230 kV bus support structure question

[snclap2]

I am retrofitting the high voltage bus at an existing substation. I am upgrading the bus from 2000 amp 40 kA to 3000 amp 63 kA by increasing the schedule from 4 inch schedule 40 to 4 inch schedule 80 extra high strength with the upgraded station post type insulators. The structural engineer believes that there will be no structural forces at the support structure that he will have to worry about. I have told him that the standards show that the force from the increased short circuit will apply to the structure at the support point of the insulators. He believes that the forces will cancel out. I believe we need a new heavier or stronger structure. Can anybody give me an idea who is correct on this one?
Steve

 

[cuky2000]

I believe the new loading conditions should be evaluated by a qualified civil engineer for the following reasons:

1- Short circuit forces increases 2.5 times for fault level from 40 kA to 63 kA.
2- Dead wait of bus (assuming Copper) increases 33% for large bus thickness from schedule 40 to schedule 80.
3- The aging and degradation of steel structure,foundation, anchoring, bolts, etc should also be considered in the analysis specially if rust or other degradation is observed.

I understand that the support structure should be designed no only for the extreme load but also designed for a load combination of extreme wind and short circuit.

Wind forces may not be a governing factor since the outside diameter of the schedule 40 and 80 are the same (OD=4.5&quot. However the moment in the base will be increase substantially do to the substantial increase of the SC forces.

Good engineering practice and common sense suggest an evaluation of the structural design with the new conditions for expected design life of the upgraded substation.

 

[jghrist]

If the bus supports are three-phase supports, your structural engineer is correct as far as the short-circuit forces are concerned. The short-circuit forces will pull the phases together or push them apart, depending on the fault type. There is no net short-circuit force on a three-phase support.

If the bus supports are single-phase, there will definitely be an increase in the shear and moment forces on the structure.

The weight will increase, but this is usually not a factor in structure strength.

 

[motorman]

I suggest that your structural engineer spend some time at thissite:

http://www.cda.org.uk/megab2/elecapps/pub22/index.htm

 

[cuky2000]

Jghrist,

Even thought it is possible to use 3-phase bus support for 230 kV, most installation at this voltage is a single stand. Furthermore, old installations use extensively bolted lattice structures.

In all cases, if the actual loading condition change, is a good idea that engineer in record of the upgraded project be able to verify by an acceptable proceedure if the installation meet the present conditions in accordance with updated standards.