Experimental Study On Breakdown Characteristics Of Air Gap Under AC/DC Composite Voltage

China has formed the largest DC transmission system in the world after several decades of development of DC industry.With enhancement of voltage level,the parts such as fittings,shields and insulator of converter valve which is an improtant equipment in the high voltage direct system,suffered greater volume of DC/AC composite voltage and harmonic voltage.Discharge or flashover may occur due to the high electric field.In order to optimize the insulation level of the converter valve under composite voltage,the air gap breakdown characteristics under the composite voltage are studied in this paper.The rod plane electrode is used as the high voltage experimental electrode in this paper.Series and parallel AC/DC compounded experimental circuit are designed.Features of series and parallel AC/DC compounded experimental circuit are compared through the simulations and experiments.A composite AC/DC test platform based on AC-DC parallel circuit are built.Respectively,the positive voltage compounded with AC voltage and the negative voltage compounded with AC voltage breakdown characteristics are investigated in the base of the parallel AC-DC parallel test platform.The negative voltage compounded with AC voltage is connected with the rod electrode when the research of breakdown characteristics of the negative voltage compounded with AC voltage is carried out.The breakdown characteristics affected by the compound ratio of AC and DC component,the gap distance and the radius of rod electrode are researched.The experimental results show that the AC and DC composite breakdown voltage increases first and then decreases with the increase of the negative DC component.The breakdown of composite voltage under different AC and DC components can be divided into two cases,that is,positive peak breakdown and negative peak breakdown.The negative voltage compounded with AC voltage is connected with the plane electrode when the research of breakdown characteristics of the positive voltage compounded with AC voltage is carried out.The experimental results show that the AC and DC composite breakdown voltage remains unchanged first then increases and decreases finally.The breakdown of composite voltage under different AC and DC components can be divided into two cases which is breakdown affected by positive discharge pulse and breakdown of no positive discharge pulse.Besides,breakdown voltage of no positive discharge pulse is lower.