Solid state transformer in energy conversion and distribution: a structured design framework, four-dimensional taxonomy, control strategies, and deployment roadmap

• Framework guides solid state transformer topology selection. • Taxonomy classifies solid state transformers by four metrics. • Matrix evaluates design tradeoffs and technology readiness. • Study analyzes control strategies and grid deployment barriers. The proliferation of renewable energy resources is imposing demands on electric distribution infrastructure that traditional low-frequency transformers are not equipped to fulfill. Conventional transformers operate with high energy efficiency but offer zero active controllability with no inherent power quality management and support for hybrid power flows. The solid state transformer (SST), a power electronics-based transformer incorporating medium frequency galvanic isolation, has emerged as a promising solution that enables harmonic suppression, dynamic voltage regulation, reactive power compensation, and interfacing between AC and DC segments of the power grid. Despite strong research interest spanning converter topologies, control strategies, semiconductor devices, and prototype demonstrations, the existing review treats SST in a fragmented manner. The three areas where critical literature gaps persist are: a logical progressive SST design framework that connects fundamental parameters to topology selection, a unified multi-dimensional classification system, and a structured tool for assessing technology maturity and deployment readiness across various SST solutions. This review paper addresses these gaps via five original contributions: (a) a logical SST design framework progressing from degrees of freedom through classes to topology selection, (b) a four-dimensional taxonomy classifying SSTs by integration level, functionality, technology, and application, (c) a structure evaluation matrix assessing design trade-offs, maturity, and stakeholder relevance, (d) a comprehensive review of control strategies and (e) a critical analysis of deployment barriers, alongside seven prioritized future research directions. This review provides a structured, unified, and logically progressive framework for researchers, system designers, and policy makers working in the next-generation energy conversion and management infrastructure.