Study On Digital Control For Railway Traction Inverter

Traction converter plays an important role in railway AC drive system, which directed affectes the performance of vehicle. The rail traction's capacity is larger, the switching frequency is low, and works at poor conditions, so there are many difficulties in the control. However, as a long time, the power converter and core control technologies are stillrelied on importing in our country, which significantly restricts further development of rail transportation. On other hand, due to its flexibility, repeatability and reliability, etc, digital control has replaced the analog control in the field of traction control. Therefor, this article focus on the several important issues in digital control of traction converter in order to improve the transient and steady performance of the traction drives system. Simulations and experiments demonstrate the correctness of the research results. Following are the major researchworks of this dissertation.Firstly, in field oriented control system of traction motor, there are two important factors (the rotor flux amplitude and orientation angle) that restricting the torque output performance while the variation of traction motor parameters will decline the control performance, so how to improve the vector control robustness of the system is in-depth researched. Therefore, the closed-loop control of flux and the q-axis flux model orientation angle correction algorithms are introduced based on the traditional indirect vector control in this paper, and some issues of the strategy are also discussed. What's more, two kinds of the strategy of traction motor control at neutral section are discussed in order to improve system robustness.Sencondly, the current loop digital control strategy is discussed, in order to improve the transient performance of current loop. The voltage coupling problem of vector control system is thoroughly analyzed and the complex vector PI controller is introduced to destroy the decoupling. And the different digital realized method of complex vector PI controller are discussed and compared. On this basis, the digital delay is in-depth discussed; the delay compensation strategy and a new PWM update algorithm are presented to further eliminate delay on the system.Finally, in order to improve the harmonic content at the low-carrier ratio, the optimized pulse width modulation strategy for traction converter application is discussed. The selective harmonic elimination PWM technique and harmonic current RMS minimum PWM method are studied, in particular, the calculation of harmonic current RMS minimum PWM method is showed and the comparing of two methods is explained in detail. And then, the digital beatless control compensating the fluctuating DC link voltage for AC-DC-AC converter is discussed, using the repetitive forecast and the sampling compensation method; finally, the transition method between different PWM modes is proposed.