Research On Control Strategy For The Triple Line-Voltage Cascaded VIENNA Converter

Energy and environment are two major issues facing the world today.In 2014,in order to achieve healthy and rapid economic development,the General Office of the State Council promulgated the Strategic Action Plan for Energy Development,which specifically pointed out that it is necessary to implement the industrial energy saving action plan,strictly restrict the expansion of the energy-intensive industries,curb excess capacity,speed up the elimination of backward production facilities and implement ten major energy saving projects.Among them,as one of the ten major energy saving projects to be implemented,Energy Saving Project in Motor System plays a vital role in achieving the goal of energy saving and emission reduction in China.The implementation of this project will greatly promote the development of high-voltage motors and the corresponding control systems,and in consequence make high voltage high power drive systems be used more widely in the future.In order to avoid using the bulky and expensive line frequency transformers which are usually required by the traditional medium/high voltage and high power cascaded multi-level converters.The key technologies based on triple line-voltage cascaded rectification and high frequency isolated DC/DC transmission is studied by this research for the medium/high voltage high power converter in the next generation.Considering that in a large number of practical industrial applications,unidirectional power flow is enough for the medium/high voltage high power converters,hence,with the aim of reducing the number of fully controlled power switches and the voltage stress for each power switches,simplifying the control system,improving the system reliability,the basic theory and key technology for the medium/high voltage converter in the next generation is studied by this paper with emphasis on the rectifying stage.Inspired by the line-voltage cascaded voltage-source converter(LVC-VSC)typology proposed previously by a number of references,a new typology of three phase multilevel converter based on triple line-voltage cascaded VIENNA converter is proposed by this paper,the model of corresponding equivalent switching circuit,power factor correction technology are studied fully,the voltage stress for each of the power switches,and DC voltage control for the triple line-voltage cascaded VIENNA converter are discussed as well.The principle of the Triple Line-Voltage Cascaded VIENNA Converter(LVC-VC)is studied by this paper,the operating characteristics including the relationships between the AC side currents and voltages in each Vienna rectifier in case of balanced and unbalanced transmission power flow are analyzed.Besides,modulation constraints for achieving the sinusoidal input currents is discussed also by this paper.Combined with the idea of the three-phase line current integrated control,the equivalent switching circuit models under balanced and unbalanced load are established based on the external characteristics of LVC-VC,which lays a foundation for further research on power factor correction and DC side capacitor voltage control strategy,the expansion method of LVC-VC to higher order with more cascaded cells and the corresponding mathematical relationship between AC side voltage and current are presented as well.The suitable PWM modulation strategy for LVC-VC under unity power factor operation is studied by this paper.The core formula of LVC-VC based on one-cycle control strategy is derived.Through controlling the two modulation factors in this core formula separately,utilization of DC voltage is increased and voltage stress of the power switches is reduced.Besides,the harmonic components of PWM arm voltage in LVC-VC based on the carrier modulation is analyzed quantificationally by this paper,the one cycle control strategy for LVC-VC based on carrier phase shifting strategy is designed,which makes the LVC-VC could operate under unity power factor with less harmonic components in the arm voltage.The LVC-VC power factor correction controller based on phase-shift carrier modulation and one cycle control strategy is designed as well.The mathematical relationships among DC-link capacitor voltage,modulation factor and load power is analyzed by this paper,the allowable minimum average value of DC-link capacitor voltages and the maximum value of harmonic component are calculated,which provides the theoretical basis for the selection of the power switches.In order to achieve the control of capacitor voltage on the DC side of LVC-VC,based on the mathematical model of the equivalent switching circuit,the small-signal model is derived,and the voltage controller to achieve the DC-link capacitor balancing is designed.The influence mechanism of zero-sequence current injection on the power transmission and DC-link capacitor voltage of each cascaded module is also deduced by this paper,based on which,the stable and balancing control to the DC-link capacitor voltage for LVC-VC is achieved by adjusting the active power transmission to each module.The whole control system for LCV-VC composed of three-layer control strategy is described in detail,which include:PWM modulation strategy based on the carrier phase shift one cycle control,overall DC-link capacitor voltage control,and DC-link capacitor voltage balancing control for each module.The designed control system has the characteristics of less hardware requirement,less computational burden and simple control structure.The LVC-VC low-power prototype was built,and the one cycle control strategy and DC-link capacitor voltage control strategy under balanced and unbalanced transmission power were studied experimentally.The experimental results verified the feasibility and correctness of the proposed LVC-VC topology and the built equivalent switching circuit model,which indicates that OCC based control strategy presented by this paper can effectively make the LVC-VC operate under unity power factor with stable and balanced DC side voltage for each module in cascading.In summary,LVC-VC can be used as the rectification stage of novel high frequency isolated cascaded medium/high voltage converters,in which the number of power switches and DC capacitors required as well as the voltage stress to each power switches are all reduced,in consequence makes the whole system more cost effective.