Application And Research Of MMC In HVDC Systems

MMC-HVDC has offered a new way in solving the problem of large scale wind farm and other new energy integration, increasing the capacity of the urban high voltage power grid, connecting between different power grid and supplying power for isolated island. Owing to its various merits and its technology advantage, the MMC has been designated as the key transmission equipment in the national energy science and technology planning of the 12th Five year, which needs much more technology, financial and resources inputs. In this paper, the basic model and modulation methods will be elaborated, and then we will delves into the DC faults and AC faults in MMC-HVDC in detail. In the following, simulation and experiment will be made to validate the above methods.After reviewing the basic model and modulation methods of the MMC, DC fault in MMC-HVDC will be illustrated. First the main and possible DC faults in DC network will be explained and classified, and clear assortment will contribute to a better study of DC faults. Then the Bipolar short circuit, which is regarded as the most serious faults in DC sides, is studied in detail, analyzing the current in the fault condition. Through the analyze of the over current, we will have a precise choice in equipment parameter and an accurate judge of fault current. And then analyzing the known ways, which can restrain the DC faults, exploring the merits and drawbacks, an improved sub-module is proposed to control the fault current. According to the improved module, module parameter is designed, and fault current controlling ability is testified in Matlab/Simulink.When analyzing the DC faults in MMC-HVDC, AC faults are also research hotspots in MMC-HVDC, analyzing the fault impact on the AC network. First analyzing the main fault types of three-phrase AC networks, making approximate assumptions, the internal parameters asymmetry of converter and transformer parameter asymmetry are simplified as three-phase voltage asymmetry. Then models under unbalanced conditions are established, and the input and the output power are analyzed. Before restraining the fault current, voltage drop detection is needed to monitor the voltage condition in case of faults. And then two aspects about faults control are proposed in this paper. One is about eliminating the negative sequence current, and a corresponding suppression strategy is proposed. The other is about eliminating the second-order fluctuation in DC side, and a PR based double loop control method is presented. Finally simulation is conducted in Matlab/Simulink, which verify the feasibility the method.In the last part of this paper, the experimental platform of the MMC rectifier is built, proving the reliability of the method. The platform of the MMC-HVDC is also built, achieving the effective grid-connection, delivering the power to power-grid successfully. The results of the experiment has a correspondence with the simulation results, which means the correctness of the proposed method.