Research On Fault Characteristic Analysis And Protection Strategy Of Microgrid

At present,Micro-Grid(MG)technology has become an effective means to integrate new energy power distributed generation,and promote the development and utilization of new energy,to a certain extent,ease the energy pressure.However,MG's supply and distribution of the structure of the electrical system is different from the traditional distribution network,which may lead to the traditional protection method is not applicable to the MG system.Therefore,the research on the fault characteristics and protection strategy of MG is the basis for ensuring the safe and reliable operation of MG,which has important theoretical research value and guidance significance of MG technology.The main contents of this thesis are as follows.(1)Based on the analysis of the structure and operation characteristics of the MG system,the mode of connecting different kinds of Distributed Generations(DGs)to the utility grid and the control mode of DGs are discussed in the thesis,the fault equivalent model of Inverter-based Distributed Generation(IBDG)under different protection modes of inverter is analysised emphatically.Based on it,the operation model of low voltage AC MG is established by using PSCAD simulation software.The output characteristics of common faults occur during power system are simulated and analyzed under different operating conditions of MG.(2)The MG protection characteristic is analyzed on the basis of the MG fault characteristic and the protection scheme of the contact line under the condition of MG fault ride-through operation is studied.The influence of the change of DG access position and capacity on the protection of MG is analyzed emphatically.At the same time,the influence of the change of the amplitude of the fault current on the protection under different operation modes of MG is analyzed.On it basis,the MG protection scheme under the islanding operation system based on the centralized decision-making structure of the communication system is studied.The scheme uses the Intelligent Electronic Device(IED)as the monitoring and communication components of the protection center.It adopts the improved matrix location and isolation algorithm based network structure for protection of MG.The algorithm uses the overcurrent information as the starting condition,and the fault current direction information is the fault condition.The example analysis shows that the algorithm can not only solve the problem of fault location and isolation of conventional line faults in MG system,but also can realize the function of fault location and isolation in the case of the fault occuring at the head and end of the feeder,multiple fault and missing fault information in the isolated MG.(3)The same protection strategy of different oprationg states of operating states of MG is studied.It is based on the distributed decision-making structure of the IED communication system as a framework and drawing on the principle of action of longitudinal differential protection.And then the protection strategy with defining the correlation coefficient and direction factor and fault criterion of each IED to achieve the function of IED's fast main protection and backup protection.At the same time,the isolation strategy of the MG communication interruption based on the beginning of the line overcurrent protection has taken into account.The case study shows that the same protection strategy with different operating states of MG based on the distributed decision-making structure can not only complete the line and bus fault isolation,but also to solve the IED or circuit breaker refusal case of fault isolation.At the same time,the case is given to demonstrate the effectiveness and reliability of MG fault isolation strategy under communication interruption.