A new inrush current identification algorithm based on transformer core saturation

Percentage restrained differential protection is widely used as the main protection of bulk power transformers. For any differential current based transformer protection scheme, an important problem is to distinguish the magnetizing inrush from faults to prevent the possible mal-operation of differential relays. Traditionally, the second harmonic restraint principle is adopted as the standard method to maintain the security of relays during the inrush events. In recent years, the improvement of the magnetic characteristics of transformer iron core materials led to the relatively lower second harmonic components in magnetizing inrush currents; in contrast, higher second harmonic components in short circuit currents due to CT saturation and the effect of distributed capacitance of the EHV long line connected to the protected transformer have been observed, which makes the choosing of an appropriate harmonic restraint ratio more difficult. In this paper, a fast flux restraint algorithm used to identify the magnetizing inrush current is presented. Based on the sampled voltage/current values, the algorithm calculates the changing rate of magnetic fluxes in iron core to determine the saturation status of the protected transformer. The saturation status is traced by a group of counters. The counter threshold derived from testing is used to determine if the relay should be blocked. The proposed algorithm can be applied to all kinds of transformer iron core structures and its effectiveness can be verified through the test cases.