In reviewing generator data sheets I found they typically do not list the x/r, and was hoping someone could point me in the right direction, I have on document that indicates for most machines X/R can be approximated at X/R=2*PI*f*Ta assuming that x2 is approximate the same as x”, however looking at the gen. data sheets It appears that the x2 and x” are not equal for this specific gen. [0.2625 vs 0.1655], I found another publication [reference to it C37.013 generator brkrs] that reference to calculate the x/r for the gen breaker [I assume this would also be adequate for the gen] that assumes X=x”, and R=x2/2*PI*f*Ta, with the x” and x2 not being equal this provides different results, just curious if there are any publications with reference to this or if the C37.013 is the one typically followed??
Appendix Part II Impedance Data Table 8A [from ANSI 37.010 ]
what is Ta?
Originally Posted by Mike01
In my opinion, Ta=x2/2/pi()/frq/ra where x2=negative sequence reactance and ra=stator winding resistance-it is the d.c. component time constant.
Yep, Ta would be the the armature short circuit time constant from the gen data sheet..
Mike... reur query about Ta:
A generator's current contribution to an electrical fault at (or beyond) its terminals, decays exponentially at a rate determined by the time constant, Ta.
2) Magnitude Basis.
Ta is simply the armature (stator) winding's inductance-to-resistance ratio. The inductance is found from its negative-sequence reactance, X2, while its resistance is Ra. But, the sub-transient-reactance can beused.
I am not aware of a standard, but there are typical values shown in reference documentation. Typically, in the USA, a 2-pole machine has an average value of 0.13, but ranges between 0.04 to 0.24. The 4-pole machine values are 0.20, and 0.15 to 0.35, respectively! I also believe that European machines have comparable values.
The higher value indicates, at least to me, that there is more copper than "usual!"
I used Westinghouse's T&D Electrical Reference Book (ca 1950). I'm sure you should be able to obtain data from the manufacture. However, if not, let me know and I will send you a copy of the "Typical Constants" Table!
Ta is used for faults at the generator's terminals. Ta' is used for faults beyond the generator's terminals. Ta' is the value Ta, but adjusted to include the circuit impedance between the generator and the fault location!
Regards, Phil Corso
In IEC 60909-2 /2008
Table 1 – Actual data of typical synchronous generators, motors and condensers
there are some indications about Ta –on the last column[for 2 poles generators above 50 MVA Ta=0.19-0.48]