Study On Flow Electrification Characteristics Of Oil-Paper Insulation Under Compound Electric Field

Converter transformer plays an important role in HVDC(high voltage direct current)transmission system,its reliability direct affects the safe operation of the system.The operating converter transformer not only withstands AC voltage,lightning impulse and operate-over voltage,but also DC voltage,superimposed AC-DC voltage and polarity reversal voltage.Under the influence of these voltages,there are considerable differences in the characteristics of electric field between a converter transformer and a power transformer in inner electric field's distribution,breakdown characteristics of insulation material,space charge accumulation and distribution,oil-flow electrification,which may cause abnormal discharge and breakdown of the converter transformer.In order to investigate the flow electrification characteristics of oil paper insulation under mixed electric field in converter transformer,an oil-paper tube model with concentric cylindrical electrode geometry as well as the oil-circuiting electrification experiment system was built in laboratory.AC,DC and AC superimposed DC high voltage-generating apparatus were designed according to the testing requirement.Influences of flow velocity,temperature,type and amplitude of external voltage on flow electrification were carried out with the developed testing system.Experiment result shows that streaming current increases with flow velocity linearly or exponentially related to flow status.The type and amplitude of external voltage have significant influence of streaming current.Streaming current in the insulation under DC electric and AC superimposed DC voltage is much higher than that under AC electric field and in the absence of external electric field.Streaming current increases with amplitude of external AC voltage.However,the streaming current firstly increases,then saturates and finally decreases with amplitude of external voltage under DC electric field and AC superimposed DC electric field.Temperature has significant influence on charging tendency,which is relate to the type and amplitude of external voltage.The streaming current increases exponentially with temperature in the absence of external voltage and under AC voltage.However,the streaming current shows peak effect under DC electric field and AC superimposed DC voltage.The peak point temperature decreases with increase of amplitude of applied voltage.Based on the streaming current model proposed in this paper,the testing results mentioned above can be well explained.Theoretical analysis shows that electron transfer between oil and paper and shearing action of oil flow are the main reasons of flow electrification.Streaming current in the oil is decided by migration and diffusion speed of negative ions in the paper at low temperature.At high temperature,positive ions stripped by oil flow and leakage current at the outer electrode decide the streaming current value.The temperature influences the charging tendency by affecting the ions motion speed and electric field distribution in the insulation.