Generator terminology 4

[vetlover]

I am trying to differentiate between an ac generator power factor rating and the impedance of the generator to answer a specific question. If somebody is asking the impedance of a generator, what would they be looking for exactly.

 

[davidbeach]

X_d, X_q, X_d', X_q', X_d'', X_q'', R, X₀, X₂. That is the minimum I would be looking for if asking for generator impedance. I could probably assume values for X_q' and X_q'' if the rest were provided.

 

[dpc]

The odds of getting all of the values in David's list decrease as the generator size decreases.

There is no single "impedance" for a dynamic, non-linear system like a generator. For a single value, the most useful impedance is the sub-transient impedance (X"d).

I don't think there is much direct relationship between power factor rating and the impedance.

 

[waross]

The power factor rating of a generator is arbitrary.
Suppose we have a 100kva generator. If we decide to rate it at 100% power factor, we must install a prime mover with a 100KW output (133 HP.)
If we decide to rate the generator at 80% power factor, the prime mover need only supply 80KW. (107 HP.)
The actual power factor depends on the load, not the generator.
respectfully

 

[davidbeach]

True, I didn't focus on the power factor part of the question. The generator (electric machine) has a rating (among many) of maximum amps and the prime mover (mechanical device) has a rating (among many) of maximum power. The relationship of those two determines the power factor rating of the generator installation (electrical machine plus mechanical device). If a load at unit power factor can consume as much power as the the prime mover can produce, you have a unity power factor rating. If the maximum power of the prime mover equates to a current (at unity power factor) of only 80% of the generator's maximum current rating, you will have an 80% power factor rating for the combination. That is somewhat over simplified, but gives the general idea; no relationship between the generator impedance(s) and power factor.

 

[Yuma]

If someone is asking for the power factor and the 'impedance' of a generator, maybe they are trying to figure out what the power chart of that generator looks like. This is because, given a generator rated e.g. 100 MVA;

- the power factor of that generator gives an idea on how much reactive power it can produce.

- the Xd (this is, I assume, what they are really expecting when they ask for the 'impedance') gives an idea of how much reactive power it can absorb. If it is a multipole generator, you would need the Xq also.

 

[rbulsara]

Reactance has nothing to do with reactive power, per say.
Most important of the reactances, Subtransient reactance X"d, is useful to quickly find out the maximum short circuit current the generator can deliver.

For example, the maximum SCC a generator can deliver will be FLA/X"d.

A 600A rated generator with 0.15pu X"d (or 15%) will deliver 600/0.15=4000A. There are some other variables but their effect would not be significant.