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.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:588605 |
| Date | January 2013 |
| Creators | Smith, David James |
| Publisher | Glasgow Caledonian University |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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