Modular Multilevel Converter (MMC) has gained a lot of interest in industry in the recent years due to its modular design and easy adaption for applications that require different power and voltage level, such as power transmission through HVDC. However, the control and operation of a real MMC consisting of large number of sub modules for high power high voltage application is a very challenge task. For the reason that distributed control architecture could maintain the modularity of the MMC, this control architecture will be investigate in this project.
The suitable communication technologies, which are essential to the distributed control, are discussed firstly. Two real-time communication protocols, namely EtherCAT and SPI, are analyzed with respect to the communication speed, connection topology, connection capability, synchronization accuracy and the cost. The prototype with the SPI solution has been made in the lab and has been successfully tested.
Secondly, the multilevel modulation technologies are examined. The focus is put on the phase shifted PWM (PS-PWM). Analysis of the performance of different PS-PWM, namely uniform PS-PWM and resampled uniform PS-PWM, is carried out in this project. A new method to implement the resampling technique, which is the key technique for the resampled uniform PS-PWM, is proposed and verified by the simulations and experiments.
Thirdly, the distributed control system is analyzed and modelled in frequency domain. Controller design is performed through analytical methods and Matlab tools. The sensitiveness of the distributed control system to modulation effect (phase-shifted PWM), communication delay, individual carrier frequency and sampling frequency is studied through simulations that are made in Matlab Simulink and PLECS.
Finally, the hardware design and software design of the prototype are presented. The experimental results are also documented in this report.