Detection Of Severe Ground Insulation Deterioration Before Failure in VFD-Fed Motors Using Off-line Common-Mode Voltage

Recently, the number of electrical machines operated via inverters has significantly increased. Multiple works have pointed out that 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 that deteriorating insulation quality can be detected, before machines fail and the industrial process is interrupted. A good monitoring method is one that achieves a balance between the reliability of detection and cost requirements. Monitoring methods requiring the installation of expensive and intrusive sensors cannot be justified for all machines. Therefore, a new method for the monitoring of ground insulation condition for inverter-fed machines applying common-mode voltage at motor standstill is proposed in this paper. The proposed method utilizes the commonly used sensors usually employed for differential protection and is simple in its application. It can be implemented whenever the motor is off-line for a short period of time and can screen out severe insulation deterioration that could lead to forced outages. The method has been verified experimentally in the lab on two, 380 V, 18.5 kW and 5.5 kW induction machines.