Abstract: The negative-resistance effect and open-loop modal coupling explain the mechanism about why the sub-synchronous oscillation may occur in the grid-connected wind power system from two different perspectives. For the sub-synchronous control interaction problem in doubly-fed induction generator（DFIG）-based wind farms interconnected with series compensated networks, this paper compared and analyzed the correlation between impedance interaction and modal interaction. The major contribution of this paper is the qualitative interpretation on the connection of negative-resistance effect and open-loop modal coupling. First, the connection reflects that under the condition of open-loop modal coupling the negative-resistance effect significantly weakens the impedance interaction between subsystems. Secondly, the connection was also recognized that the negative-resistance effect was a necessary condition for closed-loop modal repulsion under the condition of open-loop modal coupling. To improve the adverse impedance interaction caused by the negative-resistance effect, key issues were discussed from the perspective of breaking the open-loop mode coupling condition, such as adjusting the frequency of the open-loop mode, or increasing the damping of the open-loop mode. The mutual complementation and corroboration of the two mechanisms provide guidance for engineering practice.