| Compared with traditional two-level or three-level voltage source converters(VSCs),modular multilevel converter(MMC)has many incomparable advantages,such as 1)modular structure,which can conveniently expand the voltage and power levels by just adding the number of sub-modules(SMs);2)low switching frequency;which can significantly reduce the whole losses of MMC and MMC-HVDC system;3)high quality output waveform,which greatly reduces the requirements of ac and dc filters;4)low difficulty of voltage-sharing,because of adopting SM series structure rather than switching component series structure;5)high operation reliability,because of SM redundancy design and arm inductor direct series structure.As a result of the above mentioned reasons,MMC-HVDC is regarded as the most promising form of VSC based HVDC and also,in recent years,the most widely used VSC based HVDC form in practical projects in the word.MMC-HVDC has wide application prospects in long-distance large-capacity power transmission,asynchronous network interconnection,distributed generation into the power grid,wind farm into the power grid,no-source island,drilling platform,remote areas power supply,capacity expansion of urban grid and many other domains,and has become the focus of academe and industry.Although MMC-HVDC has so many advantages,its operation mechanism and control strategies are more complex than traditional VSC-HVDC.In order to guarantee the reliable and economical design,safe and stable operation and fault ride-through capability of MMC-HVDC,accurate operation mechanism analysis,reliable control system design as well as fast grid connection study of MMC and MMC-HVDC have significant theoretical research value and project application value.This paper mainly study the steady state operation mechanism,fast grid connection control,power decoupling control under different operation conditions,circulating current suppression and some other related problems in MMC and MMC-HVDC by theoretical analysis,simulation verification and part experiment test.(1)MMC closed-loop time-domain steady state analysis method.Accurate time-domain steady state analysis can effectively realize its performance evaluation,circuit parameters design as well as semiconductor devices selection,which is an extremely important technique in project planning and operation.A closed-loop time-domain steady state analysis method is proposed in this paper.This method first assumes that the accurate modulation function and circulating current of MMC are both unknown.Then,two closed analysis loops are constituted based on the classic circular interaction among MMC electrical quantities and modulation function.Lastly,two nonlinear loop equations are established to solve the accurate modulation function,circulating current,as well as all the other electrical quantities coupled by them.The proposed closed-loop analysis method presents high analysis accuracy in the full operation range of MMC and the analysis accuracy is irrelevant with the operation conditions.In addition,this method can realize the ac side voltage,transformer and its taps optimal design and selection.(2)The fast and accurate grid connection technique of MMC-HVDC under complex grid conditions.This technique is the basis of the normal grid-connection operation,power decoupling control and fault ride-through of MMC-HVDC.A fast differential moving average filter base phase locked loop technique is proposed in this paper.This technique incorporates a proportional component which can on-line change its value according to the input signal frequency into the traditional moving average filter based PLL(MAF-PLL).It can fast filter the lowest order harmonic to reduce the window width of MAF and then improve the locking speed of MAF-PLL.Since proportional component cannot result in phase delay,this technique will not deteriorate system stability.The proposed PLL technique,named DMAF-PLL in this paper,has simple structure and low calculation burden and it can be used in complex grid conditions,such as fault,high harmonic pollution and dc offset grid conditions to provide a guarantee for the safe operation,stable control and fault ride-through of all the converters(including MMC)and MMC-HVDC.(3)Power decoupling control strategies of MMC-HVDC under different grid conditions.The active and reactive power decoupling control strategies of MMC-HVDC under different grid conditions are studied in this paper.Detailed comparison and simulation verification between separation sequence decoupling proportional integral(PI)control strategy in the rotating frame and proportional resonant(PR)control strategy in the stationary frame are carried out.It is found in the study that PR based power decoupling control strategy of MMC-HVDC has simpler control structure and lower calculation burden because of needing no positive and negative sequence separation of electrical quantities,separation sequence control and frame transformation and inverse transformation.(4)The circulating current suppressing strategy of MMC-HVDC.Circulating current of MMC is inevitable because of its topology and operation mechanism.The production mechanism of circulating current in MMC and corresponding suppressing strategies are studied in this paper.It is concluded that traditional circulating current suppressing strategies used in MMC cannot be directly used in MMC-HVDC since it may result in dc-line voltage ripples or may not completely remove the zero-sequence circulating current component.To solve this problem,a new zero-sequence circulating current controller is proposed and corresponding improved circulating current suppressing strategies of MMC-HVDC are proposed as well.The proposed strategies have simple structure and are easy to implement.They can completely remove all the circulating current components,dc-line current and voltage ripples of MMC-HVDC under normal and abnormal grid conditions,which can greatly improve the whole system performance and stability. |
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