Optimization On Control Strategy And Control Parameters For Modular Multilevel Converter

Modular multilevel converter as voltage source converter, with modular structure, has advantages of low harmonic content of output waveform, independent control of active power and reactive power. It is suitable for new energy interconnection, isolated power supply, large city power supply, etc. As the growth of voltage class and transmission capacity, the number of submodules has been greatly improving. It encountered great challenges of modeling of MMCs in the electromagnetic transient simulation, sub-modules capacitor voltage balance control and control parameters optimization. In this paper, the electromagnetic transient(EMT) model is built up. On the basis of EMT model, capacitor voltage balance control and control parameters optimization are studied.1. In view of the large-scale modular multilevel converter, the problem of low efficiency in simulation has appeared. To address this problem, a detail equivalent model is proposed based on Thevenin’s Theorem. And a switching function model is proposed in order to further improve the efficiency of simulation based on average equivalent concept. Tradition detail model and proposed models are built in PSCAD/EMTDC which are used to verified the accuracy and efficiency of proposed models.2. A novel strategy of capcacitors voltage control based on classification is proposed, in order to reduce switching frequency and contol capacitor voltage. The method, which according to the capacitor voltage and the last state of capacitor, greatly reduces the number of capacitance in sorting, switching frequency and fluctuation of capacitors voltage through the comparison under the PSCAD/EMTDC verifies the effectiveness of the improved method.3. Control parameters are discussied in this paper according to the control strategy of MMCs. The evolutionary state assessment to adjust the PSO parameters is involved. The electromagnetic transient simulation program is linked to the proposed improved MOPSO to give the results of the optimization objective. Simulation results proof the effectiveness of the proposed method.