Research On Robust Control And Capacitor Voltage Balance Strategy For Grid-side Converter In Traction Drive System

As a bright“business card”of Chinese equipment manufacturing,high-speed railway has stood at the forefront of the global market and gradually led the development of the global high-speed railway market.On the one hand,as a critical equipment of high-speed railway,high-speed trains run for a long time in harsh climatic environments such as cross-regional temperature differences,humid heat,thunderstorms,frost,rain and snow,and also face unique working conditions such as excessive phase,pantograph net separation,occasional contact between trains and tracks.Meanwhile,the working conditions,such as traction,braking,traction and break,acceleration and deceleration,are complex and changeable.In addition,factors,such as aging and loss of power devices,are difficult to avoid,which put forward high requirements for the robustness of the control system of the grid-side converter at the front of the traction system.On the other hand,the use of on-board high-power power electronic transformer(PET)instead of bulky power frequency transformer is a promising choice to achieve the lightweight demand for high-speed trains.However,for the pre-stage of the PET,i.e.,single-phase cascaded H-bridge rectifiers,the dc-side capacitor voltage will be unbalanced due to capacitance errors,differences in line losses of each unit and load unbalanced.Without the voltage balancing control(VBC),the voltage increased by individual devices will lead to problems such as overvoltage,overcurrent,capacitance breakdown,and even out of control for the entire traction system.Moreover,dynamic performance is an important evaluation criterion for the VBC method.Therefore,taking the grid-side converter of the traction drive system as the research object,the robust control of single-phase pulse width modulation(PWM)rectifier and the capacitor VBC of the dc-side of the cascaded H-bridge rectifier are researched in detail.To enhance the robustness of control system for the single-phase PWM rectifier,H_∞loop shaping direct current control(H_∞-LS DCC)strategy is proposed.Based on constructing the expected open-loop transfer function,the expected controlled object is processed by normalized coprime factorization,and the H_∞loop shaping design is then attributed to the H_∞robust control problem for coprime factor perturbed model.According to the higher harmonic characteristics of grid-side current,the selection rules of compensator can be given.The Riccati equation is used to solve the H_∞loop shaping controller.Undesirably,the use of the normalized coprime factorization to describe system uncertainty may leads to conservative design results.Therefore,μ-synthesis direct current control(DCC)strategy is presented.It uses linear fractional transformation to perform the parameter uncertainty.By introducing the virtual uncertainty module,a general framework of theμ-synthesis control with structured uncertainty is constructed,and theμ-synthesis control problem of current-loop is further summarized.Direct power control strategy has the advantage of fast dynamic response,which is one of the research hotspots in the field of single-phase PWM rectifier control in recent years.To simplify the traditional power feedforward decoupling control structure,proportional integral modified direct power(PI-MDP)control is proposed by the relationship between the coordinate transformation and trigonometric functions.Compared with traditional feedforward decoupled power control,theαβ-dq coordinate transformation and phase-locked loop(PLL)are not needed by this control strategy.To further improve the dynamic performance of power-loop and enhance the robustness of the control system,H_∞modified direct power(H_∞-MDP)control strategy is proposed based on the PI-MDP control structure.It uses the weighting function to constrain the tracking performance and control energy,respectively.Moreover,the generalized state space of the controlled system is derived and the power controller design can then be transformed into an H_∞control problem.The traditional VBC with PI method is easy to be realized and can be extended to the multi-module H-bridge system,so it is widely used.However,this method is not suitable for high-voltage and high-power electric traction systems due to its poor dynamic performance.To enhance the dynamic performance of the dc-side capacitor voltage balancing,first-order transfer function between instantaneous power error and dc-side voltage square difference is revealed in this paper.A new feedback closed-loop system of voltage balancing system is established,and an improved voltage balancing algorithm based on PI control method is proposed.Additionally,to provide a simple application scheme for practical engineering,on the basis of the PI-MDP control structure,internal model control(IMC)strategy is adopted to design the power controller.In the proposed mathematical model of the VBC system,the specifically analysis and design for the disturbance component is not given.A VBC method based on H_∞method is thus presented.In this method,the specific indexes of system tracking performance and anti-disturbance performance can be planned into the generalized control framework through the weighting function.Moreover,considering the grid-side voltage distortion and voltage sampling error,combined with the PI-MDP control algorithm,an improved second-order generalized integration method is proposed to construct virtual signal.Meanwhile,to further reduce the debugging complexity of the voltage balancing controller,based on the proposed voltage balancing mathematical model,the IMC method is applied to the VBC system.Finally,hardware-in-loop platform and scale-down prototype are built to verify the above proposed algorithms.Compared with the traditional algorithms,the effectiveness and correctness of the proposed algorithms are proved.