  The concept of complex frequency has been recently introduced on the IEEE
Transactions on Power Systems to study bus voltage variations in magnitude and
frequency and their link with complex power injections of a power system. In
this paper, the complex frequency is applied to time-varying series
connections, namely, RLC dynamic branches, regulating transformers and AC/DC
converters. The proposed modeling approach allows deriving an explicit
expression for the complex frequency of the voltage of a certain bus as a
linear combination of three elements: net current injected by the devices
connected to the bus, adjacent voltages, and time-varying series branches. The
proposed formulation unifies the link between voltage and frequency dynamics in
AC, DC, as well as hybrid AC/DC power systems. A variety of static and dynamic
examples are presented to show the potential of the proposed formulation.
Relevant applications of the proposed modeling approach are outlined.
