Ground Grid Study for existing plant facility

[rockman7892]

I’m working on a design project at an existing industrial site which involves replacing a 480V unit substation including 480v switchgear, 2500kva transformers and incoming 27kv feeders. New 480v switchgear will be located in a mew modular building with transformers located just outside.

Customer is requiring that a grounding study be performed as part of the overall design. I’ve never seen a grounding study provided for buildings or structures in an industrial facility as grounding in such facilities is usually designed per NEC and industry standards (ground rods, ground rings,CEE’s)

The only time I have seen a grounding study performed is for new HV substations where the grounding study was performed for step/touch potential per IEEE80.

Is anyone familiar with the need or process for performing grounding study within and existing industrial facility with addition or replacement of distribution equipment? Tbt only process I’m aware of is performing fall of potential testing on existing grid to verify adequacy?

 

[7anoter4]

In my opinion you may start with
IEEE Std 142-IEEE Recommended Practice for Grounding of Industrial and Commercial Power Systems. Chapter 1 System grounding.
Since there is a transformer supply-cable or OHL- 27 kV, the ground fault on the cable, terminating or transformer will return to System [Utility] through ground, you have to check, around transformer, the touch and step potentials. If the transformer ground is separate from the building you have to check the touch potential here too.
A Ufer foundation grounding is recommended, in my opinion.

 

[cuky2000]

IEEE Std 80 can be used to do the calculation using the concrete slab as the media instead of crushed rock. The reinforced rebar could act as an equipotential mesh. Caution should be exercised if the concrete slab is isolated from the earth with a vapor barrier. Connection to the perimeter ground conductor is advised in several points to guaranty a quasi equipotential mesh.

The typical resistance and know available step and touch potential test most likely will be on the safe side because of the equipotential area on the concrete slab.

 

[Kiribanda]

1) If the 27kV is an outdoor sub you need to do a ground grid design and calculate the potentials.
2) If the 27kV feeders are cables then no need a for grounding study because no ground fault return
current is going through the soil. The transformers has to be grounded (safety ground) properly per
the applicable local codes.
3) Since the 480V swgr is sitting inside a modular bdg, the return GF currents are again going through
the 480V cables. That means no grounding study is needed.

 

[rockman7892]

Kirbanda

My understanding of requirement for grounding study is related to GF current returning through earth as you mentioned. Is there anywhere in IEEE80 or IEE142 that specifically differentiates needs for study and GPR calculation related to determination if GF current will return through earth or not?

For point #1 above Can you explain why 27kV outdoor sub would require specific ground grid calculation?

For point #2 above are you assuming that cables will contain a grounded conductor along with phase conductors or a cable shield which GF current will return to source on? What about MV service entrance cables that may only be 3-wire?

As another example I wanted to look at a typical industrial plant unit substation consisting of lets say a 13.8kV breaker in upstream switchgear feeding 13.8kV-480V transformer which in turn serves a 480V Switchgear lineup (non-service entrance). 13.8kV Switchgear is in an upstream electrical room and downstream 480V switchgear is in an electrical room with transformer located outside. If I'm correct then neither the transformer of LV Switchgear locations would require a grounding study since any fault on primary of transformer will return to upstream switchgear on 13.8kV EGC and ground fault current on secondary of transformer will return to transformer via secondary supply side bonding jumper?

 

[Kiribanda]

I should say that you have to read IEEE-STD 80 and other related tech articles to get the answer for why there is a GPR issue
when the GF current is passing through the soil.

Based on the above statement, I mentioned that if and only if the substation is outdoor, you need to calculate GPR, STEP & TOUCH potentials etc.
Based on the above statement, I mentioned that if the ground fault current is taken back to the source by the cables,
there is NO issue of GPR, STEP & TOUCH potentials etc.

 

[rockman7892]

Kiribanda

Thanks for the responses. The earth vs cable return path makes sense related to GPR

For most MV service entrances fed by cable do the cables usually carry fault back to source? I’ve seen most MV service entrances as 3-wire so I’m assuming in most cases the ground fault returns to utility source via earth?

You mention that GPR is only an issue with outdoor substations. Is that because in indoor locations the building foundation will provide an adequate equipotential grounding plane?

 

[Kiribanda]

I donot think that I can answer your questions unless I see your SLD.
Please upload the SLD marking the ground connections too.

 

[rockman7892]

Kiribanda

Attached is excerpt from SLD showing 27kV incoming feeders to new 480V unit substation. Transformers will be grounded through GEC. All equipment on secondary of transformers will have Supply Side Bonding Jumpers (SSBJ's) and Equipment Ground Conductors (EGC's) per NEC.

 

[Kiribanda]

From your SLD, I assume,
1) The gang operated grounding switch is in close proximity to the transformer.
2) The 3-phases of the lead covered cable is terminated to the line/grounding switch.
3) The lead sheath is properly grounded to the ground switch
That means the transformer and the switch consists of a small outdoor substation.
Therefore, your customer is correct and you need to do a ground grid design/ study.
You have to make sure that the GPR is within limits so that when everything is bonded together,
the GPR will not be transferred as a dangerous transferred potential down to the
480V motor enclosure at the far end of the installation.

But if the gang operated switch is remote from the transformer, you can drive four 10ft copper
ground rods at four corners and interconnect them and bonded to the switch metallic part (exposed conductive parts).
(Your local code should describe the way to do this) Then covered the area with crushed gravel
layer of 150mm. Also install a 6ftx6ft gradient control mat on the crushed stone layer for the operator to stand
when the switch is being operated. Bond the switch to the mat to ensure that the operator is not subjected to
any dangerous step & touch potentials.

 

[nhcf]

From the tagnames and configuration shown on the one-line I am pretty sure I know the plant this work is being performed at and agree that a grounding study requirement would be unusual for that size/type of plant, but there are many things unusual about that plant.

Hopefully MCC upgrades are part of the project as well.