Hybridization of energy storages (ESs) with different characteristics takes advantages of all ESs. Centralized control with high/low pass filter (HPF/LPF) for system net power decomposition and ESs power dispatch is usually implemented in hybrid energy storage system (HESS). In this paper, hierarchical control of HESS, comprised of both centralized and distributed control, is proposed to enhance system reliability. The conventional HESS centralized control is refined with implementations of online iteration, secondary voltage regulation and autonomous state of charge (SoC) recovery. ESs power references are generated iteratively to maximize the utilizations of ESs ramp rates and power capacities. Secondary voltage regulation and autonomous SoC recovery are applied to minimize bus voltage deviation and limit slack terminal SoC variation, respectively. In case of communication failure, a novel algorithm for HESS distributed control is proposed to retain system operation. Bus voltage is regarded as the global indicator for system power balance and droop relationships are imposed for ESs control. System net power decomposition and ESs power dispatch are realized with localized LPFs. SoC recovery in distributed control is implemented by tuning the threshold voltage of slack terminal. A lab-scale DC microgrid is developed to verify the proposed hierarchical control of HESS.Index Terms-Hybrid energy storage system, DC microgrid, hierarchical control, net power decomposition, ESs power dispatch.
Multiple-voltage-region control, in which the busvoltage range is divided into several regions, is usually implemented for dc microgrid operation in distributed manner. Voltage/power droop relationships are imposed for active power sharing among slack terminals. Conventionally, threshold voltages for voltage region partition are determined with fixed percentage of variation around the nominal value, which may result in unevenness of droop coefficients in different regions. If system droop coefficient is too high, significant bus-voltage step change due to load variation will occur. On the other hand, significant power sharing error among slack terminals will be induced if the droop coefficient is too low. In this paper, a compromised solution with power-capacity-based bus-voltage region partition is proposed to equalize the droop coefficients in different regions. However, the droop coefficients are determined based on the rated power capacity of system units. Busvoltage discontinuity appears when the power capacity reduces in actual implementation. To eliminate the voltage discontinuity, online droop coefficient tuning according to the real-time power capacity is implemented. Algorithms for local power capacity estimation of solar photovoltaic (PV) and battery energy storage have been proposed. A lab-scale dc microgrid has been developed for verification of the proposed methods.Index Terms-DC microgrid, droop coefficient, online tuning, power capacity estimation, voltage region partition.
Bidirectional interlinking converter (BIC) is normally configured as the slack terminal to regulate system bus voltage for dc microgrid operation in grid-tied state. In case of utility grid fault, BIC is disabled and the system changes to operate in islanded state. Localized battery energy storage (BES) voltage regulation mode (VRM) operation is activated to maintain system power balance. Operating mode changes of BES and BIC during islanding and reconnection might result in system voltage fluctuations and undesirable inrush current. Detailed transition procedure, which is crucial, has seldom been discussed in literature. In this paper, multiple-slack-terminal dc microgrid, in which both BIC and BES operate in VRM with droop control, is implemented to ensure smooth transitions between grid-tied and islanded states. BES operating mode change is eliminated, and thus, system bus voltage is actively regulated throughout the transitions. Contactor is installed at BIC dc side to decouple BIC operating mode change and grid connection. A counter is used to ensure bus voltages at both sides of contactor have been matched stably to minimize inrush current. To enhance system control accuracy, multilevel energy management system is carried out. A lab-scale dc microgrid has been developed for experimental verifications.
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