We study the impact of coupling between the communication and the power networks as it affects a SCADA-based preventive control system. Today power grids use power lines to carry control information between components in the grid and a control center using power line carrier communication (PLCC). Thus a failure in the power grid will cause a failure in the control network and may reduce the capability of preventive control that in turn increases the risk of cascading failures. We pose the problem of allocating a limited number of non-PLCC communication links (e.g., microwave links) that are immune to failures in the power grid to maximize our controllability over the grid under power system failures, so as to maximize the total demand served at the end of cascade. By formulating the problem as a nonlinear integer programming problem, we establish its hardness and identify a generic heuristic that can find an approximate solution within controllable time. We further develop a domain-specific heuristic that utilizes both graph-theoretic and power system information to achieve similar performance as the generic heuristic at a much lower computational complexity. Our evaluations based on a 2, 383-bus Polish system demonstrate that only a few non-PLCC links, when placed correctly, can substantially improve the robustness of the grid as measured by the total demand served at the end of cascade.