5.3 Setting Overcurrent Relays

Overcurrent relays are normally supplied with an instantaneous element and a time-delay element within the same unit. When electromechanical relays were more popular, the overcurrent protection was made up from separate single-phase units. The more modern microprocessor protection has a three-phase overcurrent unit and an earth-fault unit within the same case. Setting overcurrent relays involves selecting the parameters that define the required time/current characteristic of both the time-delay and instantaneous units. This process has to be carried out twice, once for the phase relays and then repeated for the earth-fault relays. Although the two processes are similar, the three-phase short-circuit current should be used for setting the phase relays while the phase-to-earth fault current should be used for the earth-fault relays. When calculating the fault currents, the power system is assumed to be in its normal operating state. However, at a busbar that has two or more transformers connected in parallel and protected with relays that do not have the facility of multiple setting groups - the ability to be adjusted to accommodate the prevailing system conditions, which is possible with numerical relays for example - then better discrimination is obtained if the calculations are carried out on the basis of each one of the transformers being out of service in turn. The same procedure can be applied to multiple circuit arrangements.