  Railway power systems operating at a nominal frequency below the frequency of
the public grid (50 or 60 Hz) are special in many senses. One is that they
exist in a just few countries around the world. However, for these countries
such low frequency railways are a critical part of their infrastructure.
  The number of published dynamic models as well as stability studies regarding
low frequency railways is small, compared to corresponding publications
regarding 50 Hz/60 Hz public grids. Since there are two main type of low
frequency railways; synchronous and asynchronous, it makes the number of
available useful publications even smaller. One important reason for this is
the small share of such grids on a global scale, resulting in less research and
development man hours spent on low frequency grids.
  This work presents an open model of a (synchronous-synchronous) rotary
frequency converter for electromechanical stability studies in the phasor
domain, based on established synchronous machine models. The proposed model is
designed such that it can be used with the available data for a rotary
frequency converter.
  The behaviour of the model is shown through numerical electromechanical
transient stability simulations of two example cases, where a fault is cleared,
and the subsequent oscillations are shown. The first example is a single-fed
catenary section and the second is doubly-fed catenary section.
