Research And Application On Unipolar Impulse Aging Characteritics Of ZnO Varistor Of AC Filter Arrester In Converter Station

As one of the most important pieces of equipment to suppress overvoltages caused by switching operations or lightning strikes,arresters are widely used in power systems due to their excellent nonlinear characteristics,huge current surge capacity and no series gap.In recent years,the frequent switching of AC filters（ACF）in the converter station of the ultra-high voltage and high voltage direct current（UHVDC/HVDC）transmission systems,resulting in the ACF arresters are frequently affected by impulse current,and the frequent impulse current can accelerate the aging of the ACF arresters,and some of the arresters are even damaged and out of operation,threatening the safe and stable operation of the converter stations.As the core elements of arresters,research of the impulse aging mechanism and impulse withstand capability improvement methods of zinc oxide（ZnO）varistors plays a significant role in the safe and stable operation of the UHVDC/HVDC transmission system.Firstly,the amplitudes and waveforms of the impulse current applied to the ZnO varistors were put forward based on the charge equivalent method,and combined with the monitored and simulated impulse currents of the ACF arresters.And the test projects of the ZnO varistors under unipolar,bipolar（negative-positive in turn）and different waveforms impulse currents were designed,the variation trends of the impulse aging characteristics and the electrical parameter of ZnO varistors under typical impulse aging tests are studied.Secondly,the damage characteristics of ZnO varistors were statistically analyzed,based on the results of the negative impulse aging tests.Moreover,the variation trends of the DC V-I characteristics in the forward and reverse directions of ZnO varistors under negative impulse currents were studied,and point out the shortcomings in the preventive test procedures of ZnO arrester.In addition,the differentiations and relationships between DC V-I characteristics and AC u-i _R characteristics of ZnO varistors during negative impulse aging tests were proposed.Thirdly,the variation trends of current waveform and harmonic characteristics of leakage current and its resistive component under the initial power-frequency reference voltage u_{1m AI}during negative,positive and bipolar impulse aging tests were compared and analyzed.Additionally,combining with the ion migration model and the variation trends of the energy-band diagram of the Double Schottky Barrier,the generation mechanism of the polarity effect of unipolar impulse aging and the mechanism of the bipolar impulse aging were studied.And explain the phenomenon of the asymmetry of the positive and negative peak values of the leakage current and its resistive component current waveforms,as well as the increase of the amplitudes of even harmonics in this paper.Also,the reason for the decrease in the aging rate of ZnO varistor under bipolar impulse current was theoretically proposed.Furthermore,an equivalent-circuit model of ZnO varistor in the small current region considering the polarity effect of unipolar impulse aging was proposed,based on the conduction mechanism of Double Schottky Barrier and polarity effect.Meanwhile,the nonlinear resistances in the forward and reverse directions in the proposed ZnO varistor circuit model were simulated,combined with the AC u-i _R characteristic curves measured results.Furthermore,the effectiveness of the proposed equivalent-circuit model of ZnO varistor considering the polarity effect was verified by comparing the waveform characteristics and harmonic characteristics of the leakage current and its resistive component current of the ZnO varistors under a voltage of u_{1m AI} obtained from tests and simulations.Finally,the variation trends of the harmonic amplitudes and harmonic ratios of leakage current and its resistive component of the ZnO varistors under a voltage of u_{1m AI} and under continuous operating voltage(0.8u_{1m AI})during the unipolar impulse aging tests were analyzed,based on the impulse aging tests and the generation mechanism of the polarity effect.Subsequently,an evaluation criterion for online monitoring of aging status of ACF arresters in converter station of UHVDC/HVDC transmission system was put forward based on the variations of the even-order harmonic characteristics,and the scope of its application was analyzed.Meanwhile,the possibility of using the accumulated charge（energy）of ZnO arrester under unipolar impulse current as a criterion for online evaluating its aging state was studied.Moreover,the differences of the aging rate of ZnO varistors under unipolar and bipolar impulse currents were compared and analyzed,from the perspective of the variations of the DC reference voltages in the forward and reverse directions,as well as the even harmonic characteristics and odd harmonic characteristics.It was further verified that bipolar impulse aging could reduce the impulse aging rate and improve the impulse aging stability of ZnO varistors and a method to improve the operating lifespan of ACF arresters in converter station was proposed.In summary,the above conclusions obtained based on the mechanism of the polarity effect of unipolar impulse aging and the bipolar impulse aging can provide theoretical support for the operation and maintenance measures,simulation analysis and online monitoring of aging state,as well as the operating lifespan improvement of ACF arresters in converter station,which is of great significance to the safe and stable operation of UHVDC/HVDC transmission system.