| Development of DC micro-grid will contribute to expanding the power systems scale, improving power quality, raising the stability of power supply and bringing inestimable economic value. However, there is volatility defect of wind or light distributed power electricity output under the influence of weather conditions and other uncertainty factors, which will directly lead to power fluctuation of micro-grid system, so that affects the normal operation of the loads. Therefore, it requires to introduce energy storage devices in DC micro-grid. Considering the power output characteristics of the distributed power, the energy storage unit that has superior features of power density and energy density are required. Therefore, by the complementary characteristics of the lead acid battery and super capacitor, the compound energy storage method in which lead-acid battery combines with super capacitor is proposed used in DC micro-grid. The compound energy storage method is used to suppress the power fluctuation to ensure the power supply of micro-grid has a reliability, continuity, and stability. And the main contents of the thesis are as follows.Firstly, the structure of the compound energy storage system of DC micro-grid is analyzed. The working principle of the lead-acid battery and super capacitor is studied. The structure of the energy storage control main circuit is discussed, and its parameters are also designed. The energy storage control method for the lead acid battery and super capacitor is put forward. The current single closed loop control method is proposed to control lead-acid battery. In addition, in order to keep the DC bus voltage steady, a control way that voltage loop combines with current loop is used to control super capacitor.Secondly, the small signal modeling is built based on the working principle of bidirectional DC/DC converter for energy storage control. The energy storage controllers of the lead-acid storage battery and super capacitor are designed, and their stability are also analyzed.Once again, the wavelet decomposition method is proposed to extract control signal for the lead-acid battery and super capacitor from power signal. Among, the slowly variable signal is applied to control the charge or discharge of the lead-acid battery. The fluctuant signal is applied to control the super capacitor to absorb rapidly changing energy.Finally, the compound energy storage method for DC micro-grid is verified under the MATLAB environment. The wavelet decomposition procedures are written by S-function to separate control signal. And the simulation model of compound storage energy system for DC micro-grid is built under Simulink/Power system. The simulation result show that the fluctuations power signal can be separated into a stable signal and a fluctuating signal by wavelet decomposition. And then the signals are used to control the lead-acid battery and super capacitor to storage energy. The simulation result show that the lead-acid battery is not only timely response to the needs of system to reasonably charge or discharge, but also the charged current is steady. The super capacitor can extract rapidly fluctuant energy of DC micro-grid. At the same time, the DC bus voltage of DC micro-grid can be kept in steady under different distributed power output and loads. Therefore, the proposed compound energy storage control method can help the energy storage system to achieve reasonable running, and meet the needs of DC micro-grid. |
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