  Local and inter-area oscillations in bulk power systems are typically
identified using spatial profiles of poorly damped modes, and they are
mitigated via carefully tuned decentralized controllers. In this paper, we
employ non-modal tools to analyze and control inter-area oscillations. Our
input-output analysis examines power spectral density and variance
amplification of stochastically forced systems and offers new insights relative
to modal approaches. To improve upon the limitations of conventional wide-area
control strategies, we also study the problem of signal selection and optimal
design of sparse and block-sparse wide-area controllers. In our design, we
preserve rotational symmetry of the power system by allowing only relative
angle measurements in the distributed controllers. For the IEEE 39 New England
model, we examine performance tradeoffs and robustness of different control
architectures and show that optimal retuning of fully-decentralized control
strategies can effectively guard against local and inter-area oscillations.
