Application of transformer-less UPFC for interconnecting synchronous AC grids

A transformer-less UPFC based on an innovative configuration of two cascaded multilevel inverters (CMIs) has been proposed recently, which is suitable for power flow control between two interconnected synchronous AC grids. The active power as well as the reactive power can be independently controlled by the new transformer-less UPFC. In addition, the semiconductor device ratings (SDPRs) for different power flow control solutions have been investigated. It can be found that cascade multilevel inverter based transformer-less UPFC has much lower SDPR when compared to MMLC based back-to-back HVDC system, indicating significant cost saving when use transformer-less UPFC for power flow control. Experimental results based on 13.8 kV/ 2MVA transformers-less UPFC prototype are shown to validate the theoretical analysis.