Practical Rules to Adjust the Numerical Overcurrent Functions of Protective Relays to Avoid Maloperation due to Inrush Currents of Downstream Distribution Transformers

This article shows practical rules which were developed for the adjustment of Numerical Overcurrent Functions (NOCF) microprocessor-based protective relays in order to avoid their maloperation due Transformer Magnetizing Inrush Currents (TMIC). The TMIC waveforms are decaying and nonsinusoidal, effects different on instantaneous, definite-time, or inverse-time NOCF; thus, specific necessary each type NOCF. Experimental methods, specifically perform tests NOCF with TMIC, applied four commercial from manufacturers. As relay has types curves, a total 26 curves included set tests. In practice, behavior cannot be assumed as known during coordination protection; therefore, based worst cases results (i.e., incorrect trip any analyzed relays). input data estimated cumulative RMS values since there is long tradition estimation. at time intervals, should divided by numerical factors proposed (to obtain suitable limits inappropriate operation). illustrate application rules, solutions two related simple example shown end this article.