Research On Fault-tolerant Control Strategy Of Modular Multilevel Converter And Prototype Development

In recent years,Modular Multilevel Converter (MMC) is a new type of voltage source converter topology, which has receives wide concern and becomes an important research focus. Modular Multilevel converter adapts modular design, its output levels is high and is flexible to be adjusted to meet the demand of different voltage levels and power levels. It eliminates the high power filter and is suitable for high power DC transmission supplying power to long distance isolated loads and renewable energy integration etc al.DC transmission system based on modular multilevel converte has been a crucial method to transimit power, lots of references are published these years.However,MMC is still not well understood, for the typical MMC topology with half-bridge sub-module, this paper includes:(1) Firstly,the structure of MMC and its operational principle are detailed analyzed. The modulation principle of nearest level modulation and carrier phase-shifted pulse width modulation are investigated.,The modulation method of N+1 levels and 2N+1 levels of output voltage was studied.The advantages and disadvantages of two modulation methods are also concluded.(2)The Control system of MMC was studied, and steady-state control strategy of MMC-HVDC system based on dq rotated coordinate is proposed.The controller adopts double closed-loop control. Inner loop current controller and outer loop voltage controller were designed in detail. DC transmission system based on Modular Multilevel Converter is nonlinear, a small signal model is designed for control strategy, and a multi-loop control system is proposed. Using the theory of discrete control, a discrete double closed-loop controller is also designed. In order to maintain system stability, automatic control theory was applied to the control system and determined the bounds of the controllers'parameters, this is worth to be used for reference to a certain extent in experiment parameter design.(3) As the basic unit of MMC,sub-module is the key to maintain MMC-HVDC system stability.Firstly,the paper introduced the common sub-module fault types of MMC and summed up different redundancy protection control methods.To solve the sub-module failure problem of MMC system with no redundancy backup and ensure MMC-HVDC meet the requirements of its safety operation,a control strategy based on energy balance of arms is proposed, the system bypasses the same amount of sub-module symmetrically,MMC operates undder unsymmetrical condition, the control method improves the utilization ratio of sub-modules.Finally, a simulation mode is built in PSCAD/EMTDC and the simulation results verifyed the validation of the proposed control method.(4) According to the theoretical analysis and the parameters design method,a small power three-phase inverter experimental prototype has been built for experimental validation.The detailed design process which contains the hardware design and the software design is given.The experimental results prove the validity and effectiveness of the control method.and the correctness of parameters design method.