Qianwen Xu

A Decentralized Dynamic Power Sharing Strategy for Hybrid Energy Storage System in Autonomous DC Microgrid

Power allocation is a major concern in hybrid energy storage system. This paper proposes an extended droop control (EDC) strategy to achieve dynamic current sharing autonomously during sudden load change and resource variations. The proposed method consists of a virtual resistance droop controller and a virtual capacitance droop controller for energy storages with complementary characteristics, such as battery and supercapacitor (SC). By using this method, battery provides consistent power and SC only compensates high-frequency fluctuations without the involvement of conventionally used centralized controllers. To implement the proposed EDC method, a detailed design procedure is proposed to achieve the control objectives of stable operation, voltage regulation, and dynamic current sharing. System dynamic model and relevant impedances are derived and detailed frequency domain analysis is performed. Moreover, the system level stability analysis is investigated and system expansion with the proposed method is illustrated. Both simulations and experiments are conducted to validate the effectiveness of the proposed control strategy and analytical results.

Design and stability analysis for an autonomous DC microgrid with constant power load

Fuel cell is a promising source in autonomous dc microgrid. Hybridization of fuel cell with battery is commonly implemented to overcome slow dynamic of fuel cell. Then battery compensates high frequency fluctuation and fuel cell provides consistent power at steady state. To achieve this objective, most control strategies require a centralized controller, which encounter reliability and scalability issues. This paper proposes a virtual capacitor droop controller to achieve autonomous dynamic power sharing at distributed level. An autonomous dc microgrid is designed to verify the proposed method. Considering the high penetration of constant power loads (CPLs) in dc microgrid and the destabilizing effect of CPLs, system stability is investigated with the implementation of the proposed controller. Both simulation and hardware experiment are conducted to validate the effectiveness of the proposed control method and analytical results.

Modeling and stability analysis of hybrid energy storage system under hierarchical control

Energy storages (ESs) are widely used in DC distribution system to compensate power imbalance. Hybrid energy storage system (HESS), which combines various ESs, can optimize energy density, power density and dynamic response. Hierarchical control of HESS is shown able to achieve dynamic power sharing among ESs with improved system reliability. However, system stability of HESS under hierarchical control, which is essential for parameter design and system operation, has not been investigated. In addition, with high penetration of constant power loads (CPLs) in DC distribution system, impact of CPL on system stability should be studied. This paper investigates stability problem in HESS with CPL under hierarchical control. A detailed small signal model for stability analysis is developed. Impacts of CPL and system parameters are analyzed. Effectiveness of the control strategy and analytical results are validated by time domain simulations.

A Decentralized Power Management Strategy for Hybrid Energy Storage System With Autonomous Bus Voltage Restoration and State-of-Charge Recovery

For hybrid energy storage system in dc microgrid, effective power split, bus voltage deviation, and state-of-charge (SoC) violation are significant issues. Conventionally, they are achieved by centralized control or hierarchical control methods with communications. This paper proposes a simple and effective strategy to solve the problem in a decentralized manner. A high-pass filter-based droop (HPFD) controller is proposed to regulate the battery converter, and a virtual capacitance droop (VCD) controller is implemented for a supercapacitor (SC) converter. The cooperation of HPFD and VCD first achieves autonomous power split that high-frequency fluctuation is buffered by SC and low-frequency power is supplied by battery. Meanwhile, the bus voltage deviation induced by the droop-based power sharing is eliminated automatically at steady state. The resulted bus voltage restoration simultaneously enforces SC SoC back to its nominal value, and, thus, ensures continuous operation of SC as a power buffer without the violation of its SoC boundary. A design guideline is developed to ensure expected system dynamics. The effectiveness of the proposed method and analytical results are validated by simulations and experiments.

A Module-Based Approach for Stability Analysis of Complex More-Electric Aircraft Power System

Moving toward more-electric aircraft (MEA) architecture, future aircraft power system will consist of a large number of power converters and motor drive loads which behave as constant power load and may cause stability issues. It is essential to ensure stable operation of the MEA power system, even though the complexity of the system hinders the comprehensive analysis of system stability. This paper presents a framework for stability analysis of a complex MEA power system. A module-based approach is proposed to provide a general and flexible solution for modeling and integrating complex systems, even under situations of system extension and reconfiguration. Using the proposed technique, a complex system model is conveniently formulated and overall system stability can be evaluated. Then, the critical stability points can be investigated. Impacts of parameters and their damping effects are identified by eigenvalue analysis and sensitivity analysis. The hybrid ac/dc MEA power system adopted by Boeing 787 is analyzed using the proposed technique. Simulations in MATLAB/Simulink are conducted to validate the effectiveness of the proposed modeling technique and the analytical results.

