The Generation and Absorption of Reactive Power
Synchronous generators can be used to generate or absorb reactive power. An over-excited machine, that is, one with greater than nominal excitation, generates reactive power whilst an under-excited machine absorbs it. Synchronous generators are the main source of supply to the power system of both positive and negative VArs.
The ability to generate or absorb reactive power is shown by the performance chart of a synchronous generator. Reactive power generation (lagging power factor operation) is limited by the maximum excitation voltage allowable before the rotor currents lead to overheating. In Figure 3.12 this is 2.5 p.u.
The ability to absorb reactive power is determined by the short-circuit ratio (1/synchronous reactance) as the distance between the power axis and the theoretical stability-limit line in Figure 3.12 is proportional to the short-circuit ratio. In modern machines the value of the short-circuit ratio is made low for economic reasons, and hence the inherent ability to operate at leading power factors (absorbing VArs) is not large. For example, a 200 MW 0.85 p.f. machine with a 10% stability allowance has a capability of absorbing 45MVAr at full power output. The VAr absorption capacity can, however, be increased by the use of continuously acting voltage regulators, as explained in Chapter 3.