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This is the second part of the series in which we looked at how the sequence network components are derived and in this part, we will be looking at three-phase fault type which is a perfectly balanced fault type. So we will be looking through this document in much more detail and let’s start reading through it.
Alright so a balanced system remains symmetrical after the occurrence of a three phase fault having the same impedance between each line and common point, and because it has the same impedance, that is why it is a perfectly balanced three phase fault and again the author states that only positive sequence current flow with the fault impedance Zf equal in all phases meaning you would have to have Zf be equal in all phases to be a perfectly balanced fault, simply add impedance Zf to usual positive sequence thevinine equivalent circuit of the system at the fault bus K and calculate the fault current from that equation meaning that if we calculate the fault current all we would need to do is to essentially take the prefault voltage in per units and divide that by positive sequence impedance per unit and you may if you have a fault impedance, you add up fault impedance their and its just a simple Ohms law to calculate the positive sequence fault current, and it says for each of the other fault types as shown in Fig 12.4 which is the previous figure, Formal Derivations of the equations for the symmetrical component currents which is the negative sequence, positive sequence and zero sequence currents are provided in the sections which follow. In each case, the fault point P is designated at Bus K meaning that it's telling you where the fault current is.
So, in this figure it's just an example of a sequence network diagram, this is the positive sequence network, this is a zero sequence network, Ya! This is the positive and this is the zero-sequence network and you can tell its positive because it has a voltage source which we would expect for a positive sequence and it seems like there is two voltage source which means it’s a two-source network, there is a source impedance here and another source impedance here, and then you have bus 1, 2, 3 and 4. And then this may be transmission line, its probably a transmission line because it has the same impedance for positive and zero sequence network, and then for the zero-sequence current you have something like this, which we would expect for a delta wye transformer right, so, I bet you that this right here is your transformer impedances, alright and then this, is your transmission line or distribution line impedance, and then this right here would be perhaps your distribution line impedance as well and then you have and then you have another transformer here which appears like a step-up transformer, step down transformer. It appears that this is a delta wye and this right here is a delta wye so this appears to be two step-up transformers. Excuse me, step down transformers, so high voltage steps it down, this is probably a distribution line and then high voltage steps it down to distribution line.
And then, of course, the actual zero sequence network for your transformers really depends on the type of transformer that you may have, So not very exciting as far as what’s going on here, it appears like the author uses an impedance method or a Z bus method to calculate the fault current but that is not in scope of this particular tutorial.
Ok in the next topic, we go to the line to ground fault which is much more exciting.