Breakdown Characteristics Of The Converter Transformer Oil-paper Insulation Under AC And Polarity Reversed Voltage

The converter transformer is the key equipment in the high and extra-highvoltage DC power transmission system. Its reliability directly affect to the safeoperation of the system. AC and polarity reversed voltage is the actual workingvoltage. This voltage is more complex compared with AC and DC voltage andpolarity reversed voltage. The oil-pressboard insulation characteristics had been notresearched in this voltage, so it had academic value and engineering significance toresearch this problem.This paper developed a set of AC and polarity reversed high voltage equipmentprimarily. On the basis of RC model of oil-paper insulation, this paper analyzedcircuit and calculates parameter, and designed AC and polarity reversed test circuit.Its feasibility was verified by simulation software. For analyzing the change rule ofoil-paper insulation resistivity under AC and polarity reversed voltage, thedissertation designed a set of current collection system which simultaneouslycollected the signal of DC and AC. This system included three modules whichincluded pre-sampling, signal processing and wireless acquisition, and did a seriesof protective and isolated works. On the basis of oil-paper composite insulationstructure characteristics, this paper designed the test model and three-electrodesystem. This paper built the AC and polarity reversed test system using laboratory’savailable equipments. The method of applied voltage used the way of fixing polarityreversed voltage component and rising AC voltage component step by step, so it canchange the amplitude of AC and polarity reversed voltage. This paper analyzed thebreakdown characteristics of oil-paper insulation under superposed voltagecombined with the results of experiment.Test results showed that the breakdown voltage of oil paper insulationdecreased when AC component increased which affected the breakdowncharacteristics of oil gap. The after current of polarity reversed component presented linear relation with polarity reversed voltage, but the initial value of absorptioncurrent grew exponentially while applied voltage increased. AC componentfollowed ohm law. It indicated that polarity reversed and space charge did notinfluence the electric field distribution in the oil-paper insulation. AC and polarityreversed voltage could be seen as two independent components which impacted inoil-paper insulation under superposition voltage. Internal electric field formed byspace charge, transient electric field formed by polarity reversed and AC electricfield would form a strong field strength in the oil, it could cause the breakdown ofoil-paper. This study can be referred to the insulation design of convertertransformer and the HVDC apparatus testing.