Solid State Transformers: A Comprehensive Review of Technology, Topologies, Applications, Research Gaps, and Future Directions

Abstract

Solid-State Transformers (SSTs), or Power Electronic Transformers (PETs), are emerging as transformative components in modern electric grids, capable of intelligent power flow control, AC/DC interfacing, and multi-level voltage regulation. While SSTs promise substantial advantages over conventional Low-Frequency Transformers (LFTs) in terms of compactness, bidirectional power flow, and integration with renewable energy sources and electric vehi-cles, their adoption necessitates a critical reevaluation of grid protection par-adigms and communication infrastructure. Unlike passive LFTs, SSTs con-tribute minimal fault current due to fast-switching semiconductors, challeng-ing conventional protection schemes based on overcurrent detection. Fur-thermore, their deployment requires robust, low-latency communication frameworks to coordinate with utility systems, raising pressing concerns re-garding protocol standardization and cybersecurity resilience. This review advances the state of SST literature by offering a thematic and evaluative per-spective—one that synthesizes converter-level advancements with sys-tem-level integration challenges. Specifically, we critique current SST archi-tectures through a multi-criteria lens involving efficiency, cost, protection compatibility, and fault ride-through, supported by comparative matrices and taxonomy frameworks. A novel contribution of this work lies in identifying the disparity between component-level maturity and system-level readiness, especially in fault isolation, thermal resilience, and coordinated control. Ra-ther than a broad technical survey, this paper adopts a focused perspective on SSTs as enablers of hybrid AC/DC smart grids. It emphasizes key innova-tions—such as advanced modulation for fault limitation, grid-compatible communication protocols, and modular multilevel topologies and maps them against evolving utility requirements. In doing so, we bridge the gap between technical feasibility and operational viability and propose a future research roadmap aligned with practical deployment milestones. The synthesis culmi-nates in a revised classification of SST readiness for distinct grid applications and outlines unresolved technical bottlenecks that warrant targeted investiga-tion.