An investigation of voltage sags by a phasor methodology

The occurrence of voltage anomalies in the electric delivery system can disrupt the performance of computers and the processes they control. The occurrence of single line to ground faults on the utility's primary lines (i.e., the most frequent type of utility faults) has a significant impact on the voltage magnitude and phase angle as seen by all the customers connected to the primary circuit. The type of transformer connection (e.g., wye ungrounded - delta) linking a customer to the primary circuit alters the primary voltage sags during primary circuit faults that seen at the customers service entrance. This paper will present the phase and line-to-line voltage sag basic equations for an ungrounded-delta transformer connection due to primary distribution single line phase faults. The basic equations presented provide the framework for evaluating the voltage sags for nine different transformer connections. The significant impact of grounded and ungrounded transformer connections on the magnitude and phase angle of the primary and secondary phase and line voltages will be revealed.