Don't be mis-led by Fig. 10 on page 21 of Vol I. These fault current magnitude comparisons are only true if impedances are disregarded, which is never done when calculating fault currents.
An example is Fig. 10(c) which shows a phase to ground fault current to be the same magnitude as Fig. 10(a), a 3 phase fault. This is only true if the impedance of the primary side is zero.
In the case of a "bolted" phase to ground fault at the secondary of the transformer, the single phase fault will always be greater than a three phase bolted fault at the same point.
It works like this:
I 3phase = E/Z1 [ E is phase to neutral; Z1 is positive sequence impedance to the fault];
I phase to gnd = 3 x E/(2Z1 + Zt) where Zt is the transfomer impedance.
If Z1 = Zt then..... I phase to gnd = E/Z1 = I 3phase, but this is never the case.
Z1 is the sum of the primary side impedance (all the way to the generator) plus Zt, so Z1 is always greater than Zt. So, I 3phase would be less than the ground fault. James Wilson, PE
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