Distributed controllers have a ubiquitous presence in the electric power grid and play a prominent role in its daily operation. The failure or malfunction of distributed controllers is a serious threat whose mechanisms and consequences are not currently well understood and planned against. For example, if certain controllers are maliciously compromised by an adversary, they can be manipulated to drive the power system to an unsafe state. We seek to develop proactive strategies to protect the power grid from distributed controller compromise or failure. This research formalizes the roles that distributed controllers play in the grid, quantifies how their loss or compromise impacts the system, and develops effective strategies for maintaining or regaining system control. Specifically, an analytic method based on controllability analysis is derived using clustering and factorization techniques on controller sensitivities.