Research on Variable Frequency Transformer: A Smart Power Transmission Technology

Abstract

The electric power systems are interconnected for the economical and reliable operation of power supply systems. An interconnection of power systems can be implemented through AC and DC links. A variable frequency transformer (VFT) has emerged as an alternative for asynchronous interconnection, like back-to-back HVDC links. Unlike back-to-back HVDC links, VFT requires reduced reactive power, offers quicker incipient transient recovery, and has coherent damping ability. Various control strategies have been proposed in the literature to achieve the controllable and reliable operation of VFT. The VFT can be scalable for high power exchange by implementing the VFT Park model. The VFT is essentially a doubly-fed induction machine (DFIM) based system that needs slip rings and brushes in the rotor circuit. Hence, brushless doubly-fed induction machine (BDFIM) based VFT systems and their various topologies are proposed in the literature. 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. 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. Moreover, a thorough literature review is done concerning VFT configuration, controls, and scaling with research papers and patents. 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.

Control Strategy and Implementation of Seamless Closed-Loop Load Transfer Mobile Prototype for 400 V Distribution Network

“…Based on series compensation technology, the developed SCLT system shows exceptional capacity in achieving both the power supply transferring operation and the power supply restoring operation. In the scenario where the voltage phase angle difference is less than 60°, compared with the back-toback full power conversion scheme [28], the adopted scheme is superior in the power capacity [16]. Moreover, when a fault occurs in the grid, the adopted scheme is more reliable due to the isolating transformer.…”

Electric Power Network Interconnection: A Review on Current Status, Future Prospects and Research Direction

“…Alternatively, the asynchronous interconnection can also be made by the variable frequency transformer (VFT). The VFT is a controllable, bi-directional transmission device that can transfer power between asynchronous power grids [96][97][98].…”

An intelligent cascaded fuzzy‐lead‐lag‐based damping control scheme incorporating variable frequency transformer and superconducting magnetic energy storage in multimachine power system

“…In the work presented, a secure connection between two networks may be established using a novel power flow controller called as variable frequency transformer (VFT) 32 . In addition to supplying regulated power supply in accordance with operator commands, the VFT also provides network operational isolation 33 . In contrast to FACTS devices, VFT improves grid stability by maintaining an ac connection's inherent capacity to respond instantly to sudden disturbances.…”

“…32 In addition to supplying regulated power supply in accordance with operator commands, the VFT also provides network operational isolation. 33 In contrast to FACTS devices, VFT improves grid stability by maintaining an ac connection's inherent capacity to respond instantly to sudden disturbances. Therefore, VFT is a suitable option for connecting a weak network to the main power system because of its intrinsic quality, which also increases the stability of the network and suppresses the LFO.…”