| Converter transformer is one of the important equipment of high voltage direct current (HVDC) transmission systems, the safety and reliablity of its insulation is very important to the stability of HVDC transmission systems. Converter transformer in HVDC systems connected to the converter valve group is different, the winding of valve side will be withstand different polarity dc, ac, dc proportion of composite electric field. The space charge/interface charge will be accumulated in the oil-pressboard interface/internal, it can affect electric field distribution of the converter transformer dielectric. The accumulation of space charge was affected the applied voltage, electric field size, dielectric temperature, moisture and other factors, it led to the electric field distribution of converter transformer is a nonlinear, anisotropic characteristics, and sometimes the special operation of polarity reversal will occurs, under these conditions the oil-paper insulation properties is closely related to space charge/interface charge. The surface flashover electric field intensity of oil-pressborad insulation is not so high as those of the body breakdown electric field intensity, so the electrical insulation properties of along the surface oil-impregnated pressboard is very key for overall insulation performance and reliability of converter transformer insulating structure. The influence of interface charge for surface creepage discharge is directly related to the insulation properties of along the surface of oil-pressboard insulation, but its mechanism is still unclear.The oil-pressboard insulation test platform under the ac/dc combined electric field was designed to be constructed, including ac/dc combined power supply device, transformer oil airtight circulation and processing device, oil-pressboard insulation creeping discharge data acquisition system, the interface charge of oil-pressboard insulation measurement platform and polarity reversal test apparatus. The interface charge measurement platform can measure interface charge accumulation and dissipation of the paper and the distribution characteristics under the non-uniform electric field, it’s minimum spatial resolution was about 1mm2, and it’s measuring sensitivity was 0.129pC/mm2. The reversal time of the polarity reversal test apparatus was level of millisencond that conforms to the actual operation of the converter transformer in polarity reversal of time.The characteristics of the interface charge for the oil-pressboard creepage model under non-uniform electric field were studied. It was obtained that the oil-pressboard interface charge accumulation and dissipation and variation rule under the ac/dc combined electric field. The study found that the polarity of interface charge in the oil-impregnated pressboard was the same polarity of DC component of the AC-DC combined voltage in AC and DC voltage (RMS) ratio of 1:1 to 1:5 combined electric field, the interface charge density increased with the more of DC component and stress time extension, with the AC component instantaneous electric field produced fluctuations similar to power frequency voltage, and the maximum value could reached 1.8 times of the charge density average value.The surface flashover voltage under AC/DC combined electric field is larger than that of flashover voltage under pure AC electric field, and the surface flashover of negative polarity DC superimposed AC voltage is higher than that of positive polarity DC superimposed AC voltage. Through the results analysis of interface charge of needle plate electrode model, the same polarity interface charge of oil-pressboard insulation can weaken electric field strength near the needle electrode, and improve the surface flashover voltage. With the increase of proportion of DC component, the apparent discharge magnitude and discharge repetition rate and cumulative apparent discharge energy and carbon traces in the development process of creepage showed a clear trend of decrease. The interface charge measurements of creepage in different stages under same proportion of combined electric field found that the interface charge growth rate of the creepage stages decreases with the increase of the total apparent discharge magnitude, it shows that the same polarity interface charge can restrain the development of the surface creepage, and creepage can weaken the interfacial charge accumulation.The flashover test results under the polarity reversal found that the interfacial charge can greatly reduce the flashover voltage of polarity reversal. And the method of evaluated the ability of interface charge accumulation of oil-impregnated pressboard was put forward by gaining surface potential coefficient k1 and k2 of the creepage flashover voltage under polarity reversal. The change of oil-pressboard interface charge density before and after the polarity reversal has been measured, It was found that the electric field produced by interface charge can strengthen the maximum synthesis field intensity of the needle electrode after polarity reversal, thus greatly reduce the surface flashover voltage. Because of the polarity reversal in the converter transformer operation, the insulation design requirements could not been reduced for the same polarity interface charge increased surface flashover voltage. The stipulated time (less than 2 min) of converter transformer polarity reversal test voltage in the IEC61378-2 standard is greater than the interface charge accumulation time of the surface structure, so the test couldn’t verified the insulating properties of surface structure. It was need to adopt CIGRE suggest abrupt change of voltage test waveforms (reverse time less than 2 ms) for review.Research on the influence factors of interface charge found the increase of temperature, moisture, the interface charge density of oil-impregnated pressboard decreases, and the the creepage discharge inception voltage slash, surface flashover voltage drop, discharge repetition rate and the total apparent discharge magnitude increase. On the surface conductivity measurement of oil-impregnated pressboard analysis, found that the increase of temperature and moisture can lead to the surface conductivity of oil-impregnated pressboard has increased dramatically, to the charge migration rate increase, makes the interface charge dissipate faster and reduces the interface charge accumulation, thus the inhibitory effect of surface creepage discharge weakened.Through the research, the evaluation methods of converter transformer oil-paper insulation performance were proposed based on the charge characteristics of oil-impregnated pressboard, designed for converter transformer insulation material selection provides a new evaluation method. |
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