This paper presents a modulation and control method for a new transformer-less unified power flow controller (UPFC) that uses an innovative configuration of two cascade multilevel inverters. The control method includes optimized fundamental frequency modulation for low harmonic distortion and high efficiency, independent control of active and reactive power transmission over lines, and dc-link voltage balance control. Experimental results using a 4160V test setup demonstrate the steady-state operation and dynamic response of the new transformer-less UPFC with the proposed control method.

1. MODULATION AND CONTROL OF TRANSFORMER-LESS UPFC
ABSTRACT
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-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. 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 dynamic-response results will be shown in this paper.