Research On Control Strategy Of Modular Multilevel Converter

Along with the power electronic technology development towards the directionof the high voltage and high power transformation, especially the voltage sourcetype inverter widely used in flexible HVDC system, the study of high-powerconverter is more and more important. However, the traditional two levels andmultilevel converter applied in the field of high voltage control is complex. Becauseof its modular structure, Modular Multilevel Converter (MMC) is an excellentcandidate for HVDC transmission. The topology, working principle andmathematical model of MMC is studied in detail.Then the circulating current isderived and the capacitance capacity is calculated based on the requirements ofadjusting time and voltage fluctuation.Secondly, through the in-depth research of the sub-module capacitor pre-charging process at the start-up time of MMC, a decouping control strategy forsub-module pre-charging is presented. With the proposed charging strategy a smoothstart-up for MMC can be realized.Thirdly, the fastest way to be derived between the arms based on the traditionalcapacitor voltage balance stategy. This paper proposed a synthesized balance controlmethod that involves leg voltage, arm voltage, individual voltage balabce controland circular current control. The proposed method can maintain capacitors voltagebalance with few harmonics in circular current.Fourthly, based on analysis of the working principle of HVDC, a decoupingcontrol strategy is proposed for achieving active and reactive power controlindependently. And then the solutions are given targeting the anomalous behaviorsof the grid voltage. An output current regulating method is presented for the aim ofstabilizing PCC voltage. In addition, a control strategy for MMC under unbalancedcondition is presented which can eliminate negative sequence current components.For the above researches, a MMC prototype is developed. The proposedmethods for the issues above have been verified in Matlab and relative experiment.The experimental results prove the validity and effectiveness of the proposed controlmethod in this paper.