State of the art of active power electronic transformers for smart gridsRoasto, Indrek; Romero-Cadaval, Enrique; Martins, Joao; Smolenski, RobertIECON 2012 : 38th Annual Conference of the IEEE Industrial Electronics Society : Industrial Electronics for Sustainable Development2012 / p. 5241-5246 : ill https://ieeexplore.ieee.org/document/6389543