Fundamental Research On The Modular Multilevel Power Conversion System

In recent years, the power electronic converter technology has a rapiddevelopment, which cause the high power energy conversion system demand growsheavily. One perfect solution to breakthrough the limitation of power semiconductordevice capacity is improving the topology of the converter itself. With itsoutstanding technical features, the modular multilevel converter (MMC) overcomesthe defects of traditional multilevel converter successlly. So it is the most attractivemultilevel converter topologies in the high-voltage high-power electric powersystem. This paper have a deeply study about the modular multilevel powerconversion system which including voltage balance control methods, modulationtechniques, fault-tolerant control strategies and the application to photovoltaicpower generation system. The specific contents are as follows:The paper firstly analyzes the problems of sub-modules capacitor voltagebalance in MMCs. Starting from the operation mechanism of MMCs, a generalizeddynamic model of the MMC is set up. Based on analysis and comparison of the twotraditional submodules capacitor voltage balance methods, this paper presented amerge network based prediction sorting algorithm, making the improved efforts onthe optimization of the sorting algorithm efficiency and operating frequencyrespectively, by using a simple capacitor voltage estimation to determine the modeselection of pulse rotation and voltage sorting, the proposed method can reduce thedependence on the hardware level, as well as cutdown the controller burden and theswitching frequency of the converter.Then the staircase modulation and scalar PWM methods for MMC are studied.For the staircase modulation, to overcome the local convergence of the Newtoniteration method in solving the switching angles, a improved hybrid particle swarmoptimization (PSO) algorithm based staircase modulation strategy for MMC isproposed. This algorithm utilizes PSO to randomly search the optimal initial valuesof switching angles, which can accelerate the iterative speed at the next step of theNewton iterative processes. For the problem of scalarization of vector modulation,this paper also presents a scalar PWM algorithm with the capacity of capacitorvoltage balance. The algorithm based on the nearest level modulation (NLM)technique, which features a linearly synthesized output voltage by means of simplearithmetic calculations of duty cycles to determinate the switching sequences andthe duration times, saving the tedious trigonometric function calculations andcoordinate transformation. Simulation and experimental results verify thecorrectness and effectiveness of the proposed modulation methods. To solve the sub-module failure problem of MMC system with no redundancybackup, a zero sequence voltage injection based fault-tolerant control strategy forMMC is proposed. The feasibility of the zero sequence voltage injection isdiscussed, and on this basis the impact on the interphase power allocation of theMMC system is revealed. In fault-tolerant mode the rated capacitor voltage of eachmodule can keep constant, improving the system transient stability under the impactof the capacitor voltage fluctuation and avoiding the swell of the interphasecirculating currents. The strategy strengthens the fault ride-through ability of theMMC, and do not need to add additional hardware cost.Finally, the control strategy of the grid-connected photovoltaic (PV) powerconditioning system based on MMC topology is studied. The technicalcharacteristics and advantages of the large-scale PV power station which adopts thegrid converter system configuration of the MMC topology are analyzed. For thedeep understanding the feasibility of bidirectional active and reactive powertransmission of the modular multilevel PV power conditioning system, thederivation process of the system operation mode based on space vector analysis isgiven, and the system operating limits which contain reactive power and harmoniccompensation are studied. For the realization of system stability control, this paperproposes a scalar PWM based active and reactive power coordinated controlstrategy. This strategy can ensure the stability of the dc bus voltage, and effectivelyimprove the utilization rate of large-scale PV power station.