Approximating the loadability surface in the presence of SNB–SLL corner points

Power system voltage security assessment is generally applied by considering the power system loadability surface. For a large power system, the loadability surface is a complicated hyper-surface in parameter space, and local approximations are a necessity for any analysis. Unfortunately, inequality constraints due to for example generator overexitation limiters, and higher codimension bifurcations, make the loadability surface non-smooth.One situation that is particularly difficult to handle is when a saddle-node bifurcation surface intersects a switching loadability limit surface. In this article we intend to investigate how several local approximations can be combined to obtain an adequate approximation of the loadability surface near such intersections.

► A method for approximating the loadability boundary of a power system is suggested.
► The method can handle intersections of SNB and SLL surfaces.
► The approximations may be used for efficient voltage stability assessment.