| Compared with the traditional two-level or three-level voltage source converter,because modular multi level converter(MMC) adopts modular design, there are no requirement of converter valves with directly series connected switching devices and problems of dynamic balancing voltage any more. At the same time, through adjusting the number of sub modules, MMC can change the voltage and power levels flexibly, extend to any level of output easily which can reduce the output voltage and current harmonic content. Modular multilevel converter based high voltage direct current(MMC-HVDC) transmission system has broaden application prospects in the renewable energy integration, power supply for the passive power grid, asynchronous grid interconnection, distribution network and etc.., Recently, MMC-HVDC has attracted more and more engineers' and scientists' attention.MMC-HVDC control strategy plays an important role in ensuring the system safe operation. Although the control strategy of two level voltage source converter based HVDC(VSC-HVDC) can be ported to MMC-HVDC, because their topology and working mechanism are different with each other, the research of MMC and DC transmission system is still necessary and has important practical engineering significances. In this dissertation, MMC-HVDC topology and mathematical model,capacitor voltage balance(CVB) and control strategies are investigated by means of theoretical analysis and simulation validations.Operation mechanism and steady state mathematical model of MMC-HVDCFirstly, two general MMC-HVDC topological modes(HVDC-Light and HVDC-Plus) and operation mechanism are analysis to investigate the control strategies. The mathematical models are built in the two different reference frames under balanced grid voltage condition. Secondly, based on the working mechanism of MMC, two modulation strategies are presented: the nearest level control(NLC) and carrier phase shifting sinusoidal pulse width modulation strategy(CPS-SPWM).Lastly, improved capacitor voltage balance algorithms are also designed with respected to these two different modulation strategies.The control strategy of MMC-HVDC system is described. Firstly, the MMC system is derived from Euler- Lagrange(EL) Model. Secondly, the control system of the MMC based on the traditional series PI regulator is analyzed, and the theory of the passive control is applied to design the controller of MMC-HVDC. Then the proposed control algorithm can realize the decoupled control and the repetitive-cycle control of real and reactive power, and maintain very good static and dynamic performances. Finally, MMC-HVDC model in PSCAD/EMTDC validated theeffectiveness and correctness of all the proposed control methods. |
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