Ian Culbert (deceased)
Iris Power-Qualitrol

Editor’s Note:  The following article was written by Ian Culbert, an engineer with Iris Power - Qualitrol in Mississauga, Ontario, Canada. It was submitted for publication by John Letal of Iris Power - Qualitrol and member of EASA’s Technical Services Committee. Sadly, Mr. Culbert passed away recently. We appreciate his contributions to the industry. 

Introduction
Due to their ever decreasing costs, variable frequency drives (VFDs) are becoming more popular for energy conservation and the reduction in inrush currents during motor starting. The most widely used type of drive today is a voltage-source, with pulse width modulated (VS-PWM) inverter, since it tends to be less expensive than other possible topologies.

In the past decade, medium and high voltage motors rated 2.4 kV to 13.8 kV fed by VS-PWM drives have become more common. Currently motors rated up to 100 MW are being designed. Motors supplied from such drives have seen premature stator winding failures since the voltage impulses from the drive can lead to rapid insulation system aging. In most cases, as the aging progresses, the partial discharge (PD) activity increases. Thus, there is a desire for on-line PD detection for such motors. 

Unfortunately measurements with conventional electrical PD detectors tend to be masked by 1000-3000 V impulses created by multi-stage VS-PWM drives. The rise time of the voltage impulses from the multi-stage VS-PWM drives tend to be 500 ns or longer. This article describes the technical issues in on-line PD detection on motors fed by VS-PWM drives, and gives an example of one system that detected the PD successfully.

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