Step Size Selection for Tap Changers in Converter Transformers for Line Commutated HVDC Systems

Abstract

This paper deals with the tap changer design for line-commutated HVDC converter transformers. It examines the impacts of tap changer switching to provide criteria for selecting an optimal step size. When switching the tap changer the firing angles in the converter change and with them the reactive power consumption. The dependence of the changes on the step size of the tap-changer is linear, so that a maximum step size of the tap changer can be calculated based on criteria for maximum allowable changes of firing angle and reactive power. For implementation into an existing AC system the analysis of tap changer switching has to consider the interaction between converter reactive power and AC voltage iteratively to derive a stable operating point of the HVDC after switching. Both effects are influenced by the reaction of the AC-voltage.