Motor Insulation Degradation Due to Switching Surges and Surge Protection Requirements

Failure investigation of four 21,000 HP synchronous motors concluded that the cause is related to voltage surges and as a result surge protection was applied to all 13.2 kV motors in the plant. (Ref.l) Further failures of three 21,000 HP synchronous motors after the installation of surge arresters indicated an inadequacy of the specified surge protection. Two of the failed motors were tested at the manufacturer´s facility to investigate the cause of failure. Test results concluded that insulation failure was caused by the combined effects of prior insulation degradation and inadequate surge protection. This paper presents an investigation into the three failed motors.