A Three-Phase Unfolding-Based Solid-State Traction Transformer for Railway Application

In this article, a solid-state transformer-based three-phase unfolding powertrain is proposed for railway traction applications. Solid-state transformers are being explored as viable alternatives to line-frequency transformers in railway drivetrains. Conventionally, their back-end employs a low-frequency, two-level pulsewidth modulation (PWM) inverter to drive the motor, which is prone to generating high pulsating torques due to switching frequency constraints. The proposed system, however, eliminates the need for PWM switching in the inverter stage and produces balanced three-phase sinusoidal voltages at the output to drive the motor. This effectively eliminates low-frequency harmonics and reduces torque pulsations. These sinusoidal voltages are achieved by generating pulsating dc voltages at the output of the dc–dc stage, minimizing the need for large dc-link capacitors. The varying voltages are then unfolded by a fundamental frequency switching inverter, which, combined with zero-voltage switching in the dc–dc stage, reduces the switching losses. This article details the operational principle and control strategy of the proposed converter, offering comprehensive insights into its functionality. To validate the effectiveness of the topology, extensive simulations are conducted at a 5-MW power level using MATLAB/Simulink. In addition, the practicality of the proposed topology is demonstrated on a scaled-down experimental prototype with a 5-hp induction motor load.