Topology Derivation And Control Strategy Of Modular Multilevel Converters

High voltage direct current(HVDC)transmission is widely considered to be beneficial for long-distance power delivery,asynchronous interconnections,and long submarine cable crossings.The voltage source converter based high voltage direct current(VSC-HVDC)system is a growing candidate of HVDC technology with the clear advantages of independent reactive power control,black-start capability,and smaller station footprint.The modular multilevel converters(MMCs)are the promising topologies for VSC-HVDC transmission system due to their modularity,scalability and flexibility.However,MMCs contain a large number of distributed sub-module capacitors,which is significantly different from other VSCs in the circuit structure.In practical HVDC projects,capacitor is the largest component in each sub-module,the investment cost of sub-module capacitor is basically the same as that of power device.Therefore,the influence of the capacitor on the construction cost,transient stability and reliable operation of the converter structure attracts considerable attention.By focusing on topology and control of MMCs,this dissertation investigates the topology derivation,modulation strategy,modelling method and control algorithm for MMCs.The achievement mainly manifests in the following aspects:Firstly,an Alternate Arm Multilevel Converter(AAMC)with half-bridge sub-module and arm phase-shift conducting modulation are proposed.The proposed topology is consisted of half-bridge sub-modules and power devices in series.By regulating the power devices in each arm,the upper and lower arms can conduct alternately.For the structure of topology,arm phase-shift conducting modulation is introduced to expand the operation boundary of the converter and realize the wide voltage range operation of converter without extra control loop.Compared with the classcial MMC with half-bridge sub-module,the sub-module number of proposed topology and modulation is reduced 18% and the total capacitance is reduced 44%.Secondly,the topology derivation of MMC about arm topology is revealed,and a novel family of modular multilevel converter topology is proposed.On the basis of fully investigating the existing MMC topologies,the level generating and switching performance of sub-modules and power devices are summarized.According to the circuit structure,the arms in MMCs are divided into DC arm and AC arm.The sub-modules and power devices with different characteristics are arranged and combined in DC arm and AC arm respectively to obtain the modular multilevel converters.The families of MMCs with different arms are derived,and the internal relationship between different topologies is found,and then a novel topology structure is proposed.Thirdly,the modelling of MMCs considering the sub-module capacitor voltage ripple is established to explore the control dynamic performance of MMCs.The effect of sub-module capacitor voltage ripple on the MMC dynamic behavior is explored and an equivalent model of MMCs is presented,which comprises the equivalent capacitors reflecting the capacitor voltage ripple.Then,decoupled current control with synchronous frequency damping is proposed.The coupling terms in the presented MMC model is offseted,which can improve the system dynamic behavior.The effectiveness of the presented model and control strategy is verified by the PSCAD/EMTDC simulations.Finally,the equivalent model of modular multilevel converter under unbalanced conditions is established,and the corresponding control strategy is proposed to improve the dynamic characteristics of system.According to the performance under unbalanced fault,the voltage ripple of sub-module capacitor is analyzed,and the equivalent model of MMC under asymmetric fault is proposed.The positive and negative sequence components under asymmetric condition are separately controlled.Furthermore,the modelling and control of converter under single-phase-to-ground fault and phase-to-phase short circuit fault are analyzed as examples.The positive and negative sequence models and control are verified by building a three-phase system in PSCAD/EMTDC simulation software.Overall,in this dissertation,the toplogy and control for MMCs are focused on around capacitor characteristics.Various modular multilevel converters are proposed to reduce the sub-module number and capacitance.And the modelling and control method of MMC considering the sub-module capacitor ripple are proposed to improve the control performance of MMC under balanced and unbalanced conditions.Finally,the simulation results validate and effectiveness of the proposed topologies,modelling and control strategy.