PI control and model predictive control (MPC) are mainly utilized in battery and supercapacitor hybrid energy storage system (HESS) of DC microgrid. Unfortunately, the regulation time of PI controller is long, while large current ripples are introduced by MPC when the system frequency is low. This paper proposes a new model predictive current control (new MPCC) strategy for HESS based on MPC and PI controllers. This method reduces current ripples, and meanwhile enhances system stability and rapidity. MPC is adopted as the outer loop, and calculates reference values for the inner PI current loop. Furthermore, the state of charge (SOC) of the supercapacitor (SC) is considered based on fuzzy control to optimize the reference power of SC inner controller. Firstly, the topology and mathematical model of HESS in DC microgrid are analyzed. Then based on PI control and MPC, the new MPCC is proposed, and compared with MPC and PI methods respectively. Finally, the fuzzy control method takes SOC of SC into account to optimize the SC reference current. Simulation and experimental results show that the proposed new control strategy could effectively decrease current ripples and dynamic processes, and entirely perform high energy density characteristic of batteries and high power density characteristic of SC.

In a DC Microgrid, the loads are interfaced through power electronic converters. These loads behave as constant power loads (CPLs) when tightly regulated. CPLs have negative incremental impedance which may cause instability. Consequently, the stability analysis is very important to ensure the whole system stable, especially in island mode. This paper investigates the large signal stability of the DC Microgrid, considering the effects of the CPLs and the storage system. Based on mixed potential theory, the large signal stability criteria are derived, and quantitatively describe the relationship between system parameters and the stability. The proposed criteria are simple and straightforward. The power of the storage system and the negative incremental impedance of the CPLs are both taken into account to ensure stability. The results indicate the criteria could guarantee the whole system stable during large disturbances.

In DC microgrids, a large-capacity hybrid energy storage system (HESS) is introduced to eliminate variable fluctuations of distributed source powers and load powers. Aiming at improving disturbance immunity and decreasing adjustment time, this paper proposes active disturbance rejection control (ADRC) combined with improved MPC for n + 1 parallel converters of large-capacity hybrid energy storage systems. ADRC is utilized in outer voltage control loops, and improved MPC is employed in inner current control loops of n battery converters. Droop control is adopted to obtain power distribution between n battery converters, and a DC bus voltage compensator is used to compensate voltage deviations and maintain constant DC bus voltage. The low-pass filter (LPF) is adopted to obtain high-frequency power as the reference for the supercapacitor converter, ADRC is also utilized in the outer power control loop, and MPC is employed in the inner current control loop. Compared with traditional observers, the voltage expansion state observer of the proposed ADRC control is independent of the system model and parameters and consequently has strong disturbance immunity, and significantly reduces voltage overshoots during power fluctuations. The MPC-based inner current control loops of n + 1 converters accelerate current response speed and significantly decrease switching losses. Simulation and experimental results indicate that utilizing the proposed control strategies, large-capacity HESS has stronger anti-interference ability, shorter regulation time, smaller switching loss, and simultaneously maintains the stability of the DC bus voltage.

Islanded DC microgrids composed of distributed generators (DGs), constant power loads (CPLs), parallel converters, batteries and supercapacitors (SCs) are typical nonlinear systems, and guaranteeing large-signal stability is a key issue. In this paper, the nonlinear model of a DC microgrid with a hybrid energy storage system (HESS) is established, and large-signal stability criteria are obtained. The HESS consists of batteries and SCs. The derived criteria reveal the influences of the filter parameters, CPL power, DG power and the proportional control parameters of the battery converter and the SC converter on the system large-signal stability. Furthermore, important large-signal stabilization methods for regulating the HESS converter’s control parameters can easily achieve the large-signal stabilization of islanded DC microgrids without extra equipment. The paper is summarized as follows: First, the topology of and control strategy for a DC microgrid with an HESS and CPLs are proposed. Then, according to the characteristics of the HESS, the DGs and the CPLs, the system is equivalently simplified. Finally, the nonlinear model and large-signal stability criteria are both derived using the mixed potential theory, and a large-signal stabilization design method for the HESS converter’s control parameters is proposed. The experimental and simulation results show the effectiveness of the proposed large-signal stabilization method.

In islanded AC microgrids, negative impedance characteristics of AC constant power loads (AC CPLs) easily introduce large signal instability to the system, while energy storage systems sometimes compensate for the dynamic characteristics of AC CPLs, and increase the system stability. Although energy storage control techniques and characteristics have gained a lot of attention, few studies have derived quantitative design guidelines for energy storage systems from the aspect of stability improvement. In order to fill this gap, this paper proposes stability control strategies for bidirectional energy storage converters considering the characteristics of AC CPLs to guarantee large signal stability of islanded AC microgrids. The presented control techniques create quantitative limits for the DC bus voltage loop control parameters of the energy storage DC/DC converter and the integral control loop control parameter of the energy storage DC/AC converter, and also interpret the positive stability influence of energy storage systems and the negative stability influence of AC CPLs. The structure of the paper is as follows. Firstly, DQ coordinate transformation is adopted, and AC microgrid nonlinear models with the energy storage system in charging and discharging states are constructed. Then, large signal models are constructed depending on mixed potential theory. Stability control strategies for bidirectional energy storage converters are obtained, and AC CPLs power, storage system equivalent resistor, and micro power source power are all taken into account. Finally, based on simulation and experimental results, it is obvious that regulating the control parameters of the energy storage converter significantly increases the large signal stability of islanded AC microgrids without extra equipment. The method is very simple and easy to implement.

In DC microgrid (DC-MG), the loads connected with converters under strict control are considered as CPLs (constant power loads). When the voltage of CPLs decreases, the current increases and the negative impedance characteristic of CPLs cause instability easily. Fortunately, appropriate control for energy storage units could improve the system stability. However, most traditional control methods for bidirectional DC-DC power converters (BDC) connected with battery storage units do not quantitatively consider the stability influences of control parameters. This paper quantitatively analyzes the stability influence of the BDC current-mode control parameters and the negative impact of CPLs and derives the control parameter determination method for BDC interfaced storage systems. Large signal stability constraints are obtained in terms of mixed potential function. According to the constraints, the large signal stability is improved when the BDC control parameter k_p increases, while the stability is degraded when the power of CPLs increases. The control parameter determination method is very effective and convenient to apply, and the appropriate parameter k_p for BDC is determined. The regions of asymptotic stability (RAS) identify that the proposed control parameter determination method could improve the system stability effectively. The determination method is fully verified by the simulation and experimental results.