Impacts of Earth Fault on Neighboring AC Motor Drives in Isolated Neutral (IT) Grid Systems

The aim of this paper is to analyze the impact of earth fault in one drive on the other neighboring drives connected in a multi motor drives application supplied from an isolated neutral or ungrounded grid system. It has been found that if one drive has an earth fault on the motor side (named as ‘faulted drive’), the fault current finds a path to charge the DC link capacitors of neighboring drive (named as ‘victim drive’) through the coupling or common mode capacitors, especially if the drive is not enabled (i.e. idle condition). It is identified that this increase in the DC-link voltage (charge-up of DC capacitors of ‘victim drive’) depends on various parameters and operational conditions such as fault resistance, magnitude of supply voltage, switching frequency of faulted drive, common mode capacitance value, and capacitance of input cable. A detailed analysis and simulations have been carried out to understand the influence of these forementioned parameters on the DC-link voltage of the victim drive. A simple solution is also proposed to overcome the impact this earth fault on neighboring drives. The study has also been verified by an experimental test at system level.