Basics of partial discharge measurement

Abstract

Partial discharge (PD) denotes a small, localized electrical discharge in the insulation between conductors, occurring when the local electric field strength exceeds a critical value. Prolonged exposure to partial discharges is known to degrade the dielectric behavior of the insulation and may even lead to complete breakdown. PD testing of high-voltage equipment and electrical apparatus is one of the most important and difficult tasks in high-voltage testing. This chapter describes the measuring instruments, the calibration methods and the test circuits for measuring the so-called apparent charge. For most apparatus, special PD measurement methods with computerized data processing have been developed, which are constantly being improved. Key words are on-site PD measurements after installation of the equipment, localization of single or multiple PD sources in spatially extended apparatus, synchronous multichannel PD measurement, VHF and UHF PD measurement techniques, permanent PD online monitoring and interference suppression methods. Partial discharges are associated with electromagnetic, acoustic, optical and chemical effects, which are also exploited for their detection and diagnosis, in particular in complex apparatus such as three-phase power transformers and gas-insulated switchgear (GIS). The PD measurement techniques at DC and impulse voltages are briefly discussed.