Transient Electromagnetic Emission Analysis Used to Study Controlled Switching of Ungrounded Shunt Capacitor Banks

Controlled switching has been used on Hydro-Quebec's high voltage (HV) network for about 25 years. Controlled switching has been generally proven to be quite efficient in controlling inrush currents and transient overvoltages. However, in certain circumstances, the system behaves in unexpected ways due to multiple prestrikes and dynamic voltage sharing between interconnected interrupters. This study focuses on switching capacitor banks with an ungrounded neutral. An innovative diagnostic system based on transient electromagnetic emissions analysis is used to confirm the presence of multiple prestrikes during energization of ungrounded capacitor bank. Simulations show that the third phase to close can be subjected to unexpected overvoltages which can result in a premature electrical making instant thus causing synchronization alarms and high inrush currents. This paper presents an analysis when switching a 120 kV 96 Mvar ungrounded neutral capacitor bank, using results obtained from the described system. Finally an optimized switching scheme is discussed.