The presence of harmonic currents as a result of switching action of inverters, intermittent sunlight, and resonances triggered by the connection of the solar photovoltaic plant to the electrical network has spread the realization of the prospective rapid depletion of a transformer's intended service lifetime owing to increased service losses and temperature rise in the active components. On the grounds of the supposed prospect of reduced transformer service lifetime under harmonic conditions, there is a growing interest by manufacturers to explore transformer design procedures that enable transformers to operate reliably under harmonic conditions. This book examines such procedures, highlighting the challenges involved in optimizing transformer performance.