A Study of the Effect of Temperature on Cavity Partial Discharges in Polyethylene (PE) Insulation

Abstract

Synthetic Polymers such as polyethylene are prevalent for high-voltage insulation applications as they offer remarkable insulating and dielectric properties. Notwithstanding precautionary measures made during manufacturing and installation processes, insulation systems are always susceptible to defects for various reasons, which constitute a significant source of Partial Discharge (PD) activity. It is a precursor to insulation degradation leading to premature failure of high-voltage equipment. PD activity is complex due to its non-stationary behaviour and multi- variance dependence. Studies in partial discharge mechanisms have received significant attention over the years to improve phenomena understanding and, in some cases, to allow conclusions to be drawn on the parameters affecting PD mechanisms. These studies have shown that different mechanisms and parameters influence partial discharge activity. In this study , experimental and analytical modelling techniques are used to explore the behaviour of partial discharge mechanisms at varying temperatures. Experimental PD measurements were carried out in accordance with the IEC 60270 standard. A test voltage of 11 kV ac was used. The test temperatures studied were 15C, 40C, 50C, 60C, 70C, 80C and 90C. Test specimens with a cavity diameter of 2.5 mm were assembled using three 1.5 mm thick polyethylene sheets sandwiched between two flat brass electrodes. Partial discharge parameters such as the charge magnitude, inception voltage and PD phase resolved pattern (PDPRP) were measured and analysed at varying temperatures. For analytical modelling, the streamer-like discharge concept is adopted to model PDIV while the apparent charge magnitude is modelled based on the induced charge concept introduced by Pedersen in the 1980s. The curve fitting approach was adopted to replicate and explain the measured experimental data. Results showed that Partial Discharge Inception Voltage (PDIV) increased linearly with temperature for the entire test temperature range. PD charge magnitude initially decreased with temperature from 15°C to 60°C and then increased from 60°C to 90°C. The evolution of PD phase resolved pattern (PDPRP) with temperature was characterised by a turtle-like pattern at ambient temperature, which transitioned into a rabbit ear PDPRP as the temperature increased to 90°C. The findings are interpreted using the mean free path effect on ionisation probability as well as the residual charge dynamics in the cavity as a function of temperature. The overall conclusion is that in polyethylene, cavity discharge characteristics respond to temperature changes. The variations in PD characteristics iv are monotonous for PDIV and non-monotonous for apparent charge magnitude as well as PDPRP. The implications of the findings are that in PD diagnosis,temperature of the equipment under test must be taken into account in interpretation of PD measurements results.