Breakdown Characteristics Of Oil And Pressboard Insulation In Converter Transformer

The converter transformer is one of key equipments in the HVDC transmissionsystem. Because of withstanding AC, DC, AC+DC(AC and DC), polarity reversalvoltage and so on, the main insulation on valve side has a higher fault ratio duringboth the factory test and the operation. A large number of experiments and electricalfield simulation studies on the oil and paper insulation of the converter transformerare carried out by researchers at home and aboard, but it is not sufficient, and asmall number of results are contradictory. Therefore, more research oncharacteristics of the oil and pressboard insulation in the converter transformer isnecessary.Mathematical analysis on characteristic of voltage distribution in the oil-pressboard insulation by RC model is carried out in this paper, and the effect of thevoltage distribution on the insulation breakdown is discussed. Both AC+DC andpolarity reversal high voltage equipments are designed. Breakdown experimentsunder AC, DC, AC+DC and polarity reversal voltage on the transformer oil, oil-immersed pressboards and the oil-pressboard insulation are accomplished withdifferent gaps and temperatures by a pair of plane electrodes as the basic model.Based on these, the breakdown mechanism of the converter transformer oil andpressboard insulation is further discussed, and the technology about the convertertransformer main insulation on valve side is analyzed.Research results indicate that, DC voltage increase and polarity reversal cancause a transient increase of the voltage on oil gaps in the oil-pressboard insulation,and the voltage amplitude depends on varying speed and duration of applied voltage,charging time constant and ratio of insulation resistance. AC+DC breakdownvoltage of the transformer oil is higher than AC breakdown voltage at anytemperature，and breakdown voltage temperature characteristics under AC voltage are different from those under DC and polarity reversal voltage. AC+DC breakdownvoltage of oil-immersed pressboards decrease rapidly along with the increase of ACcomponent, and polarity reversal can decrease breakdown voltage at lowtemperature. These prove that the oil-immersed pressboard is also a kind ofcomposite insulation which is composed of pressboards and oil holes.It has a clearmaximum value among breakdown voltage of the oil-pressboard insulation underAC+DC voltage with different AC component, which is considered to be a boundarybetween the oil-pressboard insulation breakdown caused by the oil-immersedpressboard breakdown and caused by the oil gaps breakdown. Polarity reversaldecreases breakdown voltage of the oil-pressboard insulation at low temperature, thefaster the polarity reversal is, the more serious it is. Initial values of absorptioncurrents, attenuation functions and accumulated charges of the oil-pressboardinsulation under polarity reversal voltage deviate from laws determined by the RCmodel, and it is related to the amplitude of applied voltage.It is found by summarizing experiment results that the breakdowncharacteristics of the oil and pressboard insulation under DC and polarity reversalvoltage can not be explained by the existing breakdown theory, which can beexplained by the process of space charge accumulation and dissipation and itsinfluence on the distribution of the electrical field. It is considered that the maininsulation of the converter transformer on valve side which has passed the factorytest is adequate for the steady-state operating conditions, because of the rapiddecrease of the oil-immersed pressboard breakdown voltage along with the increaseof the AC component, the flashover of oil gaps will enhance the voltage amplitudeand the AC component percentage bearing by the oil-immersed pressboard, whichmay cause the oil-pressboard insulation breakdown. Therefore, the strict purificationand operational monitor for the converter transformer oil is necessary. Enoughattention should be attached to the transient characteristic by polarity reversal and itsnonlinear relationship with the applied voltage. The rapid polarity reversal at lowtemperature must be avoided to the greatest extent. The voltage amplitude of thepolarity reversal test will be higher than that of the AC test along with the increaseof the6-impulse-bridge number, so the thickness of oil gaps should be appropriatelyincreased to bear this polarity reversal voltage.