Transformer-less Unified Power Flow Controller Using the Cascade Multilevel Inverter

The conventional unified power flow controller (UPFC) that consists of two back-to-back inverters requires bulky and often complicated zigzag transformers for isolation and reaching high voltage. This paper proposes a completely transformer-less UPFC based on an innovative configuration of two cascade multilevel inverters (CMIs). The unique configuration and control of the two CMIs as a power flow controller make it possible to independently control active and reactive power flows over a line. The new UPFC offers several advantages over the traditional technology, such as transformer-less, light weight, high efficiency, high reliability, low cost, and fast dynamic response. The transformer-less UPFC is thereby very suited for fast and distributed power flow control, such as wind and solar power transmission. A simulation model is built to demonstrate the operating principle of the proposed transformer-less UPFC.