Page 5 - Basic PD Theory

Partial Discharge for Stator Windings

Traditional Maintenance (Breakdown and Preventive)
Traditional maintenance programs are time-based and failure oriented (breakdown maintenance). Machines are taken out-of-
service at set intervals for off-line testing and maintenance. The tests and maintenance performed are routine, often based on
tradition and not on the condition of the machine. Though routine tests and maintenance are usually a good idea, the tests and
maintenance are often costly and unneeded. In addition, a standard off-line test battery is usually not sufficient to find most
machine problems. Some tests need to be conducted under normal operating voltage, thermal, and mechanical stresses to
thoroughly evaluate the machine so that off-line tests and inspections can be planned at a convenient time such as a plant outage.
Because of the desire to extend time between outages and thus routine maintenance, there is more of a risk of unexpected in-
service failures. Corrective maintenance can be extremely costly and devastating to a process.

Predictive Maintenance (Condition-based)
For assets rated 3.3kV and higher there is an alternative. Though nothing will ever replace a thorough visual inspection for
discovering problems at an early stage, there are better ways to establish a maintenance program – a method intended to reduce
outages, minimize costs related to unnecessary tests or repairs, and eliminate unexpected failures. No method is completely
foolproof, but this approach is intended to give you the most information in an economical way. A true condition-based
maintenance program analyzes all of the available information and costly specialized testing or maintenance is only performed
when the condition of the asset warrants it. The idea is to pre-plan maintenance and testing to reduce outage time, discover
potential problems before unforeseen failures, and find problems from thermal, electrical, mechanical or chemical stresses.
Ultimately, the hope is to keep the asset running longer with economical monitoring and maintenance and to only perform
additional testing and maintenance on the assets that need the most attention.

What are Partial Discharges?
For over 60 years, it has been known that Partial Discharge (PD) is a symptom of several problems caused by thermal,
mechanical (vibration and shock), electrical (voltage), environmental, and chemical stresses [1, 2]. Rarely would any one factor
occur alone; stresses combine leading to many different deterioration processes.
Partial discharges (PD) are small electrical sparks that occur when gas pockets exist within high voltage insulation [1]. Gas
pockets can occur because of the manufacturing process, thermal deterioration, stator winding movement during operation and
other possibilities. As the insulation degrades, the number and magnitude of the PD will increase. These happen because the
breakdown strength of air (3 kV/mm peak) or hydrogen is much lower than that of solid insulation (about 300 kV/mm peak).
The breakdown of gas inside the pockets creates small voltage pulses that can be detected and measured, and therefore the PD
can be monitored. Although the magnitudes of the PD pulses cannot be directly related to the remaining life of the winding
insulation, the rapid increase in the rate of PD activity or high PD levels compared to other similar windings are indicators that
visual inspections and/or other tests are needed to confirm the insulation condition. Hence a PD test assists companies to plan
the maintenance requirements of machine stator windings. For information regarding inspections and other suitable tests, refer
to Electrical Insulation for Rotating Machines [2].
Deterioration of the organic resins in the insulation materials due to PD is an extremely slow process since most stator winding
insulation systems for machines rated greater than 2300 V contain a discharge-resistant material called mica; however, the
prediction of time to failure is impossible. Eventual insulation failure will occur during a switching surge, poor synchronization,
load rejection or some other event causing insulation breakdown at the weakest point. On-line PD tests should be conducted at
regular intervals and, where appropriate or indicated, be supplemented with visual inspections and off-line testing. Consistent
and regular testing will provide the means to assist plant personnel in determining the overall winding insulation condition. The
plant’s history of a machine could be beneficial in deciding the proper course of action.
Because of this relatively slow ageing process, periodic monitoring of the PD activity makes sense. The rule of thumb regarding
the length of time between detection of significant PD and insulation failure due to damage by PD is:
• 10 years for machines over 18 kV • 2 to 3 years for 6 kV machines
• 5 years for 13.8 kV machines • Several months for 3.3 kV and 4 kV machines
The terms “coils” and “bars” are used interchangeably in the text and either of them means the stator winding.

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