New methods for offline PD diagnosis on MV cable systems

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For cable testing and partial discharge (PD) diagnostics many different test voltage wave shapes and frequencies have been established over the past years. Their application is well proven and is guided by IEC and IEEE standards/ recommendations. All off the allowed test voltage wave shapes have got their advantages and disadvantages for testing or for PD diagnosis. For testing purposes the voltage needs to produce enough stress to lead failures to breakdown. The very low frequency (VLF) waveforms turned out to be very effective and economical for that purpose on MV cables, for HV cables this method is not allowed yet, currently there are working groups available investigating the application on HV cables. For a reliable PD diagnosis voltages are needed, with waveforms close to power frequency and in its application non-destructive for the test object. A continuous increased voltage application, e.g. a VLF voltage, could cause in case of a long excitation time during PD diagnosis, unwanted breakdowns at defects even if the applied voltage is not that high like it is used for withstand testing. Damped AC voltage (DAC), which is close to power frequency (20–300Hz), is well proven to be very effective for partial discharge diagnosis and causes nearly no risk for breakdown due to the short excitation time even for critically aged cables. A new test and diagnostic system combining both, providing an effective test voltage for withstand testing and being non-destructive for diagnostic measurements, is introduced recently. This paper describes the application and comparison of the new test and diagnostic system for partial discharge diagnosis by using DAC. Furthermore true VLF cosine rectangular withstand testing with accompanying PD monitoring is discussed. It is demonstrated that cosine rectangular VLF waveform delivers comparable results of PD parameter to judge the severity and to locate PD defects in MV cable systems.