Detecting Partial Discharge in High Voltage Motors and Generators

Sean McCall, Central US Sales Manager, Megger Baker Instruments


High voltage electrical equipment presents many challenges relating to insulation and electric field management. Even when an insulation system shows good integrity when tested, it may be experiencing partial discharge events, which can be difficult to detect. However, it’s important to detect when partial discharge is occurring, as it can degrade insulation leading to actual discharge and subsequent equipment failure. With a focus on high-voltage motors and generators, this presentation considers the environmental contributors to partial discharge, and discusses the various methods for detecting partial discharge activity, using both on-line and off-line methods. On-line methods include acoustic detection, radio frequency detection, high frequency current transformers and capacitive coupling, as well as the detection of ozone gas, sometimes through olfactory means. Off-line methods include RF and capacitive approaches; but the main focus of this presentation is on the use of Resistive Division, which is a technique for capturing partial discharge events during industry-standard surge pulse testing.
The controlled application of short-duration high voltage pulses is primarily used to expose insulation weaknesses and failures between the turns of a coil. However, surge testing also offers a way to stimulate partial discharge (if occurring) and to record and quantify it. The use of PD-on-surge is presented, with examples showing PD events and how they are characterized in terms of inception and extinction voltages, as well as the filtering and processing techniques used to capture actual PD activity and avoid false positives. Analysis results in a “house plot” which is an easy-to-understand presentation of how partial discharge activity occurs with rising voltage.
The presentation concludes with a discussion of how to apply PD-on-surge testing in the contexts both of motor repair/rewind facilities and as a preventive maintenance tool in an industrial context, through periodic testing with trending analysis to observe insulation degradation over time. Finally, reference is given to the applicable industrial standards which discuss partial discharge detection in electrical equipment.

1. Attendees will understand what partial discharge in motors and generators is and how it arises.
2. Attendees will learn about using industry-standard surge testing as a method for detecting and characterizing partial discharge.
3. Attendees will learn how detecting partial discharge is an important component of predictive maintenance for high-voltage equipment.


Sean has been involved with electric motor testing and reliability for 30 years. He began his career as a Submarine Nuclear Electrician’s Mate in the US Navy at 18, from there attaining all maintenance and supervisory qualifications available to his rate and rank throughout his time in service. After his service ended in the 1997, he held a number of positions from Lead Technical Trainer to Chief Engineer to Plant Manager, in each role applying his extensive experience in numerous motor testing disciplines as well as project management, predictive maintenance and reliability, and safety. His varied background allowed him to gain working knowledge of motors, generators, pumps, propulsion, compressors, and various other classes of industrial equipment and their sub-systems. In addition, holding several customer facing primary and secondary positions allowed him to gain an overall view of processes outside of his normal responsibilities.
Since joining Baker Instruments in 2015 he has interfaced with multiple customers and projects, both domestic and international within the roles he has held. Baker Instruments is the world leader in Motor Diagnostic Instrumentation manufacture based in Ft Collins Colorado, USA. His current role is Central United States Regional Account/Sales Manager, and Systems Product Line Manager.