Research On Flexible Zero-residual-current Arc Suppression Coil

Currently, most of our country's medium voltage distribution networks use the way of neutral point to earth via arc suppression coil. According to the compensation degree of arc suppression coil to capacitance current, arc suppression coil has three working models: undercompensation, overcompensation and full compensation. In the traditional sense, full compensation means the inductive current equals to the capacitive current, which makes fault current of ground point, that is the residual current, only including the active current and the harmonic current. While the existing arc suppression coil of automatic tracking compensation only can compensate fundamental reactive component of the ground fault current, and inability to the resistive current and harmonic current. That is a shortage in the principle of arc suppression coil at present.With the development of the distribution network, with the increase of the non-linear loads and the wide use of the power electronic equipment in distribution network, the harmonic and active component of the ground fault current also enhances largely. In this case, even if fundamental component is compensated completely, so much great active and harmonic component can also maintain the arc, cause the overvoltage, a high contact voltage and a high step voltage. So carrying out the research of zero-residual current arc suppression coil is of great theoretical and practical significance. And the development of electronic power technology makes it possible to realize full compensation for the active component and the harmonic component of the ground fault current and the flexible control of arc suppression coil.First the advantages and disadvantages of all the current automatic tracking compensation arc suppression coil are introduced and their common disadvantage is pointed out. After these, the author analyses the magnitude and source of the residual current of the coal mine 6-10kV distribution network based on the test results of field. After analyzing the traditional principle of automatic tracking compensation arc suppression coil, a new tuning method, namely, extremes method combining with phase method is adopted in zero-residual current arc suppression coil. The proposed method is fast and accurate.A three-phases-five-columns-two-secondary-winding zero-residual current arc suppression coil is proposed and the equivalent circuit of this zero-residual current arc suppression coil is detailedly analysed and deduced. Regarding the inverter as VCCS and changing the control coefficient of the VCCS, the zero-residual current is realized. Considering the small dynamic compensation range and harmonic generation of the traditional three-phases-five-columns arc suppression coil during compensation, the dissertation also does great research and gives solutions. The proposed methods can made the dynamic compensation range of this zero-residual current arc suppression coil reached 100%-0% and make its harmonic current under control. Finally, the author carries out simulation and experiment, and the simulation and laboratory result both prove the correctness and effectiveness of the theoretical analysis and realization.Meanwhile the author points out that compensation effectiveness will decline and zero sequence voltage after compensation will rise with the increase of earthing point transition resistance. So if both the compensation effect and the zero sequence voltage amplification are taken into account, an effective control of zero-residual current arc suppression coil to compensate active and harmonic component of residual current should be adopted.Subsequently, the dissertation discusses relevant issues about flexible control of zero-residual current arc suppression coil and proposes the concept of arc suppression coil flexible adjustment method, which combine the advantages of the exiting two kind of adjustment mode, namely, post-set mode and pre-set mode. On the basis of deeply analyzing the compensation of zero-residual current arc suppression coil for active component in residual current, the dissertation proposes the concept of negative resistance rate in the light of different degree of compensation of zero-residual current arc suppression coil for active component in residual current. The description of negative resistance rate expands the theory of neutral to ground via arc suppression coil system further.Based on the different descend degree of harmonic and active component in residual current, this dissertation introduces a flexible control strategy of zero-residual current arc suppression coil in case of single-phase grounding with high resistance. According to the value of grounding resistance, this strategy can regulate output of the inverter, that is to say, zero-residual current arc suppression coil can compensate active and harmonic component of the residual current when the value of transition resistance is less than threshold 2, only compensate active component when the value is between threshold 1 and 2, and compensate nothing when the value is greater than threshold 1. This strategy can give attention to both small residual current and low zero-sequence voltage. According to analysis of transient process in case of single-phase grounding with high resistance, this dissertation introduces a new method to distinguish from grounding fault with high resistance which is realized by the transient AC sample value and realize the flexible adjustment and control of zero-residual current arc suppression coil. The proposed method is simple, easy and accurate.Finally, the author does simulation and experimental research on the flexible adjustment and control of zero-residual current arc suppression coil. The results showes correctness and efficiency of theoretical analysis and implementation method.