Substation sub sectionalizing

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I am currently evaluating several 345kv and 138kv GIS substations and have most of it nailed down. One question I can’t figure, what benefit is there to having midpoint busbar sectionalizing switches? My calculations show nothing is gained because they can actually reduce the availability of a busbar- however upon looking at manufacture literature and actual substations commissioned in Europe and Asia- half the time it’s common to have them.

Here is an example:

From an engineering guide…

I am not referring to the earthing switches (which are essential in any GIS configuration), but actual disconnectors (isolators) subdividing the busbars themselves. Breaking the phases. Some projects include them, others omit them- there seems to be no defining factor governing inclusion vs exclusion.


Here is some background information which may answer some typical problems you may run into:

Question 1: What are high speed earthing switches used for in GIS?

Answer 1: Is a high speed switch using motor and/or spring operated mechanism that operate faster than the regular maintenance switch and have limited breaking current capability some of them for 0.5 A. The fast grounding switch is used with dual function:
a) Provides extra safety to the maintenance personnel
b) Ground capacitance (cables, transmission lines, etc.).

Question 2: And when/what relay logic is used to activate them?

Answer 2: The circuit breaker need to be tripped first and interlocked as the regular air insulated substation (AIS). The enclosure of the GIS shall be design to withstand the thermal effects of an arc at the full rated short circuit current until the nearest protective relay has acted and tripped the upstream breaker.

Question 3: Also I’ve heard (though unsure if there is any truth) that they are used so the DC capacitance of an underground cable does not saturate the line CTs after the breaker has opened. If true I can not warp my mind around how you would coordinate BF around that… Or how you would know the breaker has extinguished its arc at that exact moment.

Answer 3: Rather than CT’s the fast earth switch protect the VT from core saturation during switching the charging current of the line

For illustration of the maintenance and fast grounding (earthing) switch, see the enclosed sketch with a typical 500 kV GIS module.