Distributed AC–DC Coupled Hierarchical Control for VSC-Based DC Microgrids

In this article, a distributed ac–dc coupled strategy for the hierarchical control of voltage source converter (VSC)–based dc microgrids is proposed. Unlike existing hierarchical control methods, this approach directly controls the ac inductor current (i_d) of the VSC's inner loop in both primary and secondary layers to achieve proportional power sharing and voltage regulation. Specifically, an ac current-based i_d-v_dc droop method is used in the primary layer, without any external dc voltage/current control loops. In the secondary layer, an ac current-based compensator is used to adjust the average dc voltage and ensure accurate power sharing among sources. In the communication layer, the global average information is obtained by transferring only one variable. Furthermore, the steady-state, dynamic, small-signal stability, and large-signal stability performance are analyzed. A fresh insight into the effect of the droop gain on system large-signal stability is provided. Finally, the analytical results have been verified through experiments on a laboratory-scale dc MG.

I. Introduction

Microgrid (MG) is generally considered to be an integration of distributed generators (DGs), energy storage systems, and loads [1]. Compared with the traditional ac counterparts, dc MGs have been recognized as an attractive option due to the reduction of power conversion stages, absence of reactive power management, and phase synchronization issues. In a common dc MG, multiple DGs are connected in parallel to a single dc bus or multiple buses, and the main control objective of the dc MG is to maintain the stability of dc bus voltage and proportional load sharing among different DGs. There are two main categories of load sharing strategies, namely active load sharing (e.g., leader-follower control, centralized control, and circular current chain control [2]) and passive load sharing (e.g., droop control [3]). As a peer-to-peer control method, droop control is usually employed to establish fundamental power sharing among different power DGs. It requires no communication links and relies only on local measurements, which is better suited to a “plug-and-play” scenario [2]. However, droop control inevitably causes voltage deviation at the dc bus. Besides, the accuracy of load sharing ratio is affected when considering the cable resistances of the dc MG.