Behavior of HV cable of power supply at short circuit and related phenomena

Abstract

Discharges in many HV loads are unavoidable at voltages close to their operational limits. Such loads may be vacuum gaps, e.g., X-ray tubes. The discharge characteristics depend not only on the state of the load, but, in the case of a vacuum gap, on external circuitry. In cabled connections, the cable length is critical. Long cables may decrease the breakdown voltage, which is mostly overlooked in literature. Selected experimental data and means of improving performance are reviewed. Regarding methods of cable connections, we consider two cases. In the first, regular connection, the cable shield is connected to ground on both sides. Then the processes in the cable can be described by conventional transmission line equations. Pattern of traveling waves developing at short-circuit conditions and overvoltages (OV) at the power supply side are shown as a function of the cable parameters. In the second case, the shield at the power supply side is grounded, and at the load side it is floating (unterminated shield connection). It is shown that conventional two-wire transmission line model is no longer applicable. PSpice equivalent circuits with lumped parameters are developed and analyzed. It is shown that the cable insulation is overstressed at the load side in unterminated shield connections, and at the power supply side in regular connections. Experimental results obtained on low-voltage models are presented.