Study of Bi-directional DC-DC Converter of Micro-grid Hybrid Energy Storage System

Hybrid energy storage can take advantage of super capacitor's high power density and battery's high energy density. At the same time, hybrid energy storage cost less. So it's suitable for micro-grid power balance control. This paper presents a main circuit structure which is Buck/Boost converters connecting super capacitor and battery with PWM (pulse width modulation) inverter to achieve bi-directional power adjustment and improve utilization of super capacitor. DC bus voltage is sensitive to and fluctuates with switching of PWM inverter's working conditions. To solve this problem a combination of voltage-current close loop control and power feed-forward control is adopted in super capacitor control while achieving fast power tracking; peak current control is adopted in battery control, this control strategy has simple control circuit and high reliability. Meanwhile peak current control can limit maximum current of battery, so it can protect battery and prolong battery life. Simulation confirmed the effectiveness of the method with voltage fluctuations within the range of 10%.