Abstract
A hybrid energy control system of a microgrid was proposed in our lab. It includes a bi-directional inverter, bi-directional dc/dc converter and maximum power point tracking converter connected through a common DC-bus. The main focus of this study is on design and implementation of an isolated bi-directional dc/dc converter, which consists of two converter stages connected in cascade. The first stage is a bi-directional full-bridge LLC resonant converter which can achieve ZVS with the resonance characteristics of the LLC, and its transformer can provide galvanic isolation. The second stage is a bi-directional buck converter which can improve the degree of freedom in tuning step-down and step-up ratio. With the division-summation (D-Σ) digital control, the second stage converter is allowed to have wide inductance variation due to the magnetic permeability of the core varying with inductor current. It can track current reference precisely and achieve constant current charging/discharging of the battery. The major contribution of this paper is that this research adopts an isolated bi-directional dc/dc converter in a hybrid energy control system. It applies the D-Σ digital control to the converter to achieve constant current charging/discharging and DC-bus voltage regulation. A 2 kW isolated bi-directional dc/dc converter prototype with input voltage 380 ± 20 V and output voltage 48 ± 6 V has been implemented. The efficiency can reach 93.9% under full load condition. The bi-directional dc/dc converter can be combined with a bi-directional inverter and a maximum power point tracker to verify the feasibility of the hybrid energy control system.