Background. Many of the modern Russian and foreign studies analyze internal faults in power transformers using simulation models with complicated matrix expressions requiring exact design data of transformers (that is often unavailable). A problem that should be addressed, therefore, is obtaining generalized simplified models of transformer analytical based on their rated values.
Materials and methods. The simplified expressions have been formed based on the model of a three-phase transformer represented as a set of mutually coupled coils using an L-shaped equivalent circuit for two-winding transformers with «wye-wye» and «wye-delta» connections to replace the mutually coupled coils.
Results. Simplified analytical formulas for two-winding «wye-wye» and «wye-delta» transformers for relay protection purposes have been formed. The regularities of changes in electrical values have been analyzed.
Conclusions. The obtained generalized analytical formulas can be used to estimate relay protection algorithm behavior (in particular, their sensivity and selectivity) in case of internal faults in power transformers using their rated values. The same procedure can be applied to other transformer types.