Space Charge Properties Of Oil-Paper Insulation During Electrical Aging Process At DC Pulsating Voltage

Converter transformer is critical equipment in the HVDC system, the insulation isvery important for the operation security. This paper analyzed the space chargeproperties of oil-paper insulation during electrical aging process at DC pulsating voltage,then designed the electrical aging model of oil-paper insulation at no uniform electricfield and uniform electric field in oils, and put up the electrical aging test platform atDC pulsing voltage. Samples of oil-paper were aged for different time or stress, andthen the regular patterns and mechanisms of charge injection amount, space chargemobility and charge distribution were compared. The main works of the author are asfollows：①Internal voltage characteristics of converter transformer were analyzed, samplesof oil-paper insulation were prepared, then used the electrical aging model of oil-paperinsulation at no uniform electric field and uniform electric field in oils, samples wereaged at an AC and DC mixed-voltage, and the ripple coefficient is1.②A systematic study on DC space charge behaviors of oil-paper insulation usingthe pulsed electro acoustic (PEA) method are performed by considering the practicaloperation conditions, and restoration and reconstruction of waveform of space chargedensity distribution were made. Space charge density under different aging degrees wascompared. Results show that homo-charge injection happened in all the test conditions.Oil-paper insulation was dominated by negative charge injection when electrical agingwas in a small degree. With increase of the electrical aging degree, chargerecombination was increased, and oil-paper insulation was dominated by positivecharge injection gradually.③The regular patterns and mechanisms of charge injection amount, space chargeinjection point and max charge density were compared. The convergence anddissipation modes of space charge of oil-paper insulation after electrical aging wererealized. Results show that number of shallow traps in the test sample after electricalaging at no uniform electric field were significantly greater than the number of deeptraps; number of deep traps in the test sample after electrical aging at uniform electricfield were significantly greater than the number of shallow traps. Dissipation rate ofinternal space charge of test samples after electrical aging was especially faster thansamples not aged.