The Development Processes Of Partial Discharge In Gas Cavity Of Oil-paper Insulation Under AC And DC Combined Voltage

Converter transformer is one of the key equipment of DC transmission systems, and suffers the fault, the reliability of insulation of converter transformer is of great significance to ensure the reliable running of DC transmission. Whose main insulation method is oil-paper insulation. Valve side winding of the converter transformer under AC, DC, AC and DC composite electric field effects in long-term, insulating paper not only suffer the electric field strength, and when the mass fraction of water in insulation board is significantly greater than 0.1%, it will make the electrical insulation strength decreased significantly, and influence insulation structure dimensions and operation life. Chromatography is a fault diagnosis method is widely used in engineering, which can effective early. Warning of transformer insulation fault state, the converter transformer insulation fault diagnosis at present still adopt the diagnostic standard of ordinary AC transformer, diagnosis method also has further verification.In this paper, according to the actual operation condition of the converter transformer, and faults often appear in the condition of converter transformer in turn and turn and in insulation paper in bubbles making process in the inevitable, development process of air gap model in AC-DC composited voltage of partial discharge pulse current method, using conventional ultra high frequency detection, ultrasonic detection and chromatography online measurement means, online collection of partial discharge signals and fault diagnosis of the insulation defects. At the same time, in order to study the effects of water on the insulation defects, further comprehensive analysis of gap models in normal conditions and higher moisture content in case of partial discharge development process, and carried on the research on the breakdown characteristics of high moisture content condition, provides the theory basis for the insulation strength of oil paper insulation, more conducive to the converter transformer fault diagnosis of oil paper insulation.Research shows that, the gas cavity model in AC-DC composite voltage partial discharge development process can be divided into 3 stages according to the characteristics of partial discharge: initial stage, development stage, near the breakdown phase; initial discharge stage discharge amount is less, less discharge number at 15, discharge development stage, discharge increases, the discharge number increase, near the breakdown phase the discharge, discharge number increased to 2000; there are also differences in the different stages of the characteristics of gas production, in the initial stage, for less, hydrocarbon gas content of the fault diagnosis of the stage of development, the gas content increases, near the breakdown stage hydrocarbon gas content appeared surge.With the increase of the DC component, the discharge intensity and characteristics of gas decreased, and in the fault development to the breakdown phase for gas production, gas C2H2 fault diagnosis of severe also decrease. According to the three ratio method, applicable in the ordinary transformer in the code, In the AC and DC under composite voltages, code "010" in the diagnosis of DC composite voltage fault types of partial discharge. There are limitations in fault diagnosis of DC composite voltage code diagnosis under AC voltage.The discharge quantity and discharge times discharge initiation phase alternating current direct current partial discharge under composite voltages in high moisture are up to 5000pC, the moisture content increases will accelerate the development of partial discharge, reduce the breakdown voltage of AC and DC composited voltage fault. Moisture content is not on board the AC voltage in oil paper insulation breakdown voltage effect, the content of the main effects of the volume resistivity of insulating board, the insulating cardboard moisture percentage reached 8%, a sharp drop in resistivity, AC and DC breakdown voltage with dips, DC component is greater, the breakdown voltage by the influence of insulating cardboard moisture more.