Typical Subtransient Reactance for Smaller Synchronous Generators?

[HenryOhm]

All, I know this has been discussed before in more general terms. Referring to the IEEE Std 141 typical reactance values, they list xd" as 0.16 per unit for 12 poles or less (salient-pole with damper windings). In my experience with low-voltage diesel-driven 4-pole gensets in the 100kW - 1500kW range, I often see xd" closer to 0.10.

I have parts of an old GE Industrial Power Systems Data Book (the page in question from 1963). It shows a Fig 1 for turbine generators that lines up well with the IEEE 141 but starts at about 2.5MVA. It shows an increase in xd" with increased size. But no similar curve for diesel driven gensets is in the tome.

Would anyone know if there is data that shows a similar relationship. More importantly, at what lower pole count than 12 and at what lower threshold in size would the IEEE 141 xd" of 0.16 be considered average or too high to use conservatively?

Thanks for any and all help.

 

[genhead]

Very small machines can vary significantly. I have just been looking at three machines from different suppliers, all in the 130-140 kva range. The X" values are 6.9%, 8.3% and 12%.