An AC-DC Coupled Droop Control Strategy for VSC-Based DC Microgrids

Abstract

Droop control is a common strategy to facilitate appropriate load sharing among different sources in dc microgrids (MGs). To endow simple control structure and fast bus voltage transient response to dc MGs with multiple voltage source converters (VSCs), this article proposes an ac–dc coupled droop control strategy, which yields the ac current (active components <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">i_d</i> ) reference of the VSCs inner loop directly in the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">dq</i> frame. In this way, the ac current ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">i_d</i> ) of the VSCs inner loop is directly linked to the load sharing performance of the dc MG. In contrast with the existing droop approaches, the proposed method does not require any extra outer dc voltage/current proportional--integral (PI) loops, which avails fast bus voltage dynamics during transients. Systematic evaluation of its performance is conducted by small-signal modeling and subsequent analysis from a single-source operation to a multisource operation. The effects of inner current loop bandwidth and droop gain on system stability are studied while feeding constant power loads. The theoretical analysis has been validated by both simulation and experimental results.