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.