State of charge balancing for distributed batteries in DC microgrids without communication networks
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ORIGINAL ARTICLE
State of charge balancing for distributed batteries in DC microgrids without communication networks Khanh Duc Hoang1 · Hong‑Hee Lee1 Received: 13 August 2020 / Revised: 12 November 2020 / Accepted: 16 November 2020 © The Korean Institute of Power Electronics 2020
Abstract State of charge (SoC) balancing and accurate power sharing have been achieved among distributed batteries in a DC microgrid without a communication network by injecting an AC signal. The frequency of the generated signal is proportional to the SoC of a predefined master battery and it is used for the other batteries as a common variable to estimate the SoC and output power of the master battery. Based on modified droop control, the distributed batteries realize a balanced SoC and accurate power sharing regardless of the differences in the line resistance. A communication system is not required. Thus, the system cost is notably reduced. Moreover, to guarantee system reliability, a new master battery selection method is provided to replace the master battery under fault conditions. The effectiveness of the proposed strategy was verified by simulations and experimental results. Keywords DC microgrid · Power sharing · Power rating · Battery system · State of charge
1 Introduction Microgrids were developed as power systems to integrate renewable energy sources (RESs), storage systems, and load devices. Microgrids can be divided into AC and DC microgrids based on the type of electricity. DC microgrids have attracted more consideration, since certain power sources (e.g., PVs and fuel cells) and modern electronic loads use DC power. Moreover, DC microgrids are preferable when compared to AC microgrids, since they have simplified power control, reduced conversion stages, and improved efficiency [1–4]. In a DC microgrid, the RESs usually cause power fluctuations, since their power varies according to weather conditions. To reduce these fluctuations, batteries are usually used to supply and store system power. However, due to differences in the battery characteristics and the line resistance in a DC microgrid, the state of charge (SoC) between batteries can become imbalanced [5], which leads certain batteries to be over-charged or over-discharged.
* Hong‑Hee Lee [email protected] 1
School of Electrical Engineering, University of Ulsan, Ulsan, Korea
To balance the SoC, batteries with higher a SoC should supply more power than batteries with a lower SoC. For this purpose, central control was introduced using a highbandwidth communication system [6]. However, this control method is hard to use when the central controller fails [7]. To overcome this, distributed control methods were proposed using a low-bandwidth communication system [8–11]. A full communication system with all of the batteries connected together was used to realize a virtual power rating concept to obtain SoC balancing with accurate power sharing [8]. Consensus algorithms were presented by considering the SoC to be a consensus variable with the aid of a sparse c
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