| Modular structure is adopted in the main part, control and protection system of modular multilevel converter HVDC (MMC-HVDC). It has the advantages which other topologies don’t have. It can improve the performance of voltage source converter HVDC (VSC-HVDC) significantly. Based on MMC’s topology, operation mechanism, mathematical model and modulation technologies, sub-module capacitor voltage fluctuation, circulation current between phases and its suppression solution and MMC’s fast simulation algorithm are researched.Firstly, MMC’s inner circulation current mechanism and its influence on AC side output phase voltage are researched. In MMC’s circulation current, the second harmonic circulation current is the most notable. Secondly, to suppress the second harmonic circulation current, three phases decoupled second harmonic circulation current suppression algorithm is proposed in this paper. Based on instantaneous power balance between AC and DC side of the converter, each sub-module capacitor voltage reference value is deduced in upper and lower arms. The reference value varies around a constant value in a power frequency cycle period, reducing the degree of sub-module capacitor voltages unbalance, and the second harmonic circulation current is suppressed. Lastly, during research on MMC’s inner circulation and its suppression solution, when the number of levels is high, simulation velocity become slow. To solve this problem, based on time-domain equivalent models of capacitor and reactor a fast simulation algorithm for MMC is proposed. Needing no approximate treatment for sub-modules, the proposed algorithm can reduce the size of nodal admittance matrix without decreasing simulation accuracy, thus the simulation velocity of MMC in PSCAD/EMTDC environment can be greatly accelerated and the proposed algorithm is well compatible with the control strategy of MMC.Respective models are built in PSCAD/EMTDC to validate the circulation current suppression solution and fast simulation algorithm. |
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