Calculation Of Short Circuit Fault Current In Power System With A Large Number Of Inverter-interfaced Sources

With the large-scale of inverter-interfaced energy sources connecting to the grid,it brings new challenges to the safe and stable operation of power system.However,the dynamic behavior of power grid under AC short circuit fault is still unclear in this tendency.In order to guarantee the safety and stability of the grid,it is urgent to study on short-circuit current of inverter-interfaced energy sources so as to provide fundamental basis for fault control and protection settings.Firstly,the mathematical model of single inverter-interfaced energy sources are established.By introducing the steady-state control strategy and low voltage ride through(LVRT)control strategy,study theoretically analyzed the transient development process of short-circuit current,then derived the calculation method of steady-state amplitude.The results show that the fully converted inverter-interfaced energy sources only has a short transient process without overcurrent,and the steady-state short-circuit current is under the control instruction.Because of the inertia link,the DFIG will have short-term transient overcurrent process,and then will enter the fault steady-state in a relatively short time.Considering isomorphic inverter-interfaced energy sources field,the DFIG wind farm with the most complex short-circuit current characteristics is taken as an example to establish the model.The external fault index and its own operating point index were selected as the cluster criterion to generate the distance function to measure the similarity between groups of machines.Adopting the aggregation hierarchical clustering method,and the reasonable equivalence method was applied to the comparison and analysis of multiple simulation examples in PSCAD to obtain the isomorphic wind farm equivalent model suitable for short-circuit current calculation.Other types of isomorphic field groups such as PMSG,photovoltaic,etc.,can learn from the equivalent method.Considering heterogeneous inverter-interfaced energy sources field including wind、PV and battery,the influence of the dominant short-circuit current is strengthened by weakening the non-correlated factors inside the field.The engineering calculation expression of short-circuit current is derived,which takes the real-time switching power of the unit,the degree of voltage sag and the reactive power compensation factor as variables.Then the heterogeneous inverter-interfaced model is established in PSCAD for simulation analysis.The fault models and fault analysis methods of individual level and field level inverter-interfaced energy sources provide a theoretical basis for the further development of inverter-interfaced energy sources technology.