Partial Discharge (PD) Measurement

This page is an overview of iTIG partial discharge (PD) measurement. For details about partial discharge detection, including causes of PD, as well as iTIG PD measurement techniques, see Partial Discharge (PD) Test Methods.

Why is Partial Discharge (PD) measurement used?

Measurement of partial discharge in motors and generators is used to assess the condition of insulation systems. Typically PD measurement tests are taken together with other test methods. PD results can provide an early warning of insulation breakdown before a surge test or hipot test fails. Measuring PD over time is a great tool to track the condition of equipment. Knowing its state can provide an opportunity to lengthen the remaining life of a motor.

For a rotating piece of equipment in use, partial discharge during normal operation or during a test is not necessarily bad, but a significant difference in PD can mean there is a problem. The standards do not define a specific level of partial discharge measurement as good or bad. However, rising PD over time, and/or a lower PD inception voltage (PDIV) over time typically indicates breakdown or weakening of the insulation.

Along with a set of other winding tests, as well as physical observations of the behavior of the motor during operation, PD measurements can be used as a tool to confirm suspicions that a motor has problems. In a motor shop or in the field, a PD measurement can be the tie breaker in the determination of pass/fail test results and determine whether or not to schedule reconditioning or replacement.

Partial discharge tests can also be used to find problems in systems powered by inverter drives (VFD or ASD).

In coil and rotating equipment manufacturing, partial discharge measurements are used for quality control to confirm that there is no PD or that the PD is below a certain level.

How PD Measurement Works

There are numerous technologies used to detect and measure partial discharge. To find the presence of PD, first a voltage is applied to the device under test (DUT) from an AC voltage source, AC hipot tester, or surge tester. A detector such as an audio receiver, RF receiver, or a capacitive coupler connected to electronic signal processing devices picks up the partial discharges and quantifies their magnitude.

Electrom Instruments checks the partial discharge measurement during a surge test. A capacitive coupler picks up the PD spikes and feeds them to an oscilloscope and computer for processing. Partial discharge levels in mV, with RPDIV and RPDEV measurements (according to IEC 61934), are displayed and recorded. These test results are automatically stored. Stored results can be compared to earlier test results if the test is part of a maintenance or reliability program for tracking the condition of the device.

Combining the partial discharge test with the surge test is a fast and cost effective way to get partial discharge measurements. No accessories, couplers, or other gadgets are required. All the required pieces for detecting PD are included inside the iTIG motor tester and winding analyzer.

iTIG PDIV and PDEV measurement
iTIG PDIV and PDEV measurement.