Approach On Arc Suppression And Fault Location Of Single Line-to-ground Fault In Distribution Network Based On STATCOM Common-mode Injection

Medium-voltage power distribution networks have continuously improved requirements for power supply reliability and power quality.Power electronic power distribution equipment is a solution that meets reliability and quality requirements,and is the current development trend of smart distribution networks.The Static Synchronous Compensator(STATCOM)based on the three-phase cascaded H-bridge converter is widely used in distribution network reactive power compensation to improve power quality.In recent years,it has also been reused for single line-to-ground fault arc suppression.STATCOM injects arc-suppression current from the bus,and has a huge compensation capacity.By multiplexing with the reactive power compensation function,it can solve the problem of high idle rate of traditional arc-suppression equipment.However,the existing STATCOM arc suppression solution requires a large number of DC sources to supply power,and the bridge arm withstand voltage increases from the phase voltage to above the line voltage,and the reactive power compensation function cannot be operated at the same time,resulting in low economic and practicality.The analysis of DC capacitors voltage equalization mechanism is lacking,the research on the cooperative control theory of arc suppression and reactive power compensation has not carried out.This thesis proposes a two-phase current injection method to realize STATCOM DC voltage equalization control in arc suppression mode;based on the use of adjustable reactance to reduce the withstand voltage requirements of three-phase bridge arms,STATCOM differential-common-mode decoupling control is proposed to achieve arc suppression and reactive power compensation at the same time;with the characteristic frequency common-mode signal injected by STATCOM,a fault location method based on the interphase unbalance characteristic of the common-mode signal is proposed,then the fault location is accurately and efficiently determined,and the arc suppression and fault location are seamlessly superimposed on the same hardware platform.The specific research points are as follows:(1)Aiming at the problem that the existing STATCOM arc suppression scheme requires a large number of DC sources for power supply,a two-phase current injection arc suppression method considering DC voltage equalization control is proposed.Based on the mechanism of common-mode arc suppression current,through current phasor decomposition analysis,it is demonstrated that only two non-fault phases inject arc suppression current to achieve voltage balance of independent DC capacitors.Furthermore,the arc suppression current and DC capacitor voltage controllers are proposed,the control parameters suitable for different transitional resistances are optimized.The simulation verifies that the two-phase injection method can quickly extinguish the arc.A 10 k V megawatt real prototype and a centralized parameter simulation distribution network experimental platform are built to verify the effectiveness of the proposed method.(2)In order to solve the problems of the existing STATCOM arc suppression scheme and the two-phase injection arc suppression method that the bridge arm withstand voltage rises and cannot be compensated simultaneously,an active and passive hybrid arc suppression method with STATCOM neutral point series adjustable reactance grounding is proposed.Passive devices are used to share the common-mode voltage caused by ground fault,effectively reducing the bridge arm withstand voltage requirements.The differential-common-mode decoupling control strategy of STATCOM is proposed to realize the superposition injection of arc suppression current and reactive power compensation current.The differential-common-mode decoupling controller is designed,and the control parameters are optimized according to the distribution line parameters measured online.Through simulation analysis and low-voltage prototype experiment,the feasibility of the proposed hybrid arc suppression method is verified.(3)Aiming at the problems of low accuracy and high misjudgment rate in traditional signal injection locating methods,the common mode component control is expanded,and the characteristic frequency common mode current is superimposed in the STATCOM arc suppression mode,and the fault location based on the interphase unbalance of the characteristic frequency common mode signal phase is proposed,then the adverse effects of signal shunt and attenuation is effectively avoided.Analyzing the distribution of injected common-mode signals in three-phase lines,the interphase unbalance characteristic of the common-mode signal flowing upstream of the fault point of the faulty feeder is revealed,then criteria for fault line selection and location is proposed.In order to enhance the sensitivity of the criterion and reduce the interference to the operation of the distribution network,a fault location signal selection method adapting to different ground resistances and system parameters is proposed,and a positioning signal superimposed injection controller is designed to realize the simultaneous operation of arc suppression and fault location.Comparing with the traditional method through simulation,it is proved that the proposed method is more sensitive and reliable,and the experimental verification of the 10 k V megawatt-level prototype is realized.The above research results lay a theoretical foundation for the application of STATCOM to single line-to-ground fault arc suppression and fault location in the distribution network,and also provide technical support for the real realization of STATCOM’s multi-functional multiplexing,making it a key device for power electronic distribution networks.