Stability Enhancement of Electric Power Systems by Employing Variable Frequency Transformer

Abstract

Economical and technical advantages inherent in the coordinated operation of power systems resulted in large-scale grid interconnection. The major concern regarding the interconnection of the neighbouring power systems with relatively weak tie-lines is the appearance of poorly damped electromechanical oscillations. Inter-area (IA) oscillations reduce system stability and transmission capacity, and without an efficient damping mechanism, these oscillations can cause system failure and even lead to large-scale blackouts. In this work, the impact of variable frequency transformer (VFT) on the IA oscillations of a two-area power system connected in series with the tie-line is investigated. The VFT is a recent power transmission technology that offers continuous control over bi-directional power flow with no harmonic injection. The VFT is made up of a rotary transformer for continuously regulated phase shift, as well as a drive system and control that regulates the power flow through the VFT by adjusting the angle and speed of the rotary transformer. The VFT is controlled to increase system damping, and consequently suppress inter-area oscillations. A comprehensive model of the interconnected power system including VFT is developed in the MATLAB/Simulink environment to study the efficacy of the proposed scheme.

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

“…Even at standstill, the dc drive system is designed to provide torque in order to transmit power via the VFT. [39][40][41] A VFT may be reduced to the series circuit of an equivalent reactance and an ideal phase shifter in the steady state, without taking into account the influence of VFT circuit resistance. During normal operation, VFT is subjected to frequency and voltage variations caused by disturbances in the two power systems.…”

“…The torque causes the rotor to turn until the power flow demand is met, and the torque direction determines the power flow direction. Even at standstill, the dc drive system is designed to provide torque in order to transmit power via the VFT 39–41 …”