Investigation Of Air Pressure And Humidity On Corona Onset Characteristics In Wire-Plane Electrodes Under AC-DC Composite Electric Fields

In recent years,with the rapid development of China’s power grid,large capacity AC and DC hybrid transmission projects will be formed nationwide for grid-connected power generation in the future.This erection method will make the AC/DC composite electric fields superimposed on the conductor surface,and the effect of the composite voltages will have a great impact on the corona discharge of the conductor.In China,the terrain is complex,the west is dry and rainy,the terrain is high and steep,while the south is wet and rainy.All these environmental factors will affect the corona discharge of the conductor.Therefore,the influence of air pressure and humidity on the corona characteristics of the conductor under composite voltages are studied in this paper.In this paper,calculation models of composite corona voltage considering the influence of air pressure and humidity are established firstly.Corona discharges under AC-DC composite voltages are divided into four types according to the voltage type and polarity of the conductor at the corona onset side.The corona voltage calculation models of the DC side conductor and the AC side conductor are established respectively through the corona onset criterion of the conductor by using the charge simulation method and the charge emission and migration.The effects of air pressure and humidity on ionization,attachment coefficient,photon absorption coefficient and effective ionization coefficient are analyzed.Then,the calculation accuracy of the model under different air pressure,wire radii and AC-DC composite voltages are verified by the research of some scholars and experimental data.In the four corona types,the corona onset voltage of the conductor all decreases with the decrease of air pressure under the action of composite voltages.For the equivalent positive corona,the composite corona electric field of the conductor will decrease by 8.4%for each 0.1 atm drop in air pressure;For the equivalent negative corona,the composite corona electric field of the conductor will decrease by 7.9%for every 0.1 atm drop in air pressure;Under composite voltages,the existence of non-corona side voltage makes the composite corona electric field increase with the increase of non-corona side voltage.The density of space charges under composite voltage are larger than that under single voltage.The decrease of air pressure increases the density of space charge,photoionization rate and net electron generation rate.At last,in the humidifying chamber with adjustable relative humidity of 20%～90%,the experiments of conductor under different humidity are carried out based on the four-channel signal acquisition experimental platform of AC and DC voltage and current.The experimental results show that under the four corona types,the corona onset voltage decreases with the increase of humidity,and decreases with the increase of the voltage of the conductor on the non-corona side.Under the fixed composite voltage ratio and relative humidity,the corona onset voltage of the four types of corona onset methods in ascending order is the negative corona onset at AC side,positive corona onset at AC side,negative corona onset at DC side and positive corona onset at DC side.Under the four corona types,the corona onset voltage on the rising side of humidity is slightly higher than that on the falling side,and the voltage on the falling side is about 2.01%lower than that on the rising side.As for the micro aspect,the increase of effective ionization coefficient and average ionization ability due to the increase of humidity is the main reason for the decline of corona onset voltage.The increase of humidity produces more space charges,increases the photoionization rate and net electron generation rate.The effect of decreasing 0.1 atm on photoionization effect and net electron generation rate is about 8-10 times that of increasing 10%relative humidity.Under different humidity,the experimental value is in good agreement with the calculated value.Under the action of the AC-DC composite voltages,the law of the influence of humidity on the corona onset voltage is further explained.