Dynamic Grid Power Routing Using Controllable Network Transformers (CNTs) With Decoupled Closed-Loop Controller

Increases in system loads and in levels of penetration of renewable energy, together with limited investment in transmission infrastructure, are fostering the need for a smarter and more dynamically controllable grid. Flexible ac transmission systems devices can be used to dynamically control the grid and more efficiently route power and thus mitigate these stresses, but such devices are either too complicated and expensive for implementation or incapable of independently controlling active and reactive powers. A controllable network transformer (CNT) has a fractionally rated direct ac/ac converter and was introduced as a simpler and more cost-effective solution to realize dynamic power control between two areas. The CNT utilizes the dual virtual quadrature source (DVQS) technique to change both the line voltage amplitude and phase angle, thus enabling a dynamic power control; however, the control variables defined in this technique have a cross-coupling effect between active and reactive powers. In this paper, the CNT operating ranges with and without considering line resistance are analyzed; then, a decoupled closed-loop controller is designed to achieve independent active and reactive power control based on a reference power control command. To address the possibility of power overshoot in a CNT with DVQS, a hybrid open-loop/closed-loop proportional-integral controller is also proposed. Simulations and experimental results are given to verify the controller design.