  The magnetoquasistatic simulation of large power converters, in particular
transformers, requires efficient models for their foils windings by means of
homogenization techniques. In this article, the classical foil conductor model
is derived and an inconsistency in terms of circuit theory is observed, which
may lead to time-stepping instability. This can be related to the
differential-algebraic nature of the resulting system of equations. It is shown
how the foil conductor model can be adapted to mitigate this problem by a
modified definition of the turn-by-turn conductance matrix. Numerical results
are presented to demonstrate the instability and to verify the effectiveness of
the new adapted foil conductor model.
