Closed-loop control and data- recording of a modular-multilevel converter (MMC)

University essay from KTH/Skolan för elektroteknik och datavetenskap (EECS)

Abstract: Modular multilevel converters (MMCs) are the preferred converter solution in flexible ac transmission systems (FACTS) and high-voltage direct current (HVDC) applications. This is due to the high quality of the voltage and current signals, lower overall losses, and fewer problems with switching-related EMI. However, without an efficient and fast data recording system, the sampled data from current and voltage measurement boards can cause long latencies in the control system and make it difficult to analyze the operation of MMCs. In this thesis, a filed programmable gate array (FPGA)-based closed-loop control, and a high-speed data recording system is developed for a low-power singlephase MMC prototype. In the prototype, a data-transmission scheme based on the RS485 (TIA/EIA- 485) standard exists. This protocol offers a robust solution for transmitting data over noisy environments. A direct memory access (DMA) scheme is utilized to transmit sampled data from the programmable logic (PL) to the processing subsystem (PS) in the Zynq-7000 SOC. Moreover, an asymmetric multiprocessing (AMP) mechanism was implemented on the two processor cores in the PS. The first processor controls the power transmission to and from the power grid, and the second processor runs the ethernet application to transmit sampled data to the computer using MATLAB. For the closed-loop control of this MMC prototype, a phase-locked loop (PLL), a proportional resonant (PR) current controller, and an energy control loop for capacitor voltage balancing and control are implemented. The results showed that the output power of this single-phase MMC prototype is under control and each sub-module capacitor voltage is balanced and charged to the desired value. The sampled data can be recorded from the computer through the implemented data recording system at 25.6Mbps. Moreover, a dynamic oscilloscope function is developed in MATLAB using this online data recording scheme. 

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