Capability curve


Capability curve of an electrical generator describes the limits of the active and reactive power that the generator can provide. The curve represents a boundary of all operating points in the MW/MVAr plane; it is typically drawn with the real power on the horizontal axis, and, for the synchronous generator, resembles a letter D in shape, thus another name for the same curve, D-curve. In some sources the axes are switched, and the curve gets a dome-shaped appearance.

Synchronous generators

For a traditional synchronous generator the curve consists of multiple segments, each due to some physical constraint:
  • at the right part of the curve, the generator is constrained by the heat dissipation in the armature. The heating is proportional to the sum of squares of active and reactive currents, at the near-constant voltage it is closely proportional to the sum of squares of MW and MVAr, therefore this part of the curve resembles a section of a semicircle with the center at ;
  • at the upper part of the curve operation requires higher voltage on the output of the generator and thus higher excitation field. The rotating excitation winding has its own field heating limit;
  • at the bottom of the curve the magnetic flux constraints in the stator cause heating of the magnetic core at the stator end .
The corners between the sections of the curve define the limits of the power factor that the generator can sustain at its nameplate capacity. In practice, the prime mover is designed for less active power than the generator is capable of, so a prime mover limit changes the constraints somewhat, operation at real power levels between zero and the minimum is not possible even with these designs.

Wind and solar photovoltaics generators

The inverter-based resources need to have reactive capabilities in order to contribute to the grid stability, yet their contribution is quite different from the synchronous generators and is limited by internal voltage, temperature, and current constraints. Due to flexibility allowed by the presence of the power converter, the doubly-fed and full-converter wind generators on the market have different shapes of the capability curve: "triangular", "rectangular", "D-shape". The rectangular and D-shapes of the curve theoretically allow using the generator to provide voltage regulation services even when the unit does not produce any active energy, essentially working as a STATCOM, but not all designs include this feature. The fixed speed wind turbines without a power converter cannot be used for voltage control. They simply absorb the reactive power, so a switched capacitor bank is usually used to correct the power factor to unity.
Older PV generators were intended for distribution networks. Since the current state of these networks does not include voltage regulation, the inverters in these units were operating at unity power factor. When the PV devices started to appear in the transmission networks, inverters with reactive power capability appeared on the market. Since the power limit of an inverter is based on the maximum total current, the natural shape of the capability curve is similar to a semicircle, and at full capacity the real power always needs to be lowered if the reactive power is to be produced or absorbed. Theoretically the PV generators can be used as STATCOMs, although in practice the solar plants are disconnected at night.

Effects on electricity pricing

For a synchronous generator operating inside its D-curve, the marginal cost of providing reactive power is close to zero. However, once the generator's operating point reaches the corners of the D-curve, increasing the reactive power output will require reduction of the real power. Since the electricity markets payments are typically based on real power, the generating company will have a disincentive to provide more reactive power if requested by the independent system operator. Therefore the reactive power management is separated into an ancillary service with its own tariffs, like the Reactive Supply and Voltage Control from Generation Sources in the US.