Impact Analysis Of Nanoparticles On Polyimide Insulation Performance Under High Frequency Electric Stress

Polyimide(PI)is a promising,widely used insulating material in modern high voltage equipment such as high frequency power transformer(HFPT),because of its magnificent electrical,mechanical and thermal properties.There are numerous factors which are responsible for polymeric insulation degradation and ultimate failure,partial discharge(PD)is considered to be a vital cause among them which can lead the material towards its electrical performance deterioration and failure.Continuous PD activity in the air gap between the high voltage(HV)electrode and the insulating material originates the number of free electrons and other space charge species at polymer insulating material's interface,resulting in the formation of a conductive path between the source and the ground electrode by means for electron avalanche raised by electrons,this may finally cause the insulation breakdown.Due to the continuous discharge activity,chemical and structural distortion occur inside the insulating material which significantly affects the insulation performance.For the sake of investigating the influence of nanoparticles on polyimide insulation performance,2wt.%,3wt.%and 5wt.%PI nanocomposite samples were fabricated adopting in-situ polymerization technique.Electrical aging tests were conducted on PI and PI nanocomposite samples.The samples were exposed to continuous high frequency electric stress and the discharge signals were obtained.The obtained original discharge signals during the whole test period were noisy and it was needed to apply a signal denoising technique to get the genuine PD signal,therefore,the wavelet sym8 signal denoising technique was adopted.The obtained PD characteristic parameters include average and maximum PD amplitudes,total PD number per cycle and second.Phase resolved partial discharge(PRPD)patterns were also obtained for analyzing PD densities.Fourier transform infrared spectroscopy(FTIR)analysis was conducted on the eroded area around the ball electrode due to continuous PD activity for aged PI and PI nanocomposite samples in order to investigate the degree of damage to their chemical composition.On the other hand,a scanning electron microscope(SEM)based microscopic study was conducted on both unaged and aged PI and PI nanocomposite samples for investigating the contribution of nanoparticles on the degree of their structural damage.Furthermore,the effect of nanoparticles on the track path formation at nanocomposites insulation surface under high frequency sinusoidal voltage stress was also investigated in this work to explain the mechanism of electrical tree channel propagation in PI nanocomposites.Based on obtained results,it was found that the nanoparticles offer significant resistance to Partial discharge activity in polymer nanocomposites.The number of PD events and the PD maximum amplitude were decreased proportionally with nanoparticles concentration in PI/TiO2 nanocomposites.The nanoparticles start to agglomerate when the addition of nanoparticles exceeds a certain amount.Nanoparticles provide a great hindrance to material's chemical composition and structural damage by suppressing the space charges accumulation,this is why the PI nanocomposites experienced less structural and chemical damage in compression with base PI.Nanoparticles act as barriers against the formation and growth of electrical tree channels resulting it takes longer time to electrical tree to propagate in PI nanocomposites than that in PI(i.e.tree propagation time in PI<PI/TiO22wt.%<PI/TiO2 5wt.%.).Consequently,PI nanocomposite insulation were found more robust against high frequency electric stress.