What is a Generator capability curve?

The generator capability curve described the capability real and reactive power capability of a generator. Real power is plotted on the horizontal axis, while reactive power is plotted on the vertical axis.

A reactive capability curve consists of three curved segments. One segment is the arc of a circle centered at the origin of the reactive capability curve. Because the radius of that circle is the apparent power, S (in MVA), it is based on the thermal heating limitations inherent in the stator winding and reflects the fact that the stator limitation is based on current alone.

The second segment is an arc of a circle centered on the Q axis - the arc joins the positive Q axis with the constant MVA portion of the curve, and defines the upper boundary of reactive power OUT of the generator. It is the arc of a circle because it also reflects current-based heating; the critical difference is that the limitation described is that of the rotor winding.

The third segment joins the negative Q axis (representing reactive power into the machine) with the constant MVA portion of the curve. This segment reflects end-ring heating while in underexcited operation.

When you change the tap on the generator step up transformer, you will change the reactive output of the generator. Remember that reactive (VARS) always flow downhill in voltage - from higher voltage to lower voltage. So if you change the tap on the transformer to produce a lower open-circuit secondary voltage, the reactive output of the generator will increase. Conversely, if you change the tap to cause a higher open-circuit secondary voltage, the reactive output of the generator will decrease.