Quantification of transient current signals in the viewpoint of overcurrent relays

Transient currents are quite common in power systems. They can be due to both faults and nonfault switching events. In the case of nonfault switching events, such transient currents should not lead to overcurrent relays operate. This paper seeks to study the transient currents in the viewpoint of overcurrent relays. Starting from relay operation principles, the possible impact of transients on overcurrent relays is analyzed. Quantification is made based on their detrimental effect on relays, i.e. the potential risk of relay mal-trip caused by these transients. Cases on both static and digital relays are analyzed. An impact severity curve is developed for each transient current to assess its impact on parameter selection of each relay characteristic type. Collection of these curves leads to formation of transient current database for overcurrent relays.