Research On Optimal Control For Flexible Traction Power Systems Of Urban Rail Transit

As an electric energy-driven,fast and convenient way of public transportation,urban rail transit has played an important role in improving the environment and alleviating traffic congestion.With the rapid increase of domestic operation mileage and higher electricity consumption,energy conservation and emission reduction as well as improving the level of intelligence becomes critical to the sustainable development of urban rail transit system.However,in the traditional uncontrolled diode based traction power system,the diode rectifier has become a major obstacle to the bidirectional free flow of energy between the AC medium voltage ring network and the DC contact network,and it cannot regulate and control the AC side power factor and the DC side voltage.This paper introduces a voltage source converter(VSC)based on pulse width modulation(PWM)in the traction power system of urban rail transit,upgrading the traditional traction substation to a bidirectional substation,and builds a flexible AC/DC traction power system.This paper has conducted thorough research on three key technologies including DC impedance reconstruction,distributed reactive power compensation,and AC/DC optimal power flow in order to achieve comprehensive system optimization and energy efficiency improvement.A virtual impedance technology is proposed to reconstruct the output impedance of the DC system to improve the energy efficiency and stability.Aiming at the problem of DC system regenerative braking energy distribution,a virtual resistor is constructed through droop control,so that regenerative braking energy can be absorbed by multiple tractions,and the utilization rate of regenerative braking energy in the AC power grid is improved.The VSC DC small signal model is established,considering the large-scale variation of the steady-state operating point and the influence of the filter inductance energy storage,the optimal controller parameters are designed to improve the stability of the VSC control system;To solve the oscillation problem when the train runs in the constant power region,the characteristic root distribution and stability law of the system is analyzed based on the space state equations approach,and the high-frequency impedance is built through the DC current linear feedback,which improves the stability of the DC system.Simulation results show the effectiveness of droop control and damping control.A distributed reactive power compensation method in traction power systems is proposed,which utilizes the VSC in traction substations for distributed reactive power compensation to replace SVG in the main substation in order to reduce equipment investment cost and simplify system structure.The steady-state characteristics of the AC system are analyzed,and on this basis the detailed implementation of distributed reactive power compensation is presented: the load is time-varying during line service time,and the power information is collected by the control center in the main substation to generate the reactive power adjustment amount through the closed loop control.And it is sent to the traction substations to realize dynamic compensation via communication system;during non-service time,the load is relatively stable,there has large amount of capacitive reactive power generated by the power cable,and the optimal compensation parameters are generated based on the offline optimization of the power flow model to achieve the minimum transmission losses.In addition,as the the VSC operating range is constrained by the DC voltage,an over-modulation suppression strategy is proposed.The over-modulation caused by the low DC voltage condition is suppressed by adjusting the VSC reactive power,and the system performance is improved.The effectiveness of the distributed reactive power compensation method is verified by simulations.An optimal power flow control method for flexible traction power systems is proposed to reduce the overall energy consumption of the system.The coupling model of the AC/DC power system including VSC is established,and the multi-trains dynamic operation model is established according to the train power characteristics.Considering the DC voltage droop control,the power flow distribution of the AC/DC coupling system is analyzed,and the optimal parameters of the no-load voltage and equivalent internal resistance of the traction substation are calculated,and the comprehensive energy consumption of the flexible traction power system is minimized,and the train electrical braking capacity is maximized.The validity of the proposed method is verified by actual subway line simulation examples.