| At present,the on-board transformer of electric locomotive mostly adopts industrial frequency transformer,which has the advantages of simple manufacturing process,low price and high reliability.However,it also has the disadvantages of large volume and weight,high loss during no-load operation,easy to cause grid voltage and current distortion,and can not meet the needs of rapid and lightweight electric locomotive.With the development of high-power electronics and conversion technology,power electronic traction transformer(PETT)is proposed.It not only has the function of conventional transformer,but also has the edge of high efficiency,light weight,low environmental pollution and high power quality.Thus PETT is the future development trend of traction locomotive.However,PETT has the characteristics of constant power load,saturation nonlinearity,poor overvoltage and overcurrent capacity.In addition,its working conditions are complex during the operation process and may suffer various large or small disturbances.Thus a variety of stability and overvoltage problems will happen when PETT interacts with the traction network.First,when multiple locomotives are connected to the grid at the traction station simultaneously,low-frequency oscillation may occur due to the impedance mismatch between locomotives and traction network.Second,when the input voltage of the locomotive drops,the DC voltage may be unstable due to the power limitation of the converters of locomotive,and the system cannot automatically return to normal operation even after the fault is removed.Third,when the locomotive passes through the neutral section,the breakers switch on load,which is easy to cause overvoltage at the neutral section and input terminal of locomotive.These problems will cause malfunction of protection system,failure of Electric Multiple Units(EMU)converter,traction substation overvoltage tripping,burning of traction substation charging module and EMU DC side capacitor,and other accidents,which will seriously endanger the safety of locomotive and power grid.Therefore,it is of great significance to study the key technologies of stability control and overvoltage suppression of power electronic train-network coupling system under different working conditions according to the characteristics of PETT,which can provide theoretical support for the stable and safe operation of locomotive.The research contents of this paper are as follows:(1)Aiming at the lack of accurate mathematical model of power electronic train-network coupling system in the existing literature,a unified impedance model of PETT based on extended description function method is proposed and verified.Firstly,the unified impedance of LLC resonant converter is established and simplified at low frequency.Then,using the simplified model of LLC converter,combin ing the front cascade H-bridge and its control,the overall impedance of PETT is deduced.Furthermore,when the parameters difference of cells in PETT is small,it can be simplified as a single-phase rectifier(i.e.equivalent model of conventional locomotive)through equivalent transformation,which shows that conventional locomotive and PETT locomotive are essentially the same.Finally,the correctness of the model is verified by impedance measurement in the simulation.The derived impedance of PETT locomotive,combining the equivalent impedance of traction network,can be easily used to analyze the low-frequency stability of train-network system accurately,and lay a foundation for the mechanism analysis of system low-frequency oscillation.(2)In order to analyze the role of traction network and electric locomotive in system low frequency oscillation and reflect the interaction between them intuitively and clearly,an RLC equivalent circuit model is proposed in this paper.Firstly,the negative resistance and capacitance characteristics of electric locomotive are analyzed.By impedance transformation and circuit equivalence,multi-input multi-output(MIMO)locomotive impedance is converted into single-input single-output supersynchronous and subsynchronous impedance.Then,the RLC circuit model is established based on the system supersynchronous impedance.By analyzing the equivalent impedance parameters and resonance of RLC circuit,the effects of traction network and locomotive on the low-frequency stability of the system are studied.In addition,the influences of parameters of traction network and locomotive on low-frequency oscillation are also studied.Finally,the analysis results are verified by simulation and Hardware-in-the-loop(HIL)simulation.Compared to MIMO impedance model,the proposed RLC circuit model can not only explain the LFO phenomenon in high-speed train-network system intuitively and clearly,but also simplify the stability calculation process.(3)In view of the inaccuracy of locomotive transient stability region and lack of stability improvement method under network voltage dips,a detailed discrete iterative model of the system is established,and a current feedforward method is proposed.Firstly,the transient process and cause of the locomotive voltage instability when the network voltage drops are analyzed.Then,considering the main circuit,control loop and over modulation of the system,a detailed discrete iterative model of train network system is established.By using this model,the system transient response of the train,key parts and stability region under different parameters are analyzed.Then,the large signal stability boundary of the system is analyzed based on the mixed potential function,and a current feedforward method is proposed to improve system stability.Finally,the correctness of the model and the effectiveness of the improvement method are verified by simulation and HIL simulation.(4)Aiming at that analysis of the influential factors of overvoltage in the neutral section during the process of auto-passing neutral section is unclear and the cost and reliability of the suppression method cannot be taken into account meantime,the transient process of auto-passing neutral section and the influences of traction network,neutral section and locomotive on the overvoltage ar e systematically studied,and an active-passive composite suppression method is proposed.Firstly,the transient process of auto-passing neutral section and mechanism of overvoltage are analyzed.Then,based on the system equivalent circuit,the effects of parameters of neutral section and locomotive on overvoltage are analyzed in detail,including capacitance of neutral section,power,ac side inductance,control method and parameters of locomotive.In addition,by analyzing the characteristics of overvoltage in the neutral section and the system influential parameters,an active-passive composite suppression method combining passive absorption circuit and active suppression control of locomotive is proposed,which can achieve low cost and improve the reliability of suppression of overvoltage.Finally,the correctness of theoretical analysis and the effectiveness of suppression method are verified by simulation and HIL simulation. |
