  The shifted frequency-based electromagnetic transient (SFEMT) simulation has
greatly improved the computational efficiency of traditional electromagnetic
transient (EMT) simulation for the ac grid. This letter proposes a novel
interface for the co-simulation of the SFEMT model and the traditional EMT
model. The general form of SFEMT modeling and the principle of analytical
signal construction are first derived. Then, an interface for the co-simulation
of EMT and SFEMT simulation is proposed based on rotational invariance
techniques. Theoretical analyses and test results demonstrate the effectiveness
of the proposed method.
