Modulation and control of a class of modular multilevel converters for high voltage direct current (HVDC) transmission systems

The voltage-sourced converter (VSC) based high-voltage direct-current (HVDC) transmission technology is one of the most promising technologies for (i) expansion of the power networks for large cities and in-feeding the city centers, (ii) grid integration of renewable energy resources, i.e., hydropower, wind farms, and solar plants, (iii) long-distance bulk-power transmission, (iv) interconnection of asynchronous power grids, and (v) electrification of isolated power loads, islands, and oil and gas stations. Among the existing VSC topologies, a class of modular multilevel converters (MMCs) is the most promising topology due to its modularity and scalability. However, there are a few technical challenges associated with the control of MMCs including (i) balancing the submodule (SM) capacitor voltages at their desired values without creating unnecessary SM switching transitions and sacrificing the efficiency, (ii) reducing the circulating currents flowing through the three phases of the MMC. Although the circulating currents have no effect on the ac side of the MMC, if not properly eliminated/minimized, increase the ripple amplitude of the SM capacitor voltages, rating values of the converter components, and power losses, (iii) reducing the magnitude of the dc-line voltage/current ripple, and (iv) handling the dc-side short circuit faults. In MMC-HVDC systems, in case of a fault occurrence on the dc side, the Integrated Gate Bipolar Transistors (IGBTs) are blocked. However, the diodes provide a current path for the fault current from the ac side to the dc side until the ac circuit breakers open. Therefore, the MMC by itself does not provide dc-fault-handling capability. This research proposes a model predictive control (MPC) strategy that takes the advantage of a cost function minimization technique to address the above-mentioned challenges associated with the control of the MMCs. A discrete-time mathematical model of the MMC-HVDC system is derived and a predictive model corresponding to the discrete-time model is developed. The predictive model is used to select the best switching states of each MMC unit based on evaluation and minimization of a defined cost function associated with the control objectives of the MMC-HVDC system. Time-domain simulation studies in the PSCAD/EMTDC environment for various operating scenarios have demonstrated/validated the effectiveness and superiority of the proposed methods, as compared to the existing solutions