Study On Accumulative Effect Of PP Film For Power Capacitor Under Impulse Voltage

As an important equipment to compensate reactive power and protect electrical system from overvoltage,power capacitor frequently suffers impulse voltage.Statistics show that,impulse voltages power capacitor may suffer one year can be as more as ten thousand times.The repeated overvoltage could damage the polypropylene film(PP film)in the capacitor and reduce its insulating property,which would threaten the safe operation of electrical power system.Therefore,in this article,we studied the failure regularity and damage characteristic of PP films under the accumulation effect of impulse voltage.(1)The online monitoring results showed that the impulse voltage took on the quality of oscillation and damping,which is significantly different from the double exponential impulse voltage waveform IEC standard recommends.To obtain the breakdown characteristics and V-N characteristics of PP film under the impulse voltages with different waveforms,we founded the repeatable test platform for double exponential impulse voltage waveform and damped alternating oscillation impulse voltage waveform.(2)The breakdown characteristics of PP film under double exponential impulse voltage and the influence of wave front/tail time were determined.We found that,within certain range,the breakdown voltage of PP film decreases when the wave front time decreased or the wave tail time increased.The fitting equation of the relationship between U50 and wave front/tail time was u=17.43-4.27×0.3386a+2.79×0.9950 b.The accumulative breakdown characteristic of PP film under double exponential impulse voltages was also obtained by experiment.The results were analyzed using twoparameter Weibull distribution model.Then we got the V-N characteristic curves of PP film under double exponential impulse voltage.We also investigated the influence of wave front/tail time of impulse voltage to the V-N characteristics.According to the results,the times PP film withstanding impulse voltages declined when the wave front time decreased or the wave tail time increased within certain wave front/tail time.The breakdown characteristics and V-N characteristics of PP film under damped alternating oscillation impulse voltage were also determined by experiment.Similarly,we investigated the influence of frequency and time to half value of impulse voltage of damped alternating oscillation impulse voltage.And the results showed that the breakdown voltage and the number of repeated impulses to breakdown at the same amplitude of impulse voltage decreased as the frequency decreasing or the time to half value of impulse increasing within limits.We also investigated the difference between the V-N characteristics of PP film under double exponential impulse voltage and damped alternating oscillation impulse voltage.Results showed that the accumulative effect of PP film under standard lightning impulse was more obvious compared to the damped alternating oscillation impulse voltage.(3)The surface microtopography variation of PP film after the accumulative effect was determined using the scanning electron microscope(SEM)and the atomic force microscope(AFM),the surface roughness of PP film increased obviously after the accumulative effect.After 200,400,600 times of impulses,the surface arithmetic average height Sa of PP film were 1.31,1.51,1.80 times bigger than the initial value respectively.We investigated the dielectric parameters variation of PP film after different times of impulses.Result showed the real part of relative permittivity εr′,the dielectric loss factor tanδ and the conductivity σ increased with the increase of impluses.As the accumulative effect led to the increase of new polar material and thus led to the increase of εr′,tanδ and σ.We also studied the thermostability of PP film after different times of impulses.Results showed that the initial temperature were 309°C,352°,358°C,366°C,375°C respectively after 0,100,200,400,600 times of impulses,while the end temperature were 480°C,482°C,483°C,485°C,486°C.The reason was that the accumulative effect led to the breakage of the weak link of molecular chain,while the remainder molecular chain was more stable and the heat stability of medium changed.The main molecular structure and functional groups variation by infrared spectrometer and X-ray diffraction.Result showed that the main molecular structure and functional groups within PP film did not change significantly.Analysis suggests that the breakage of the weak link has no evident effect on the variety and scale of the functional group as the main chemical bond was C-C and C-H.