Online UHF measurement of partial discharge in stator windings using microstrip patch antennas
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Date
2019
Authors
Partyka, Michael
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Abstract
Partial Discharge (PD) is the electrical discharge resulting from the partial dielectric breakdown of voids in electrical insulation. PD is a phenomenon that can be measured from conducted currents or radiated emissions and can be used to help evaluate the condition of rotating machine winding insulation. PD occurs in stator winding groundwall insulation due to the voltage stress in voids left by the manufacturing process, but it also occurs when other defects are present in the insulation system. PD from each type of defect in a stator winding produces a unique phase resolved partial discharge (PRPD) pattern which is presented as a plot of PD pulse magnitude against power frequency phase. Using PRPD patterns, different stator winding defects can be characterized by PD pulse polarity, phase location, and cluster shape.
On stator windings, it is typical to perform online PD measurements in the VHF range
using high voltage capacitors to couple high frequency currents from the generator’s ring
bus, terminals, or isolated phase bus. It is also typical to perform online PD measurements
in the UHF range with near-field sensors placed underneath stator slot wedges which couple induced PD currents. Existing stator winding PD measurement systems require a machine outage for installation which can be prohibitively expensive. It would be advantageous to use antennas for online PD measurements since no outage would be required and installation would be simple. Currently, there is not enough existing research to determine whether antenna-based methods are feasible.
This thesis investigates antenna-based UHF techniques for online PD measurements.
Rectangular microstrip patch antennas were designed and fabricated with resonant frequencies of 900, 1500, and 2450 MHz. These antennas were used to detect and quantify PD on single Roebel bars in a laboratory setting and on the stator winding of an operating hydrogenerator. This was done by placing each antenna near the specimens, acquiring pulses with a digital oscilloscope, and presenting PRPD patterns from these acquisitions. Based on the agreement between PRPD patterns from these tests and those obtained with a commercial instrument, PD was successfully measured with the antennas on every specimen.
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Keywords
UHF,Patch antenna, Microstrip patch, Partial discharge, Stator winding, Hydrogenerator, Turbogenerator, Synchronous machine