Investigation on Fault Current Characteristics of Droop-Controlled Inverter With Virtual Impedance Emulation

The droop-controlled inverter can provide a more stable operation in weak grids, but it suffers from an overcurrent under fault conditions and may damage power devices. As a solution, the virtual impedance control has been widely utilized in existing research for the overcurrent limiting, which however mainly focuses on the function realization and parameter design, leaving its effects on fault current characteristics unrevealed. To fill this void, a fault current calculation method of such control is proposed in this paper, and the impact of key parameters, such as magnitude and structure of the virtual impedance, and droop coefficients, on the fault current is investigated. Calculation results show that with the same virtual impedance magnitude, the transient fault current grows as $\boldsymbol{X}_{v} / \boldsymbol{R}_{v}$ reduces, and the steady-state fault current changes parabolically with an increasing $\boldsymbol{X}_{v}/\boldsymbol{R}_{v}$. Moreover, higher active power-frequency droop coefficients induce an elevated transient fault current, while its steady-state value does not change significantly. On the other hand, a higher reactive power-voltage droop coefficient will reduce both the transient and steady-state fault currents.