PLL-Less Three-Phase Droop-Controlled Inverter with Inherent Current-Limiting Property

In this paper, a novel droop control method for three-phase grid-connected inverters is proposed to guarantee closed-loop system stability and an inherent current-limiting property without the need of a PLL. The inverter is connected to the grid via a filter and a line. Based on the synchronously rotating dq frame modelling and nonlinear ultimate boundedness theory, it is analytically proven that the proposed control scheme maintains the inverter current below a certain upper bound. This current limitation is guaranteed independently of the grid, line and filter parameters; thus increasing the controller robustness. In addition, asymptotic stability of the desired equilibrium point of the closed-loop system is guaranteed under different values of the proposed controller gain. To verify the effectiveness of the proposed nonlinear control strategy, extensive simulations are realized using Matlab/Simulink, where both the stability and the current-limiting property of the controller are validated.