Study on three-level DC-DC converter for microgrid energy storage system

With the rapid rise of new energy generation in microgrid, energy storage receives much concern in dealing with the instability of distributed generation units’ output power. As the interface of storage device and power grid, the performance of the energy storage converter directly affects the operation efficiency, reliability and output power quality of the microgrid. Due to the two-level structure of the traditional DC/DC converter has the disadvantages of serious switching loss, low voltage stress and low efficiency, a new type of three-level DC-DC converter is used in this paper. This topology requires no high frequency isolation transformers and secondary circuits, small size, high efficiency and high power density, and each switch tube voltage stress is half of the input voltage, and it can utilise parallel structure to implement fast charge and discharge requirements of super power capacitor. The three-level DC-DC converter topology and operating principle are introduced in detail. The modulation mechanism is analyzed in the case of balance and unbalance of neutral point voltage. Based on the above, a new type of three closed loop control strategy is proposed, which is composed of a double closed-loop and an extra voltage closed-loop. The former is used to maintain stable DC bus voltage, and the latter is to handle the inherent neutral point voltage offset problem of three-level structure in the process of energy storage units charge and discharge. A simulation model of energy storage system is built in MATLAB, and the simulation results validate the feasibility and effectiveness of the three-level converter structure and control strategy.