Stator insulation weak spot localization on a 6 kV synchronous motor

This work presents the results of a stator insulation weak spot localization measurement on a 6 kV 5.5 MVA motor. The motor in question was build in 1953 and its class B insulation system has been in operation ever since. After 50 years of running, the motor was removed and a new one installed. Visual inspection of the stator didn´t show any serious damage, tear or wear. The owner intends to use the motor as a replacement motor in case of an emergency. To increase motor reliability, a stator insulation system replacement is considered. Since the visual inspection didn´t show any particular reason to replace the entire insulation system, several diagnostic measurements were performed. This paper describes the results of a PD localization measurement performed on this motor. This novel PD localization technique uses the time of flight of the PD signal through the stator core to pin point the actual PD site of every PD occurrence. The measurement technique and its results will be discussed in-depth. The PD localization measurement detected a region in the insulation system with an elevated PD activity. This localized elevated PD activity supports a decision on which region of the insulation system is best suited for replacement. This is in contrast to replacing the entire insulation system.