : 1. The plug setting PS can be done at 100%, i.e. PS = 1.0 A.

(ii) At relay A the maximum load current, assuming 25% overload is (160+80)+0.25(160+80) = 300 Assuming 1 A relay to be used, the CT ratio can be selected to be 300 : 1. The plug 1)setting can be done at 100%, i.e. PS = 1.0 A. (b) Deciding the time-multiplier settings: Starting from the most remote relay R B (i) Since R B does not have to maintain selectivity with any other relay, it can be made to operate the fastest. Thus the TMS of R B can be selected as 0.1. (ii) Now, to maintain selectivity between R A and R B, the following constraint must be met:

The operating time of R B for maximum fault just beyond bus B can be found from For maximum fault at B, fault current = 3000 A on primary side which becomes (3000/100) = 30 A secondary. Since plug setting is done at 1.0 A, The TMS of R B has already been set at 0.1. Substituting these values, we get