Hazardous Temporary Overvoltages From Self-Excited Large Induction Motors—A Case Study

Large induction motors, when they become disconnected from their synchronizing source and isolate with part of the high-voltage system, become self-excited as long as their inertia keeps them spinning and there is enough capacitance from the system to provide the magnetizing current for the machine and maintain air-gap flux. These induction motors, operating as self-excited induction generators, temporarily backfeed the islanded subnetwork. Under these conditions, they act as a weak inductive source and can excite near fundamental frequency series resonance due to interaction with the high voltage system shunt capacitance, such as from long lines or shunt capacitors used for network voltage support. This paper uses an actual case study to provide insight into the phenomenon of self-excitation with dynamic overvoltage with induction motors. A 60-Hz analysis of equivalent sequence circuits was able to predict the occurrence of overvoltage problems and this was then confirmed by detailed studies using an electromagnetic transient program. A protection solution to avoid backfeed was implemented to safeguard the system against these overvoltages from occurring.