Insulation Monitoring in Medium-Voltage Propulsion Electric Machine with SiC Based Variable Speed Drive

Variable speed drives (VSDs) are typically employed to control the propulsion electric machines in modern electric ships with integrated power system (IPS) architecture. The pulsed voltage excitations produced by VSDs subject the machine insulation system to severe electrical stresses, and cause concerns over the long-term reliability of the electric machine. The next electric ship technology enabler, silicon carbide (SiC) power semiconductor, intensifies the stress on the insulation system by introducing an even faster voltage switching speed dV/dt. Traditionally online partial discharge (PD) measurements have been used as the indicator of machine insulation condition. The use of VSDs, however, has proposed substantial challenges to the detection of PDs, especially in medium- to high-voltage electric machines.

This paper introduces an experimental machine insulation test platform employing 10-kV SiC-based inverter as the excitation source, medium-voltage form-wound coils as testing samples, and various sensors with post-processing techniques to effectively measure PDs. The experience of using five detectors catering to four different PD physical manifestations is demonstrated. Details are offered for other insulation test setups to adopt this effective system. The PD occurrence behavior, the insulation condition evaluation, and the effect of dV/dt are investigated and summarized. The conclusions offer insights for the design and installation of VSDs, particularly the ones using SiC power semiconductors, in electric ships with a focus on the insulation reliability of propulsion machines.