Study of high frequency transient overvoltage caused by cable-transformer quarter-wave resonance

摘要

When the substations are separated from the overhead line by an underground cable, surge arresters are usually installed at the junction between the overhead line and the cable. Typical insulation coordination studies are conducted in order to decide if another set of surge arresters is required at the entrance of the substation. In this paper, a less classic issue is investigated. It studies the conditions of occurrence of strong overvoltages due to the interaction between a quarter-wave underground cable and the HV/MV power transformer, following a lightning strike. These overvoltages may appear on the secondary side of the transformer even if at the primary side they are well below the lightning withstand voltage of the equipment. The theoretical background is presented and the high frequency models of the system's component are detailed. In particular, a high-frequency model of the transformer is developed. An EMTP-like program is used to simulate the time domain electromagnetic behavior of the system and to investigate the different resonance frequencies. It is shown that, in configurations where the transformer is unloaded, the cable exhibits a quarter-wave behavior at specific lengths and frequencies which leads to high overvoltages. The connection of the load modifies the frequency response of the system and eliminates the risk of appearance of these critical overvoltages.