Abstract
Compared with the AC power system, HVDC can transmit electricity over long distances and high power without line inductive reactance and capacitive reactance. Modular Multiple Converter or MMC has been used in VSC-HVDC from 2002. It includes a large number of sub modules to improve the level and capacity of high-voltage transmission, and reduces the switching frequency and power losses effectively. In this project, firstly I analyzed the structure of MMC and each sub module, and built a circuit model of 11-level MMC in MATLAB/Simulink. Secondly, I used Nearest Level Modulation, or NLM to control the cutting-in and cutting-off of all the sub modules. Thirdly, I designed an LCL filter to decrease the harmonics of MMC's output voltage and current, and connected the MMC to a 330kV grid through MATLAB/Simulink. Then I designed a double current feedback controller to control the d and q components of the grid current. Next, I researched the state-space model of MMC connected to the grid with double feedback current control and derived the state space equations of this model. Finally, I compared the waveforms of grid current, capacitor voltage and current, and DC current between the switching circuit model and state-space models. Simulation result shows that with proper control, the d and q components of the grid current can flow to their desired values. This means that the design of double feedback current controller and LCL filter is effective. Moreover, simulation results of the MMC state-space model are consistent with its switching circuit model. There are two main contributions in this project: 1) The LCL filter is firstly used in MMC interfacing grid. It can get high quality output voltage and current waveforms with relatively few sub modules (just 11 SMs in this project). 2) The double current feedback control is firstly used in MMC-LCL-grid system. The grid current becomes controllable under this technique. 3) The state space model of MMC-LCL-grid with double feedback control is also worked out. Its simulation results are consistent with that in switching circuit model.