Per‐Unit System and Fault Calculations

Fault calculations pertain to the determination of current and voltages with their relative phase angles at various points in the power system network; following the occurrence of an abnormal condition such as a fault or open circuit. This chapter focuses on the per‐unit concept and will focus on the application of fault studies to determine adequate relay settings. In the per‐unit system, various physical quantities such as kVA, voltage, current, and impedance are expressed as a decimal fraction of base quantities. The need for symmetrical component theory was derived from the need to provide a systematic way of calculating unbalanced faults or short circuits to properly understand, design, and implement protection systems. Power system equipment represents different values of impedance to positive sequence, negative sequence, and zero‐sequence voltages. Zero‐sequence currents have a path to flow if the neutral of the Y‐connected windings is grounded with the presence of a delta winding.