# Xsource vs Xd''

Hello all,

I've noticed that in a lot of salient pole sync. generator models, the load-flow parameter Xsource is equivalent to the dynamic parameter Xd'' = Xq'', the sub-transient reactance.

This is corroborated in the link below (which I don't have enough karma to show), where it states X2 = Xd'' + Xq'' / 2 (I've assumed X1 = X2, the positiive sequence source impedance): http: //www.ee.uidaho.edu/ee/power/jlaw/COURSES/ECE523/F07/HANDOUTS/SG/s21b.pdf

Forgive me as it has been a while since I've taken a formal machine modelling course.

So my question is theoretical: why is it that the sub-transient reactance, a short time-frame measurement, is the same as the steady-state source impedance? On one hand, this makes sense to me from a short-cicuit calculation stand-point. But from a load flow stand-point, aren't we trying to best capture the reactive power losses of the synchronous generator? Or do we build the losses into the reactive power limits, and Xsource has no effect in the load flow solution. In steady-state, we're really only concerned with the voltage, and P and Q injections at the terminals, and from what I see, Qout from the Gen is equal to Qin on the transformer branch (Xsource not modelled in steady state). Then in dynamics, we use Xd and Xq etc. Perhaps I've answered my own question...