Certain sophisticated boundary conditions offer powerful modelling features for many
electrotechnical models. Their application, however, wipes out the particular structure of the
system matrices which occurs when structured grids are applied. In this paper, this effect is
prevented by considering the unconstrained system together with additional constraint equations
or by projecting the algebraic system using an oblique or orthogonal projector. The numerical tests
indicate that the efficiency of sparse iterative solvers can be preserved even in the presence of
complicated boundary relations. The described projections are also advantageous for formulations
derived for unstructured grids.