Power system resonance excited by VFD ground fault

This paper investigates the resonance phenomenon produced by a voltage source PWM variable frequency drive (VFD) on an ungrounded delta system. Many drives are installed on ungrounded delta systems without isolation transformers or other means to provide inverter ground reference. A drive output ground fault will not be detected by either the VFD or conventional power system ground fault protective relaying. Without a ground reference, the drive continues to operate with the motor phase grounded. This condition modulates the power system neutral and excites a system resonance when the system loading and damping are optimal. Since this ground condition can go undetected for long periods of time, there is a high probability the system will experience optimal resonance conditions. This condition excites a triplen harmonic above the 30th and produces amplitudes of 2.6 times voltage. The resonance bursts were recorded for as long as 200 milliseconds and a secondary resonance was recorded for longer periods. The secondary resonance is almost as destructive but lower in amplitude. These resonances will fail many types of electrical equipment. The problem may go undetected or unresolved for a long time. This paper presents the excitation mechanism, resulting resonance and ways to avoid the problem.