Overvoltages in Medium Voltage Industrial Systems Associated with Switching High Voltage Transformers Connected Through Insulated Cables

This work presents and discusses the solutions to overvoltages experienced by the medium voltage system (13.8 kV) in an existing refinery resulting from the switching of power transformers supplied by a high voltage system (230 kV) through insulated cables. The analysis demonstrates the influence of the other feeder cables and the transient interaction between them and the switched feeder, due to their high capacitances, leading to the increase of overvoltages to values over the insulation withstand. These overvoltages exhibit high frequency and high amplitude and are primarily transferred from the high voltage side to the medium voltage side mainly through transformer capacitive couplings. This process may lead to insulation breakdown in medium voltage equipment, as confirmed by simulations and practical observations. Solutions for overvoltage mitigation, such as surge arresters and pre-insertion resistors applied to switching circuit breakers, are presented.