Medium Voltage Substation Yard 1

[timm33333]

I have couple of questions regarding a 34.5kV substation yard of an industrial facility, it contains a 34.5 kV outdoor switchgear and a 34.5kV outdoor oil type transformer.

The substation is on the load side of the utility meter, so technically it is in the jurisdiction of NEC. Can we still use the clearances mentioned in section 11 of NESC for doing the physical design of this substation? Also can we use NESC section 11 for fence-grounding, or should the fence-grounding be done as per NEC 250.194. Thanks

 

[davidbeach]

Those questions would be much better answered by the local AHJ.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[jghrist]

You have to follow the NEC if it is stricter than the NESC. I would follow the NESC for clearance and grounding unless the NEC requires greater clearances or more grounding.

 

[timm33333]

Thanks. As the grounding system of the substation yard would be connected to the grounding system of the rest of the plant, so I think we have to only calculate the minimum size of the ground conductors, and the calculations of touch (and step) potential are not required, is it correct?

 

[jghrist]

A 34.5 kV fault could create hazardous step- and touch-voltages because some of the current will return to the 34.5 kV source through the earth. Safety should be assured in outdoor ac substations through the application of IEEE Std 80 - Guide for Safety in AC Substation Grounding.

 

[jghrist]

NEC Rule 250.194 requires grounding to limit step, touch, and transfer voltages. Alternate designs for fence grounding performed under IEEE Std 80 are permitted under engineering supervision.

 

[timm33333]

I am not able to figure out that what would be the required thickness of the crushed rock for the substation grounding. Annex D of IEEE-81 talks about different methods of measuring the resistivity of rock, but it does not mention the thickness of the layer of rock.

Also table 7 of IEEE-80 mentions different types of granite rocks, which of these is most common? Thanks

 

[bacon4life]

Every utility near me uses specs a specific rock from a single gravel pit in the region. Perhaps you can find out what a local utility uses?

 

[timm33333]

Yes will check with utility.

 

[mls1]

One of the most practical substation design guides is the RUS "Design Guide for Rural Substation" publication 1724E-300. You can get it from the below link. It has specific recommendations such as minimum layer of 4" with 3000 ohm-m resistivity.

https://www.rd.usda.gov/sites/default/files/UEP_Bulletin_1724E-300.pdf

 

[jghrist]

Calculation of tolerable step- and touch-voltage includes the thickness of the crushed rock surface. The required thickness depends on the calculated touch-voltage of the grid as well as the resistivity of the rock. 4-6 inches is common. Thicker surfacing makes driving in the substation more difficult.

 

[cranky108]

There is more to substation rock then resistivity. I have seen 6 inch of rock become somewhat like quick sand for traffic in a substation. The rock size, and the number of fines is somewhat of an art that a civil engineer is better equipped to handle.
Truthfully, most utility electrical engineers don't do a good job with this.

 

[jghrist]

The rock size, and the number of fines is somewhat of an art that a civil engineer is better equipped to handle.

The number of fines can be critical for resistivity. Wet crushed rock with fines will have low resistivity. Left up to a civil engineer, you may get crusher run with a lot of fines, which will make a nice surface to drive on but not provide the insulation qualities for electrical safety. I agree that electrical engineers with little practical experience may botch things up, but electrical engineering knowledge is also needed.

 

[bacon4life]

The shape of the rock also matters. Naturally occurring rocks have rounded edges whereas mechanically fractured rocks have angular surfaces that lock together better. The rock we use has almost no fines but still locks together fairly well.

 

[timm33333]

Thanks. IEEE-80 figure B-2 shows a square substation yard for grounding, but it does not tell that how to determine the physical size (length X width) of the yard.

The substation has one transformer, one switchgear, and one control panel. Should we get the actual dimensions of the equipment (transformer, switchgear, control panel) and then add clearances for guarded/unguarded equipment as mentioned in table 124-1 Part A of NESC, and in this way determine the physical dimensions of the yard?