Dielectric Properties Of EPR And Partial Discharge Mechanism Occurring In Cavities And Along Surface Of EPR

The dissertation is supported by the National Natural Science Foundationof China, namely “Study on Insulation Fault Mechanism and Residual LifeEstimation Methods of High-voltage Mining Cable”(No.51377113). SyntheticEthylene Propylene Rubber (EPR) with excellent dielectric properties andspecial weathering and water resistance has been widely used as insulationmaterials for middle-and high-voltage power cables, such as undergroundcables and submarine cables in Europe and America, as well as mining cablesand marine cables in China. When EPR insulation materials is always exposedto the high temperature, the physical and chemical changes will take place,which results in the degradation of insulation and the change of dielectricproperties. Moreover, the cables insulated with EPR will inevitably containsome defects which are possibly introduced into the cables duringmanufacturing process and installation. The defects could be the source ofpartial discharges and surface discharge, which will lead to insulationdegradation and premature failure. Consequently, research about the change ofEPR dielectric properties due to the external conditions and the degradationmechanism of EPR due to the internal defects has gradually attracted theattention, which is pertinence and has realistic significance. In order tounderstand the effect of temperature and thermal aging on the dielectric strengthand permittivity of EPR insulation and the characteristics and change rule ofpartial discharge due to the defects, the dissertation pays attention to theresearch about the dielectric properties, cavity discharge, surface discharge andsurface breakdown phenomenon of EPR.According to the aforementioned problems about EPR used as cablesinsulation, the dissertation, which is with the EPR insulation of mining cables as the object of research and combines theory, simulation and experiments, paysattention to the research about the dielectric properties, cavity discharge, surfacedischarge and surface breakdown phenomenon of EPR. The research content isas follows:The research on dielectric properties, which is the basic characteristic ofinsulation, is helpful to reveal the macroscopic properties of insulation frommicrocosmic mechanism. In the dissertation, the EPR samples was firstly madeand the dielectric frequency and temperature spectroscope was measured usingthe broadband dielectric spectroscope analyzer. The dependence of complexpermittivity, complex conductivity, dissipation factor and complex electricmodulus on frequency and temperature was investigated. Characteristicparameters, such as temperature, the relaxation time, the fractional shapeparameters, the real part of complex electric modulus in static and highfrequency, were extracted. The activation energy was calculated according to thepolarization peak of electric modulus. It could be found from experiment resultsthat the relative permittivity, dissipation factor and conductivity of EPR relatedto temperature and frequency. The dielectric dispersion phenomena could beobserved on the spectroscope of the electric modulus which follows the DEBYEformulation. Meanwhile, the experiments of breakdown voltage, which had beenperformed under the applied voltage in the temperature range from ambienttemperature to200°C, showed that the breakdown electric field strength of EPRwas a temperature dependence and decreased at the speed of about33(V/mm)/°C. It could be found form the comparison of dielectric strength andfrequency spectroscope in the different thermal aging experiments that thethermal aging at the temperature of less than140°C rarely affected the dielectricproperties of EPR. The research result about the dielectric properties of EPR isof significance to comprehend the EPR characteristic under different condition,utilize and improve the electrical properties of EPR.Cavity, as one of the typical partial discharge defects in extruded powercables, will lead to insulation degradation. Study on the formation, developmentand change of cavity discharge is importance to comprehend the degradation process of EPR insulation. Therefore, the relationship between the electric fieldstrength or partial discharge inception voltage and the type and dimension ofcavity was analyzed in the theory. The electric field and space chargedistribution of cavity within EPR were simulated before and after partialdischarge happened. It was found that the electric field strength distribution ofcavity in the insulation depended on the type and dimension of cavity and therelative permittivity of insulation. Moreover, partial discharge in the cavity wasclosely related to both the electric field distribution and the space chargedistribution. Then, the samples with the different dimensions of artificialspherical cavities in EPR were made. Partial discharge in the cavity at differenttemperature was measured and analyzed. The theoretical amplitude of partialdischarge inception voltage was calculated. The effect of temperature on cavitydischarge was also investigated. Based on the analysis of cavity dischargemechanism, the time dependence of the electric field strength was simulatedbefore and after cavity discharge, which confirmed that the experiment resultwas consistent with the calculation and simulation result. It was also found thatthe distribution, amplitude and statistic quantities of partial discharge due to thecavity were closely related to the dimension and position of cavity, temperatureand applied electric field strength. The results of cavity discharge support thetheory and experiments data for diagnosing the defect type of EPR andevaluating the life of cables.Surface discharge, as the other reason for the degradation insulation, willlead to higher pulse overvoltage and high temperature which damages insulationand electric apparatus. The distribution of surface electric field and space chargeof EPR under plane to plane electrode, needle to plane electrode and rod to ringelectrode was simulated and analyzed respectively. The breakdown voltage andelectric field strength, surface discharge inception voltage and distribution alongthe surface of EPR in three different electrode structure was also measured andanalyzed respectively. It was found from the experiment results that spacecharges were accumulated at the boundaries between electrodes and EPR atwhich the surface discharge taken place. Meanwhile, surface discharge voltage of EPR displayed obvious polarity effect, i.e. the surface discharge voltageamplitude under negative polarity DC voltages was smaller than that under ACvoltages and positive DC voltages; breakdown voltage amplitude under negativepolarity DC voltages was larger than that under AC voltages and positive DCvoltages. The process and waveforms of surface discharge under differentelectrode was slightly different, which is related to the gap distance of electrodesand applied electric field strength. The results of surface discharge reveal theformation reason and development process of discharge along the surface ofEPR, which is significance to restrain the surface discharge occurring in thecable joint and improve the electric field distribution.