An Improved Nonlinear Droop Control Strategy in DC Microgrids

Droop control has drawn widespread attention and various nonlinear droop characteristics have been developed in dc microgrids. This article proposes an improved nonlinear droop control strategy, which uses the difference between the squared nominal voltage and the squared dc voltage as the droop input and generates the ac current reference directly from the droop characteristic. Compared with the traditional dual-loop droop control strategy, this method can provide faster transient response due to the elimination of the external dc voltage or current loop. And the bus voltage can be regulated very compactly, which is an optimization of steady-state performance. In this article, the stability assessment that violates the constraint conditions of the conventional impedance-based stability criterion is investigated, which considers the number of right-half-plane poles of the open-loop transfer function. The Nyquist criterion of generalized Bode plot is used to analyze the system stability. An adaptive current sharing strategy is proposed to improve the current sharing accuracy in multisource operation. This article also discusses the comprehensive design process for determining droop gain. Finally, experimental results validate the theoretical analysis.