Positive, Negative, Zero Sequence networks

[mjmarkey1979]

I'm currently dusting off my 10 year old power knowledge, and more specifically unbalanced fault analysis which hasn't been used for a while as I've been in the building services game for the last 5 years (so ohms law and the volt drop equations was about all you had to remember).

I remember that the majority of the time positive and negative sequence components are usually equal but with their vectors shifted and that Igf = 3VLN / (Zo + Z1 + Z2 + ZG). Where Zg is the impedance of the ground return path including any arcing (if that's applicable of course).

If you know the earth fault current from a site report and the fault level of the grid supply point of say 2500 MVA, is it possible to derive from that the Zo + Z1 + Z2 components based on the assumption that Z1 = Z2 and from the grid infeed impedance (Zs) say 0.04 based on a 100MVA base.

Sorry gents, but I could really do with a sanity check and more insight into something I don't normally do everyday, it's a new position and making a good impression with this crowd and me stressing on this stalling on this is not giving me the clarity I need.

Thanks in advance.

 

[HornTootinEE]

Good questions, but if my memory serves, the impedance should be (Z1 + Z2 + 3*(Z0+ZG)) the derivation of which can be shown in the transposition matrices that convert phase currents to sequence currents and vice versa.

As far as finding your individual Z values assuming Z1 = Z2, I don't see another way to split those out, unless you have one of them already. Z1 and Z2 is usually just your physical impedance, i.e 1/0 wire has an R of A and a jX of B, that sort of thing. You could probably estimate within a very reasonably close value for Z1 and Z2 just using feeder length and wire size/type. This would ignore mutual impedances (ZAB, ZBC, ZCA) but depending on feeder length, those could easily be negligable.

This all assumes you are just looking for the thevenin impedance at the point of fault looking back into the system, which is where you reduce the sequence networks anyway for your current calculations...

Hopefully that helps, see if your library or another contact has a copy of Blackburn's system protection book. That is a goody.

 

[mjmarkey1979]

Thanks HornTootinEE but in this office, everybody hoards any specialist text like the last chocolate in the tray. Due to me dealing with Power Generation side, we don't have the demand for purchase of DigiSilent, ETAP etc. packages. This could be a good cause for highlighting its need though.

 

[HornTootinEE]

Yeah, if you need to do calcs by hand, Blackburn is a great start.

I believe he also covers generator protection.

 

[odlanor]

I guess this is an aproxx