| With the development of distributed generation and micro-grids, the application of energy storage system is helpful to the stability of intermittent energy generation, improve efficiency of grid, ensure the operation security of power system and promote the large-scale application of renewable energy. In order to realize the power stability in DC micro-grids and meet the requirements of high voltage large capacity in wind power generation system, and take the advantages of distributed energy storage system, bidirectional DC/DC converter in distributed energy storage system has become a hot research topic.A novel staggered three-level bidirectional DC/DC converter is proposed in this paper, which is very suitable for wind power generation system with high voltage large capacity in DC micro-grids. And the converter is simple, efficient and reliable. The operation principle is introduced in detail in this paper. Considering the factors causing flying capacitor voltage instability, the voltage control strategy of flying capacitor is presented in details. In order to improve the steady and dynamic performance, state space average method is adopted to establish accurate mathematical model for bidirectional DC/DC converter. The DC bus voltage regulator, current regulator and the flying capacitor voltage regulator are designed.The State-of-Charge(SOC) of each energy storage module in distributed energy storage system may not be the same. In order to achieve dynamic power sharing among different energy storage modules, the previous SOC-based droop control method is improved. So the improved SOC droop control method for distributed power optimal allocation is proposed in this paper. The different SOC droop expressions are adopted in charge mode and discharge mode, so that each storage module tend to absorb or output reasonable power according to its SOC value. The power sharing among different energy storage module was achieved by using this method. According to the state of energy storage system and wind power, the smooth transition control strategy for charge mode and discharge mode in energy storage module is researched. Power bidirectional transmission is implemented and the smooth transition for charge mode and discharge mode is realized.Based on theoretical search and simulation, the experimental prototype of storage module with staggered three-level bidirectional DC/DC converter is built and tested. The typical waveforms reflecting working principle of three-level bidirectional DC/DC converter are obtained. The improved SOC droop control strategy and smooth transition control strategy for charge mode and discharge mode are also experimented. The system working state are experimented when one energy storage module is cut-in or cut-off. The experimental results show that dynamic power sharing among different energy storage modules can be realized. The theoretical analysis and the effectiveness of the control strategy are verified. |
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