Research On Control Strategy Of Throughthrough Multilevel In-phase Power Supply Device

The existing electrified railway mainly uses the traction power supply mode of single-phase power frequency,which has problems such as negative sequence,reactive power,electric phase division and harmonics.In order to balance the three-phase load of the public grid,the single-phase traction network adopts the power supply mode of phase sequence rotation,which will lead to the existence of electric phase division area in the grid,resulting in the loss of the speed and traction force of the electric locomotive,and reduce the operation efficiency and safety of the electric locomotive.At present,passive compensation is mostly adopted,which cannot cancel the electrical phase division and realize the through-through in-phase power supply for the whole network.The rapid development of power electronics technology provides a new development idea for through-through in-phase power supply.Through-through multilevel in-phase power supply device adopts all-power electronic transformation structure to replace the traditional traction transformer,which can completely cancel the electrical phase division,reduce the traction force loss of electric locomotive,and proactively manage the power quality problem.In this paper,a power unit control strategy is studied based on a through-through multilevel in-phase power supply device.The main contents are as follows（1）The through-through multilevel in-phase power supply topology is analyzed.This topology adopts a cascade multilevel structure,consisting of back-to-back H-bridges to form power units,and n power units are cascaded.A single power unit is a single-phase back-to-back H-bridge structure,and the input side of the power unit is the single-phase voltage after three-phase to single-phase splitting.The output of the inverter side cascade can provide the single-phase AC required by the electric locomotive operation,and realize the through-through in-phase power supply of the whole network.The scheme adopts the active governance method to solve the electrified railway power quality problems.Compared with the traditional traction power supply topology,the symmetric structure of the through-through multilevel in-phase power supply device topology can eliminate the negative sequence current,and the AC to DC transformation can isolate the reactive power and negative sequence current between the grid and the grid.Through the detailed harmonic analysis of the power unit,it is concluded that the double frequency disturbance caused by the instantaneous power fluctuation of the DC bus will make the modulation voltage contain odd harmonics through the modulation link,and then transfer to the grid side,resulting the input current i _N containing odd harmonics,thus affecting the power quality of the grid.Similarly,the odd harmonics of the inverter side modulation voltage will also affect the power quality of the inverter side after the cascade.（2）Aiming at the problem of system performance degradation caused by disturbances such as double frequency of the rectifier side of the power unit,the voltage-current feedforward control strategy was studied.Based on the voltage-current double closed-loop control,the three disturbances,namely DC bus voltage v_d,load current i₀ and power unit input voltage v _N,were fed forward to the control system.The voltage and current feedforward disturbance suppression control strategy of single-phase PWM rectifier is obtained to improve the dynamic performance of the system.The simulation model was built in Matlab/Simulink,and the steady state and dynamic simulation experiments were carried out on the voltage and current feedforward control strategy of the power unit.The experimental and simulation results verified the effectiveness of the strategy.（3）The basic structure of the inverter side of the through-through multilevel in-phase power supply device is studied,and the working principle of the single-phase cascade inverter is deduced.The unipolar frequency doubling carrier phase shift modulation mode is adopted to study the output voltage control strategy of the through-through multilevel in-phase power supply device on the inverter side.The single-loop and double-loop control strategies of the output voltage of the voltage source inverter are compared,and the main circuit of the through-through multilevel in-phase power supply system and the inverter side control model are built in Simulink platform.The proposed control strategy is verified by simulation,and the results prove the effectiveness of the dual-loop control strategy for voltage source inverter.（4）Based on the proposed scheme,a hardware-in-the-loop simulation is carried out to verify the feasibility and correctness of the disturbance suppression control strategy for single-phase PWM rectifier and the voltage double-loop control strategy for inverter.