Study On Characteristics Of Space Charge Distribution Of Nano-modified Liquid Dielectric Under Impulse Voltage

The security,stability,and reliable operation of power system are largely dependent on the liquid dielectric excellent electrical properties, especially the insulating breakdown strength of liquid dielectric.The breakdown process of liquid dielectric is related to the electric field distribution. The internal electric field distribution of dielectric is affected by the space charge.So how to measure the electric field and space charge distribution characteristics of liquid dielectric has important theoretical value and practical significance.Some nanoparticles uniformly dispersed in a liquid dielectric in an appropriate amount can improve the dielectric breakdown strength of the dielectric.The effect of nanoparticles on the space charge distribution and transportation is believed to be an important factor to improve the breakdown strength of dielectric.So far,studies of nano-modified mechanism to liquid dielectric are mostly the theoretical discharge simulation and the space charges behavior measurement by using the pulse electro acoustic technique(PEA) under DC voltage.Compared with DC and AC voltage,the duration of impulse voltage is short.Whether the effects of nanoparticles on the space charge under impulse voltage are consistent with it under AC or DC voltage remains need test to verify.In this study,the non-conductive Al2O3 nanoparticles were chosen to modify the propylene carbonate, which has a high Kerr constant(1.1×10-12m/V2). The impulse breakdown voltage of propylene carbonate before and after modification was tested to study the influence of non-conductive nanoparticle modifications on the impulse breakdown properties of liquid dielectrics.Test results indicate that the negative impulse breakdown voltage of modified propylene carbonate increases by 26.42% compared with that of pure propylene carbonate.Moreover, we measured the space charge distribution and transportation characteristics of propylene carbonate before and after modification under the impulse voltage by using the electro-optic effect method.Analysis shows the nanoparticles added to the liquid dielectric cause traps that can capture fast-moving charges.The fast moving charges can be converted to slow-charged nanoparticles.Therefore,the effective moving distance of free charges in modified propylene carbonate is shortened,which causes most of the space charges to accumulate near the electrode of the same polarity.The electric field in the vicinity of the parallel plate electrodes is weakened by the space charge accumulation near the plates.Hence,the charge injection is obstructed from the electrode in a dielectric liquid.Space charge density is lower and the electric field is more uniform in modified propylene carbonate than that in pure propylene carbonate. It also reduce the energy of fast moving charges obtained from the electric field and inhibit streamer propagation.The dielectric breakdown strength of modified propylene carbonate is improved.The space charge distribution and transportation characteristics of liquid dielectric can be measured under the impulse voltage by using the Kerr electro-optic effect method.However,the traditional high Kerr constant measurement system could not be applied to low Kerr constant liquid dielectrics such as transformer oil, which is commonly used in power system.For these reasons, the traditional high Kerr constant measurement system has been improved in this paper.The major improvement measurements are extended plate and improvement of the light-receiving device sensitivity.The measurement accuracy of the improved system is been verified by experiment.In this study, the Fe3O4 nanoparticles were chosen to modify the transformer oil.The impulse breakdown voltage of transformer oil before and after modification was tested to study the influence of nanoparticle modifications on the impulse breakdown properties of transformer oil.Test results indicate that the negative impulse breakdown voltage of modified transformer oil increases by 26.42% compared with that of pure transformer oil. Moreover, we measured the space charge distribution and transportation characteristics of transformer oil before and after modification under the impulse voltage by using the improved electro-optic effect method. The results showed that: The space charge is less in nano-modified transformer oil compared with pure transformer oil.Different space charge injection volume leads to the different electric field distribution in pure transformer oil and nano-modified transformer oil. The electric field is more uniform in modified transformer oil than that in pure transformer oil.Analyst show that the adding nanoparticles introduced a "trap",the fast-moving charge is likely to be captured by "trap" and the fast-moving charge is converted into low mobility charged nanoparticles which inhibit the space charge generation and streamer development,improving the insulation of transformer oil breakdown strength.