X/R Ratio Calculations - Which sequence X/R to use?

[acog]

I'm performing some calculations for a substation and have been presented with the following system information at the point of connection:

3ph fault (5.53kA phase & positive sequence), X1/R1=4.8, R1=0.016pu, X1=0.077pu

2ph-G fault (6.75kA, 6.99kA phase), (3.2kA, 4.4kA, 1.2KA, zero, pos, neg) X1/R1=4.8, R1=0.016pu, X1=0.077pu, X0/R0=45, R0=0.0007pu, X0=0.031pu

1ph-G fault (6.94kA phase), 2.31kA (sequence: pos=neg=zero), X1/R1=4.8, R1=0.016pu, X1=0.077pu, X0/R0=45, R0=0.0007pu, X0=0.031pu


My question is, do I use the positive sequence impedance X/R ratio (4.8) or the zero sequence X/R ratio (45) for decrement factor calculations such as IEEE80 section 15.10, IEEE605 Equation 18.

Do I only use the zero sequence value for earth faults, and all other faults use the positive sequence value? 45 seems very high for a zero sequence X/R ratio...

Thanks in advance

 

[7anoter4]

I think you have to take into consideration all the reactances and resistances involved in short-circuit.
That means: Sum [R1+R2+Ro+3*Rf]/Sum[X"1+X2+Xo] for phase-to-ground short-circuit.
Rf is the estimated resistance of the fault in ohm (normally it is assumed Rf = 0)
IEEE-80/2000 15.2:
c) For each fault, based on its duration time, tf, determine the value of decrement factor Df to allow for the effects of asymmetry of the fault current wave (see 15.10).
IEEE-80/2000 15.10:
The X/R ratio to be used here is the system X/R ratio at the fault location for a given fault type. The X and R components of the system subtransient fault impedance should be used to determine the X/R ratio.