This continuously-varied phase shift can cause miss operation for the current-based differential protection relay by generating false trip signal. Phase-shifting transformers (PST) are exhibited to continuous variations in phase angle. Furthermore, detailed mathematical modeling of the VFT system is also carried out along with the numerical simulations under both steady-state and various fault conditions using PSCAD/EMTDC software. Moreover, a thorough literature review is done concerning VFT configuration, controls, and scaling with research papers and patents. A comparison of VFT with its counter technology back-to-back HVDC link is also elaborated, which suggests VFT a better option for asynchronous grid interconnection. In this paper, a comprehensive review is carried out, which includes: power system networks interconnection philosophy, power flow controllers used in the interconnected networks, FACTS controllers, and HVDC link with their technical merits and limitations. Hence, brushless doubly-fed induction machine (BDFIM) based VFT systems and their various topologies are proposed in the literature. The VFT is essentially a doubly-fed induction machine (DFIM) based system that needs slip rings and brushes in the rotor circuit. The VFT can be scalable for high power exchange by implementing the VFT Park model. Various control strategies have been proposed in the literature to achieve the controllable and reliable operation of VFT. Unlike back-to-back HVDC links, VFT requires reduced reactive power, offers quicker incipient transient recovery, and has coherent damping ability. A variable frequency transformer (VFT) has emerged as an alternative for asynchronous interconnection, like back-to-back HVDC links. An interconnection of power systems can be implemented through AC and DC links. The electric power systems are interconnected for the economical and reliable operation of power supply systems.

In this study, we propose the construction of an asymmetrical controllable phase-shifting transformer in order to achieve flexible control. These features must be considered, taking into account power system conditions such as the voltage variability profile and active power transfer demand. The symmetrical unit had the lowest power losses and a stable output voltage level compared to the asymmetrical one, which increased the output voltage, while the quadrature voltage also grew. All phase-shifting transformer types had a significant impact on the transmitted active power, but each type had different features. Simulation results were then used to compare the electrical properties and efficiency of all the types of phase-shifting transformer considered. Laboratory transformer parameter data were used to prepare asymmetrical and symmetrical phase-shifting transformer simulation models. Research was performed on a laboratory phase-shifter unit with longitudinal and quadrature voltage regulation, and then these results were correlated to the simulation model equivalent. This study is therefore focused on the efficiency question with respect to electrical energy parameters. However, the efficiency of phase-shifting transformers has not been analyzed, especially not with regard for their various types. Their positive influence on power flows has been proved in the literature. Phase-shifting transformers are effective elements used to control power flows in many power systems. The ACPST also allowed adjusting longitudinal and quadrature voltages therefore, this PST type can be installed in places where high flexibility is required, especially in systems with high renewable energy sources penetration. This dependency is a result of the greater ACPST angle compared to the classical APST. ACPST achieved the same power transfer for the lower output PST voltage, which is its main advantage. APST construction always resulted in higher output voltage than the input one. Based on the ACPST simulation model, the APST model was built for the given work conditions, the influence of both PSTs were compared. This allowed verifying the results of the real life and computer simulations. The parameters of the MATLAB model were then adjusted to parameters of the real life PST model. Then the 30A laboratory PST device was built in the laboratory. The MATLAB simulation model of the ACPST was built to obtain influence of PST in selected models for selected parameters. In this manuscript, the results of the asymmetrical PST (APST) with the in-phase PST (called asymmetrical controllable PST-ACPST) were compared, allowing to control both longitudinal and quadrature voltage. Phase shifting transformers (PSTs) are currently widely used in power systems to control power flow.

Phase shift controller setup