Research On Modular Multilevel Converter And Its Control Schemes

Modular multilevel converter(MMC),compared to traditional multilevel converter topologies,presents a series of distinctive features,such as very high voltage and power ratings(±400k V/500MW),high efficiency(higher than 99%),modular structure,scalability,easy maintainance,high reliablity,and high-quality output voltage waveform.Due to these characteristics,MMC has recently bec ome the most attractive topology for medium-and high-voltage applications and one of the hottest research topic in the fields of power electronics.It has revolutionized the HVDC technology and significantly pushed the development of offshore wind farms.Meanwhile,MMC has also been found promising to be applied in medium-voltage motor drives and other power-electronic applications.However,as MMC is still a relatively new topopoly,its basic principles and many technical details such as modulation and energy distribution have not been fully studied yet.On the other hand,there are a lot of challenges associated with its start-up precharge,submodule fault protection,and capacitor voltage ripple suppression.These unclear principles and remaining challenges are hindering the development and wide-spread application of MMC.Against this background,chapter 2 of this thesis starts with the basic operating principle of MMC.The phase-shifted carrier modulation is analyzed by using Fourier series expansion,revealing the influence of carrier displacement angle on the voltage and current harmonics.Based on this analysis,the optimum displacement angles are specified for the voltage harmonics minimization and the circulating current harmonics cancellation,respectively.On the other hand,through analysis of power transfer within MMC,the control basis for capacitor voltage balancing is derived and a hierarchical balancing controller is proposed in which the control layers are decoupled with each other.Moreover,a three-phase MMC prototype has been built in the laboratory and the structures and parameters of this prototype are introduced.Chapter 3 is aimed at investigating the control of MMC under two special conditions: start-up precharge and submodule failure.Firstly,the precharge requirement of MMC submodule capacitors before put into operation has been analyzed,then two closed-loop precharge control methods are proposed for MMC,respectively,with respect to start-up from the dc-side and the ac-side.By means of feedback control,the precharging current is regulated at a constant value thus the submodule capacitors can be linearly charged,which significantly reduces the start-up time and avoids inrush current.On the other hand,a comparative study between cold reserve and hot reserve mechanisms of the MMC submodules is carried out.The circuit characteristics of MMC under submodule failure is analyzed,and the complete fault diagnosis and tolerance control method is proposed.All these proposed schemes have been verified experimentally on the MMC prototype.One of the major obstacles limiting the widespread application of MMC is that the heavy,bulky,expensive capacitors are necessitated by each submodule to keep the capacitor voltage ripple within reasonable limits.Hence how to reduce the capacitance of MMC becomes the key factor to save cost,volume,particularly in the case of driving motors at low speeds.In chapter 4,a modified MMC topology is proposed by optimizing the connection between submodules,in which capacitor voltage ripple of the top,middle,and bottom submodules can be significantly reduced.Moreover,a second-order capacitor voltage ripple suppression method is proposed.In terms of the more severe fundamental capcaitor voltage ripple,it is indicated that this voltage ripple is in proportion with the dc-link voltage of MMC.Therefore another novel MMC topology is proposed by adding a series switch in the dc link,combined with current control,which can effectively decrease the average value of MMC dc-link voltage,substantially suppressing the fundamental capacitor votlage ripple.The above suppression schemes have both been tested on the MMC prototype,showing the potential as a cost-effective solution for motor drives.Chapter 5 investigates the derived MMC topologies,aiming to expand the application of MMC.Firstly,MMC is utilized as a series HVDC power tapping device.By employing full-bridge submodules,the MMC power tap can supply the rural communities that otherwise have little or no access to electricity.In addition,by connecting two MMCs in a front-to-front structure,it can be used as a dc power flow controller,which can solve the uncontrobility problem of meshed lines in future HVDC grid.Finally the MMC-based dc ice melting device is introduced,indicating its advatages compared with traditional thyristor-based devices.Furthermore,an optimized capacitor voltage control and the STATCOM operation mode are proposed,to fully utilize the power capacity of MMC de-icer.These reaserch have been verified by simulation or experimental results,which are promising to find practical usage in the future.