Control Strategy Of Traction Rectifier Based On Virtual Synchronous Machine And Stability Analysis

PWM rectifier,which has been widely used due to its high power factor and low harmonic content and bidirectional energy flow,is used in high-speed trains to convert AC to DC.Usually,PWM rectifier adopts switching devices which have high response speed and can be flexibly controlled,but switches lack inertia and damping,then the traditional algorithm can not support the traction network and has poor performance when the traction network has faults.In the field of distributed generation,some scholars have proposed virtual synchronous machine,which imitates the external characteristics of synchronous generators,such as droop,inertia and damping characters,to reduce the potential risks and challenges to power system under high permeability of power electronic equipment.A new control strategy of traction rectifier is proposed in this paper,which imitates the external characteristics of synchronous machine,based on the idea of virtual synchronous machine.The proposed method can achieve dynamic balance between source and load,it can support the traction network when the traction network is abnormal and improve the stability of the system.Firstly,the basic principle of virtual synchronous machine is briefly introduced in this paper.The two-level traction rectifier is equivalent to synchronous machine,and the mechanical model,electrical model and electromagnetic model of virtual synchronous machine suitable for traction rectifier are deduced.Aiming at the single-phase topology of traction rectifier,the power calculation of traditional virtual synchronous machine is improved,which is calculated by second-order generalized integral,then a traction rectifier control strategy based on virtual synchronous machine technology is obtained.Secondly,due to the wide attention paid to the stability problem,the harmonic state space model,which is a typical linear time-varying periodic model,and its stability analysis method is introduced in this paper.A detailed model of the proposed control strategy based on virtual synchronous machine technology is derived.The average model of the topology,control loop and closed-loop system in time domain is established,and the average model is linearized near the minimum periodic trajectory,Then the linearized model is transformed into the harmonic state space model in frequency domain to obtain the final harmonic state space model of the system.On top of that,the hardware-in-the-loop test system based on RT-LAB+DSP TMS320F28335 is introduced.The basic functions of the proposed control algorithm,such as the unit power factor operation and the DC side voltage tracking the given value,are verified by the simulation of MATLAB/Simulink and the hardware-in-the-loop test.The characteristics similar to synchronous machines are verified in this paper,the rectifier can autonomously participate the regulation of traction network when the voltage amplitude and frequency of the traction network fluctuates.At last,the stability of the proposed control method is analyzed by means of MATLAB/Simulink simulation,hardware-in-the-loop test and theoretical calculation of the eigenvalue trajectory of the system matrix,which verify the accurate of the harmonic state space model.