Edward Corr

Thermoplastic materials aging under various stresses

The most popular cable insulation material used is XLPE due to its excellent electrical and thermal properties. However, it does not lend itself to ease of recycling. As a result of an increase in concern worldwide regarding environmental protection, it is the objective of this work to investigate whether a thermoplastic material could be used to replace XLPE for cable insulation. Among thermoplastic materials, HDPE is regarded as one with the most similar properties as XLPE. Although it is clear that the performance of polymeric material changes with different stresses, especially polymer nanocomposites aging process under AC electric field stresses, there are also not many publications on how a superimposed AC voltage would affect the insulations performance in HVDC power systems. This paper reports the dielectric properties of HDPE under thermo-electrical stresses. DC stress with and without a superimposed AC stress were applied in the experiments undertaken. The degradation of materials with change in frequencies are summarized and discussed.

Long term testing and analysis of dielectric samples under DC excitation

This paper details testing conducted under DC conditions on a dielectric sample containing internal voids. The DC testing was conducted using a ramp method to vary the voltage applied to the dielectric sample. The dielectric sample was de-energised for a week prior to two separate identical ramp tests and the results are presented showing the variability of PD activity. After the second ramp test an additional ramp test was performed in quick succession and PD activity was reduced, emphasizing the importance of de-energising the sample between tests. A major challenge associated with void type dielectric samples is ensuring that repeatable results are generated and possible approaches are discussed.

PD activity in void type dielectric samples for varied DC polarity

This paper discusses the DC testing of a dielectric sample (with voids) under DC conditions. The ramp test method was employed to assess whether the tests were repeatable. Three DC ramp tests were performed in quick succession. The polarity of the first/third tests was positive and the polarity of the second test was negative. The resultant partial discharge activity was analyzed for each series of tests and compared for positive and negative conditions. The results show that the inclusion of a negative ramp between two positive ramps enabled similar PD activity to be recorded in the first and second positive ramp tests. The negative cycle was used to re-initialise the test sample and allows trapped charge to migrate enabling similar PD activity in the subsequent positive ramp test. Changes in PD behavior were observed in the distribution of PD data between the tests.

Analyses of partial discharges in dielectric samples under DC excitation

The main focus of the paper is to develop a better understanding of partial discharges under DC excitation. Partial discharges studied will initially be limited to discharges from well-defined discharge sites. These include corona, surface discharges and internal voids. The samples are first tested under AC excitation as a sense check to ensure the samples yield the expected PD events. The samples were then subjected to DC excitations where the PD events were recorded and subsequently analysed. A number of analysis techniques will be applied to potentially enable the identification and classification of the type of PD event occurring in the DC system under investigation.

Aging behaviour of polypropylene under various voltage stresses

Practical HVDC systems have superimposed AC or transients and the synergistic effect of these factors on polymer aging would be of interest. Although it is clear that partial discharges under AC stress will gradually degrade the insulation behavior of a polymeric material, there are not many publications detailing the effect of superimposed AC voltages on polymer performance in a HVDC power system. Assuming polypropylene (PP) is suitable for use as electrical insulation, this paper considers the behavior of PP under various voltage ratios and temperatures. Factors which cause the degradation of PP will be summarized and explained. To simulate the working condition, electro-thermal aging equipment will be used. Fourier transform infrared spectroscopy - attenuated total reflection (FTIR-ATR) measurement and dielectric spectroscopy measurement will be carried out before and after aging.