Shunt Capacitor Energization with Vacuum Interrupters - A Possible Source of Overvoltage

The energization of a large capacitor bank with vacuum interrupters coupled with the simultaneous failure of a load transformer on a radial feeder prompted this study. The study reveals a source of overvoltage accompanied by sudden and severe voltage reversals at the transformer terminals. This phenomenon is characterized by the momentary interruption and subsequent reignitions of the two predominant transient high-natural-frequency oscillatory capacitor closing currents. The momentary interruptions take place at numerous higher natural-frequency current zeros, which occur during the short interval between the prestrike of the arc and the metal-to-metal closure of the contacts and at the first lower natural- frequency current zero, which occurs either during the short interval between the prestrike of the arc and the closing of the metallic contacts, or during the contact separation due to mechanical contact bounce, or both. Analysis is in two parts, field tests and PACE computer analysis. The computer studies duplicate field test results and permit the evaluation of various combinations of overhead and underground circuit parameters and corrective measures.