Pengfeng Lin

An Integral Droop for Transient Power Allocation and Output Impedance Shaping of Hybrid Energy Storage System in DC Microgrid

Power allocation in hybrid energy storage systems (HESSs) is an important issue for dc microgrids. In this paper, an integral droop (ID), inspired by the electrical characteristics of capacitor charging/discharging process, is proposed and applied to a cluster of energy storages (ESs) with high ramp rates. Through the coordination of the ID and conventional V-P droop, the transient power allocation in HESSs can be intrinsically realized in a decentralized manner. The high-frequency components of power demand can be compensated by the ESs with ID, whereas the ESs with V-P droop respond to the smooth change of load power. Additionally, the ID coefficient can be designed according to the nominal ramp rate of the ESs with slow response, which helps to extend the lifespan of the HESS. On the other hand, to easily assess the stability of the system feeding constant power loads, a minimum relative impedance criterion (MRIC) is developed. Based on MRIC, it is revealed that the proposed ID can shape the output impedance of the HESS and stabilize the entire system. The feasibility and effectiveness of ID are verified by both simulations and experimental 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.

Stability Design of Single-Loop Voltage Control With Enhanced Dynamic for Voltage-Source Converters With a Low LC -Resonant-Frequency

Voltage-controlled voltage-source converters (VSCs) are widely applied in islanded microgrids, dynamic voltage restorers, and uninterruptible power supplies, etc. Typically, an LC filter with a large filter capacitance is employed to improve the quality of output voltage and system robustness against external disturbances. Nevertheless, a large capacitance may lead to a low LC-resonant-frequency and cause unstable voltage control of VSCs when a single-loop proportional-resonant (PR) or resonant controller is used. This paper proposes a novel design of single-loop voltage control, where a negative proportional gain is adopted in the PR voltage controller, and it is revealed that the voltage control can be stable even with a low LC -resonant-frequency, e.g., lower than one-third of the system sampling frequency. To tackle the poor dynamic issue introduced by the negative proportional gain, this paper further proposes a closed-loop zeros configuration-based control method, which can flexibly tune the system dynamic response without affecting its stability. Finally, simulation and experimental results are provided to verify the effectiveness of the proposed method.

An integral-droop based dynamic power sharing control for hybrid energy storage system in DC microgrid

Power sharing performance is a critical issue of hybrid energy storage system (HESS) in autonomous DC microgrid (MG). In this paper, a novel integral droop (ID) is proposed to mimic dynamic characteristics of the capacitor by using energy storages (ESs) with quick response. The main advantage of the proposed controller is that dynamic power sharing among ESs is automatically realized in the decentralized level. For a given HESS, ESs with the proposed ID enables to compensate fast power change while ESs with conventional voltage-power (V-P) droop provide low frequency components of power demand. With coordinated control between ID and V-P droop, high/low pass filters (LPF/HPF) are intrinsically formulated in HESS in order to obtain reasonable dynamic power allocations among ESs. Matlab/Simulink model of HESS is established for the verification of ID controller, in which the impacts of different ID coefficients on transient performances of the system are analyzed in detail. Finally, the effectiveness of proposed ID is experimentally validated on a HIL (hardware in loop) HESS platform.

Modified strong tracking cubature Kalman filter for LiFePO4 storage system

Lithium Iron Phosphate (LiFePO4) batteries have obtained extensive interests for the high energy density, little contamination, and ready availability. To enhance the compatibility of the batteries in electrical systems, the accurate estimation of the state of charge (SOC) is remarkably significant. Conventionally, Kalman filter algorithm and its derivations can be utilized for SOC estimation. To obtain SOC values more precisely and faster, this paper proposes a modified strong tracking cubature Kalman filter (MSTCKF). The estimation algorithm robustness is strengthened with the adoption of eigenvalue decomposition, which ensures the positive definiteness and symmetry of the priori error covariance matrix. Corresponding simulations are provided to compare the SOC estimation accuracy of MSTCKF, strong tracking cubature Kalman filter (STCKF), cubature Kalman filter (CKF) and extended Kalman filter (EKF). By setting different initial SOC errors, the results show that MSTCKF averagely performs more accurate than STCKF, CKF, and EKF by 19.82%, 26.74%, and 46.63% respectively.

Improved virtual capacitive droop control for hybridization of energy storages in DC microgrid

Hybrid energy storage system (HESS) facilitates the integration of various energy storages (ESs) with different ramp rate. Virtual capacitive control (VCC) is used to ensure the idle condition of supercapacitors (SCs) when HESS is in steady state and dynamic power sharing in transion. Improved virtual capacitive droop (IVCD) is proposed in this paper by introducing DC bus voltage regulation for batteries. Compared with VCC, HESS under IVCD eliminates redundant SCs SOC regulation controller and realizes SOC recovery by automatically transferring power from the battery to SCs in transient state. Therefore, both SCs SOC recovery and DC bus voltage regulation are concurrently achieved. System stability is also studied with parameters variation under IVCD control scheme. The effectiveness and feasibility of the proposed IVCD are verified in Matlab simulations. Advantages of IVCD are demonstrated by exhaustive analysis of simulation results.