Influence Mechanism Of Electrified Railway Traction Load Connected To Power Grid On The Operation Of Adjacent Wind Farm And Its Control Strategy

With the rapid development of electrified railway and the continuous maturity of wind power generation technology,the impact of electrified railway traction load connected to the grid on the operation of adjacent wind farms has received extensive attention.The single-phase power supply mode of the electrified railway traction power supply system will inject a large amount of negative sequence currents into the power system,and the special transmission mode of the electric locomotive will inject a large amount of harmonic currents into the power system.The negative sequence and harmonic currents will cause power quality problems such as three-phase unbalance and harmonic distortion of the grid voltage,which threaten the safe and stable operation of the power system.Since the wind farms are mostly at the end of the grid,the imbalance and harmonic distortion environment of the grid voltage caused by the electrified railway traction load will threaten the stable and reliable operation of large-scale wind farms.The thesis considers the situation that the electrified railway traction load and the doubly fed wind farm are connected to the same point of common coupling,and the influence mechanism of the electrified railway traction load connected to the grid on the nearby doubly fed wind farm is analyzed.To reduce the negative impact of electrified railway traction load connected to the grid on the doubly fed wind farm,an improved sliding mode control strategy based on railway static power conditioner is proposed,which significantly improves the power quality of the electrified railway traction power supply system.Meanwhile,for improving the operation capability of the doubly fed wind power generation system under unbalanced grid voltage,a coordinated control strategy of the double fed wind power generation system is proposed.Under the premise of ensuring the normal operation of the doubly fed wind turbine,the output quality of the doubly fed wind power generation system is improved.The main research contents and contributions of the thesis are summarized as follows:?1?The simulation models of electrified railway traction load and doubly fed wind power generation system are built,and the influence mechanism of electrified railway traction load connected to the grid on the operation of adjacent doubly fed wind farm is studied.By analyzing the mathematical model of electrified railway traction load,the electrified railway traction load model with the V/v traction transformer and SS_6B electric locomotive is built and simulated based on PSCAD/EMTDC.The main reason for the negative sequence and harmonic currents generated by the operation of the electrified railway traction load is obtained through the theoretical analysis.A simulation model of the doubly fed wind power generation system is built to achieve stable operation of the system under ideal grid voltage conditions.The united model including electrified railway traction load and doubly fed wind farm is established,and the effects of negative sequence and harmonic currents caused by electrified railway traction load on the operation of doubly fed wind farm are obtained through the theoretical calculation and simulation analysis.?2?In view of the power quality problems such as negative sequence and harmonics in V/v traction power supply system,a control strategy based on railway static power conditioner compensating for reference current rapidly and accurately is proposed.Firstly,the generation mechanism of negative sequence and harmonic currents are revealed based on the V/v traction power supply system.Besides,the topology and compensation principle of the railway static power conditioner are analyzed,and the mathematical model for railway static power conditioner is established and the control equation of the compensation current is derived.Then,the control strategy of railway static power conditioner based on improved sliding mode is proposed,which realizes the accurate tracking of the output current of static power conditioner and then improves the power quality of the V/v traction power supply system.The proposed strategy which is proved to be effective by the simulations in PSCAD/EMTDC.?3?For improving the operation capability of the doubly fed wind power generation system under unbalanced grid voltage,a coordinated control strategy of the double fed wind power generation system is proposed.Under unbalanced grid voltage,the stator and rotor currents of a doubly fed induction generator?DFIG?are unbalanced,which influences the safe and stable operation of the machine,and double frequency ripples will appear in the output power of the doubly fed wind power generation system.The operation mechanism of the doubly fed wind power generation system with a series grid side converter?SGSC?is analyzed firstly in this paper,and the control strategy for the SGSC aiming at maintaining the stator voltage balance is proposed,which ensures the balance of stator and rotor three-phase currents.On the basis of the system with a SGSC,the mathematical model of the parallel grid side converters?PGSC?under unbalanced grid voltage is analyzed,and the double frequency power control equation is established.Combined with the double frequency ripples generated by the SGSC,a control strategy based on a reduced-order resonant power compensator for the PGSC is proposed,the goal of simultaneously suppressing the double frequency ripples of the output active and reactive power of the system is achieved.The proposed strategy which is proved to be correct and effective by the simulations in PSCAD/EMTDC.