Operation and analysis of an improved transformerless unified power flow controller

In this paper, operation and analysis for an improved transformer-less unified power flow controller (UPFC) is presented. As is well known, the conventional UPFC that consists of two back-to-back inverters requires bulky and often complicated zigzag transformers for isolation and reaching high power rating with desired voltage waveforms. To overcome this problem, a completely transformer-less UPFC based on an innovative configuration of two cascade multilevel inverters (CMIs) has been proposed in [1]. Although the new transformerless UPFC offers several advantages over the traditional technology, such as transformer-less, light weight, high efficiency, low cost and fast dynamic response, its performance has been limited for some operation points and in some scenarios, the rating of the transformer-less UPFC suffers a lot. In light of this, an improved transformer-less UPFC is proposed in this paper. The benefits of the improved UPFC include, besides the advantages of transformer-less UPFC already possessed, same functionality of original structure, no more hardware needed, more flexible operation and less converter rating. This paper presents rating analysis and operation principle for this new transformer-less T-shape UPFC (TUPFC).