Influence of automatic control of a tap changing step-up transformer on power capability area of generating unit

Abstract A synchronous generator is a source of real and reactive power, which can be conveniently regulated over a wide range of values. The real and reactive power of a synchronous generator are limited by the reactive power capability curve, which is determined by the following constraints: stator (armature) current, rotor (field) current, load angle (owing to steady-state stability), the temperature in the end region of the stator magnetic circuit, turbine power, and the terminal voltage of the generator. Large generating units operate on a high-voltage network by using step-up transformers, which can be equipped with on-load tap changers. This article discusses how controlling the transformation ratio of step-up transformers can enlarge the area surrounded by the reactive power capability curve. Equations suitable for such an analysis are derived, and a numerical example is presented. A new control algorithm for tap changing is proposed.