The Reactive Capability Curve

The Reactive Capability curve gives an alternative explanation why a synchronous generator that is operated below and within its rated power factor will induce more heat dissipation.

Figure 5 is a typical reactive power capability curve for an actual generator. This is also the curve that dictates the power factor rating of a generator, which is usually 0.8. It shows the maximum reactive power that synchronous machine can absorb or supply.

Figure 3: Graphical Derivation of the reactive capability curve showing field current as the limiting factor at lagging power factor operation

Figure 4: Graphical Derivation of the reactive capability curve showing prime mover power as the limiting factor at leading power factor operation

Figures 3 and 4 are the graphical derivation of the reactive capability curve of a synchronous generator with the assumption that the terminal voltage and output KVA are held constant. Figure 3 shows that if the generator is operated at a power factor below its rated value, the field heating limit is actually exceeded, thus, incurring, abnormally, higher heat dissipation than its normal operation at rated power factor. Ideally, at rated power factor (0.8), the output KVar is still in correct proportion with the output KVA, and the heating is still in the allowable range.