Optimal Transformer Tap Selection Using Modified Barrier-Augmented Lagrangian Method

The optimal tap selection of transformers directly connected to generators is one of the two significant industry problems. The generator stepup and auxiliary transformers generally are equipped with no-load (fixed) taps that are infrequently changed. Their optimum positions need to be determined for meeting power system discrete states over long periods, covering the annual light-load, peak-load, and emergency conditions. The other and related problem is the use of design reactive capability of generators rather than their actual operating limits. This paper reports on application of a modified barrier-augmented Lagrangian (MBAL)-based nonlinear optimal power-flow method for the optimum selections of the transformer tap positions and the voltage set points of the generators within their over and underexcitation operating limits. The feasibility of the method is demonstrated using a 160-bus test system in operation at a midatlantic utility. It is shown that the method minimizes the deviations of the system bus voltages from the unity while meeting the power system equality and inequality constraints under lightand peak-load conditions.