| Cross-linked polyethylene(XLPE)is widely used as an extremely important insulating material for various high-voltage transmission and electrical insulation applications due to its excellent electrical,chemical and mechanical properties.With the rapid development of China's economy and the rapid expansion of power grids,oil-paper insulated cables,EPR insulated cables,and overhead lines are gradually replaced by cross-linked polyethylene cables,which are becoming the main body of urban power transmission and distribution systems,and also are widely used in high-voltage and ultra-high-voltage lines.Along with this,the stability,reliability,and safety of power system operations have become increasingly prominent.Under this background,it is important to develop technology for detecting the pressure resistance of UHV power cables,to understand the types of common defects in power cable insulation,and to define the mechanism of withstand voltage breakdown of power cables,and to ensure the safe and reliable operation of power systems.In this paper,two typical defects are prepared to simulate the common defects in the installation and operation of the main insulation of the power cable.The withstand voltage characteristics and breakdown of the XLPE power cable with air-gap and tool-mark defects are studied by transforming the parameters including the attenuation frequency and amplitude decay time of the oscillatory through the damping oscillatory wave test platform.And then,the equivalence of the insulation voltage characteristics and power frequency voltage of the XLPE cable under the action of a damped oscillatory wave voltage was verified.Firstly,a test platform for the voltage oscillation of a damped oscillatory wave was built,and two kinds of XLPE power cable insulation samples with typical defects were prepared.By analyzing the principle of generating DC-oscillating damped oscillating wave and describing the physical meaning of each physical quantity of oscillating wave voltage,an oscillating wave generation test platform with a frequency range of 20-300 Hz was established through reasonable parameters of inductance,capacitance and resistance.Power cable defect specimens containing pin gap defects,and tool-mark defects were manually prepared based on the analysis of mechanical defects of common power cables.Power cable mechanical defects can be simulated at different scales and depths by designing specific processing methods.Secondly,the breakdown test of XLPE power cable with needle air gap defect and tool mark defect was carried out.The pressure breakdown characteristics of XLPE power cable under two kinds of defects are studied by selecting 8 frequency points in the range of 20-300 Hz based on the experimental platform for damping oscillatory waves.The breakdown characteristics are carefully analyzed by the breakdown theory and the microscopic appearance diagram of the breakdown process.It is indicated that there is a strong dependence on the breakdown characteristics of the XLPE power cable with the oscillatory wave voltage.The oscillation wave withstand voltage characteristics of the air gap defect and the tool mark defect have similar changes.In the air gap defect and the tool mark defect condition,the breakdown voltage increases with the increase of the frequency in the range of 20-300Hz,and reaches the peak at about 100Hz;after the peak point,the breakdown voltage decreases under the action of the oscillating wave.It is considered that the breakdown of needle air gap defects and knife mark defects is due to the electron bombardment on the XLPE insulation and the interaction of heat.It is found that the breakdown voltage increases first and then decreases with the change of the attenuation coefficient through changing the attenuation coefficient of the damped oscillatory wave.This is mainly due to the change in mechanical force and the rate of change of mechanical force.In addition,it is found that the voltage breakdown voltage of power cables with air gap defects is higher than that of power cable samples with tool mark defect defects.The analysis suggests that the XLPE breakdown performance is a weak point probability breakdown.The larger the spatial scale of the defect is,the higher the probability of the weak point is.Therefore,the breakdown voltage of XLPE samples with tool mark defects is much lower than that of XLPE samples with air gap defects.The equivalent coefficients between breakdown voltage of the damped oscillating wave and commercial frequency sinusoidal wave was defined.The equivalent properties of the damped oscillation wave and the working frequency sine wave are studied in two typical defects.It is indicated that the equivalent breakdown coefficient of the damping oscillation wave and the power frequency sine wave is between 1.06 and 1.70.It is in good agreement.Through related research,we can better understand the insulation breakdown process and mechanism of XLPE cable insulation of oscillating wave voltage under the condition of mechanical defects,which is of great significance for preventing the occurrence of power cable faults or fault operation and maintenance. |
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