Independent real and reactive power flow control without sensing receiving end voltage in transformer-less unified power flow controller

Unified Power Flow Controllers (UPFCs) are the most versatile Flexible AC Transmission (FACTS) devices. Yet, they are seldom used for power flow control owing to the high cost and size of the Zig-Zag transformers. Recently, a transformer-less UPFC concept has been introduced that uses cascaded multilevel inverters to match the line voltage levels and the configuration does not need isolation. Thus, both the zig-zag transformers and the isolation transformers can be eliminated from the system. The control and functionality of the series and shunt inverters in this proposed topology are quite different from that of the conventional UPFC. This paper proposes a sensor-less control scheme to achieve independent real and reactive power flow control using the transformer-less UPFC. Typically, UPFCs are installed near the sending end which is far away from the receiving end. This implies that the receiving end voltage cannot be sensed locally. The control scheme proposed in this paper does not need information about the receiving end voltage in order to control power flow. Additionally, it is also made immune to the variations in sending and receiving voltage vectors. Detailed discussion of the control block architecture forms the crux of the paper. The proposed control scheme is also validated by means of numerical simulation.