Research Of High Power Modular DC-DC Converter Control Technique

With the successful construction of HVDC projects and widely used around the world, experts and scholars research on the HVDC more deeply. It can be imagined that the proportion of using HVDC transmission technology for power transmission network in the future will increase. However, for HVDC transmission project which has been built or under construction so far, the voltage level is very different. Currently, the DC grid voltage in the world has not established uniform standards. If we want these different voltage levels of the DC circuit to connect to form a network and to improve the operational flexibility of the power grid, DC-DC converter is an essential equipment.Since the restrictions of voltage level of power electronic devices, now high-voltage power converters commonly adopt multi-level converter technology or series and parallel technology of modular power converter. However, as the voltage level increasing, the shortcomings of DC-DC converter which based on traditional multi-level technology appear; the complexity of circuit configuration and control strategy significantly raises; the system’s reliability is poor; especially the scalability is terrible. Modular multilevel converter (MMC) has same features with cascade converter, but whose devices don’t need directly series connection. Which has a modular structure, easy to implement multi-level number and modular design, well suited for high-voltage power occasion.To achieve high power DC voltage conversion, modular multi-level converter was introduced into the DC-DC converter in this article. First, modular multi-level converter MMC working principle and sub-modules working mechanism are introduced; DC-DC converter topology and mathematical model based on MMC were detailed analysis. Because of the defect of the traditional three-phase MMC Converter short circuit fault, introducing DC-DC converter based on MMC with seperation and describing how it works. Secondly, in order to increase the operating frequency of the converter and to reduce volume of arm reactors and sub-module capacitor, and to reduce switching losses of IGBT, we proposed an improved nearest level modulation strategy. Then based on the mathematical model of unbidirectional power transmission DC-DC converter and bidirectional power transmission, the way how to realize their control was researched. According to their own characteristics, their controller were designed, and finally to verify the correctness and effectiveness of the analyzed control strategies that were simulated in PSCAD/EMTDC simulation software.