Diagnostic Measurement and Modelling of Electrical Insulation Based on Very Low Frequency High Voltage Excitation

Abstract

Electrical insulation of high voltage (HV) power equipment plays essential roles in sound functioning and reliability of power systems. Examining the insulation condition through various diagnostic testings such as dielectric response (DR) and partial discharge (PD) measurements may be able to reveal the presence of defects and degradations in the insulation. Very low frequency (VLF- 1 Hz or lower) applied voltage has emerged as a promising diagnostic tool as it significantly reduces the required reactive power from the test supply. However, the existing interpretation knowledge at conventional power frequency (PF) 50 Hz cannot be directly applied to understand test results in the VLF range. This is the main motivation of the research which explores the dielectric behaviours and associated physical processes under VLF excitation. For dielectric response, experimental studies were carried out on short sections of medium voltage service-aged cross-linked polyethylene (XLPE) cables to diagnose the bulk insulation condition, such as the measurement of dissipation factor, polarisation and depolarisation current, frequency domain spectroscopy, activation energy etc. Experimental results show that dielectric behaviours of electric insulation are influenced by several factors including the excitation frequency, voltage amplitude, ambient temperature, dipolar processes (e.g. conduction and polarisation) etc. An empirical physical model describing the loss-factor measurement based on well-known dipolar theories is developed and verified by experimental results. Different partial discharge processes (e.g. cavity, surface and corona) are also investigated at both VLF (0.1 Hz) and 50 Hz applied voltage. Measurement results are presented with the phase-resolved image and show that discharge characteristics (inception voltage, magnitude and repetition rate) are strongly dependent on the applied frequency. Based on the finite element analysis (FEA) method, a dynamic model to simulate the discharge behaviours in the cavity or on the degraded surface is developed to examine the frequency dependence. The main contributions of this research include the measurement and modelling of both the dielectric response and partial discharge in electrical insulation. The research findings provide valuable information to understand the diagnostic characteristics at very low frequency excitation.