Offline Common-Mode Voltage Based Inverter-Embedded Groundwall Insulation Testing for Motors

Recently, inverter-operated electrical machines have increased significantly. According to multiple works, insulation aging occurs faster for inverter-fed machines due to voltage overshoots and uneven voltage distribution caused by high dv/dt switching at several kHz. This necessitates better insulation monitoring methods so deteriorating insulation quality can be detected before machines fail and industrial process is interrupted. Good monitoring methods achieve a balance between detection reliability and cost requirements. Monitoring methods requiring the installation of expensive/intrusive equipment cannot be justified for all machines. Therefore, a new method for monitoring of ground insulation condition for inverter-fed machines is presented here. It utilizes commonly used sensors usually employed for differential protection and is simple in its application. It can be implemented before/after the motor is started/stopped and can screen out severe insulation deterioration preventing machine failure. The method has been verified experimentally on a 380 V, 18.5 kW induction machine.