Research On The Control System Of Modular Multilevel Converter Based On Optical Fiber Multiplex Techniques

As the rising of the High Voltage Direct Current Transmission based on Voltage Source Converter (VSC-HVDC), the converter with high voltage and high power rate becomes the focus of attention again, and the Modular Multilevel Converter (MMC) is the one which stands out because of its high-modular, high-quality and high-reliability. Recently, research about MMC still emphasize on control, such as modeling, sub-module capacitor voltage balancing algorithm, internal circulating current inhibition, however, the research on control system architecture design of MMC and its implementation are rare, for this reason, this paper choose control system as its research object. The main contents are shown as below:1) The topology, operating principle and modulation strategy of MMC are theoretical basis of its control system. The2-level control system architecture is made up by the host-controller level and the auxiliary-controller level, but its capacitor voltages are not isolated and difficult to sample. To solve this problem, an additional sub-module-controller level is added to make the3-level control system architecture. Each sub-module-controller needs2downside optical fibers receive PWM and protect signal separately from the auxiliary-controller,1upside optical fiber send sub-module information, and the total number of optical fibers is propotional to the number of the sub-modules. If the PWM modulation is achieved by the sub-module-controller level instead of the auxiliary-controller level, each sub-module-controller will only need1downside optical fiber to receive the modulated information, while synchronizing these sub-module-controllers has added its control system complexity.2) This paper proposes a3-level control system architecture based on optical fiber multiplex techniques, which including the host-controller level, the phase-controller level and the sub-module-controller level. The host-controller is the core of the whole system, each phase-controller is responsible for its phase unit’s PWM modulation and sub-module capacitor voltage balancing algorithm, while each sub-module-controller is used to generate driving signals of swithes, protect and monitor its sub-module. The command informations, such as PWM and protect signal, from the phase-controller to each sub-module-controller share one downside optical fiber after they are modulated into different rectangular pulses, so the optical fibers are saved. In addition, as to the PWM modulation is achieved in the phase-controller level, the system only needs synchronize these phase-controllers, which is quite easy to achieve.3) Based on the above, a MMC prototype is designed and built, the number of its sub-modules in a single bridge arm is8, and finally, the proposed architecture’s feasibility is verified by experiments. Fistly, DSP and FPGA are introduced. Secondly, after analyzing the necessity of synchronization among phase-controllers and its methods, information interaction between different controllers and relavent FPGA design are emphasized. Thirdly, take the prototype as an example, system parameters design of N, C and L are given, so as to the important parts of hardware design of each controller’s board. At last, inverter experiments are done by using this prototype, key waveforms under both steady state and dynamic state are given.