The conventional unified power flow controller (UPFC) that consists of two back-to-back multipulse inverters requires bulky and complicated zigzag transformers for isolation and reaching high voltage/high power. However, the zigzag transformers are usually very expensive, lossy, bulky and prone to failure. Moreover, they are slow (up to minutes to steady state after energizing) in dynamic response due to magnetizing current, and are thus not suited for fast-changing power flow control of intermittent and sporadic wind and solar power application. A completely transformer-less UPFC based on an innovative configuration of two cascaded multilevel inverters (CMIs) has been recently invented. The unique configuration and control of the two CMIs as a power flow controller make it possible to independently control active and reactive power over a transmission line. The new UPFC represents a technological breakthrough and offers several enabling advantages that the traditional technology cannot provide, such as being completely transformer-less, light weight, high efficiency, high reliability, low cost, and possessing a fast dynamic response. This dissertation reveals detailed modeling, control and analysis of the innovative structure of transformer-less UPFC: 1) UPFC power flow control, such as voltage regulation, line impedance compensation, phase shifting or simultaneous control of voltage, impedance, and phase angle, thus achieving independently control both the active and reactive power flow in the line; 2) dc capacitor voltage balance control for H-bridges of both series and shunt CMIs; 3) modulation of the CMI for low total harmonic distortion (THD) of output voltage and low switching loss; 4) fast system dynamic response. The UPFC functionality with proposed control method is verified at 4,160 V.Furthermore, applications of transformer-less UPFC to solve real-world problems are analyzed and experimental verified based on 13.8-kV/ 2-MVA setup. One demonstration is to use transformer-less UPFC for interconnecting two synchronous grids with large phase difference. The proposed transformer-less UPFC can realize grid interconnection control, independent active and reactive power control, dc-link voltage balance control, etc. Furthermore, 1-pu equipment can effectively compensate system with phase difference as large as 30°. Another example is to install the transformer-less UPFC into a congested grid to release the transmission grid congestion. The transformer-less UPFC is able to control bidirectional power flow and make parallel operation possible for two different feeders. Both detailed theoretical analysis and functionality test with proposed control strategy are addressed in the dissertation. The innovative transformer-less UPFC has enormous technological and economic impacts on controlling the routing of energy over the existing power grid. The enabling technology of modularity, scalability makes it easy installation anywhere in the existing grid. Furthermore, the transformer-less UPFC helps maximize/ optimize energy transmission over the existing grids to minimize the need for new transmission lines. As a result, this will increase the transfer capability of the grid, combined with the controllability and speed of operation of the devices, thus will enable increased penetration of renewables and demand response programs. Finally, it will reduce transmission congestion and increase dynamic rating of transmission assets.
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