Optimal Control Strategy For Converter Of MMC-HVDC

Recent decades have witnessed the popularity of modular multi-level converters(MMC)in the field of flexible HVDC transmission,owing to their significant advantages in modularized structures and flexible extension.In a certain MMC-based flexible HVDC system,appropriate MMC control strategies have a crucial impact on the reliability and efficiency of its operation.Aiming at optimizing its operation,this thesis systematically investigates optimized MMC control strategies for flexible HVDC systems from three aspects,i.e.,circulating current suppression,capacitor voltage balancing,and fault tolerance control.As for studying large-scale MMC optimization and control,accurate and fast electromagnetic transient simulations serve as the cornerstone of verifying the efficacy of the study.In order to mitigate the heavy computational burden of the conventional simulation algorithm,an improved algorithm is tactfully developed to speed up MMC-related electromagnetic transient simulations.In particular,by correcting the capacitor currents of MMC sub-modules at switching points,the processing efficiency of switching points is dramatically improved.In addition,when a converter is blocked,according to its blocking characteristics,it is approximated as an uncontrollable half-bridge circuit.A comparative study on an MMC-based HVDC system with 10 MMC sub-modules illustrates the effectiveness and efficiency of the proposed improved algorithm.Based on it,a fast electromagnetic transient simulation model for the real-world Xiamen flexible HVDC transmission demonstration project is constructed,acting as a fundamental test bed for the subsequent studies.In terms of MMC circulating current suppression,how circulating currents come out is firstly anatomized from the theoretical perspective.In an MMC-based HVDC system,deviations of the upper and lower arm voltages between the actual output values and the ideal reference values are the root of circulating currents.In essence,such deviations are caused by the fact that the traditional nearest level modulation strategy fixes the total number of MMC sub-modules put into operation on the upper and lower arms,being unable to eliminate the impact of the actual fluctuations of capacitor voltages.With this cognition,a superimposed approximation modulation(SAM)strategy is designed to suppress circulating currents.Based on the real-time fluctuations of capacitor voltages,the number of MMC sub-modules to be put into use is dynamically adjusted.In this regard,circulating current suppression is achieved at the modulation strategy level.Moreover,on the basis of the law of energy conservation,it is proved that no second-harmonic current exists on bridge arms with the SAM strategy adopted.Regarding capacitor voltage balancing control for MMC sub-modules,focusing on overcoming the defects of conventional control strategies that have high switching frequencies and large amounts of sorting calculation,a novel frequency division-based voltage balancing control strategy is put forward.Specifically,it reduces the frequency of capacitor voltage sorting to a much smaller value than that of triggering control,and optimizes the frequency of capacitor voltage sorting based on theoretical analysis.Furthermore,with the incorporation of the SAM strategy,a combinational control strategy of MMC capacitor voltage balancing is proposed.It has no impact on the output characteristics of the bridge arms,yet being capable of reducing the switching frequency without additional harmonics introduced into the output voltages of the bridge arms.With respect to fault tolerance control for MMC sub-modules,stemming from the physical mechanism of their post-fault asymmetric operation,an optimized fault tolerance control strategy is devised with the help of energy prediction of the bridge arms.Concretely,with instantaneous energies stored in MMC sub-modules predicted at each time instant,the varying capacitor voltages of the sub-modules are calculated and fed into the controller in real time.Even with no additional circulating current suppressor,this strategy has its own strength in suppressing the asymmetrical fundamental and second harmonics of circulating currents.Besides,it is not merely limited to handling fault conditions.Instead,it can be extended to normal operating conditions as well,achieving second harmonic suppression without supplementary circulating current suppressors and thus reducing the complexity of large-scale MMC control.All the three types of optimized control strategies were tested on the above-mentioned realistic Xiamen flexible HVDC transmission system via electromagnetic transient simulations,whose results demonstrate their effectiveness and efficacy.