Observability analysis and estimation of transformer tap settings

This paper presents a numerical algorithm for observability analysis in a state estimation model which treats transformer tap settings as state variables and provides for turns ratio and phase-shift angle measurements. The proposed model is (transformed to a conventional node frame of reference formulation, by introducing two fictitious buses and branches land a pair of zero active and reactive pseudo measurements for each transformer, whose tap is modeled as a state variable. For an unobservable system, observable islands are determined and multiple measurement placement is accomplished in a noniterative manner. The method is applied to both voltage and phase-shifting transformers. The method is illustrated by a test system.