Partial Discharges are symptoms of insulation degradation that continually promotes the deterioration of the insulation condition, eventually leading to permanent failures. The PD event itself is not dangerous but it is the state of the discharge activity that can lead to unforeseen failures. Hence continuous monitoring of PD enables the prevention of these unforeseen failures that result in economic losses. High voltage equipment that has been installed more than forty years ago is highly susceptible to PD since it is reaching the insulation end life. For the case of underground cables, continuous PD monitoring will help avoid unplanned outages and the need for the immediate replacement of faulty cable sections that will incur large costs. Traditionally, PD diagnostics have been carried out through offline methods, whereby the cable specimen is removed from service during the diagnostic tests. On the other hand, online PD diagnostics are preferable since the services are not disrupted. However, there are some major challenges that come with the online approach. For instance, heavy noise interference, the detection and location of PD through the interpretation of measured signals. Presented in this Thesis are presented novel contributions to the area of online PD detection for underground HV cables. The work encompasses aspects of physical data acquisition procedures and the post-processing signal processing algorithms. A new online PD data acquisition unit equipped with pre-processing and signal conditioning is presented. The system has other features that take into account the difficulties of logistics as well as issues related to the regulations and protocols of the utility. The primary aim of this developed system is to produce a PD database that will be used for research purposes. Since applied wavelets for the field of PD diagnostics have been popular amongst researchers, it was investigated further in this Thesis. Following from the data acquisition system, large data sizes required intensive processing. A new wavelet-based algorithm combined with the use of Higher Order Statistics is presented. This algorithm enables the simplification of data signals to highlight potential PD activity resulting in the reduction of manual examination of wavelet algorithms. The conventional wavelet algorithms applied in the literature generally referred to a specific approach of the wavelet implementation i. e. the decimated approach. However, it was found that the non-decimated approach has several advantages with regards to PD signature detection and PD location. The application of both approaches and their comparisons is applied to simulated data as well as online field data. Finally, the analysis of online PD data acquired from the new data acquisition system is presented. Several PD characterisation processes are applied and positive results were generated. Aspects relating to the physical environments of test site are also included. The challenges and experiences gathered in this research project are described.

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A novel approach for extracting information about the insulation integrity of a dielectric from Partial Discharge (PD) signals is described. The approach is based upon chromatic methodologies and involves addressing PD signals with three processors having non-orthogonal (overlapping) responses. The outputs of the three processors are transformed into a number of chromatic parameters designated x, y, z and H, L, S. x, y, z represent the proportion of the signal from each of the three processors. H, L, S represent respectively the dominant phase, effective strength and notional spread of the PD signals. A range of Partial Discharge signals obtained by various researchers worldwide have been analysed using the chromatic approach. These include investigations on various dielectrics (epoxy resins, impregnated oil, Perspex ... etc) which exhibited complete insulation breakdown, as well as PD signals generated from different PD sources have been analysed using the approach. Changes in the values of the chromatic parameters are shown to reflect various PD trends and to provide early indications of incipient insulation failure and PD source discrimination. The approach provides a high degree of traceability in relating the chromatic parameters to the raw PD signals

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Esters of palm kernel oil have been synthesized and characterized for application as electrical insulating oil. Since such fluids serve both as insulants and coolants, tests were performed to characterize the chemical, physical and dielectric properties. Accelerated ageing tests were carried out to mimic conditions in a power transformer. Alkyl esters were synthesized from laboratory purified palm kernel oil: methyl ester was prepared through a transesterification reaction; an epoxidation reaction involving the methyl ester produced with insitu per-acetic acid in the presence of a catalyst to produce corresponding epoxy alkyl ester. This was then reacted with acid anhydrides under nitrogen in the presence of catalyst. This opened the epoxy rings of the epoxy alkyl oleate and linoleate components to attach side hydrocarbon chains. The characteristics and composition of the products were monitored using GC-MS and FTIR spectroscopy. The synthesized ester possessed a flash point that satisfied the specified minimum flash point for mineral oil, higher thermo-oxidative stability, and high and reliable breakdown strength from the distribution of breakdown data (43 kV/mm). It has a melting point of about -7˚C which is higher than mineral oil. An ester-paper sample which was aged in a pressure vessel under nitrogen and metal catalysts exhibited improved performance compared with the mineral oil-paper system. The rate of decrease in the tensile strength of alkyl ester-paper within the ageing condition was significantly lower than mineral oil-paper. Impregnation of cellulose paper with ester increased the charge storage capacity of the insulating paper. A strong low frequency dielectric dispersion (LFD) was observed which became more pronounced with ageing. Quasi-DC conduction was deduced to have dominated the low frequency response of alkyl ester-paper. The characteristic breakdown strength of the aged paper samples in comparison with the unaged sample did not show any significant difference. It is suggested that the synthesized ester is as good as mineral oil in many ways and a viable alternative fluid for cooling and insulating power transformers.

Outdoor high voltage power plant is exposed to environmental pollution which is an important cause of failure. Knowledge of conditions leading to flashover is essential to design insulation levels. Discharges on the surface of polymeric insulators are one of the ageing mechanisms responsible for eventual failure. In this thesis, we examine water droplets on insulator surfaces. Chapter 1 discusses pollution on the surface of high voltage insulators, and chapter 2 considers water droplets on insulator surfaces. Chapter 3 reviews recent work in this area. Chapter 4 presents the aims of the present work to examine partial discharges at the edge of water droplets and to investigate droplet vibration with an applied ac field. Chapter 5 identifies the apparatus and procedures that were developed. Chapter 6 describes an experimental study of electrical breakdown at the edges of sessile water droplets on a PE surface subject to ac electrical stress up to of 2.0MV/m at 50 Hz. The study involves observing the motion of water droplets using a high-speed video camera operating at 3000 frames per second whilst electrically detecting any partial discharge activity. The significance of droplet vibration on electrical stress enhancement is investigated along with the effects on partial discharge activity. Chapter 7 describes test sample geometry and electric field modeling using finite element analysis. Chapters 8 and 9 describe an experimental study of breakdown at a sessile water droplet on a planar, polymeric, insulating surface subject to ac stress, parallel to the insulator surface, up to 2. OMV/m. The contact angle between droplet and surface was varied by controlling the physical properties of the droplet and by inclining the insulator plane from the horizontal. A theoretical model is developed which shows that it is possible to sustain partial discharges in the air around a droplet above the polymer surface.