Effect of shielded distribution cable on very fast transients

Fast transients in power systems can be generated by switching of vacuum and SF6 insulated devices as well as by solid state devices such as those used in variable speed drives. Transients with ns risetimes can be generated which, in a cable-connected system, propagate down the cable to inductive devices such as motors and transformers. In general, the amplitude of such surges is not out of the ordinary; however, the very short risetime can cause unacceptable voltages across the first turn of an inductive device. A cable with high frequency loss does not generally decrease the amplitude of such surges appreciably but can lengthen the risetime substantially by absorbing high frequency energy from the surge. This reduces the voltage across the first turn of inductive devices and thereby protects them from damage and failure caused by such surges. In the present contribution, we quantify the high frequency losses and effect thereof on very fast transients for four types of shielded, 15 kV distribution cable, three made from various EPR compounds and one made from TR–XLPE.