Hybrid energy storage system and associated converters examination for DC distribution

DC distribution main consist of power electronics components and converter. Energy storage system is also required to maintain the voltage stable for DC bus. This paper discusses the structure of a hybrid energy storage system (HESS) of battery and ultracapacitor. A new bidirectional DC-DC buck/boost converter used for energy transfer between DC bus and HESS is also analyzed in detail. It not only can operate bi-directionally, it can also provide three different working modes: buck, boost and buck-boost modes. The buck and boost mode can provide lower and higher voltage transfer ratio than traditional Buck and Boost converters respectively. Its buck-boost mode can provide positive output voltage. A cell balancing circuit with one center-tapped inductor and two active switches is also introduced in this paper. Its advantages of simple structure and energy fast-transferring make it more suitable for ultracapacitor string. Experimental and simulation results verified the feasibility of the two power converter circuit and HESS.