Fault Current Calculation for Inverter-interfaced Power Sources Considering Saturation Element

The fault current of converter-interfaced renewable energy sources (CIRESs) is mainly determined by the control system of the inverter, so the traditional calculation method of short-circuit current based on voltage-flux is no longer applicable. A typical control system of CIRESs is established by coordinate transformation from abc frame to dq frame. Through the tuning of PI parameters, the second order system is reduced to the first order system, and the fault current is calculated using transfer function. After that, the impact of the saturation element on transient current is analyzed. The fault current at this time is divided into two stages according to whether the saturation element is working or not, and the specific moment when the input signal exits the saturation region is calculated. When the input signal enters the saturation region, the output of the saturation element is a constant value, so the short-circuit current can still be seen as a step response. At the second stage, the fault current can be regarded as the superposition of zero input response and zero state response. The classical method of linear control theory is used to calculate the fault current for the system with a nonlinear link, so it is easy for industrial application due to the small amount of calculation. In addition, the transient current peak value and its occurrence time calculated by this method are basically equal to the simulation results, and this proves the effectiveness of the proposed method.