Research Of Technologies On Active Full-compensation In Resonant Grounded Systems

With the acceleration of the industrialization in modern society,the size of distribution network is getting larger.As a result,the majority of existing passive arc suppression coil still cannot realize the accurate compensation for the system capacitance current.The residual fault current is still sizable.In addition,the large number of non-linear electrical equipment are used in modern power system and the working environment of transmission line gets worse,which make the content of higher harmonic component and resistive active component increase obviously in single-phase earth fault current.At this time,even fundamental reactive component is totally compensated,the residual fault current is still large and the problems of arc grounding over-voltage and phase-to-phase short circuit are easily to happen.Therefore,it is necessary to develop a new active full-compensation arc suppression device,which can compensate for the ground fault current and minimize its value effectively to realize the reliable arc extinction on the basis of accurate measurement of the reactive component,harmonic component and active component of fault current.On the basis of the above background,this paper analyzed the operating characteristics of resonant grounded system,discussed the new change of single phase to ground fault current in distribution network,pointed out that using the traditional passive arc suppression coil cannot guarantee the reliable self-extinction of grounding arc.First of all,square wave signal injection method used to real-time measure the capacitance phase-to-earth current is put forward.In this method,a square wave signal with certain energy is injected into the secondary side of the voltage transformer belonged to the arc suppression coil.Then parallel oscillation under natural resonance frequency is generated.As a result system capacitance to earth can be figured out by detecting the resonance frequency at this time.Compared with others,this method has a short measurement period and can be implemented easily.Subsequently,according to the fact that fifth and seventh harmonic component affect the extinction of grounding arc,harmonic component measurement method is proposed by analyzing the relationship between fault harmonic current and the displacement voltage of neutral point.In this method,in the case of total capacitance of the system known only,make FFT decomposition of the displacement voltage of neutral point,then harmonic component of corresponding frequency can be figured out.Finally,the source of the active component in the fault current is introduced.And the method of measuring active component based on the damping ratio is proposed.By measuring the natural damping rate,the value of active component can be indirectly obtained through this method.Based on the accurate measurement of three kinds of fault current components,a new active full-compensation arc suppression device is improved,including main arc suppression device,auxiliary active compensation device and measurement control unit.Main arc suppression device can accomplish the task of limiting the displacement voltage of neutral point and fully compensating the fundamental reactive component by the control mode of ‘pre-tune and with-tune’.In auxiliary active compensation device,single phase voltage source inverter is used,by the control method of hysteresis comparison PWM,compensating current is produced by the given inverter control command current.Thus the suppression of harmonic component and active component is realized.The measurement control unit,controlling the main arc suppression device and auxiliary active compensation device,can detect three kinds of fault current components by applying related measurement methods.Finally,the three kinds of measurement methods and active full compensation arc suppression device are simulated and verified by the platform Matlab/Simulink.The simulation result proved the correctness and validity of the measurement methods and arc suppression device described in the paper.