Research On Some Key Technologies Of Neutral Flexible Grounding In Distribution Networks

Most of the faults in the distribution networks are caused by single-phase grounding fault.Because the fault current of the small current grounding system is small and does not change the rigid structure of the load power supply,it is conducive to promoting the recovery of the instantaneous fault and improving the power supply reliability of the distribution networks.However,with the improvement of the cabling rate of modern distribution networks and the wide use of non-linear loads,the single-phase grounding fault current is increasingly difficult to meet the conditions of reliable arc extinction,which is easy to develop and cause serious consequences such as arc grounding over-voltage,interphase or three-phase short circuit fault,and at the same time,the excessive ground current will also seriously harm the safety of people and equipment.Neutral flexible grounding is the expansion of the resonant grounding method,which can make the small current grounding system become flexible and controllable,and greatly improve the compensation effect of the broad-band fault grounding current,which is expected to become an important development direction of the rapid disposal of the distribution networks grounding fault in the future.Under this background,this paper focuses on the new active arc suppression method of neutral flexible grounding system,mainly from three aspects: transient response optimization of flexible grounding,power frequency current compensation and harmonic grounding current compensation.The key technologies such as arc suppression for flexible grounding fault are studied and improved by combining theoretical analysis,simulation verification and dynamic model experiment.In order to solve the problem that the traditional resonant grounding method cannot realize the compensation of transient process,and the transient process of arc suppression coil may also cause adverse effects on arc suppression,a transient response optimization method based on flexible grounding method is proposed.When a single-phase grounding fault occurs,the active inverter injects the transient compensation current of active and inductive components into the neutral point,so as to optimize the transient response during the fault.Theoretical analysis and simulation results show that the active component of transient compensation current can accelerate the attenuation of transient process and restrain the amplitude of transient over-voltage;the inductive component is used to compensate the transient current of arc suppression coil,so as to reduce the arc building rate or make the arc extinguish quickly and reduce the probability of developing from transient fault to permanent fault.In order to solve the problem that the residual current of the fault point cannot be fully compensated in the small current grounding system,which affects the arc suppression performance,an active arc suppression method based on the closed-loop control of zero sequence admittance parameters is proposed,which does not need to be used in line selection or phase selection.The change rule of zero sequence admittance parameters before and after the fault of flexible grounding system is deduced theoretically,and it is used as the basis of parameter closed-loop control.The proposed method reduces the measurement accuracy requirements of the system for zero sequence transformer,and is suitable for the fault conditions of different grounding transition resistance,and realizes the fast processing and reliable arc suppression.In addition,the influence of unbalanced current on parameter measurement and active arc suppression is systematically analyzed,and an improved parameter measurement method suitable for the unbalanced situation of the system is proposed,as well as a new unbalanced current compensation method to avoid the system over-voltage caused by the full compensation of grounding current.In view of the problem that the harmonic component of the fault grounding current affects the reliable arc suppression caused by the large harmonic content of the distribution networks,this paper focuses on the influence of different harmonic sources on the grounding current,and proposes a new harmonic grounding current compensation method based on the principle of zero sequence current parameter ratio by analyzing the flexible change law of zero sequence current on the basis of the new flexible grounding mode.The proposed method adopts zero sequence current closed-loop control,considers the influence factors of harmonic impedance,and is not restricted by the types of harmonic sources and the location of fault points,so as to further improve the accuracy of wide-band grounding current compensation,which is conducive to further realizing the protection objectives of zero residual current and reliable arc suppression.In order to verify the feasibility of neutral flexible grounding,a set of experimental platforms for simulating 10 k V distribution networks is built,and the field experiments are carried out for the key technical requirements of different fault types,such as fault current compensation.The experimental results show that good effect of compensating grounding current,which verifies the correctness of the relevant theory and design.