Comparison of Low and High Frequency Partial Discharge Measurements on Stator Windings

Abstract

Partial discharge (PD) testing has been used for over 60 years primarily as a method to assess condition of the stator winding insulation in motors and generators rated 6 kV and above. More recently it has also been used by some machine manufacturers as a means of assuring the quality of the insulation on single winding elements (coils and bars). Although both on-line and off-line tests mainly use a high voltage capacitor to detect the PD, the PD measuring systems in use work either in the low frequency (LF) regime (less than about 1 MHz) or in the very high frequency (VHF) (30-300 MHz) range. By reference to several international standards, published work as well as some experiments described in this contribution, the advantages and disadvantages of the two approaches are compared. Based on this work, it is now clear that off-line PD tests should be done in the LF range. For on-line tests, either method may be used, but use of the VHF method has become more widespread with machine end users, since the owners themselves can perform and interpret the results with a relatively low risk of false indications.

Partial Discharges Monitoring for Electric Machines Diagnosis: A Review

“…Another reason for performing the test in low frequency is that, depending on the location of the sensor relative to the PD source, the high-frequency components of the PD signal induced by the external high voltage source are likely to be attenuated. Therefore, the greater the detection frequency range, the greater the PD detection accuracy [27][28][29][30][31].…”

“…Online measurements of PDs are taken during commissioning and normal operation to analyze the winding insulation and whether it can meet operational reliability standards. For online measurement, IEC 60034-27-2 [32] indicates that any frequency range, whether low (less than 3 MHz), high (3 to 30 MHz), very high (30 to 300 MHz), or ultra high (300 to 3000 MHz), can be used to detect PD; however, measurements of PDs in higher frequencies have greater advantages due to the smaller noise presence in that band [27,30]. On the other hand, high-frequency signals tend to be attenuated more easily, and thus are more effective at detecting PDs close to the excitation terminals.…”