On-line Partial Discharge Measurements in an Operating Generator Stator Winding Using UHF Antennas

On stator windings, it is typical to perform online partial discharge (PD) measurements in the VHF range using high voltage capacitors to couple high frequency currents, and in the UHF range using near-field sensors placed adjacent to stator slot wedges to couple induced PD currents. The installation of conventional stator winding PD measurement systems can be challenging to utilities because it requires the generator to be out of service, and it also requires specialized labour to carefully install the sensors and associated wiring without damaging the asset. It would be advantageous to use antennas for online PD measurements since an outage may not be required and installation would be simple. This paper investigates antenna-based UHF techniques for online PD measurement, using rectangular microstrip patch antennas designed and fabricated with resonant frequencies of 900, 1500, and 2450 MHz. These antennas are used to detect and quantify PD on single Roebel bars in a laboratory and on the stator winding of an operating hydrogenerator. This is done by placing each antenna near the specimen under test, acquiring pulses with a digital oscilloscope, and generating PRPD patterns and time-frequency plots. Agreement with test results obtained with a commercial instrument shows that PD can be successfully measured with UHF antennas for every specimen tested.