A-frame bus in substations

[Overunity_1]

Forgive my ignorance here as I am an Electrical Engineer, but I have a question regarding using A-Frame bus between rigid bus pipes in a substation. Are these A-Frames considered structural, the same as an insulator? If so how are you calculating the force on the lower bus? Is there anyway to do this without FEA? I've gone through IEEE605-2008 and its my understanding that this standard doesn't apply to A-frame connections as you need to implement FEA to model the flow of short circuit current through the A-frame itself.

I can't seem to find any advantage over using an A-frame bus pipe setup over just dropping a strain bus tap down, unless these can be utilized as a structural support, relying on the insulator strength from the lower bus insulators.

 

[dauwerda]

Yes, typically when A-frames are used it is so that they can also be relied upon to provide structural support to the high bus.

When you ask about forces what do you have in mind? Gravity/dead load, wind load, ice load? All of those can be modeled and calculated fairly easily without any type of FEA. Are you talking about short circuit forces? I certainly can't speak for everyone, but my experience has shown that A-frames aren't typically considered when calculating and designing for short circuit forces other than for providing support. That is, loads that could be directly induced in the A-frame by short circuit are ignored, instead the design considers what it must support if the high bus (that the A-Frame is supporting) has short circuit loads induced in it.

 

[Overunity_1]

Yeah when I'm referring to short circuit forces. That is interested that these are ignored to be honest. The short circuit forces can be quite high, and of course propagate through the A-Frame. Is IEEE605-2008 the best document to use when trying to calculate the cantilever force applied to an insulator on a lower bus that is supporting a higher bus? If not what's the best way to calculate this?

 

[dauwerda]

IEEE 605 is what is used to determine what the short circuit force is, based on phase spacing, fault current, etc.
Figuring out what load the insulator sees is generally just simple statics.
Lets say you find that the short circuit force is 20lbs/ft and your A-frame is supporting 30ft of bus. The load at the top of your A-Frame is 20x30 = 600 lbs
This load transfers down to the low bus and essentially results in a shear and a moment (through a force couple where one leg of the A is in tension and the other is in compression) in your low bus. This force is generally assumed to need to be resisted by the insulator(s) directly below the A-Frame (which is conservative for insulator design as some of that moment will be resisted through bending of the lower bus). So, in the picture below, the insulator would need to be able to resist a direct shear of 600 lbs at the top and a moment of 600lbs x 8ft = 4800ft-lbs. So at the base of the insulator the moment would be 4800ft-lbs + 600lbs x 4ft = 7200ft-lbs.

I believe insulator manufacturers generally provide their bending strength as a cantilever strength, where instead of reporting an allowable moment, they report the allowable shear load applied to the top of the insulator. To convert this to an allowable moment, you would just multiply the reported cantilever strength by the insulator length. So if you have a 48" insulator with a 4500lb cantilever strength, the allowable moment is 4500lbs x 4ft = 18000ft-lbs

 

[Overunity_1]

Ah thats very helpful thank you! Does it matter where the insulator is located or how many insulators are on the bottom bus or is it safe to assume the applied force is equal across both insulators on the bottom bus?