High Step-Up/Down Transformerless Multilevel Converter for Renewable Energy Applications

This thesis focuses on a high step-up/down transformerless dc-dc modular multilevel converter (MMC) that would be applicable to dc power systems. The design achieves high voltage ratios for interfacing renewable energy sources such as photovoltaic and line interactive Uninterruptible Power System (UPS) systems. The circuit topology provides for high step-up/down dc-dc conversion ratios using an MMC approach operating in resonant mode in order to improve overall efficiency. This topology operates to step-up the input voltage with 1:10 or larger conversion ratio. As a bidirectional converter, it also provides step-down capability at the same voltage ratio (10:1 or greater). The MMC circuit system consists of an upper and lower set of cells. The number of the upper cells is N, and the number of the lower cells is M. Phase-shift pulse width modulation (PS-PWM) is used to control voltage and power flow. PS-PWM with high duty cycle is generated to ensure that all the capacitors are connected except for one of them, which is out of the connection. A MATLAB/Simulink(TM) and LTspice simulations for the proposed topology are presented. Moreover, PV and UPS systems with the proposed topology are simulated using MATLAB/Simulink(TM). In photovoltaic application systems, a closed loop control system is represented for voltage regulation in case there is a change in the input voltage. In UPS application, closed loop controllers for charging and discharging batteries are presented.