Research On Control Method And Energy Management Strategy Of Battery-Supercapacitor Based Hybrid Energy Storage Systems In DC Microgrids

With the recognition of the importance of renewable energy sources and their great potential as major energy sources in the future,the penetration of renewable energy in the microgrid is increasing.However,the intermittency,randomness,and unpredictability of renewable energy sources lead to the mismatching of power generation and load demand when they are connected to the microgrid,which affects the overall power balance in the DC microgrid and thus,makes it unable to operate stably.At present,the most effective means to solve the above problems is to deploy an energy storage system in the microgrid,which absorbs/releases unbalanced power to maintain the overall power balance in the network.Hybrid energy storage systems have attracted extensive attention because it integrates various energy storage devices with different characteristics and realizes complementary advantages.This paper carries on the research of hybrid energy storage systems,focusing on the control method and energy management strategy of lithium battery and supercapacitor-based hybrid energy storage system.The specific contents are summarized as follows:1.A digital predictive control for hybrid energy storage systems based on the deadbeat control is proposed.With the proposed control,the optimal duty ratio can be generated according to the current voltage and current value in the single sampling cycle.Therefore,the DC bus voltage can keep stable under different disturbances,meantime,the dynamic performance of the hybrid energy storage system and the stability of the DC microgrid can be improved effectively.Moreover,the lithium battery responds to the average current demand while the supercapacitor responds to the transient current fluctuations,which reduces the cycle times and current stress of the battery and thus,prolongs its life.Compared with the conventional proportional-integral control,the proposal has the advantages of faster recovery and less overshoot of DC bus voltage,and no need for PI parameters.2.An energy management strategy with two degrees of freedom for the hybrid energy storage system based on SOC feedback is proposed.Two degrees of freedom including an adaptive high-pass filter cut-off frequency f_c and a charge/discharge coefficient k_b are introduced to adjust the charge-discharge current of the supercapacitor and the lithium battery respectively according to their state-of-charges,which avoids overcharging or over-discharging and reduces the current stress of the battery during the shutdown process.Then,three operating modes of the hybrid energy storage system are proposed,including power-sharing mode,battery-only mode,and extreme mode.The proposed energy management strategy can realize the safe and stable operation of the DC microgrid,the seamless transitions from the power-sharing mode to the other two modes,and faster energy recovery of the supercapacitor.3.A simplified DC microgrid platform was established in the laboratory and the control of the whole platform was realized by using the TMS320F28069 DSP chip.The proposed methods are verified by the practical hardware experiments,and the experimental results are consistent with the theoretical analysis and simulation results,which illustrate the effectiveness and superiority of the proposed digital predictive control based on deadbeat control and energy management strategy with two degrees of freedom based on state-of-charge feedback.