DC Microgrids—Part I: A Review of Control Strategies and Stabilization Techniques

This paper presents a review of control strategies, stability analysis, and stabilization techniques for dc microgrids (MGs). Overall control is systematically classified into local and coordinated control levels according to respective functionalities in each level. As opposed to local control, which relies only on local measurements, some line of communication between units needs to be made available in order to achieve the coordinated control. Depending on the communication method, three basic coordinated control strategies can be distinguished, i.e., decentralized, centralized, and distributed control. Decentralized control can be regarded as an extension of the local control since it is also based exclusively on local measurements. In contrast, centralized and distributed control strategies rely on digital communication technologies. A number of approaches using these three coordinated control strategies to achieve various control objectives are reviewed in this paper. Moreover, properties of dc MG dynamics and stability are discussed. This paper illustrates that tightly regulated point-of-load converters tend to reduce the stability margins of the system since they introduce negative impedances, which can potentially oscillate with lightly damped power supply input filters. It is also demonstrated that how the stability of the whole system is defined by the relationship of the source and load impedances, referred to as the minor loop gain. Several prominent specifications for the minor loop gain are reviewed. Finally, a number of active stabilization techniques are presented.

I. Introduction

Environmental concerns and reduction of fossil fuel reserves gave rise to a growing increase in the penetration of distributed generators (DGs) that include renewable energy sources (RESes), energy storage systems (ESSes), and new types of loads like electric vehicles (EVs) and heat pumps in the modern power systems. However, these new components may pose many technical and operational challenges should they continue to be integrated in an uncoordinated way, as is the case today. Appearing in large numbers and scattered across the large geographical areas of interconnected networks, some of the most prominent problems that they can introduce in the system's operating conditions include deteriorated voltage profile, congestions in transmission lines, and reduction of frequency reserves [1].