Bus support spacing for high fault duty.

[Revitup]

I have been doing some initial calculations for the maximum bus support spacing for a 230 kv swithcyard. the expected 3 phase sym fault duty is 70kA. my calculations show the max spacing is 12 feet this is too close. are there any suggestions of how to increase this and still be able to obtain an insulator with sufficient cantilever strength?
thank you

 

[jghrist]

Increase phase spacing. Use actual max anticipated fault current (if the 70kA is higher). Use high strength insulators. Use three-phase bus supports if the problem is with the steel instead of the insulators.

 

[burnt2x]

There was a paper trying to standardize practices by the AIEE which tabulated the recommended bus spacing up to 1175 kV BIL (1954). If you want, try searching for it on the Standard Handbook for Electrical Engineers - Substation Design chapter.

 

[Kiribanda]

Revitup,

Since you have mentioned about cantilever strength of bus support insulators, in your post, I am raising this question.

Appreciate very much if you could explain what is meant by "UNDERHUNG CANTILEVER STRENGTH" for a post type or cap and pin type insualtors?

Thanks in advance

 

[7anoter4]

According to IEC 865 Fm3=miuo/2/pi()*sqrt(2)/2*ip3^2*lng/am where:
Fm3 is the maximum force acting on the central main conductor
ip3 is the peak value of the short-circuit current
lng is the maximum center-line distance between supports along the bus.
am is the effective distance between main conductors [between parallel buses]
am=a distance between coplanar main conductors center-line for single circular cross-section
Bending force on the supports:
Fd=Vf*Vr*alpha*Fm3
Vf =ratio of dynamic and static force on supports
Vr=ratio of stress for a main conductor with and without three-phase automatic reclosing
From table 2 maximum Vf*Vr for three-phase short-circuit is 2.7
alpha=factor for force on support - for simple supports [see table 3] =0.5
For a certain conductors and supports you may reduce lng [distance between supports along] or increase the am [distance between parallel buses] in order to reduce the bending force on the supports [as pointed out by jghrist]

 

[Revitup]

Appreciate very much if you could explain what is meant by "UNDERHUNG CANTILEVER STRENGTH" for a post type or cap and pin type insualtors?
this is the rated strength of a particular insulator when installed in the underhung position (the position that is supported from above and not below) typically the normal insstallation is supported from below

 

[7anoter4]

The cap and pin types [disc type, longrod type and other like these] doesn't have underhung cantilever strength but only post type or pin type has it.
The pin type does not take main transmission line strain (tension), and functions as a jumper line insulator.
So, only post type has underhung cantilever strength.
Line post insulators are cantilever support members, with ratings defined as follows:
Specified Cantilever Load (SCL) is the ultimate cantilever strength rating of the line post insulator. SCL is identical to the minimum average breaking load (ABL) rating in previous catalogs.
Reference Cantilever Load (RCL) represents the maximum recommended load in cantilever that a post insulator is designed to withstand during its life span. RCL equals 50 percent of the SCL, and is identical to the insulator’s maximum working load (MWL) and maximum design cantilever load (MDCL).
For more information see:

http://www.pfisterer.com/download_download/d_6710.pdf