Hello,
I'm performing a fault study on a small independent distribution system. The software I'm using requires me to specify the 3 phase fault current and the short circuit ratio (X/R) for a substation (i.e. generator) and the corresponding thevinen sequence impedances (positive and negative) are calculated for me.
I have received some information from the generator manufacturer -> X''d = 0.12, X2 = 0.22, X0 = 0.07, (X/R) = 0.35, p.f. = 0.8, 600 V, 250 kW, Y-config. Along with Xd, X'd, X''q, ......
When I use the above info. and calculate various faults by hand (i.e. line to line, line to ground, etc.) and compare it with what the computer gives me, the values are off quite a bit.
For example -> the method that I'm using for calculations is,
3 phase;
Irated = (250/0.8)/(sqrt(3)*0.6) = 300.7 A
Ip.u. = 1/X''d = 1/0.12 = 8.333
I(3 phase short) = 300.7*8.333 = 2505.83 A
for line to line -> Ia1 = 1/(X2+X''d)
then I determine Ib (Ib=a^2Ia1+.....)
for line to ground -> Ia1 = 1/(X2+X1+X''d)
etc.
Like I said before, with the software that I'm using, I enter the 3 phase fault current and the X/R ratio and it computes the positive and negative thevenin sequence components.......not the zero component. I realize that this zero component will alter my line to ground and 2line to ground values.
Can someone tell me how to determine this value (zero sequence) ? Since it's Y-connected, is it same as positive ?
How does this program actually calculate the thevenin impedances by hand, knowing only the 3 phase fault current and the short circuit ratio ?
Any other ideas, or help would be much appreciated
Louie
I'm performing a fault study on a small independent distribution system. The software I'm using requires me to specify the 3 phase fault current and the short circuit ratio (X/R) for a substation (i.e. generator) and the corresponding thevinen sequence impedances (positive and negative) are calculated for me.
I have received some information from the generator manufacturer -> X''d = 0.12, X2 = 0.22, X0 = 0.07, (X/R) = 0.35, p.f. = 0.8, 600 V, 250 kW, Y-config. Along with Xd, X'd, X''q, ......
When I use the above info. and calculate various faults by hand (i.e. line to line, line to ground, etc.) and compare it with what the computer gives me, the values are off quite a bit.
For example -> the method that I'm using for calculations is,
3 phase;
Irated = (250/0.8)/(sqrt(3)*0.6) = 300.7 A
Ip.u. = 1/X''d = 1/0.12 = 8.333
I(3 phase short) = 300.7*8.333 = 2505.83 A
for line to line -> Ia1 = 1/(X2+X''d)
then I determine Ib (Ib=a^2Ia1+.....)
for line to ground -> Ia1 = 1/(X2+X1+X''d)
etc.
Like I said before, with the software that I'm using, I enter the 3 phase fault current and the X/R ratio and it computes the positive and negative thevenin sequence components.......not the zero component. I realize that this zero component will alter my line to ground and 2line to ground values.
Can someone tell me how to determine this value (zero sequence) ? Since it's Y-connected, is it same as positive ?
How does this program actually calculate the thevenin impedances by hand, knowing only the 3 phase fault current and the short circuit ratio ?
Any other ideas, or help would be much appreciated
Louie