Modelling, simulation development of PEA system for space charge measurements at high electric fields

Abstract

Insulation design of High Voltage Direct Current (HVDC) cables at high voltages is quite challenging. At higher electric fields peculiar space charge formation might take place in polymeric insulation. One of the effects of space charge is to distort the electric filed distribution, which may result in a high electric stress, locally. The local stresses may lead to local damage, degradation or initiate breakdown of insulation. Therefore, measurement of space charge is considered very important. There are many non-destructive techniques available in literature and can be categorized as optical, thermal and acoustic techniques. Out of these, acoustic techniques are very popular among researchers. Among acoustic techniques Pressure Wave Propagation (PWP) and Pulsed Electroacoustic (PEA) are widely used. For the measurement of space charge at high electric fields, PEA is very popular among dielectric researchers due to its simplicity, low cost and ability to measure space charge in thick samples and also its immunity to corona discharges. Although the system is popularly used by research community, the literature is scantily available on the design and development. It was observed that, there was a need of modelling the PEA system so that various design related issues could be easily understood by developers. In the present work, a PEA system was successfully modelled by taking analogy between transmission lines and acoustic wave propagation in solids. Then it was successfully simulated using circuit simulation software. Followed by modelling and simulation, a 50kV PEA system was developed, and some experiments were performed on low density polyethylene (LDPE) film samples. The experiments were conducted using developed PEA system at 25ºC on 100µm LDPE film samples under applied electric fields of 100 - 150kV/mm. It was observed that the formation of space charge or packet charge depend upon time duration of step voltage application. Further it was observed that the average conductivity continuously decreased from about 2×10-14Ω -1m -1 at 10 minutes under 100kV/mm to 10-15Ω -1m -1 at the end of 30 hours (150kV/mm). At about half an hour, when the packet charge subdued the conductivity was ~10-14Ω -1m -1 (100kV/mm). In addition, the thesis also presents how to estimate certain characteristic parameters of space charge formation using latest theoretical models. It is believed that the work is useful for the researchers for easily understanding the PEA system and also for developing a new system. In all, about three journal papers are expected out of the thesis while one of them already published.