Research On Flexible Fault Current Limiting Method In Medium Voltage Distribution Network

With the development of power system,short-circuit current level in a distribution network is increasing.An excessive short-circuit current may not only exceed the breaking capacity limit of circuit breaker,but also endanger the safety of distribution equipment and threaten the stability of a power grid.So,research on early short-circuit fault detection and then limit it is very important and has great significance.The thesis summarizes domestic and foreign research on short-circuit fault detection and early fault current limiting technology.Fault characteristics in medium voltage distribution network are analyzed,the wavelet detection method is proposed used in early short-circuit fault detection of medium voltage distribution network,which is optimized by a Neural Network.Method proposed in the thesis finds the applicable wavelet function by NN training and learning,high order detail component is used as detection criterion,which obtained from wavelet transform of the current.Due to flexibly of expanding,the H bridge converter is very suitable used in higher power field.With the ability of current or integrated voltage injection,and good harmonic characteristics and dynamic response fast,it has become a research hotspot in harmonic control and reactive power compensation.Single-phase split voltage type H bridge circuit used to limit short-circuit current in medium voltage distribution network is proposed for the first time in the thesis.Split voltage H bridge flexible fault current limiter(SVHB-FFCL)is construct and used in medium voltage distribution network to limit short-circuit fault current flexibility.Single phase SVHB-FFCL coupling with the network by several capacitor,these capacitors connected in series to reduce the voltage pressure.Each capacitor is connected in parallel with the output of inductance filter circuit of the H-bridge converter bank,the bank composed of several H bridge converter cascade or multiplied meeting the withstand level of voltage and current of power electronic switching devices.Based on above study,PSCAD/EMTDC simulation model of a medium voltage distribution network is built,different algorithm program of short-circuit fault early detection are written with MATLAB.ntification effect of the optimization wavelet detection method proposed in the thesis with other traditional detection ones in short-circuit fault,which verify the superiority of the method.Based on the optimized wavelet detection method,the prototype of short circuit fault detection in power distribution network is built and verified in the physical simulation system of distribution network.The experimental results further demonstrate the effectiveness and practicability of the proposed method.PSCAD/EMTDC is used to build the simulation model in the medium voltage distribution network,based on the short-circuit fault detection results,control input SVHB-FFCL to carry out thecurrent limit.The simulation results show that the split type H bridge flexible limiting technology,adapt to different types of fault location,feeder and the structure of distribution network operation mode changes,can realize flexible transient fault current suppression,to reduce the harm of short circuit current,circuit breaker to reduce the burden of study,provides a new idea for engineering application.