Partial discharge (PD) techniques have been used for many years for detecting incipient insulation faults in power transformers. However, reliable electrical techniques for locating the PDs still need to be developed. This lack of development is caused by the difficulty in extracting information from PD terminal signal measurements and relating this information to PD location. The main objective of this thesis is to develop an electrical PD location technique for use in power transformers. The overall approach consists of three elements. First, a computer simulation model is developed. Simulated PD pulses are injected into the winding at all locations at which a PD might occur. A file of results is created of all the terminal responses to the injected PDs. Second, a PD is injected into the transformer winding undergoing experimentation at some specific site, and the PD signals measured at the terminals are then analysed and the characteristics relating to the location of the PD extracted. Third, algorithms are constructed for matching the file of calculated results with the experimental results for the purpose of identifying the location of the PD in the transformers. A computer simulation model was specifically developed for studying the characteristics of PD propagation in power transformer windings. The model includes a lumped-element, multi-winding transformer model and a PD waveform model. Simulation analyses are carried out to define the transfer functions from the PD locations to the measuring terminals. Features contained in the spectra of the PD terminal signals relating to PD location are extracted. The effect of different winding construction, and terminal connections, on PD propagation are studied. Experimental tests were performed on full-size transformer windings to investigate the differences between the PD terminal signals produced when a PD pulse was injected at different locations. Experimental results were obtained for a multi-winding 220kV transformer winding and for a single 110kV transformer winding. Finally, PD location algorithms are developed based on comparing the characteristics of the PD signals measured at the transformer terminals experimentally, with those obtained from simulation analysis. Feature template matching methods and artificial neural network techniques are developed. It is shown that for the 110kV transformer winding, the feature template matching PD location algorithm predicts the location of the PD source with an accuracy better than 5% of the winding length. However, for high power, high voltage, multi-winding transformers, further refinements of the feature template matching PD location algorithm are required. Likewise the artificial neural network PD location algorithm, whilst showing promise, also needs further research to be applied to such transformer windings. In brief, the work presented indicates that the overall approach used to locate PDs in power transformers is sound. Methods, techniques and algorithms have been developed which with further refinement will allow substantial advances to be made in this field.

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High Voltage (HV) Alternating Current (AC) bushings are a fundamental component of a power transformer, however, they represent over one-quarter of transformer failures. The majority of bushings in-service are of the ail Impregnated Paper (OIP) condenser type and this design is analysed in this work. The main fault modes of bushings are due to 1) Partial Discharge (PD) activity, 2) surface contaminants, and 3) moisture ingress, and in this research each of these areas are investigated using a combination of Finite Element Method (FEM) and experimental techniques. For the PD fault analysis, nine previously unevaluated OIP/grading foil defect geometries that may occur within a bushing condenser system are proposed and their electric fields modelled to assess the regions most susceptible to PD activity. Additionally, the Partial Discharge Inception Volt ages (PDIVs) for each defect geometry are theoretically calculated. Concurrent PD measurements are made on these defects to establish their general characteristics using three measurement methods: Phase Resolved Partial Discharge (PRPD) apparent charge, direct PD current pulse, and Radio Frequency Interference (RFI). The relationships between these three measurement methods are presented, where it was found that all methods can provide an indication of the relative PD magnitude change over time, but only the PRPD apparent charge technique was suitable for PD type diagnostics. Additionally, a FEM model of the discharge process is developed which provides a new method to approximate the time-domain parameters of PD current pulses from bushing cavity type geometries. For the surface contaminant fault analysis, a unique FEM model of an actual bushing in service is presented and used to evaluate the electric fields associated with varying internal and external surface contaminant scenarios. The identification of the areas most susceptible to higher stresses which are at risk to discharges and insulation deterioration are reported. For the moisture ingress fault analysis, a novel Dielectric Frequency Response (DFR) model is proposed for a OIP bushing, which provides an more sensitive method to estimate the moisture content within a condenser as compared to power frequency measurements. The modelled and measured DFR dissipation factor and capacitance results are compared and analysed, showing that changes in moisture content down to 0.2% can be identified using the proposed model as compared to 1.0% using power frequency methods. Additionally, this new DFR model provides an improved ability to distinguish a localised region of high moisture content from a uniformly distributed moisture content case by a steeper dissipation factor gradient at lower frequencies.

The thesis considers using measurements of Ultra High Frequency (UHF) signals from Partial Discharges (PD) to detect and characterise defect signals in multi-source environments. PD is the result of partial bridging of the insulation medium between electrodes, and can be a precursor to eventual flashover. UHF monitoring techniques became predominant in monitoring Sulphur Hexafluoride (SF6) filled Gas Insulated Substations (GIS). There are some particular characteristics of a PD that make monitoring in this range of signals advantageous, such as fast propagation velocities and low attenuation. These attributes make the technique very suitable to determining the type of PD source and location of the defect with accuracy. The research presented in the thesis uses three UHF sensors to detect PD signals from defects. By simultaneously capturing the UHF signals on three channels of a high bandwidth oscilloscope the time of arrival and signal strengths at the three sensors can be used to form Partial Discharge Cluster Maps (PDCMs), which are one of the main contributions of this research. These enable multiple defect sources to be identified and the defect location to be accurately determined. The application that will be considered in most detail is the use of UHF techniques to characterise defects in High Voltage (HV) oil-filled transformers. This is a relatively new area of research and the thesis discusses the challenges the complex internal structure of a transformer presents. The capability to locate and characterise multisources is shown to be successful. An on-line system was developed for plant assessment. Using this system, the technique was successfully tested on operational transformers and these results are also presented in the thesis.

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