| Modular Multilevel Converter (MMC) has the advantages of lower switching losses, lower harmonics of output voltage waveform, lower requirement for switch devices, higher reliability and so on. This technology has gained more and more applications in practice. This paper is mainly focused on the study of the basic theory, control strategy, modeling and simulation, the coordination operation of Multi-terminal HVDC (MTDC) for a MMC-HVDC transmission system.First, the operation mechanism of MMC is analyzed and a mathematical model of AC three phase circuits under d-q coordinate is derived to facilitate the design of controllers. Based on the model, double closed-loop control method is introduced for active power loop and reactive power loop in converter control layer. The valve control layer, which has functions of circulation suppression, nearest level modulation and capacitor voltage balance, is also illuminated detailedly.Then a 2-terminal 11-level MMC-HVDC transmission system is built in PSCAD/EMTDC. The effectiveness of controllers is verified by the step responses of control targets. The system can quickly track the dynamic variations and exhibits good performance. The average-value model (AVM) of MMC presented uses controllable voltage sources and controllable current sources to imitate the dynamic behavior of MMC, in which the inner difference among sub-modules is ignored. Simulation results show that the detail model and AVM model have the same trend during the transient process from the point of power system research. AVM model can be used to accelerate the simulation speed greatly without much sacrifice in simulation accuracy.Finally, a voltage margin control method is investigated in a multi-terminal MMC-HVDC system and the corresponding controllers are designed. The simulation results in a 5-terminal system indicate that the voltage margin control can realize the voltage takeover successfully between the master station and the slave station after the master station losses the regulation ability owing to reaching the power limitation or is out of service, so the system can continue to operate stablely. |
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