Research On Control Strategy Of Three-level DC Converter In Rail Transit Regenerative Braking System

With the rapid development of economy and the rapid advancement of urbanization,many cities in China are carrying out large-scale rail transit construction.But with the development of rail transit,reducing energy consumption has become an urgent problem to be solved.In this context,the energy generated during train braking in rail transit can be recovered and used by the regenerative braking energy(RBE)absorption device,which not only solves the problem of voltage fluctuation of traction network during train braking,but also can recycle the energy and reduce the energy waste.In the regenerative braking system of rail transit in this paper,the main research direction is the super capacitor energy storage type RBE absorption device and Control strategy of converter circuit.Firstly,introduce the super capacitor monomer,and then analyze and design the super capacitor bank;Secondly,the working principle of the converter in the regenerative braking system of rail transit is analyzed,and its characteristics are derived and analyzed.and through the comparison with the two-level bidirectional converter,highlights the advantages of the three-level bidirectional DC-DC converter(TLBD converter),and the parameters of TLBD converter are designed.Then,The core content of this paper is to research on the control strategy of TLBD converter in the system.In the traditional control strategy of TLBD converter,double closed-loop control is the most used and most effective method.so use a super capacitor current of the inner loop,traction network voltage outer loop of the double closed loop control strategy,At the same time,in order to better maintain the stability of the flying capacitor voltage,a control loop of the flying capacitor voltage is also added.In view of the above control method,the system is mathematically modeled by using the state space averaging method to obtain the corresponding transfer function,and then the stability of the double closed-loop control effect are analyzed by using the complex frequency domain Bode graph response method.However,due to the complex parameter setting of the PI controller in the double closed-loop control,referring to the application idea of model predictive control(MPC)in power electronics,the TLBD converter is controlled through the combination of MPC and PI control.This control method can not only reduce the number of PI controllers,but also reduce the difficulty of control,In addition,it also reduces the current ripple of the output inductor,and the control effect is better.For the above-mentioned control method,the prediction model is established through its state-space equation,and then the optimal switching state of the TLBD converter is obtained through appropriate objective function evaluation,so as to realize the optimal control of the system.Finally,Through the simulation test on Matlab simulation software and the experiment on the hardware-in-the-loop simulation platform,the simulation and experiment of the two control strategies of the whole system are completed,The results verify that the two control methods can achieve the corresponding control effect for the whole system,and have certain validity and reliability.Compared with the traditional double closed-loop PI control,the optimized control method has the same control effect,which reduces the number of PI controllers,reduces the amount of calculation,reduces the complexity of control,and reduces the current ripple of the output inductor.