Saturated Sub-transient reactance of a generator?

[Karmicpower]

We have a synchronous generator of ABB make. There are two values of Xd,Xd' and Xd" ; different vaues for saturated and unsaturated. Could you guys tell me what exactly is saturated and unsaturated for a synchronous generator?

 

[Marmite]

Saturated figures are calculated by applying a short circuit to a machine operating at rated open circuit voltage. As the current is several times the full load value, saturation is present and the reactance measured is the saturated value. This is also known as the rated voltage value.
In some cases the test is made with a reduced voltage to reduce the mechanical strain caused by a direct short circuit. The voltage is chosen so that the initial current is approximately its full load value. Saturation is much reduced and the reactance measured is the unsaturated (Rated current)value.
Regards
Marmite

 

[EddyWirbelstrom]

The saturated sub-transient reactance x"d sat must be used when calculating the initial symmetrical rms short-circuit current contribution from the generator.

 

[wolf39]

Unsaturated reactance values can be determined by application of short-circuit tests to synchronous generators, excited to reduced terminal voltages of up to 20%. In this region it can be expected that the relation between applied test voltages and resulting short-circuit currents is linear, i.e. the magnetic generator components (mainly stator core and field poles) can be regarded as unsaturated. Unsaturated transient and subtransient reactances can be calculated by the designer quite accurately. For short-circuit calculations, however, saturated reactances must be used. Saturation of magnetic generator components reduces reactance values, thus increasing short-circuit currents considerably. Saturation effects on transient and subtransient reactances are very difficult to determine. As a rule of thumb saturation factors of between 0.8 and 0.9 can be applied.

In contrast, the determination of saturated synchronous reactances is easy if the no-load characteristic is known.

Regards

Wolf

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