The Influence Of Nanoparticles On Corona Discharge Characteristics In Transformer Oil Under DC Voltage

As the key equipment of Ultra High Voltage Direct Curent(UHVDC)transmission system,the insulation status of converter transformer is particularly important for the safe and reliable operation of power system.The converter transformer needs to withstand DC voltage.As the main insulation material of converter transformer,the improvement of transformer oil insulating strength is of great significance to reduce failure.Nanoparticle modification provides a new idea for it.The nanopartical modified transformer oil has excellent electrical performance,but the research on its discharge characteristics under DC voltage is insufficient.As the main form of partial discharge in transformer oil,the continuous development of corona discharge will damage the insulation.Therefore,the development of corona discharge in transformer oil and the influence mechanism of nanoparticles on corona discharge are necessary.to more deeply research.In this paper,titanium dioxide nanoparticles and silicon dioxide nanoparticles were prepared,and the corresponding transformer oil-based nanofluids were obtained by two-step method.Then,the breakdown voltage and partial discharge inception voltage(PDIV)characters of pure oil and transformer oil-based nanofluids under DC voltage were measured.The results show that the negative breakdown voltage and negative PDIV under DC voltage are increased by adding nanoparticles,and the effect of TiO2 nanoparticles is more obvious.But the nanoparticles have little effect on positive DC breakdown voltage.The measurement platform for corona discharge of transformer oil in needle-plate gap was established,by which the images,electrical signals and optical signals of corona discharge under DC voltage were measured.Then the effects of nanoparticles on the characteristics of corona discharge were studied.The results show that corona discharge under negative DC voltage is inhibited by adding nanoparticles.The corona area is reduced by adding nanoparticles,and the effect of TiO2 nanoparticles is more significant.There is a correspondence between the optical pulse and the current pulse.Both the frequency and amplitude of optical pulse and current pulse are inhibited by adding nanoparticles.The trap distribution and ion mobility of transformer oil-based nanofluids were measured.The addition of nanoparticles reduces the trap energy level and increases the trap density.In addition,the ionic mobility is significantly reduced by adding nanoparticles.On this foundation.The development model of corona discharge in transformer oil-based nanofluids under DC voltage is proposed,and the role of charge transport is explained,which reveal the modification mechanism of nanoparticles for corona discharge under DC voltage.Under the negative DC voltage,electrons are captured on the surface of polarized nanoparticles,reducing the number of electrons.Meanwhile,a large number of shallow traps are introduced by nanoparticles.And the electrons emitted from the cathode escape from the ionization region more rapidly due to the shallow traps.These two processes will hinder the collision ionization between electrons and oil molecules and reduce the probability of electron avalanche.More importantly,polarized nanoparticles capturing electrons results in the formation of slowly moving negatively charged nanoparticles,increasing the number of negative ions equivalently.So the distortion of the electric field at the tip of the needle is reduced by adding nanoparticles,which will help to restrain the corona discharge.In addition,the penetrating streamer channel between electrodes is difficult to form,increasing the breakdown voltage.