Digital Simulations and Field Measurement of Transients Associated with Large Capactior Bank Switching on Distribution Systems

Several unexplained and unusual arrester failures have been noted on some Canadian power systems. Data collected from one SCADA system has correlated these failures with the deenergizing of large shunt capacitor banks at the next higher voltage level. Laboratory experiments on a typical capacitor switch, digital transient simulations of a typical system where failures had occurred, and staged transient field tests on that system were undertaken to resolve the problem. The laboratory experiments showed that voltage escalation due to multiple restrikes and high frequency current clearing is unlikely for the air magnetic breaker involved. However, the simulations and field tests illustrated the possibility of high transient voltages being coupled through a delta-wye transformer following a single severe restrike. These high frequency and low frequency transients could have caused the arrester failures. It was found that surge arresters placed at the capacitor bank location would not effectively reduce the transferred surge. A solution being pursued by one Canadian utility is replacement of the air magnetic breakers with restrike-free circuit breakers for improved capacitor switching performance.