Modal-based voltage stability analysis of low frequency AC transmission systems

The low frequency AC (LFAC) transmission, in which a power system is operated at a low frequency, i.e., below 50/60 Hz, is superior to the conventional 60 Hz system in terms of power transfer capability. In addition, due to low operating frequency, the line reactance is reduced and thus voltage drops along the line are decreased. The low frequency transmission thus offers a higher voltage profile for a power system. In other words, an LFAC system can be more voltage stable in comparison to the conventional 60-Hz system. This paper intends to focus on the voltage stability of an LFAC system. The theoretical foundations of a two-bus system are first discussed based on eigenvalue. A modified stability index calculation is also introduced for low frequency transmission to estimate the system stability accurately. The simulation results from a practical system verify that the LFAC transmission has great benefits over the 60 Hz system in terms of power transfer capability and voltage stability.