Research On Optimized Control Methods For Grid-side Converter In Railway Electric Traction

In recent years,China's high-speed railway has experienced a rapid development,and the“four vertical and four horizontal” network has been completed.As of the end of 2019,China's high-speed railway ranked first in the world with a total mileage of 35000 kilometers.As a landmark equipment of high-speed railway,the electrical multiply units(EMUs)operate in extremely complicated conditions,such as traction,breaking,starting and stopping,frequent acceleration and deceleration,which results in an extremely severe operation environment for its electric traction drive system.Grid disturbance,load step change and electromagnetic interference challenge the control strategies of the gird-side converter.Thus,in order to improve the steady-state control precision and dynamic response of the grid-side converter,this paper studies the direct power control(DPC),model predictive power control(MPPC)and grid voltage sensorless control for single-phase two-level pulse-width modulation(PWM)rectifier,and the capacitor voltage balancing control for single-phase cascaded H-bridge rectifier.Specific contents are as follows:Firstly,the operation principle and mathematical model of single-phase two-level PWM rectifier are analyzed.On the basis of the instantaneous power theory of single-phase system,the power mathematical model of the adopted rectifier is established,which lays the foundation for the research of control methods in the following chapters.Then,the basic principles of conventional table-based DPC and power feedforward decoupling DPC schemes are analyzed in detail.Combining the features of these two DPC schemes,a constantfrequency-hysteresis DPC scheme is proposed in this paper,which can achieve constant switching frequency while retaining the fast dynamic response of hysteresis controllers.The principle of conventional finite-control set MPPC and MPPC with power error minimization are analyzed in detail.And the essential relationship between MPPC with power error minimization and predictive power control is presented.Then,in terms of the terminal trajectory of modulated voltage vector,a MPPC scheme with elliptic trajectory optimization of the modulated voltage vector is proposed in this paper.To solve the parameter sensitiveness problem of MPPC schemes,a quantitative relationship between the predicted powers and their references considering inductance mismatch is analyzed in detail.And an inductance on-line estimation method is developed according to the analysis.In order to realize grid voltage sensorless control,a grid voltage estimation method with model reference adaptive system(MRAS)is proposed in this paper.According to the basic principle of MRAS,the mathematical expressions of active and reactive powers in reference model and adjustable model are modeled.Then,the amplitude,frequency,and angle information of the fundamental component in grid voltage can be estimated.With the combination of the grid voltage estimation method and DPC scheme with power feedforward decoupling,the grid voltage sensorless control strategy with MRAS is presented.In order to tackle the inductance sensitivity of the proposed scheme,an inductance on-line estimation method is developed.Simultaneously,to solve the startup problem of control system,a system parameter initialization method is presented,which can realize the smooth transition of the adopted system from uncontrolled mode to PWM mode.For the single-phase H-bridge rectifier,its operation principle and mathematical model is introduced.And the whole control system of CHB rectifier is divided into two layers.The upper layer control system only controls the grid current and total dc voltage or average voltage,while the lower layer control system regulates these capacitor voltages to be balanced among H-bridges.Then,the coupling effect of voltage balancing control on the upper layer control system is analyzed in detail,and the expressions of duty cycle modifications to eliminate the coupling effect is presented.Based on the H-bridge power model,a quantitative solution for reactive power balancing is presented to design the reactive duty cycle modifications.To avoid the use of PI controllers in individual voltage closed-loop,and enhance the dynamic performance of voltage balancing control,a dynamic references design method is adopted to evaluate the active duty cycle modifications.According to the above analysis,an improved capacitor voltage balancing method is proposed by designing the active and reactive duty cycle modifications of each H-bridge.Due to the limitation of the modulation index,the effective voltage balancing area of the proposed method is analyzed,and the constraint of the active duty cycle modifications is presented.To verify the effectiveness and correctness of those proposed methods,simulations and experiments are carried out on Matlab/Simulink environment and scale-down experimental platform,respectively.