Research On PWM Technology Used In Power Flow Controller

Considering the problems existed in the electric traction power supply system of electrified railway, such as abundant negative sequence,harmonics and reactive power, and the lost of traction speed and power which are restricted by the neutral section insulator between adjacent power supply sections, a new type of cophase traction power supply system is built by combining YN,vd connection transformer with PWM controlled three-level IPFC(Integrated Power Flow Controller). The system can comb-out three-phase unbalance basically, and provides minimum harmonic content by using a three-level main circuit, thereby, the inconsistency between railway department and electric power department will be solved radically. The remove of the neutral section insulator conform to the trend of the High-Speed Passenger Service and the Heavy Cargo Service.This paper mainly studies the core devices of cophase traction power supply system-the three-level PWM control technology of IPFC, the control strategy determines the performance of overall system. In this paper, the structure of cophase traction power supply system and the performance of the YN,vd balance transformer are firstly discussed, the structure of the double-level IPFC and the control strategy general framework are analyzed, on this basis, a diode-clamped three-level main circuit of IPFC and the high-frequency switching function model are established, the principle of balanced transformation is analyzed, the relationship between instruction currents and load currents of IPFC is gained. Then focused on the three-level PWM control strategy for main circuit and its neutral-point potential balancing control strategy are discussed. The article uses the active integrator with a mean separation method to produce the required instruction current signals of the three-level IPFC. By comparative analyzing five kinds of three-level PWM modulations, then the classic three-level SPWM modulation method is chosen to real-time tracking the compensation instruction signal, a control method of neutral position with voltage pre-feed for the SPWM modulation is combined to control neutral-point voltage balance, and a good balance transform effect is obtained for power flow controller.Finally, the double-level and three-level PWM power flow controller are modeled and simulated under MATLAB/SIMULINK in this paper, the comparative analysis is discussed. The simulation results show the structure of the power flow controller, comprehensive compensation current generating method, SPWM modulation and its neutral-point potential balancing control are proved to be true and feasible. The results also show that the double-level and three-level all have their advantages and disadvantages, when using, it should to be selected according to the actual situation.