Improved overcurrent protection using symmetrical components

Transient currents are quite common in power systems. They can be presented for both faults and switching events. In the case of switching events, such transients should not cause an overcurrent relay operation; therefore a dependable and secure relay response becomes a critical matter. Meanwhile, proper techniques must be used to prevent undesirable relay operations due to transient current and harmonics of current in the power system. This paper uses the concept of symmetrical components to discriminate fault from nonfault events. For this purpose, a criterion function is proposed using the above-mentioned components. The advantage of the proposed algorithm is shown by simulation of some typical transient currents due to transformer energizing and induction motor starting using EMTDC/PSCAD software.