switch station grounding- when is it simply a site not a substation

[bacon4life]

The recent thread238-222409 on substation grounding brings up the question of when an installation become a substation vs. a switchyard vs. just some equipment inside a fence.

Obviously if it has a power transformer, it is a substation and needs a full ground grid.

If you put a fence around a high voltage switch, does it become a substation, thus requiring a ground grid and fence grounding? How about a couple of HV motor operated disconnects and a small communications hut? Perhaps the boundry is if there are fault interupting devices?

 

[dpc]

I don't see that there is much difference between a substation and a switching station when it comes to grounding requirements. The main issue is limiting the ground potential rise (and step and touch potentials in particular) and this is a function of system voltage and fault current.

With a large transformer, you may have more available fault current, but it would certainly be possible to have high step and/or touch potential in a switching station.

 

[bacon4life]

If it is simply a matter of limiting GPR, then it seems like many transmission cooridors should be fenced and grounded. A transmission fault would cause GPR at the site of the fault and at the transformer (ground source). A remote switch or remote switching station would not see any significant GPR.

For non ground source locations, is the need to ground actually driven by the likelyhood of utility workers being present during a fault? Otherwise what is the difference between a switching station, and the nextspan of wire just outside the switching station?

 

[dpc]

If the fault was inside the switching station, there could substantial touch/step potential.

I think you are putting too much emphasis on the location of the ground source.

 

[davidbeach]

In the typical substation you have transformers that can be a source of ground current resulting is high voltage gradients in the ground around the transformer and as you move out further the gradients become lower. The fence is there not because of GPR issues but as a means of protecting all the high value stuff in the yard. On the right of way of a transmission line the voltage gradient during a ground fault is much lower than in a substation and the likelihood of step potential problems is small enough to not place the public at risk. I wouldn't want to be touching a steel line structure when the line flashes over there or near by, but fencing all structures would be a non-starter.