In the last years, a great effort has been paid in
the exploitation of renewable energy resources, an attractive
solution for satisfying the growing energy demand. This is
considered one of the most interesting challenges in the
future for guaranteeing the sustainability and for
dramatically reducing the environmental impact. In this
context, one of the most crucial aspects to take into account,
for effectively allowing the competitiveness of renewable
resources with respect to the conventional ones, is the
improvement of the capability of the existing transmission
systems. In fact, as a consequence of the renewable resource
massive employment, transmission congestions could arise,
compromising the reliability of the overall power system.
At this aim, multiphase power transmission is a natural
candidate to increase the capability of the existing corridors,
avoiding in this way the search for land availabilities for
new overhead transmission lines. A viable alternative
solution to traditional three-phase transmission lines is
provided by four-phase transmission lines. In recent papers
some advantages with respect to the traditional systems
have been discussed. However, some modeling problems
have to be deeply investigated for demonstrating the worth
of employing the four-phase transmission system. More
specifically, the problem of the interface between the threephase
section and the four-phase one is very complex, since
it involves a proper description of the special transformers
required. A detailed mathematical model of the special
transformers and their connections is needed for
understanding the relationship between the symmetrical
components of the two different systems. This is also
indispensable for properly describing the transient stability
margins against large disturbances as short circuits. In the
paper, starting from the equivalent mathematical
representation of the special transformers in terms of nodal
admittance matrix, the authors derive some fundamental
results which could be very useful for extending some
standard methodologies and for properly evaluating the
robustness characteristics of this new kind of transmission
system. More specifically, the proposed methodology allows
to outline the sensitivity of the system to some transformer
parameters and to perform robust transient stability
analysis in spite of semizero and zero sequence transformer
parameters uncertainty.
