  Wide-area dynamic studies are of paramount importance to ensure the stability
and reliability of power grids. The rising deployment synchrophasor and other
sensing technologies has made data-driven modeling and analysis possible using
the synchronized fast-rate dynamic measurements. This paper presents a general
model-free framework of inferring the grid dynamic responses using the
ubiquitous ambient data collected during normal grid operations. Building upon
the second-order dynamic model, we have established the connection from the
cross-correlation of various types of angle, frequency, and line flow data at
any two locations, to their corresponding dynamic responses. The theoretical
results enabled a fully data-driven framework for estimating the latter using
real-time ambient data. Numerical results using the WSCC 9-bus system and a
synthetic 2000-bus Texas system have demonstrated the effectiveness of proposed
approaches for dynamic modeling of realistic power systems.
