Minimum Spacing Between Busbars

[MEdgarCI]

Good Day,

I'm designing a system where we have a 3-phase busbar system (at 385V) from a PV inverter penetrating into the LV side of an MV transformer. Off of these bus bars, we are cabling up to connect to the transformer spades, and the number of runs is causing us to stack our lugs back to back. Even though the feeder is 385V, the equipment manufacturer is requiring any cabling to be 2000V rated, as there is a total potential of 1700V during equipment operation.

I'm being asked to verify minimum spacing between the busbars, as there is a concern by connecting our lugs (1000kcmil) back to back, we may get too close to bare live parts. Specifically, I am looking for a section in the code which dictates the minimum spacing between those busbars at 2000V.

I don't think the cabinet itself counts as an auxiliary gutter, so 366.100(E) is inapplicable, but the closest item I've come across which even discusses this. 408.56 only describes up to 1000V, and of course this isn't a cable tray, so 392.80(2) doesn't work, and everything else I've come across online or in the Code discusses higher MV values, or spacing between overhead transmission/distribution.

Thank you in advance.

 

[Jraef]

... Even though the feeder is 385V, the equipment manufacturer is requiring any cabling to be 2000V rated, as there is a total potential of 1700V during equipment operation. ...

This statement confuses me. What the heck does this mean? If you are at "385V" (an odd number to start with), that would be the phase-to-phase voltage potential, or if it is the phase to neutral votlage potential, then the resulting phase-to-phase would be 666V (oooh... bad juju...)

Anyway, bus spacing within equipment is not dictated in the NEC, it is going to be an ANSI design spec for equipment, but even then it is a PERFORMANCE spec, not a list of absolutes. You are responsible for making it so that there are no problems, how you get there is up to you. There are however "tried and true" numbers you can go by.

Read this, it explains it very well.
https://www.powellind.com/sites/dow...Bus Spacings in Metal-Enclosed Switchgear.pdf

 

[Carultch]

Good Day,

I'm designing a system where we have a 3-phase busbar system (at 385V) from a PV inverter penetrating into the LV side of an MV transformer. Off of these bus bars, we are cabling up to connect to the transformer spades, and the number of runs is causing us to stack our lugs back to back. Even though the feeder is 385V, the equipment manufacturer is requiring any cabling to be 2000V rated, as there is a total potential of 1700V during equipment operation.

I'm being asked to verify minimum spacing between the busbars, as there is a concern by connecting our lugs (1000kcmil) back to back, we may get too close to bare live parts. Specifically, I am looking for a section in the code which dictates the minimum spacing between those busbars at 2000V.

I don't think the cabinet itself counts as an auxiliary gutter, so 366.100(E) is inapplicable, but the closest item I've come across which even discusses this. 408.56 only describes up to 1000V, and of course this isn't a cable tray, so 392.80(2) doesn't work, and everything else I've come across online or in the Code discusses higher MV values, or spacing between overhead transmission/distribution.

Thank you in advance.

The inverter manufacturer probably has a document explaining the specific transformer requirements for this application. This isn't an ordinary transformer to specify. Make sure you include those in the specification. One might relate to the issue of busbar spacing.

I don't know the answer, and I'm not certain whether the answer exists in the NEC, but I would like to add some clarification to the way you've stated the situation. I've been down this road before with similar inverters and found this voltage requirement very confusing.

To clarify, the nominal voltage is 385V phase-to-phase during ordinary operation. Yes, this is unusual, since it is the "natural voltage" of the inverter power electronics before any transformers.
It has the capability to spike at 1.7 kV phase-to-phase. Very brief (perhaps microseconds), but it is still a voltage spike that could occur.

 

[Tony S]

Have you considered the fault current rating for this set up? Spacing it a minor point when dealing with high fault currents. You have to add both the transformer and inverter fault currents.

 

[Carultch]

Have you considered the fault current rating for this set up? Spacing it a minor point when dealing with high fault currents. You have to add both the transformer and inverter fault currents.

Do inverters even have fault currents that are anywhere near the order of magnitude of the medium voltage connected transformer?

If so, where do I find that information? I've had to fill this out on interconnection apps, and had no idea where to find it.

 

[MEdgarCI]

I agree, this particular inverter is an odd one, but I've come across several inverter manufacturers who use oddball voltages on their output.

We have all of the design aspects surrounding the pairing of the inverter to the suitable MV transformer addressed, this was just the last piece of the puzzle, and it was more for a sanity check than anything.

Much appreciated.

Cheers.

 

[Bugman1400]

Do inverters even have fault currents that are anywhere near the order of magnitude of the medium voltage connected transformer?

If so, where do I find that information? I've had to fill this out on interconnection apps, and had no idea where to find it.

That info is typically in the data sheets for the inverter. Most inverters supply no more than 200% of FLA. This is because, if the DC or transformed AC is faulted, the fault current is limited by the inverter circuit which will instantly limit or clamp any current over 200%. It is not like a rotating generator where there is a DC field that collapses. Which inverter are you using? If you need help with the LGIP Interconnection Request from FERC 2003-C, let me know. I am filling out a few for solar developers myself.

 

[Phil Corso]

MEdgar...

Separation due to voltage is a relatively minor concern. The principal problem is related to the placement of insulators/supports. They must withstand the stresses exerted by both repelling and attractive forces while carrying short-circuit currents, especially the first-cycle peak (which can be quite asymmetrical)!

It is an exacting science requiring knowledge of breaking, compressive, and tensile loads of bus-bar, insulators, support, and spacer components! In fact, proper design factors, number more than 50! Examples are dimensions (of course), moments of inertia, material yield, etc!

I strongly suggest you contact firms that deal with professionally designed Bus-Bar!

Regards, Phil Corso

 

[Tony S]

Do inverters even have fault currents that are anywhere near the order of magnitude of the medium voltage connected transformer?

If so, where do I find that information? I've had to fill this out on interconnection apps, and had no idea where to find it.

Depending on what the transformer is feeding in to, sizeable fault currents can back feed.

Transformers aren’t bothered which way the power is going.

Getting it designed and built by a specialist is your best option.