Study On High-Voltage High-Power DC/DC Converters For Grid-Connected Renewable Energy Sources

The common problem of the renewable energy sources, such as wind and solar, is the fluctuation and intermission of output power, and the connection of large scale of the renewable sources to the power grid is a huge challenge for the traditional electrical equipment, grid structure, and operation. DC grid, as one of the solutions to the aforementioned issues, is an emerging and promising approach which has drawn much attention recently. As the connectors between the renewable energy sources and HVDC grid, the high-voltage high-power DC-DC converters are one of the key equipments in the dc grid. The main parts of this dissertation are as follows:1. A new LC resonant converter is proposed in the dissertation, which is suitable for grid-connected renewable energy sources. The converter can achieve high voltage-gain using LC parallel resonant tank. It is characterized by zero-voltage-switching (ZVS) turn-on and nearly ZVS turn-off of main switches as well as zero current-switching (ZCS) turn-off of rectifier diodes, moreover, the equivalent voltage stress of the semiconductor devices is lower than other resonant step-up converters. The operation principle of the converter and its resonant parameter selection is presented in the paper. The operation principle of the proposed converter has been successfully verified by experiment results.2. High gain DC/DC converters are required in the applications of DC-grid collection of renewable energy and energy storage system in high voltage dc grid. This paper studies a high gain DC/DC converter based on modular multilevel converter (MMC) technology, which adopts the H bridge structure in the low-voltage side and the MMC structure in the high-voltage side. The converter not only reduces the voltage stress of each switch in high-voltage side, but also simplifies the structure of the low pressure side, taking into reliability and economy of the converter. The working principle of the converter, the nearest-level-control (NLC) modulation mode of the MMC and the voltage balance control strategy based on the capacitor voltage sequencing are presented in the paper. Simulation results verify the working principle of the converter.3. The other two kinds of DC/DC converters in the DC grid system are studied, which are the single-bipolar conversion DC/DC converter and the multi-port DC/DC converter. The topology, operation principle and control method of each converter are introduced. Then the simulation model of a four terminals multi-voltage level dc grid system including different kinds of DC/DC converters is built. By simulation, it is proved that the single-bipolar conversion DC/DC converter not only has the ability of voltage control, but also can connect renewable energy sources to bipolar operation DC grid system. The multi-port DC/DC converter can connect the DC buses with different voltage levels, and has the ability to control power flow.