Implementation of Hybrid PWM with Smooth Transitions between Modulation Modes for Railway Traction Field-Oriented Control System

High-power medium voltage frequency converters used in railway applications are typically operated at low switching frequency to minimize IGBT switching losses and maintain junction temperature within acceptable limits. Simultaneously, these converters require minimal harmonic distortion of the stator current, as high harmonic content adds additional losses in the induction motor. Achieving that requires specialized pulse-width modulation (PWM) techniques like selective harmonic elimination (SHE) PWM. Combining SHEPWM with closed-loop vector control for high dynamic operation typically involves an observer, however observers underperform at low frequencies. In order to address this issue, the paper proposes a field-oriented control (FOC) method with a hybrid PWM. The main feature of this novel approach lies in the fact that it utilizes both current model and observer, ensuring smooth transitions between modulation regimes. Validation tests on a downscaled laboratory setup have demonstrated its performance in steady-state and transient conditions.