Research On Voltage Control And Circulation Suppression Strategy Of Modular Multilever Converter

With the widely used for the wind power, solar power and other renewable energy power generation, there must need higher requirements for the high-voltage and the long-distance power transmission. The multilevel converter become an effective measures for the high-voltage and high-power conversion due to its low requirement of stress for the device and the high quality of the output waveform. Among the numerous multilevel topologies, the Modular Multilevel Converter-MMC is widely used for the high voltage direct current due to its modular design of the structure, easy to expand, with its multi-level output, reducing the harmonic content and output of electromagnetic interference, no passive filter and transformer; with common dc bus and easily implement for back to back structure. In recent years, MMC has gradually applied in other areas, such as motor drives, static var compensator, electric traction, distributed power generation and other fields.Firstly, this paper introduces the research the status for MMC, conducted a study of its topology, operation principles, obtain its equivalent model with the AC side and the DC side. The current applied to the MMC modulation schemes are introduced and compared their advantages and disadvantages. On this basis, the MMC voltage control scheme and circulation suppression program conducted in-depth research.In voltage control, voltage balancing problem analyzed during startup and steady state voltage control case. During the startup process, this paper presents a startup program for the used in special occasions single-phase MMC system; this proposed scheme is based on the basis of energy control, the program can be divided into two phases, phase controlled rectifier and energy control stage. Energy control stage, mainly using the energy of the upper and lower leg, and the energy difference between the upper and lower leg to control the sub-capacitor voltage module.It analyzed the energy conversion process under this program and verified the feasibility of the program for the start in simulation. For the steady-state operation, the paper describes the arm level and the sub-module level of capacitor voltage balancing scheme in detail. One option switching mechanism is proposed based on the improved nearest level modulation. And it also verifies the optimization for the device switching frequency base on ensuring a good balance for the capacitance voltage in this mechanism.Then this paper analyzes the mechanism of circulation generated, the circulation of the coupling of different orders on the basis of full consideration of the sub-module capacitor voltage fluctuations. It also gives the detailed derivation of the steady-state model of circulation. Then on this basis, the circulation PR suppressor is designed. The two separate harmonic components based on the notch filter and verify its feasibility through simulation.Finally, based on the above theoretical analysis, a single-phase MMC 16 modules experimental platform is designed and built. It also gives the design methods for the main circuit parameters, the block diagram of the overall structure for the platform and the features for the digital control system. It verify the start scheme, the option switching mechanism for the voltage balance and the circulation suppressor by experiments...