State of Charge Balancing for Distributed Battery Units Based on Adaptive Virtual Power Rating in a DC Microgrid
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ORIGINAL ARTICLE
State of Charge Balancing for Distributed Battery Units Based on Adaptive Virtual Power Rating in a DC Microgrid Khanh Duc Hoang1 · Hong‑Hee Lee1 Received: 15 November 2019 / Revised: 27 May 2020 / Accepted: 2 July 2020 © The Korean Institute of Electrical Engineers 2020
Abstract This paper proposes an adaptive virtual power rating method for state of charge (SoC) balancing among distributed battery units (BUs) in a DC microgrid. The virtual power rating is flexibly determined according to the SoC to obtain the droop gain of BU, and the balanced SoC is achieved by means of the modified droop controller. Because an accurate power sharing among BUs is satisfied by using only virtual power rating, SoC balancing performance is consistently ensured regardless of the line resistance difference. Moreover, the voltage restoration to keep the grid voltage at a desired value is easily realized without PI controller, and the proposed control strategy is implemented based on the distributed control method with simple low-bandwidth communication. The system stability is investigated, and the performance of the control method is demonstrated through both simulations and experiments. Keywords DC microgrid · Power sharing · Power rating · Battery system · State of charge · Voltage regulation
1 Introduction Conventional power generation methods have some problems such as fossil fuel scarcity and greenhouse gas emissions. To deal with these problems, renewable energy sources (RESs) have been intensively developed, including wind power, photovoltaics (PV) and fuel cells. Microgrids are an effective solution to integrate these RESs and loads. Because various power sources and modern loads in microgrids have DC coupling (e.g., fuel cells, PVs, and LEDs), DC microgrids have recently received more attention and are considered as an efficient method compared with AC microgrids [1–6]. Furthermore, there are no issues linked with reactive power and synchronization, and the control schemes are simpler with higher efficiency because of the absence of transitional AC power conversion stages. In a typical DC microgrid in Fig. 1, RESs such as PVs and wind power generate power fluctuation because of their stochastic and intermittent properties. To guarantee the stable operation of a DC microgrid, battery units (BUs) are normally applied to mitigate the power fluctuations of RESs.
* Hong‑Hee Lee [email protected] 1
University of Ulsan, Ulsan, Republic of Korea
BUs operate in charging mode to absorb excess power from the RESs when their power generation is larger than the consuming power. Oppositely, when the power from RESs is not enough for the load, BUs operate in discharging mode to supply the remaining power needed for the grid. In a DC microgrid, the connections of sources and loads are widely distributed. Thus, BUs are generally located at different places in the grid and operate with different charging or discharging power due to the voltage drop induced by line resistance [7]. These conditions result in an
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