Turbine-generator steady-state reactances

The variation of the steady-state axis reactances of a 500 MW generator with load and power factor has been obtained from measured load points. At rated power, the direct axis reactance can be as much as 30% below the unsaturated value, and measured values of quadrature-axis reactance can vary by 20% over a range of power factors. The finite-element method has been used successfully to simulate the same load points. In these calculations, the absence of experimental error, and the ability of the method to show distributions of flux and saturation, have allowed a clearer insight into the complex phenomena that cause reactance variation. On load, the rotor body carries field leakage flux on the d-axis, and mutual flux displaced from it. These fluxes act together to give a saturation pattern about an intermediate axis. Thus, when current acts on one axis (d or q), flux is produced on both axes, although the cross-axis effect is only about one-tenth of the self effect. Each operating point may be described by effective values for the three conventional reactances (xd, xq and xa) used in a normal phasor diagram, or by six more basic reactances (three self- and three cross-axis reactances), each of which is load dependent. The variation of these reactances with load angle, at 100% and 50% of rated power, has been calculated, and its significance is discussed.