Does Partial Discharge mean the end is near!?!

In this article, we examine a generator which was experiencing significant Partial Discharge (PD) and the results of testing performed to understand the expected effects on the winding life.

A 150 MWe generator had experienced a tripling of the PD value on the “B” phase, over a period of approximately 21 months.  Based on the PD testing, EME was asked to conduct bar testing to gain further insight on the extent of degradation which the insulation had experienced and implications to the remaining life of the winding.

The bars selected were top bars from slot #1 and #2. These locations are the Phase B line coil and the coil adjacent to it.  The bars were successfully removed and some of the side packing and patches of corona treatment had been severely degraded.   A process specification was developed which performed Voltage Endurance and Partial Discharge Testing.

In addition to the in-service bars which were removed, the site also had 2 spare bars.  The objective of the testing was to utilize the spares to develop a baseline condition and compare the highly degraded in-service bars. The Voltage Endurance was performed on Bars #1, #2, and the spares in accordance with IEEE 1043 and IEEE 1553 for 13.8 kVAC, although the rated voltage in this case is 14.4 kVAC.

What is interesting and somewhat unexpected is that all of the bars were able to sustain a 30kVAC for 405 hours at a temperature of 99-100°C, and that based on these results we could expect a remaining life of 20 years!  This information was critical to the customer and enabled him to make an informed decision that rewinding of the generator could be postponed for approximately 3 years, which would provide ample opportunity to implement a scheduled rewind as compared to a forced outage effort.

Check-out the video of the bars being tested at 50kVAC after completion of the voltage endurance tests!

If you’d like to learn more about this program and our recommendations, please don’t hesitate to contact us.

To get more detail, please contact Alan Spisak at