A Decentralized Control Strategy for Autonomous Transient Power Sharing and State-of-Charge Recovery in Hybrid Energy Storage Systems

This paper proposes a decentralized power management strategy for hybrid energy storage systems to achieve transient power sharing and state-of-charge (SoC) recovery simultaneously. A virtual capacitance droop control strategy with an autonomous SoC recovery loop is proposed for energy storage (ES) with fast dynamic response, and the conventional virtual resistance droop control method is employed to regulate ES with slow dynamic response. The hybrid battery/supercapacitor (SC) system is taken as an application example. With the proposed method, load power is autonomously split into high-frequency and low-frequency parts to be compensated by SC and battery. Meanwhile, SC SoC is automatically recovered and this enables continuous operation of the SC as a power buffer without mode change or performance tradeoff. A design guideline is developed to ensure desired transient power sharing dynamics and SoC recovery with negligible interactions. Both simulations and experiments are conducted to validate the effectiveness of the proposed strategy and analytical results.

Time-delay Stability Analysis for Hybrid Energy Storage System with Hierarchical Control in DC Microgrids

Hybrid energy storage system (HESS) plays an important role in the operation of dc microgrids which have attracted significant research attention recently. The hierarchical control is widely adopted for the coordination of multiple energy storages in a HESS. As the hierarchical control comprises the centralized and the decentralized control levels, the time delays during signal transfer processes between two control levels may significantly affect HESS operation and may lead to instability. In this paper, considering the multiple delays in the hierarchical control processes, the maximum delayed time (MDT) is defined to assess the stability margin for a HESS. An accurate and effective method based on small signal stability model is then proposed to determine the MDT of a HESS to maintain its stability. The effectiveness and correctness of the proposed method are verified using a lab-scale dc microgrid.

A Decentralized Control Strategy for Economic Operation of Autonomous AC, DC, and Hybrid AC/DC Microgrids

Economic operation is a major concern for microgrids (MGs). System operation cost is optimized when the incremental costs (ICs) of all distributed generators (DGs) reach equality. Conventionally, economic dispatch of DGs is solved by centralized control with optimization algorithms or distributed control with consensus algorithms. To improve the reliability, scalability, and economy of MGs, a fully decentralized economic power sharing strategy is proposed in this paper. As frequency is a global state in ac MG and dc bus voltage serves as a natural indicator in dc MG, a frequency-IC droop scheme is proposed for ac MG, a voltage-IC droop scheme is proposed for dc MG, and a normalization scheme is proposed for hybrid ac/dc MG. By using the proposed technique, ICs of DGs reach equality with the convergence of the system global indicator (frequency or dc bus voltage). Then power sharing of each DG is automatically achieved based on its relevant IC function and the total operating cost can be optimized without any communication or central controllers. The proposed approach is implemented in an ac MG, a dc MG, and a hybrid ac/dc MG in MATLAB/Simulink to verify its effectiveness.

A decentralized control strategy for economic operation of autonomous AC microgrids

Economic operation is a major concern for microgrids. Conventionally, economic dispatch of distributed generations (DGs) are solved by centralized control with optimization algorithms or distributed control with consensus algorithm. To improve the reliability, scalability and economy of microgrids, a fully decentralized economic power sharing strategy is proposed in this paper. The proposed method is based on frequency/incremental cost droop (f/IC) characteristics and incremental cost (IC) functions of DGs. ICs of DGs reach equality with the convergence of system frequency. Power dispatch of each DG is automatically achieved based on its relevant incremental cost function. Therefore, by using this method, the incremental cost of each DG will reach equality autonomously and the total operating cost can be optimized without any communication or central controllers. Simulation platform of an autonomous AC MG with three DGs is built in Matlab/Simulink to verify the effectiveness of the proposed method.

Distributed coordination for economical operation of power park with multiple microgrids

Microgrid is widely implemented for system with high penetration of renewable energy sources. Multiple microgrids can be interlinked to form a Power Park which can be used for wider area applications. Centralized power park control, in which one of the microgrid operate as the voltage/frequency regulator while the power exchange of the rest microgrids with power park network, is determined by the central controller. However, system operation relies heavily on the voltage/frequency regulator, communication network and central controller, whose failure will result in malfunction of the whole system. Droop based control (frequency/active power (f/P) droop and voltage /reactive power (V/Q) droop) enables system operation in distributed manner without the central controller and communication link. Besides, system bus voltage and frequency are regulated coordinately. Droop based control is a fully distributed control without considering system operation cost. In this paper, a modified droop control is proposed. Instead of implementing the conventional f/P droop control, frequency/Incremental Cost (f/IC) droop control is implemented to ensure minimized system operation cost. MATLAB/Simulink model has been developed for verification of the proposed control algorithm.