Protecting High-Voltage Motors Against Switching Overvoltages

In an industrial plant equipped with HV motors, several severe stator failures occurred in spite of the installation of MOA (metal-oxide arrester) surge arresters. All of these motors were switched with vacuum circuit breakers. The purpose of this paper is to demonstrate the necessity of complementary protection against overvoltages by adding C-R (capacitor-resistor) surge suppressors. This paper will first give an overview of the types of switching surges that can occur in this network configuration. Secondly, network simulations using EMTP/ATP software are presented. The object of these simulations is to calculate the overvoltages created during motor switching, to study the evolution of these overvoltages as a function of cable length and motor power, and to check if additional protection needs to be installed on these motors. The comparison is made between the simulated voltages and the motors withstand voltage capability defined in the IEC 60034-15. It can be concluded that even with two MOA surge arresters, the protection of the motor is not guaranteed. A full motor protection is provided by installing C-R surge suppressors thus eliminating multiple reignitions. This paper will give installation recommendations for both the C-R surge suppressors and the MOA surge arresters.