###### Abstract
However, the overall scheme  
Figure 1: General configuration of a DC microgrid. involves multiple control loops, and this is quite inconvenient for controller design. In [10], the reference voltage and virtual impedance were devised as two piecewise functions of local SoC so that coordinated operation could be achieved between ESSs and the utility grid. Moreover, \(I\)-\(V\) SoC-based droop approaches have also been applied to inner current loops for better dynamic response [15; 16]. Deriving from inverse droop, the balancing method in [17] adaptively modifies the reference voltage by adjusting the frequency-power curve. But power losses are also neglected for SoC estimation. Through sparse communication, three-layer hierarchical control schemes [18; 19] have been presented to cope with the issues of bus voltage deviation and inconsistent line resistances. But additional balancing factors [18] or control loops [19] require an elaborate design to ensure system stability. Some active balancing strategies are also used to achieve SoC consistency. For hybrid ESSs with different parameters, the event-triggered control theory was used to acquire balanced SoC levels in [20]. But complex principles may cause inconvenience for engineering applications. Based on the remaining energy of all batteries and the output signal of the voltage loop, a common time reference was produced to indirectly change the individual reference currents for an equal SoC in [21]. For modular multilevel ESSs, SoC balance was accomplished by directly adjusting the reference currents of inner loops in [22]. But large-signal stability analysis should also be provided for proper controller design because the supercapacitor voltages decrease obviously during the balancing process. For output-series hybrid ESSs, the cooperative control of a three-port converter was studied in [23]. The problems of imbalanced SoC and voltages are addressed, while excellent transient performance cannot be guaranteed in the absence of an inner current loop. A simple structure, high efficiency, and boosted output voltage are the main virtues of dc-dc boost converters, leading to their extensive applications in microgrids. Based on interfacing boost converters, the novel contributions of this article are summarized as follows:  
1. A SoC-balancing strategy based on the current distribution ratio (CDR) is presented for multiple ESUs interfacing with boost converters by adjusting the reference currents. 2. Current saturation is used to shorten the SoC equalization time by fully utilizing the current capability and enabling a large accelerating factor. The impacts of current limits and the accelerating factor are analyzed, and their selection rules are provided. 3. The controller design is given to guarantee sufficient margins during discharge. 4. The adaptive current limits that vary in proportion to the sum current are utilized for series applications to prevent unexpected step reference currents under conditions of load change. Comparisons with representational balancing methods are listed in Table 1. The main advantages of this strategy are described below:  
1. _High Precision of SoC Equalization without Extra Sensors_: In boost converters, the inductor current is also the ESU current. Instead of approximate estimation, these two currents are precisely measured by a single current sensor in each PCU. Then, the real-time SoC can be directly calculated after the initial SoC is determined. This makes the proposed strategy a cost-effective solution for accurate SoC equalization. 2. _Applicability for Parallel and Series Scenarios_: In parallel scenarios, this strategy distributes the load current to mitigate SoC deviations. In series scenarios, it regulates individual voltage gains for SoC consistency while the sum voltage is unchanged. 3. _Accelerated Balancing Process_: The current capacity of the converter is fully exploited by introducing saturation. Thus, high current deviations are maintained for a long time, and this leads to the fast elimination of SoC differences.