Study On Fault Characteristics And Line Protection Of Independent Microgrid

Microgrid are developed rapidly as an effective form of use of renewable energy and distributed generation.The independent microgrid does not have electrical connection with the conventional power grid.It only provides power supply to the load by means of internal distributed power supply,and is often used in remote islands far away from the main network.The power supply form and current limiting link of the inverter interface in the independent microgrid make the short-circuit current amplitude much smaller than that of the conventional power grid.The impact of the load current on the fault current distribution is obvious,and the topology gives the line current a greater possibility of bidirectional flow.Traditional protection of distribution network is difficult to operate reliably.Therefore,it is urgent to study the fault characteristics and protection of the independent microgrid.Mastering the microgrid fault characteristics is the basis of protection study.The detailed models of diesel generator,the constant-voltage-frequency-control power supply and the constant-power-control power supply are established,and the fault output characteristics of different types of power supplies are theoretically analyzed.The output current constraint and the outlet voltage constraint of the inverter-type power supply are analyzed.Considering the difference of operation mode,power supply type and fault severity,the fault output expressions of the constant-voltage-frequency control power supply and the constant-power-control power supply are obtained.The simulation verifies the correctness of power fault characteristics analysis under different conditions.Operation performances of traditional relay protections are analyzed.The difference of fault current contribution ability of power supply on both sides of the line,fault resistance and load current make the sensitivity of current differential protection operation decrease or even make the protection reject.Distributed power supply can increase the measurement impedance of distance protection,which may result in rejection.The difference of fault current contribution ability of the power supply makes it difficult to calculate the current protection setting value,which can not meet the requirement of selectivity.Furthermore,an improved scheme of microgrid protection including power differential protection and longitudinal impedance protection is proposed.The correctness of the proposed scheme is verified by the simulation results.Further research on the new principle of microgrid protection is carried out.Analytical expressions of equivalent positive sequence fault component impedance for different types of power supply branches in microgrid are derived.The effects of microgrid load before fault,output current amplitude after fault and external equivalent impedance on the impedance angle of equivalent positive sequence fault component are analyzed.Based on the positive sequence current fault component of line and distributed photovoltaic power,the impedance angle of equivalent positive sequence fault component is proposed.The method of judging the fault direction by comparing the phase of source current before fault,and using the sudden change of photovoltaic branch current combined with the sudden change of photovoltaic power reference value to construct the start-up logic,forming a locking protection action scheme,which can accurately identify the faults inside and outside the region and quickly remove different types of faulty lines in the smallest range.