Short circuit loads to structure 1

[structural-eng]

I'm looking for more information regarding the amount of force created by a short circuit condition that the structural steel of a bus support should be able to resist. I am a structural engineer and my task is to design the structure and the foundations for a substation. I have a copy of IEEE 693 but there's just enough electrical verbage in it to make it unclear to me that I can determine the loads without additional information from the electrical engineer. I have asked the electrical engineer that we work for about short circuit loading on a couple of occasions, as it seems to me that the force is very dependent on the layout of the bus and other electrical conditions, and I never get a straight answer but instead they tell me that it can't govern for the particular project.

We generally are very conservative with the structure and foundation design for their substations because they like to be able to re-use them so I think we would be covered for whatever the short circuit force is but I would like a better understanding. Exactly what information would I need from the electrical in order to determine the force? I generally just get an electrical layout and the bus size which I use to determine wind (and seismic if it applies)loads.

I've read some other posts that seem to indicate that the loads to the structure may be equal and opposite but there may be additional shear loads to the foundation that need to be considered.

Any guidance would be appreciated.

 

[jghrist]

You need IEEE 605-2008 - IEEE Guide for Bus Design in Air Insulated Substations. This guide provides the calculation tools for bus design. Based on these calculations, the engineer can specify the bus size, forces acting on the bus structure, number of mounting structures required, and hardware requirements.

 

[structural-eng]

I do have a copy of IEEE 605-2008 but I'm not designing the bus, the electrical engineer is. I am only designing the structure (steel beams and columns) and the foundations. Should the electrical engineer being giving me the short circuit loads based on their bus design?

 

[cuky2000]

Yes, the electrical engineer should provide a short circuit current for the bus design, the X/R ratio at the sub (for the decrement factor), the phase clearances and the geometry of the bus structure. It is a good engineering practice to prepare design basis parameters to allow all disciplines to provide their input.

As a general criterion, If the SC available at the sub is known, usually designer adds 10%-20% for future grow butt not to exceed the SC rating of the lowest SC RMS rating of equipment such as circuit breaker, disconnect switches or the design standard that the project is following.

An alternative approach, although often conservative, the SC current for design purposes could be assumed as the RMS rating of the selected protective device (breaker or fuses) usually rated 31.5 kA, 40 kA, 63 kA, etc.

Take advantage of the structure manufacturer that usually could provide the load and moment at the base and the anchor bolts pattern for the foundation design. It is common to specify concrete with a strength of 3 to 4 kips for foundation design.

Enclosed is an excerpt regarding SC in a substation that I hope help.

_{Suggested considerations for structural design:
a) Consider horizontal & vertical forces load combination suggested by ASCE-113 & IBC Code(SC, wind, weight, snow, seismic, etc)
b) Horizontal forces such as SC & wind load for structure with 3 Insulators mounted in a common beam do not impact the foundation design (e.g. post insulator, disc switches, CCVT, PT, SA, etc.)
c) SC load on 3 phase equipment bushing does not impact the foundation design (e.g. dead-tank breaker, transformers, etc).
d) SC & wind forces for single stand bus support or equipment mounting will produce a reaction & moment on the base. Therefore, this needs to be considered in the foundation design.}

 

[jghrist]

The electrical engineer should be able to furnish the short circuit, wind, and weight loads at the supports as part of the bus design.

 

[7anoter4]

In my opinion, what an electrical engineer could give you it is the force acting on the conductors.
From here you have to use this in order to calculate the structure.There are some publications
which provide more for instance
IEC 60865-1/2011
Short-circuit currents – Calculation of effects – Part 1: Definitions and calculation methods
chapter 5.5 Structure loads due to rigid conductors .
In 60865-2/2015 they are some example how to calculate[ for instance]:
6.4.2 Conductor stress and forces on the supports
However I don't think it is something about foundation calculation.
See also -for instance:

https://ccaps.umn.edu/documents/CPE...rsShouldKnowaboutSubstationRigidBusDesign.pdf

 

[bacon4life]

Yes, the EE should be giving you info, though not all EEs are great at structural calculations. Consider designing the structures to exceed the breaking strength of the insulators.