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 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 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 to using these three coordinated control strategies to achieve various control objectives are reviewed in the paper. Moreover, properties of DC MG dynamics and stability are discussed. The paper illustrates that tightly regulated point-of-load (POL) 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 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. Index Terms-DC microgrid (MG), local control, coordinated control, impedance specifications, stability. NOMENCLATURE Acronyms AVP Adaptive voltage positioning. BLDC Brushless DC. CC Central controller. CPL Constant power load. DBS DC bus signaling. DCL Digital communication link. DG Distributed generator. DPS Distributed power system. EET Extra element theorem. ESAC Energy storage analysis consortium. ESS Energy storage system. EV Electric vehicle. GM Gain margin.

DC power systems are gaining an increasing interest in renewable energy applications because of the good matching with dc output type sources such as photovoltaic (PV) systems and secondary batteries. In this paper, several distributed generators (DGs) have been merged together with a pair of batteries and loads to form an autonomous dc Microgrid (MG). To overcome the control challenge associated with coordination of multiple batteries within one stand-alone MG, a double-layer hierarchical control strategy was proposed; 1) The unit-level primary control layer was established by an adaptive voltage-droop (VD) method aimed to regulate the common bus voltage and to sustain the states of charge (SOCs) of batteries close to each other during moderate replenishment. The control of every unit was expanded with unit-specific algorithm, i.e. finish-of-charging for batteries and maximum power point tracking (MPPT) for renewable energy sources (RESs), with which a smooth on-line overlap was designed; 2) the supervisory control layer was designed to use the low bandwidth communication interface between the central controller and sources in order to collect data needed for adaptive calculation of virtual resistances (VRs) as well as transit criteria for changing unit-level operating modes. A smallsignal stability for the whole range of VRs. The performance of developed control was assessed through experimental results.

DC microgrids (MGs) have been gaining a continually increasing interest over the past couple of years both in academia and industry. The advantages of DC distribution when compared to its AC counterpart are well known. The most important ones include higher reliability and efficiency, simpler control and natural interface with renewable energy sources (RESs), electronic loads and energy storage systems (ESSs). With rapid emergence of these components in modern power systems, the importance of DC in today's society is gradually being brought to a whole new level. A broad class of traditional DC distribution applications such as traction, telecom, vehicular and distributed power systems can be classified under DC MG framework and ongoing development and expansion of the field is largely influenced by concepts used over there. This paper aims firstly to shed light on the practical design aspects of DC MG technology concerning typical power hardware topologies and their suitability for different emerging smart grid applications. Then, an overview of the state of the art in DC MG protection and grounding is provided. Owing to the fact that there is no zero current crossing, an arc that appears upon breaking DC current cannot be extinguished naturally, making the protection of DC MGs a challenging problem. In relation with this, a comprehensive overview of protection schemes which discusses both design of practical protective devices and their integration into overall protection systems is provided. Closely coupled with protection, conflicting grounding objectives, e.g. minimization of stray current and common mode voltage are explained and several practical solutions are presented. Also, standardization efforts for DC systems are addressed. Finally, concluding remarks and important future research directions are pointed out.Index Terms -DC microgrid (MG), power architectures, protection and grounding, standardization. 0885-8993 (c)

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