The Breakdown Characteristics Of Oil-paper Insulation For Converter Transformer Under Low Temperature

The building of global energy interconnection is being discussed by our country,with “One Pole and One Equator” as its core,which puts forward higher requirements for electricity transmission grid with large capacity and long-distance.DC transmission is widely used in high voltage long distance power transmission for its advantages of high transmission capacity,small grounding and low operation loss.Convert transformer is the key equipment of HVDC transmission grid.Compared with common power transformer;the valve side major insulation in convert transformer sustains AC composite DC voltage.Nowadays,oil-paper compound insulation is still used in extra-high voltage convert transformers and its insulating property is not only related to its electric field distribution,but also related to its moistures and temperatures.Considering the possible operation faults of the converter transformer in alpine areas,it's necessary to make further research in breakdown characteristics of oil-paper insulation in converter transformer under low temperatures to guarantee the reliable operation of convert transformer.Firstly,the dielectric parameter measuring system for transformer oil and oil-impregnated paper at low temperatures was established,and the relative dielectric constant and bulk resistivity of transformer oil and oil-impregnated paper with different moistures were measured under temperature range from-40 to 20? ?.The measured results show that temperature and moisture have less influence on relative dielectric constant of transformer oil and oil-impregnated paper.However,they have great influence on bulk resistivity,which leads to the ratio change of bulk resistivity between oil-impregnated paper and transformer oil.According to the typical oil-paper insulation model,the expressions of the electric field distribution for oil-impregnated paper and transformer oil was derived,and the rules of the electric field distribution in oil-impregnated paper and transformer oil with different moistures was obtained by numerical methods under different temperatures.Then,the platform was established used for breakdown test under low temperature condition in the laboratory,which can generate high voltages with waveform of the DC superimposed AC.Base on the above platform,the experiment researches on moisture influence on the breakdown characteristics for transformer oil and oil-impregnated paper under the complicated electric field were carried out with temperature range from-40? to 20?,and breakdown mechanisms for oil-paper insulation under the complicated electric field at low temperature were analyzed based on the test data and simulation results of electric field distribution.The results show that the breakdown electric field for transformer oil and oil-impregnated paper with different moistures temperatures in different DC contents change in different ways with different temperatures.It's the result of temperatures,moistures combined with DC contents.The breakdown voltage of oil-paper compound insulation in different DC contents also changes in different ways with different temperatures.Temperatures affected the breakdown characteristics of oil-paper compound insulation in compound electric field at low temperatures through changing the breakdown voltage and voltage allocation for oil and oil-impregnated paper.With the decreasing of temperature,the dielectric strength of the oil-paper insulation is changed from depending on oil to oil-impregnated paper with high DC content.At this point,the breakdown characteristics of oil-paper compound insulation are decided by breakdown characteristics of oil-impregnated paper at low temperatures.