The Effect of HV Impulses on Partial Discharge Activity and on the Dielectric Response in Oil-impregnated Paper Insulation
Abstract: This work investigates how HV impulses affect the behavior of partial discharge (PD) activity and the low voltage dielectric response of oil-impregnated paper insulation. It also investigates how the change in the PD activity is related to the degradation level of oil-impregnated paper insulation. In order to accomplish these objectives, the ageing were done under three different electrical stress conditions, i.e. HV impulses following an early stage 50 Hz AC PD activity, a prolonged PD activity at a 50 Hz AC stress alone, and a combination of HV impulses and a prolonged PD activity at a 50 Hz AC stress. In order to predict the level of deterioration caused by each ageing stress condition, the dielectric spectroscopy (DS) measurements in a frequency range of 1.0 mHz to 1.0 kHz were performed before and after subjecting a test object to each of the ageing stress conditions. The investigations were mainly done on the test samples consisting of a cavity deliberately introduced between the layers of oil-impregnated paper. Additionally, the investigation about the effect of HV impulses alone on the DS results was done on aged oil-impregnated paper transformer bushing. The PD experimental results presented in this thesis indicate that HV impulses below the impulse breakdown stress following an early stage AC PD activity will neither cause a significant change in phase resolved partial discharge (PRPD) patterns nor damage oil-impregnated paper insulation to a level that can be noticed with visual observations. On the other hand, a prolonged PD activity at a 50 Hz AC stress can cause the change in PRPD patterns by decreasing the total PD charge and the number of PD pulses, but cannot quickly damage the oil-impregnated paper insulation as it would do when it is combined with HV impulses. In addition to that, the results show that the combination of both, HV impulses and a prolonged PD activity at a 50 Hz AC stress can cause a high drop in the PD parameters (total PD charge and number of PD pulses). The DS results show that HV impulses below the impulse breakdown stress following an early stage 50 Hz AC PD activity will not cause a significant increase in the real part of the complex capacitance and in the dissipation factor as they will do when they are combined with a prolonged PD activity at a 50 Hz AC stress. Further, the dielectric spectroscopy results obtained every three hours during the ageing of oil-impregnated paper insulation by a prolonged PD activity at an AC stress show that the dissipation factor will increase, but the PD parameters (total PD charge and the repetition rate) will decrease with time of PD application. For a case of the aged oil-impregnated paper transformer bushing, HV impulses of amplitudes up to 200 kV did not result in the change in the dissipation factor curve before removing insulating oil from the bushing. However, after removing about 2.5 liters of insulating oil from the bushing, HV impulses resulted in the change in the dissipation factor curve. The magnitudes of the dissipation factor curves appeared to be much higher in the middle frequencies region, i.e. the frequencies between 10 mHz and 100 Hz. After refilling the bushing with the same insulating oil, the loss peak shifted towards the higher frequencies. To understand how the ageing by-products initiated by PDs in the small cavity can modify the geometry of oil-impregnated paper insulation; the model of oil-impregnated paper insulation, comprising of a small cavity, was implemented in Finite Element Method (FEM) software (COMSOL Multiphysics 4.2a). The comparison between the simulation and experimental results show that PD by-products will result in two zones, i.e. aged and unaged zones, and the aged zone will grow with time of PD application; thereby increasing the dissipation factor. On the other hand, in order to interpret the change in the dissipation factors for the dielectrics in aged oil-impregnated paper transformer bushing after had been exposed to HV impulses, a model of a part of the condenser body (oil-paper insulation) was also implemented in the FEM software (COMSOL Multiphysics 4.2a). To model a condition of low insulating oil level in the bushing, a part of oil subdomains was replaced with the air dielectric properties. A comparison between the simulation and experimental dissipation factor curves indicate that HV impulses will produce the by-products (ions), which will increase the conductivity of air when the bushing has low insulating oil level. On refilling the bushing with the same insulating oil, the insulating oil will take these ions and the reactions between the aged insulating oil by-products (such as acids) and the ions, may produce more ions, thereby increasing further the conductivity of the insulating oil.
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