Aspects of PD measurement in hybrid gaseous insulation at DC voltage

Abstract

The increased application of HVDC components in electrical energy systems has led to new challenges in diagnostic testing and measurement of electrical insulation. In addition to other diagnostic methods Partial Discharge (PD) diagnosis at DC voltage is currently subject to much discussion worldwide. This is because the established and approved procedures, methods and evaluation techniques at AC voltages cannot be so easily applied under DC conditions. This paper deals with some problems associated with PD measurement, which can be carried out in hybrid gaseous insulation using DC testing. In principle, such insulation types are mostly characterized by the presence of insulating (solid) materials. These interfaces can become electrically charged, should an electric field be present within the insulation, e.g. by the application of an external electric field during PD testing. For the study of such phenomena, a simple needle-plane electrode set with an insulating sheet was investigated at DC voltage with different polarities and electrical stress durations. It could be shown, that the charging process on the insulating sheet already takes place at test voltages below the measured PD inception voltage level. The charge intensity on interface, and as a consequence, its impact on the electrical field conditions e.g. the inception field strength, depends both on polarity and applied voltage duration. It is assumed, that any charging process below the (self-sustaining) ionisation level, characterized by the electrical inception field strength, should be caused by natural ionisation processes which take place in the test object. This assumption was supported by different results obtained if the test set was under open- and closed- volume condition. In the same manner, the results obtained were confirmed by electrical field calculations for different charging conditions on the insulating sheet and applied voltage polarity. The results described in this paper lead to the main conclusion that the charging ability of interfaces in hybrid gaseous insulation must be considered if PD measurement at DC voltage should be performed. For a deeper understanding of these processes further investigation, e.g. in a more compact insulation type appears to be necessary.