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Gantry dropper configuration
2

Gantry dropper configuration

Gantry dropper configuration

(OP)
Currently designing a 220 kV SS, some designs implement gantry beam underhung insulator (I or sometimes V insulators) to support dropper from line side insulator string down to surge arrestor or disconnector.

The image below shows I string used for droppers as well as gantry jumpers.



I understand that for jumpers it is to avoid conductors encroaching clearances, however, for the dropper (The bay on right above), the dropper can go directly to the equipment below (mostly a voltage transformer).

what is the criterion for underhung string? is it to avoid clearances encroachment during wind blows? to slightly reduce the short circuit tensile forces on the equipment terminals below ( I don't see any major force reduction with U/H string)?

any clues?

RE: Gantry dropper configuration

I've heard the argument that if one of the gantry insulators or cleats break, the underside one will prevent the whole span from falling into the energized bus bar below. Good way to prevent an entire section or substation from being cleared.

RE: Gantry dropper configuration

A general rule of thumb suggests that for 220 kV AIS dropper in gantries as follow:
• Underhung insulator is not required up to 10 m height
• For taller gantries, let’s say 19 m and above underhung insulator is suggested.
Considering the design wind speed, conductor tension, area, etc, the swing angle could be estimated by calculation or using software such as PLS-CADD.
See sample of substation layout typically used for 220 kV nominal voltage.


RE: Gantry dropper configuration

(OP)
Great cuky2000, where to find swing angle equation for wind blows?

RE: Gantry dropper configuration

Assuming the insulator has the same swing angle as the conductor, a simplified formula for swing angle is as follow:
∅=tan-1(dc.F/12Wc)
where: dc = conductor diameter in inches; wc = weight of conductor in lbs./ft. F = wind force.

RE: Gantry dropper configuration

(OP)
Reference?

The swing angle will be useful for the two phases next to gantry legs. I think with wind blows, the inter phase clearances are usually kept intact as the three phases move together with the blow and return back and forth.

RE: Gantry dropper configuration

The source from the previous post is from the USDA Rural Development, BULLETIN 1724E-200 DESIGN MANUAL FOR HIGH VOLTAGE TRANSMISSION LINES.
Additional information could found from a good transmission / substation line design. One reference that personally I use is Overhead Power Lines: Planning, Design, Construction by Friedrich Kiessling, Peter Nefzger, Joao Felix Nolasco, Ulf Kaintzyk. This reference has equation with and without insulator and graphic with swing angle as a function of wind speed.
Beware that for substation application, those equations need to be reconfigured to consider the drop down conductor tension without exceeding the maximum pulling forces on the connected equipment pad below the gantry.

RE: Gantry dropper configuration

@cucky2000, where did you find that graphic? Its very informative and I am genuinely interested. :)

RE: Gantry dropper configuration

Mbrooke: Below is an excerpt with additional details of the swing angle equation and graph of a swing angle vs. wind wpeed. I hope this help.

RE: Gantry dropper configuration

It does, huge help! Thank you for sharing this :)

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