Busway Zero Sequence Impedance

[davidbeach]

Does anybody have any good data on busway zero sequence impedances? I'm looking for conclusive information on low voltage busway, copper or aluminum bus bars, with an aluminum housing. Conrad St. Pierre's book, table B-8, has a set of Z0/Z1 ratios for steel housing. Those ratios have a fairly wide range, do not reflect the differences between R0/R1 and X0/X1, and are for steel rather than aluminum housing.

The impedance data on the web sites of the major manufacturers does not address this issue, at least not directly. They do provide the housing resistance, presumably there would also be some reactance but at a much lower level than the case with steel housing.

Thanks,

David Beach

 

[alehman]

It will vary depending on manufacturer and configuraiton. Suggest consulting your local manufacturer's rep, who will probably refer you to a factory engineer.

 

[davidbeach]

The answer back from one factory was that Z0=Z1. An approximation perhaps, but not as accurate as I would hope.

David Beach

 

[dpc]

I'm pretty doubtful that z0=z1, but I also doubt that you're going to get anything else any better.

In the EasyPower library data, only R1 and X1 are used. A Z0/Z1 multiplying factor is applied based on an old article by R.H. Kaufmann ("Let's be More Specific About Equipment Grounding", Proceedings of the American Power Conference, 1962 - this is a great paper, BTW.)

Factors vary considerably depending on bus rating and type of busway (feeder vs plug-in) and Z0/Z1 ranges from about 3 to over 50! I suspect this was based on a steel housing, but I don't think aluminum housing is going to bring it down to 1.

Hope this helps a little.

 

[davidbeach]

The Kaufmann article is the source cited by St. Pierre for his multiplying factors.

I've a situation where a high Z0 coming off a 2MW, 480V, generator would very, very useful, and busway would make the connection far easier to construct than pipe & wire.

Generator by itself, or utility by itself, the ground fault values are quite manageable. But generator in parallel with the utility presents a very different picture. The utility has a fairly low system Z1 and relatively high system Z0 while the generator has a high Z1 but a very low Z0. When paralleled, the combination of the low utility Z1 (and Z2) in series with the low generator Z0 results in ground fault currents upwards of 30kA greater than the sum of each by itself. 40 feet of 8 sets of 500 kcmil cable has enough Z0 to drop the available SLG fault current by nearly 20kA. I wouldn't need much of a Z0/Z1 ratio on busway to make it work, but it has to be a number that leaves me sure that the gear will not see a possible SLG fault in excess of its ratings.

The engineering software really needs to address ranges of impedances, using the low end of the range for fault studies and the high end of the range for load flow studies. As long as there can only be one value for impedance, do you want the one the gives the worst case fault study (and leaves you with too much voltage drop in real life) or the one that gives you the worst case voltage drop (and who knows what kind of interrupting performance)?

I think that most of what we see for component impedances is based on the high end of the range to allow for good voltage drop calculations, which would also be why Z0 seems so often ignored. How many systems are there out there that are subject to interrupting faults in excess of their ratings because the system actually has a lower impedance than was used during the original fault study?

David Beach

 

[dpc]

I agree that there is an inherent conflict between modeling for short circuit calcs and modeling for power flow. Another issue is always what conductor temperature to specify.

But my experience is that short circuit calculations are generally always conservative because so many impedances are ignored: equipment buses, connections, circuit breakers, fuses, starters, etc. Not to mention the actual fault impedance.

The analysis software can't model data that doesn't exist or isn't available. EasyPower allows you to specify any X0 and R0 you want for busway, if you don't like the ratios from the Kaufmann data. But you have to get the data from somewhere.