Isolated Multilevel DC-DC Converters for Interfacing Battery Energy Storage Systems to Grid and Electric Vehicles

DC-DC converters are utilized for shifting DC voltage level and controlling power flow between two DC ports. DC-DC converters find applications in multiple aspects from consumer electronics such as cell phones, laptops etc. to large data centers, BESS connection with DC link for grid forming inverter, supplying power to auxiliary components from HV battery in electric vehicles, for optimizing output of PV strings, charger circuits for electric vehicles etc. One of the key challenge in designing these converters is that these converters are required to handle high amount of power, have high efficiency and power. Multilevel DC-DC converters were developed with the aim of reducing device stress and improving efficiency. Although multilevel converters have the aforementioned benefits, the balancing of multilevel capacitors is not as straightforward. Thus, appropriate modulation of the converter becomes crucial for multilevel DC-DC converter design. This thesis proposes DC-DC converter topologies for integrating battery storage systems with DC link for grid integration and electric vehicles. All the converters are galvanically isolated between high side and low side circuits. They also feature soft switching for devices leading to high efficiency operation and multilevel voltage on the high voltage side. A bidirectional current fed DC-DC converter is proposed first, as an interface between BESS and DC link of the grid connected inverter. The converter features natural commutation of transformer’s leakage inductance current through device modulation. The second topology features a voltage fed version of the BESS grid interface converter for a variable DC link between propulsion battery and edrive inverter for electric vehicles. The converter is bidirectional, soft switched and features low noise, high efficiency operation. The third topology is unidirectional rendition of the propulsion battery-inverter interface for electric vehicle fast charging. But through modification of switching frequency and transformer, capable of achieving quasi resonance operation. This leads to ZCS for rectifier diodes which is crucial in high current high power applications e.g. fast charging. Detailed analysis for the modes of operation of the converters is presented. The converter design constraints have also been addressed. Performance of the converters is then evaluated through simulation models and laboratory prototypes.