Relay Cordination

Consider four number of substation, Substation A, Substation B, Substation C, Substation D. Here Substation A is generation station and B, C and D are distribution stations. In this, if the fault (short circuit or earth fault) occurs in Substation D means, the substation D relay has to operate, instead of that, the substation A relay operated means such system said to be poor relay coordinated power system. It causes the total power system shutdown or unnecessary zone trips. Because, there is no fault on substation A, B and C but the substation A operates unnecessarily. In order to avoid such relay operation, we have to set co-ordination between all 4 substations..