Partial Discharge Measurements by Electrom Instruments

The voltage stress required to generate Partial Discharge (PD) comes from the high-frequency surge test pulses produced by the iTIG II Motor and Winding Analyzer. These pulses are similar in rise-time to the pulses produced by inverter drives and simulates what the motor sees during operation very well.

The iTIG II measures Repetitive Partial Discharge Inception Voltage (RPDIV), Repetitive PD Extinction Voltage (RPDEV), and maximum PD during the Partial Discharge test.

Partial Discharge is a broad-spectrum signal and can be measured in a wide range of frequency bands. Electrom’s iTIG II Motor Analyzer measures PD in the 1MHz to 30MHz range.  This means the partial discharge measurement is done in a range where the signal has relatively high energy compared to measurements done in higher frequency ranges, such as in the GHz range.

PD test results as seen on the iTIG II D

PD test results, including RPDIV and RPDEV, as seen on the iTIG II D

No Accessories Needed: No accessories are required for partial discharge measurement with the iTIG II. The PD coupler is internally in the iTIG II, and the normal output leads used for other tests are also used for partial discharge measurements.

Fully Automatic: The partial discharge measurements can be part of a fully automatic sequence of tests, including everything from inductance, impedance and low resistance measurements to Hipot Step Voltage tests, surge tests, and everything in-between.

All Electrom iTIG II models can be upgraded to include partial discharge measurements. The upgrade may involve hardware and software, or only software.

How Partial Discharge measurement results are used to analyze motor conditions  

Low Voltage Motors:

For low voltage motors, there should be no partial discharge at the normal surge test voltages of 2E+1000V or less. This makes the analysis very easy. If PD is found, the motor has insulation that is breaking down, or the motor is contaminated with a significant amount of “dirt” and/or moisture. The contamination level should already be known from the Insulation Resistance test, and if very “dirty” the motor should be reconditioned before conclusions are made about partial discharge levels.

When a low voltage motor is used with an inverter drive, PD indicates that the system has problems that eventually will fail the motor. Actions should be taken to improve the system to extend the life of the motor. These improvements can include those mentioned in the section called “PD effect on the motor” found here. Re-impregnating the stator can also extend the life of the motor.

If the surge test is done at normal surge test voltages, the detection of PD is an early warning because it is done at a voltage above the peak sinusoidal voltage. If action is not taken, the motor will eventually suffer from turn to turn arcs, and/or arcing to ground with complete breakdown.

Medium Voltage Motors and Inception Voltage:

Medium voltage motors may have some partial discharge that is acceptable at normal surge test voltages but may also have none. If PD is detected, the question is, will it change over time? Since the PD amount is somewhat variable, it is not the best measure to track, at least not as the only value tracked.

The best value to track is the Repetitive Partial Discharge Inception Voltage, or RPDIV.
The RPDIV is defined by the IEC 61934 standard as the voltage at which PD is detected in 5 of 10 consecutive surge pulses.

To find out what the Inception Voltage is, the surge test voltage is automatically ramped up in small steps, and partial discharge is measured at each voltage step. This is done for each phase in a 3-phase motor. The RPDIV is likely to be different from phase to phase. Maximum PD is also displayed and stored during this test.

If PD inception is reached, the key is to track it over time. If it does not decrease significantly, the PD is not harming the insulation or has not started to weaken it yet.

If the partial discharge inception voltage starts dropping, it is a warning that the insulation is breaking down, or that motor contamination is increasing. By its nature, there is some inherent randomness to the PDIV value so it is important to make sure the results are statistically significant by looking for a consistent, multi-point trend or a substantial decrease.

It is important to note that surface PD tends to increase with contamination and moisture on the windings. Therefore, it is best to do the PD tests during similar humidity conditions. Megohm results from Insulation Resistance tests should be taken into consideration in the analysis.

The iTIG II motor analyzer with PD will automatically store all test results including PD, RPDIV and RPDEV so the results are easy to compare and track over time. This can be done with reports that are generated, by scrolling through past tests in the iTIG II or in the TRPro report program on a PC, and by reviewing test summary data generated by the iTIG II in an Excel spreadsheet or database program.