Modular multilevel high-voltage inverters are core components of rail vehicle traction systems, and their control performance directly determines the energy efficiency and operational reliability of traction drives. The primary objective of this study is to overcome the inherent limitations of traditional voltage model flux observers—specifically waveform distortion and DC component interference—which severely degrade the precision of rotor flux-oriented vector control (FOC) systems. To address this problem, an improved voltage model flux observer with limited compensation is proposed in this paper. The observer introduces Cartesian-polar coordinate transformation and flux linkage amplitude limiting with constant angle in the feedback loop, which effectively eliminates waveform distortion and significantly reduces DC component interference in flux observation. Based on the nonlinear mathematical model of three-phase asynchronous motors, we established both closed-loop and sensorless vector control systems for modular multilevel high-voltage inverters. Systematic simulation verifications were conducted under working conditions of no-load startup, abrupt load change, and motor forward/reverse rotation. Our findings quantitatively demonstrate the superiority of the proposed method: the voltage and current harmonic contents are drastically suppressed, achieving a highly optimized total current distortion rate (THD) of exactly 1.3%. Furthermore, the sensorless control system achieves exceptional speed tracking with a maximum transient error of less than 40 r/min during reversal and negligible steady-state fluctuations. The proposed improved flux observer is well-suited for rotor flux-oriented vector control systems, providing robust theoretical and simulation support for enhancing the energy efficiency of rail vehicle traction systems in high-voltage engineering scenarios.
Li et al. (Fri,) studied this question.