Research Of Flexible Short Current Limiting Method For Distribution Network

In recent years,with the growing share of distributed generation,distribution network has been expanded continually,the structure of which is becoming more and more complicated.It results in a sharp increase in short circuit current which may exceed the interrupting capacity of the circuit breaker in some cases.When it happens a short circuit fault,the short circuit current will reach its peak value within about half cycle,which has enormously adverse impact on the grid and threatens the distribution equipments and network.Thus the detection of short circuit faults in incipient stage and limiting of the fault currents have practical significance and research value.In this paper,the research status about the fast detection and limiting of short circuit faults is summarized in detail and the short circuit fault characteristics in distribution network are analyzed in depth.Combining the instantaneous value of current waveforms with its high order detail component via wavelet transform as criteria,an early detection method of lOkV medium voltage distribution networks is proposed.The detection results of this method and other algorithms are compared.In addition,in order to improve the anti-interference and adaptability,an IIR digital low pass filter is designed as preposed filtering unit.Cascaded H-bridge(CHB)multi-level converter based on multilevel PWM technique is suitable for occasions with high-voltage and high-power and has the ability to inject comprehensive current or voltage into grid.This device has a superior harmonic characteristic of output signal and its response speed is quite rapid.For these reasons,CHB converter has become a research hotspot in the fields of harmonic inhibition and reactive power compensation.In this paper,the multi-level converter using Three One-phase CHB topology has been applied in short current limiting field,the principle of flexible short current limiting for the distribution network is studied and the control scheme of fault current limiter(FCL)is proposed.Aiming at limiting the peak value of short current,some suitable signals is selected as feedback in the control system which is designed based upon different modulation strategies and control algori:hms,to make the converter generate appropriate current.On the basis of analysis on principles about the fast detection and limiting of short circuit faults,distribution network software simulation system and CHB flexible fault current limiter simulation model are built using MATLAB/SIMULINK software.The program of early detection algorithm is written using MATLAB,too.In addition,the waveforms of short currents in distribution network physical simulation system are collected.The early detection method is applied in software simulation waveforms and physical simulation waveforms of short currents respectively.The simulation and experiment results both show that the methods adopted in this paper can realize millisecond level fault detection and can be used in practical distribution network system.According to the early detection results,FCL is controlled to put into operation in the software simulation system.The simulation results show that flexible short current limiting technique can suppress the peak value of short currents so as to relieve the burden of circuit breaker,mitigate the hazards attributed to the short circuit current.In a word,this method provides a novel thought to practical engineering applications.