Battery Integrated Modular Multifunction Converter for Grid Energy Storage

Most of the conventional converters are retrofitted for grid energy storage systems and, therefore, theoretical analysis, mathematical models, control development are not part of the converter design. Although multilevel converters are suitable for energy storage applications, existing multilevel converters have many drawbacks such as balancing of dc sources, undesired circulating currents, and hard switching of the devices. This paper proposes a battery integrated modular multifunction converter consisting of several modules that are connected in parallel and series with their soft dc link connected to the grid using a three level inverter switching at the line frequency. The zero voltage switching of the devices and soft dc link voltage, which eliminates the need for electrolytic capacitors, are some of the key attributes of the converter that also produces three phase currents with low THD without any bulky filters. Mathematical model and controller design are presented to describe the operation of the converter. The validity of the converter operation is further demonstrated using experimental and simulation results of a 1 kW laboratory prototype.