Modelling of tap-changer transformers in an energy management system

Tap-changer transformers are used in a power system to influence the voltage profile as well as the distribution of power flow. The coupling of the two transformers-the main transformer and the tap transformer-establishes a complex power system element. The correct modelling of this device has turned out to be critical for the accurate estimation and calculation of the power system state. This paper introduces an improved model which simulates the behaviour at the different tap positions and loading conditions. The paper focuses on the positive sequence system of a regulated connected group consisting of a three-winding main transformer and a two-winding tap-changer transformer. Model verification against test reports and experiences gained from using the new model in an EMS are documented.