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In this paper, a modulation and control method for the new transformer-less unified power flow controller (UPFC) is presented. As is well known, the conventional UPFC that consists of two back-toback 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. The new UPFC offers several advantages over the traditional technology, such as transformer-less, light weight, high efficiency, low cost and fast dynamic response. This paper focuses on the modulation and control for this new transformer-less UPFC, including optimized fundamental frequency modulation (FFM) for low total harmonic distortion (THD) and high efficiency, independent active and reactive power control over the transmission line, dc-link voltage balance control, etc. The new UPFC with proposed control method is verified by experiments based on 4160 V test setup. Both the steady-state and dynamicresponse results will be shown in this paper.Index Terms-Flexible ac transmission systems (FACTS), unified power flow controller (UPFC), transformer-less, multilevel inverter, power flow control.

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