| The stable DC voltage and rational load power sharing are the critical issues in Voltage Source Converter Multi-Terminal Direct Current(VSC-MTDC) system. Compared to conventional two-terminal high voltage direct current(HVDC) system, the system is more flexible and suitable for energy internet and renewable energy integration due to its realization of multi power supply, multi load carrying and reduction of the ratio of solar and wind power restriction. Meanwhile, modular multilevel converter(MMC) has been widely applied in VSC-HVDC systems. The MMCs feature simple configuration and high capacity expand ability. Hence, it can be used to tackle the obstacle of limited power rating in semiconductor devices. Since a huge number of modules are used in MMC, the modulation strategy plays an important role in its safe operation. Thus, an in-depth analysis has been done in terms of the control algorithm of MMC in this thesis. At the same time, focusing on energy conversion of MMC, the corresponding modeling and modulation technique have been studied in detail. The detailed contents are as following:Firstly, based on the radial and meshed configurations of VSC-MTDC system, a modified droop control method is proposed to improve the power distribution. Based on the conventional droop control method, both the voltage fluctuation and power distribution have been taken into account. The DC bus voltage and the output power are controlled by proportional-integral(PI) controllers, which could output two compensating components. Besides, via low bandwidth communication network, DC voltage restoration and rational power distribution with power line impedance mismatch are implemented simultaneously. The stability of the proposed method for different network configurations and line impedances are validated.Secondly, aiming at VSC-MTDC system, an optimal power distribution method is proposed based on the above modified droop control method. Based on the desired optimal power flow and analysis of the power loss of the system with π-type transmission line, the DC voltage and power distribution are optimized. The output voltage of each DC terminal and power dispatch results are derived by considering the criteria of minimum power line loss. Then, by combining the results with the formulas of calculating DC voltage reference of droop control, the optimal proportion is determined for suitable power sharing. At last, the system stability is verified by taking the power line parameters and communication delay into account.Thirdly, the modulation issue of MMC is discussed in this thesis. Based o n the modeling of MMC, it can be seen that the modulation ratio is shown by the amplitude of the line-frequency component. Hence, the maximum value of the modulation ratio should be reduced to avoid over-modulation. To achieve this goal, the modulation method with inserted compensating component is proposed to reduce the line-frequency component in the MMC modulation ratio. By properly designing the capacitor in each submodule of MMC and considering the operation range of the converter, the relationship of the compensating components used for circulation current mitigation and the capacitor voltage ripple is revealed. The feasible range of the maximum modulation ratio of MMC is derived. The proposed method can effectively suppress the circulating current and avoid over-modulation, which can further ensure the stable operation of MMC.Finally, in terms of the VSC-MTDC system with two-level converters, an in-depth discussion in regards to the control diagram of single-phase fault at the AC side is conducted in this thesis. A generalized method is proposed to mitigate the influence induced by single-phase fault. The method of signal delay is used to achieve positive and negative component decomposition of grid-connected current. The current reference values are derived in different operation modes. Proportional-resonant(PR) controllers are employed to control grid-connected current. Afterwards, the equivalent impedance model is accomplished based on multilevel VSC-MTDC system. The relationship between the derived impedance model and the compensating component in the modulation index and the capacitor value in the submodule are further analyzed. Then, based on the DC side impedance model, the procedure of DC voltage ripple propagation from one DC terminal to another is analyzed considering the fault at one terminal. The impact of different parameters in the power transmission system is discussed as well. |
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