MMC-HVDC Topology Research And Modeling Simulation In RTDS

Modular Multilevel Converter (MMC) based HVDC has obvious technical advantages, so it has broad application prospects and opens up a new direction for the development of the felexible HVDC technology. However, there are a large number of power sub-modules (SMs) in the conventional MMC, causing that quantities of data are needed to collect and process. It results in the complicated hardware construction of control system. A novel MMC topology based on nested-loop mechanism (NLMMC) is proposed. The key work of this paper is to introduce and analyse the topology construction and basic principles, and to design corresponding suitable controller. Moreover, MMC closed-loop modeling method under RTDS circumstance is investigated, and the validity of modeling method and the feasibility of the novel topology and it’s control strategy is validited.First, MMC’s topology features and multilevel generation mechanism are introduced. Two modulation methods and it’s feature are analysed. Capacitor voltage fluctuation mechanism and general balancing method are also analysed. Optimization of balancing method from two aspects are carried out.Next, the NLMMC topology is proposed. The new topology construction and operation principles are analysed. Selection methods for relevant parameters are derived. SMs coordinating control method for the novel method is designed. Module contribution ratio (MCR) is introduced for measuring the converter’s multilevel output capability, and thus proves the novel topology’s better capability.Then, MMC controller’s hardware construction is analysed. NLMMC topology’s specific design principle to minimize the number of SMs is obtained. Considering SMs coordinating feature in novel topology, formulas for calculating converter loss are derived. The novel topology’s design principle to minimize converter loss is obtained.Finally, considering the characteristics of small time step simulating circumstance in RTDS, MMC closed-loop modeling methods under RTDS circumstance is posed. The feasibility of the modeling method is verifyed by setting up a conventional MMC model successfully. Then, NLMMC model is setted up by the above method and the novel topology’s feasibility and the control strategy’s effectiveness are verifyed.