| With the rapid development of rail transportation in our country,a large number of Electric multiple units(EMUs)adapting to practical needs have been put into operation.The standard EMU spectrum represented by "Fuxing" is developing towards the light weight,the integration of main and auxiliary.In recent years,the country has vigorously built the railway source-network-vehicle-storage integration system.In the integrated system,the stability of the most basic and important component of the EMU must be guaranteed.However,it has been recorded that the intermediate DC link voltage of EMU has been oscillating and unstable for many times during the static adjustment test in the main engine plant.Voltage oscillation instability will not only affect the safe and stable operation of the EMU itself,but also pose a serious threat to the normal operation of the EMU-traction network coupling system.Therefore,this thesis carries out theoretical research on the oscillation instability of DC link voltage in the middle of EMU traction converter system during the in-plant test,with the overall goal of revealing and suppressing the oscillation instability of DC side voltage in traction converter system of long series "Fuxing" EMU.Based on the study of the generation rule,physical mechanism and stability analysis of the DC side voltage oscillation instability problem of the four-quadrant converter,a feasible suppression scheme adapted to the current EMU design is proposed.The main work of this thesis can be summarized as follows:(1)The internal frequency domain impedance model of traction converter system is established.Considering the voltage loop controller,current loop controller,system delay and other modules,the output impedance model of traction converter system in dq coordinate system and the load input impedance model of EMU under different load power are established,which provides a theoretical means to analyze the voltage oscillation and system stability caused by the internal coupling of traction converter system.(2)The impedance model of EMU-traction network coupling system is established.Considering the modules of voltage loop controller,current loop controller and system delay,the output impedance model of traction network side and the input impedance model of EMU side under synchronous rotating coordinate system are established,which provides a theoretical means for the analysis of the voltage oscillation and system stability caused by the coupling of EMU and traction network.(3)The mechanism of voltage oscillation instability and the influence of different factors on the system stability are revealed.The voltage oscillation instability in the static test in the factory is caused by the constant power load such as the traction inverter of the EMU and the negative damping characteristic of the EMU when it is connected to the traction network.Due to the negative damping characteristic of the constant power load and the influence of the secondary resonant circuit,the instability of the coupling system inside the EMU may result in voltage oscillation,which is directly manifested in that the equivalent impedance ratio does not meet the relevant stability criterion,resulting in the insufficient stability margin of the system.When the loop gain of the system composed of the output impedance of the traction network and the input impedance of the EMU does not satisfy the generalized Nyquist criterion the risk of voltage oscillation and instability at the coupling point of the vehicle network coupling system is high.Based on the small signal average model established in synchronous rotating coordinate system,the instability mechanism of the system can be analyzed,and then the influence rules of different factors on the system stability can be analyzed according to the negative damping expression of EMU input impedance.(4)A voltage oscillation suppression method based on virtual impedance control strategy is proposed.The key to restrain voltage oscillation instability is to change the input and output characteristics of EMU traction converter system equivalent.For the voltage oscillation instability caused by the internal impedance mismatch of EMU traction converter system,a virtual resistance control strategy including compensation is designed.The virtual resistance can effectively adjust the output impedance of the four-quadrant converter.Furthermore,in order to prevent the steady-state error caused by current feedforward from increasing,a compensation link using integral controller is designed to make the DC side voltage track the given value effectively.For the voltage oscillation instability of the DC side caused by impedance mismatch of the vehicle network coupling system,an active damping control strategy with the introduction of band-pass filtering is designed,which can effectively adjust the input impedance of the EMU side and make the system loop gain meet the stability criterion.Among them,the introduction of bandpass filter is to select the frequency range in which the virtual impedance is applied and reduce the influence of the proposed control strategy on the dynamic performance of EMU in other frequency bands.Considering the practical engineering application,the above two strategies are combined and some parameters are optimized.Simulation and experimental results show that the proposed control strategy can effectively suppress the oscillation instability of intermediate DC link voltage,which provides a solution to enhance the stability of EMU and vehicle network coupling system. |
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