Exploration of Transients by Switching Capacitors

Induced control circuit transients from energizing the first 230-kV shunt capacitor banks at Pinnacle Peak Substation damaged solid-state devices, caused flashovers at terminal blocks and initiated false supervisory control indications. Simulated, reduced voltage and full voltage switching of 230-kV capacitors were performed to explore the transient problem. The nature of the transient, its transfer mechanism to control circuits, how to reduce it at the source, and how to improve immunity of control circuits to the transient were studied. Simulation tests used actual power circuits in an inservice substation, which were impulsed at 20 kV. Reduced voltage tests were made on an isolated system at 30-50% voltage. Full voltage tests were made on a 230-kV interconnected system. Plastic-jacketed and lead- sheathed control cables and copper-braid- shielded coaxial cable specimens in a variety of grounding and shielding techniques were tested. Radio frequency, electromagnetic and electrostatic components of control circuit transients were measured separately. Transients up to 7,500 volts on unshielded cables were indicated, and frequencies-from 6-80 kHz and 0.4-7 MHz were recorded. Power circuit lumped parameter oscillations and distributed parameter propagation and reflection-type transient components were observed. Unshielded control cables were protected by grounding spare conductors and aluminum trench covers, paralleling two No. 4/ 0 bare ground cables, and installing 0.1 ¿F bypass capacitors. Switches with 1 602 resistors per phase were used to suppress transients at the source. With these measures, control circuit transients were reduced to 65 volts for energizing 230-kV capacitors on an interconnected system.