Protection Key Techniques Of Islanded AC Microgrids Considering Characteristics Of Distributed Generators And Multiple Busbar Topology

Microgrids with consumption capacity of renewable energy sources and islanded operation capability can effectively enhance the power supply reliability and power quality of distribution networks in the case of the large-scale penetration of renewable energy sources such as wind power and photovoltaics.They are an important mean to solve the problem of reliable power supply in islands and remote areas.However,as the multi-bus systems,microgrids has the characteristics of various types of distributed generators,flexible operation mode and network topology as well as complicated neutral grounded modes.It leads to that the fault characteristics of microgrids are much different from those of traditional power system.Especially for the islanded microgrids,due to no energy support of utility grid as well as the weak overcurrent capability and complicated output characteristics of inverter-based distributed generators(IBDGs),the microgrid system presents the phenomena of small inertia,weak damping and bidirectional power flow.Thus,the islanded microgrids suffer from the challenge of safe and reliable operation under short-circuit faults.In order to enhance the power supply reliability of islanded microgrids,this paper focuses on the fault characteristics and fault ride-through control of device-level distributed generators and the fault characteristics and protection of system-level islanded microgrids.The research work are as follows:(1)With respect to the protection problem caused by the complicated fault response and large fault inrush current of IBDGs,the fault current characteristics of different voltage-controlled IBDGs are revealed.Two fault ride-through control strategies,current limiting control and mode switching control,are proposed.According to the output characteristics of different voltage control strategies,the equivalent fault model of voltage-controlled IBDGs is established.The iterative calculation method is used to mathematically describe and quantitatively calculate the output current of IBDGs.In addition,the main influencing factors of fault current and the time required to reach the maximum allowable value(i.e.,the maximum short-circuit fault tolerance time of IBDGs)are analyzed.On this basis,the current limiting control and mode switching control are proposed to rapidly restrain the fault inrush currents of different voltage-controlled IBDGs.(2)With respect to the protection problems caused by the complicated output characteristics of IBDGs and the small system scale and flexible network topology of islanded microgrids,the fault characteristic differences between internal and external faults are revealed.And a non single-phase to ground fault main protection method based on time-domain waveform feature is proposed.Considering the influence of the fault characteristics of distributed generators and the network topology of microgrid systems,the phase differences between positive-sequence current fault components at both ends of the fault line and non-fault line are analyzed.Also,the feature differences between internal and external faults are pointed out.On this basis,the polarity and time information of the initial peak of positive-sequence current fault component are extracted by using mathematical morphology algorithm.The main and auxiliary criteria are constructed to form the double-ended protection scheme for the non single-phase to ground faults of islanded microgrids.The proposed double-ended protection has relatively low requirements on communication bandwidth and data synchronization,which can identify internal fault within 15 ms.Therefore,it is used as the main protection of islanded microgrid to realize fast and reliable fault line isolation under non single-phase to ground faults.(3)With respect to the protection problem caused by the large differences in the output characteristics of different types of distributed generators and the unobvious differences in the fundamental fault current and impedance when the fault location moves along the feeder with short length and small X/R ratio,a non-single-phase ground fault backup protection method based on active harmonic injection is proposed.Assuming that all IBDGs can be equivalent to constant voltage sources in the harmonic layer,the phase differences between harmonic voltage and current on the fault and non-fault lines of the islanded microgrids with looped and radial network topologies are analyzed,respectively.The phase characteristic differences between internal and external faults are pointed out.On this basis,a single-ended protection method for the non single-phase to ground faults of islanded microgrids is proposed,including the harmonic injection control of IBDGs and the design of protection criteria.By adding harmonic injection control loop,the output characteristics of IBDGs are unified in the harmonic layer and the required harmonic components are injected into the microgrid system.The protection criteria are constructed based on the sign feature of harmonic phase difference.The proposed single-ended protection only requires local fault information,and as the backup protection of islanded microgrids,it ensures the detection and isolation of non-single-phase ground faults when the main protection fails.(4)With respect to the difficulty in fault detection and isolation caused by the weak single-phase to ground fault current and complicated fault characteristics of neutral ineffectively grounded islanded microgrids,a single-phase to ground fault protection method based on zero-sequence voltage and current waveform feature is proposed.The single-phase to ground fault characteristics of the islanded microgrids with different neutral ineffectively grounded mode and network topologies are analyzed.It is found that the looped microgrids have a virtual terminal that the zero-sequence current is equal to zero.When the high frequency virtual terminal locates on the non-fault line and fault line,the fault characteristic differences between internal and external faults are revealed.On this basis,based on the skewness signs of the high frequency zero-sequence voltage and current at both ends of the line,and the fundamental amplitude of the difference between calculated and actual zero-sequence currents at another terminal of the line,the fault line identification criteria are constructed to realize single-phase to ground fault line detection and isolation.(5)Based on the research results of the fault analysis and protection strategies of IBDGs and islanded microgrids,combined with the non single-phase to ground fault and single-phase to ground fault starting criteria,the overall protection scheme of islanded microgrids covering all types of short-circuit faults is proposed.The RT-LAB closed-loop test system for a 10 k V islanded microgrid is built to verify the effectiveness of the protection scheme under various fault conditions.This paper focuses on the safe and reliable operation requirements of islanded microgrids under short-circuit faults.From the device level distributed generators and the system level islanded microgrids,four research contents are carried out,which include the fault characteristic analysis and fault ride-through control of voltage-controlled IBDGs,the double-ended main protection and single-ended backup protection for non single-phase to ground faults,and the single-phase to ground fault protection of islanded microgrids.The overall protection scheme of islanded microgrids covering all types of short-circuit faults is formed,providing security for promoting the application of microgrid technology.