Decoupled closed-loop power flow control for the controllable network transformers (CNT)

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. FACTS 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 power. 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 Sources (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 coupling effect between controlling active and reactive power. To overcome this coupling limitation, a fully decoupled closed-loop controller for a CNT to achieve independent active and reactive power control is derived and tested in this paper.