Research On Flexible Current Arc Suppression Method For Single-line-to-ground Fault In Distribution Networks

In China,the neutral point of distribution networks is mainly grounded by the neutral non-effective method.Under the single-line-to-ground(SLG)fault condition,due to the line-to-line voltages are symmetrical,the power supply for customers is not affected.However,SLG is often with arc over-voltage,which can easily cause an insulation breakdown,so it is necessary to deal with it as soon as possible.With the increase of the scale of the cable lines,the phase-to-ground capacitance and leakage resistance are increased continuously,which make the proportion of active component in the ground-fault residual current larger.Moreover,the extensive use of power electronic equipment has also increased the proportion of harmonic components in ground-fault current.Traditional arc suppression coil is hard to achieve the ground-fault arc suppression,so it is necessary to develop active arc suppression technology to realize total compensation of ground-fault current.According to different application scenarios,this paper proposes three kinds of flexible current arc suppression methods,namely three-phase injection type,neutral point injection type,and hybrid neutral point injection type,and corresponding flexible arc suppression device(FASD).The reference value of closed-loop control of flexible current arc suppression method is calculated from the neutral point voltage and the insulation parameters of distribution networks.A measurement method based on twice injections is deduced.Under the regular operation of distribution networks,the insulation parameters measurement of distribution networks is achieved by adjusting the injection current twice.In the process of ground-fault arc suppression,the controller is susceptible to the perturbation of system parameters,and the ground-fault arc has the characteristics of non-linear,non-periodic,and random,so FASD requires higher robustness of the controller.Compared with the traditional PID control algorithm,the back-stepping control(BSC)method is a recursively designed non-linear feedback control with solid robustness.It can restrain the perturbation of system parameters.The reference value of the injection current is related to the faulty phase power supply voltage.It contains a differential operation,so it is susceptible to the disturbance and noise of sampling signals.The second-order generalized integrator phase-locked loop(SOGI-PLL)algorithm is employed to obtain the amplitude and phase of faulty phase power supply voltage for eliminating the effect of disturbance and noise.Combined with SOGI-PLL and BSC control algorithms,this paper proposes the flexible current arc suppression method based on the BSC-SOGI-PLL algorithm,which can realize the ground-fault current steady-state suppression.The traditional PID control algorithm is used in the FASD grid connection process,which is easy to cause the overshoot phenomenon due to the asynchronous voltage and current.The segmented proportional-integral-differential(SPID)control algorithm is proposed to adjust the control parameters at different stages of transient and steady-state for suppressing the overshoot of the controller in the transient state.Combined with SOGI-PLL and SPID algorithms,this paper proposes the soft grid-connection(SGC)strategy based on the SPID-SOGI-PLL control algorithm for achieving FASD transition smoothly from the stand-alone mode to grid-connected mode.For verifying the effectiveness of the insulation parameters measurement method and different control algorithms,the simulation models which have FASD with three topological structures and distribution networks under SLG fault are built by MATLAB / SIMULINK software.Cascaded H-bridge multilevel converter combined proposed improved distributed commutation modulation(IDCM)strategy and control algorithms is selected in this paper.The physical and industrial prototypes of FASD have been developed and verified the effectiveness of arc suppression on the 10 kV physical simulation platform and 10 kV real experimental platform,respectively.The software simulation and experimental results indicate the proposed flexible current arc suppression methods are feasible and effective and have the merits of low ground-fault residual current and fast response for ground-fault current and slight transient fluctuation.