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Abstract
A novel distributed reinforcement learning (DRL) strategy is proposed in this study to coordinate current sharing and voltage restoration in an islanded DC microgrid. Firstly, a reward function considering both equal proportional current sharing and cooperative voltage restoration is defined for each local agent. The global reward of the whole DC microgrid which is the sum of the local rewards is regarged as the optimization objective for DRL. Secondly, by using the distributed consensus method, the predefined pinning consensus value that will maximize the global reward is obtained. An adaptive updating method is proposed to ensure stability of the above pinning consensus method under uncertain communication. Finally, the proposed DRL is implemented along with the synchronization seeking process of the pinning reward, to maximize the global reward and achieve an optimal solution for a DC microgrid. Simulation studies with a typical DC microgrid demonstrate that the proposed DRL is computationally efficient and able to provide an optimal solution even when the communication topology changes.
Highlights
DC microgrids attract increasing attention in recent years for two reasons: 1) on one hand, the emerging diversity of distributed generators (DGs) includes a majority of DC generators, such as photovoltaics (PVs), fuel cells (FCs) and energy storage systems (ESSs); 2) on the other hand, there will be more and more DC loads, such as electric vehicles (EVs), DC relays, in future smart grids [1,2,3]
A DC microgrid has several advantages compared with an AC microgrid: 1) it has less loss from power transformation because there are no AC/DC converters, 2) it avoids some problems often occurring in an AC microgrid, for instance, harmonics and synchronization; and 3) it can have improved power quality and reliability, because reactive power compensation is not needed from the power supply [4,5,6]
Droop control is generally accepted as an effective solution for DC microgrids, and such applications of droop control have been investigated in many papers [7, 8]
Summary
DC microgrids attract increasing attention in recent years for two reasons: 1) on one hand, the emerging diversity of distributed generators (DGs) includes a majority of DC generators, such as photovoltaics (PVs), fuel cells (FCs) and energy storage systems (ESSs); 2) on the other hand, there will be more and more DC loads, such as electric vehicles (EVs), DC relays, in future smart grids [1,2,3]. A distributed control scheme can be regarded as a feasible solution for DC microgrids in this study Another problem that needs to be considered in a DC microgrid is to coordinate the following two objectives in which exist inherent contradictions: 1) to implement voltage restoration in DC buses; and 2) to realize accurate current or load sharing in a DC microgrid. Inspired by distributed control and the RL algorithm, a novel distributed RL (DRL) approach for a DC microgrid is proposed and investigated in this study It can achieve the same control performances as a centralized control scheme while overcoming some of its problems. 2) Proposal of an evaluation method using a global reward discovered locally, which can be used to evaluate the control performance of DRL considering both equal proportional current sharing and cooperative voltage restoration for an islanded DC microgrid. The rest of this paper is organized as follows: Section 2 presents a brief introduction to hierarchical control of a DC microgrid and the distributed consensus method through pinning control; Section 3 elaborates on the proposed DRL, including its reward function, the distributed consensus method through pinning control, and its detailed control process; the proposed DRL is simulated and investigated with a typical system in Section 4; and conclusions are presented
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