We use λ for flux linkages, v for line-to-neutral voltages, i for currents, r for winding resistances, X for reactances, ω for angular frequencies, and δ for angles against the infinite bus. The subscripts d and q denote the direct axis and the quadrature axis, respectively, subscript a denotes the armature (stator), subscript r denotes the rotor, subscript f denotes the filed, and subscripts kd and kq denote the direct axis and the quadrature axis, respectively, for the dampers.