Abstract This paper presents a high‐power‐density solid‐state transformer (SST) designed for 25 kV alternative current railway applications, delivering a 3 kV direct current output to traction inverters. The SST is composed of sub‐modules with the adoption of 1.7 kV insulated gate bipolar transistor and SiC metal oxide field effect transistor (MOSFET) in an input‐series–output‐series structure, providing higher power density than conventional topologies that rely on high‐voltage switches (>1.7 kV). Because these high‐voltage SiC MOSFETs are still expensive and not fully commercialised, the proposed approach offers a more cost‐effective alternative. A total of 42 converter cells ensure a highly modular and scalable design, with precise synchronisation and high‐speed control achieved through an EtherCAT‐based communication network. Additionally, a distributed control algorithm is introduced, mitigating excessive dependence on the communication link for module‐level operations. The effectiveness of the entire system—including the design and control schemes—has been experimentally verified, ranging from individual converter cells to a reduced SST prototype of up to three sub‐modules. These results confirm the feasibility and advantages of the proposed SST in terms of power density, cost efficiency and reliability for railway traction applications.

Due to the difference between inverter-side and grid-side currents in theLCLfilter, an Active Power Filter (APF) with an Inverter-side Current Feedback (ICF) configuration may struggle to accurately supply harmonic currents caused by nonlinear loads. In a large-capacity APF operating at low switching frequencies (5 kHz or less), the resonant frequency of theLCLfilter is typically designed to be 2.5 kHz or less, which is below the recommended maximum harmonic current order of the 50th(3 kHz) to meet harmonic regulations specified by grid standards. As a result, meeting these grid standard recommendations becomes challenging for a large-capacity APF with ICF configuration. To address this issue, this paper proposed a method for modifying the harmonic current reference based on the transfer function of the difference between inverter-side and grid-side currents, enabling accurate harmonic current supply even with the ICF configuration. Since the proposed method depends onLCLfilter parameters, a detailed performance evaluation accounting for parameter variations was conducted, and the effectiveness of the proposed method was validated. Through simulations and experiments, it was confirmed that applying the proposed method effectively improves both Individual Harmonic Distortion (IHD) and Total Harmonic Distortion (THD).