Power electronic transformers for utility applications

The concept of realizing a small size “solid-state” transformer has been discussed for some time. A fairly straightforward approach to accomplish size reduction in a transformer feeding a conventional rectifier-inverter system is to introduce an isolated DC-DC converter in the DC link, isolation being provided by a high frequency transformer. So also, several topologies that employ AC-AC converters connected on primary and secondary sides of a high frequency transformer to reduce the size and weight of the magnetic core have been reported in literature. Such AC-AC converters need switches with bi-directional voltage blocking and current carrying capability, which are commonly realized with pairs of gate turn-off devices such as insulated gate bipolar transistors (IGBT). This paper explores the possibilities of employing AC-AC switched mode power converters in combination with reactive elements to realize a chopped AC link, thereby decreasing the required magnetic core size for voltage transformation and isolation. A primary advantage of this approach is that, the static power converter needs only six devices to implement a three-phase electronic transformer, which makes it an economical solution. Operating principles, magnetic design and other practical issues are discussed. Detailed computer simulations accompanied with experimental verification are presented in the paper