Phase Shift Control Scheme of Modular Multilevel DC/DC Converters for HVDC-Based Systems

Abstract

Among the different multilevel topologies, the modular multilevel converter (MMC) has now become a subject of intense research, where it offers some interesting and useful features. In this project, DC/DC converters are proposed for the HVDC-based systems to reduce the transmission losses. The full-bridge converters and three-level flying capacitor circuits are combined and integrated by employing the phase shift control scheme which can be easily applied to achieve 0-voltage and proposed for the high step-down and HVDC-based systems, and also which has a capability to generate the high-quality power under various conditions. Importantly, in this project the voltage auto-balance ability among the cascaded modules is achieved by the flying capacitor which removes the additional components or control loops, and it also allows the operation at higher frequencies and at higher input voltages without scarifying the efficiency. The neutral point clamped (NPC) converters and flying capacitor-based converters are the major multilevel topologies for the high-power and high-voltage applications. Zero-voltage switching (ZVS) performance for both the leading and lagging switches can be provided to reduce the switching losses by adopting the phase-shift control scheme. The time sequence of the leading leg in the phase-shift-controlled full-bridge converters is kept constant, and only the phase of the lagging leg is shifted to regulate the output voltage. A high DC voltage is required for the DC-based distribution and micro-grid systems to improve the delivery power capability which reduces the transmission losses. The switch voltage stress is reduced, and thus, the circuit reliability is enhanced in this project. The MMC concept can be easily extended to N-stage converter to satisfy the high-voltage applications with low-rated voltage switches. The circuit operation and converter performance are analyzed by simulations. The result of input voltage sharing across the capacitors is of 300 V. Thus, the input voltage auto-balance is achieved excellently. The input voltage is stepped down to 53 V. The modes of operations and converter performance are analyzed and simulated by using P-Sim software. The three-stage converter is designed to increase the step-down ratio when compared to two-stage converter. This operation is similar to the two-stage converter.