13.8kV impedance grnd SDS grounding conductor

[coulter]

Greetings Ladies and Gentlemen: I am stumbling. I am not particularly familiar with impedance grounded, 13.8kv, SDS.

Existing Installation:
The service is 138kv. The substation xfm belongs to the customer. xfm is 20MVA, 8%, 138D/13.8Y, impedance grounded, 167ohm. 500' of 3 parallel - 3c/350CU w/#2 g, MC-HL to 13.8 OCP. xfm sec OCP is 800A. The MC-HL sheaths are terminated at the OCP end and are not terminated at the xfm end. Yes, they should have been, but they aren?t. I am told the plan is to eventually terminate the sheath - not sure how, it would be very difficult to get a TMC fitting over the conductors. (However, that is another issue)

Q: What code section(s) determine(s) the size of the grounding conductor from the xfm to the OCP?

Here are my thoughts:
1. It's not a service, it's an SDS.
2. So, starting at 250.30, Exception: sends us to 250.36 or 250.186.
3. 250.36 is for 480 to 1000V, so that doesn?t fit
4. 250.186 Impedance Grounded Neutral Systems, kv and over, shall comply with:
A. Location - okay we are there
B. Identified and insulated - okay done
C. System neutral connection point - okay
D. Equipment grounding conductors. ?EGCs shall be permitted to be bare, and shall be electrically connected to the ground Bus and grounding electrode conductor.?

Humm. Nothing about the size.

The next place we get a clue is:
5. 250.190, Grounding of Equipment, ?All non-current carrying metal parts ? shall be grounded.? Still nothing about size.

I?m thinking the code is telling me there shall be an EGC, but I have to pick the size. Which makes sense, one might think at 13.8kv the installations are engineered, not cookie cutter.

Engineeringly speaking, the grounding impedance limits the current to 47A. The protective relays take the xfm out in a second or so. The installed #2 in each cable is sufficient

The real problem:
I have a third party inspector (not an AHJ - but with sufficient corporate support we will be mandated to follow) that is suggesting the grounding conductor from the substation xfm to the OCP should meet 250.122. Which would mean a 1/0 for an 800A CB. (I?d likely size for a 1000A CB, just cause we could set the CB up to that if future loads were added).

The cable armor (sheath) was not terminated at the substation end, so there is no credit for the extra grounding conductor equivalent cross sectional area. So, the installed #2 is small by 250.122 criteria.

carl

 

[jtester]

I'm not sure I understand the configuration. Does the grounding conductor at the OCP go thru relaying, or is that detection somewhere else. I guess even better, What are you doing with the grounding conductor at the OCP?

If it is functioning as an EGC, I am used to seeing connections from each piece of substation equipment down to a buried ground grid. I've never seen individual EGC's run in medium voltage substations. They would create dangerous touch potentials.

Jim T

 

[coulter]

I'm not sure I understand the configuration. ...

I've tried to attach a sketch

 

[coulter]

Wonders of wonders - the sketch attached

See if my explanation makes any more sense looking at the sketch.

carl

 

[ryan_618]

What is the impedance of the impedance? Do the math (I=E/R) and figure out how much current will flow under the first fault condition. Be sure the conductor is sized to carry the current. Although I don't agree, the NEC calls this conductor a neutral. Neutrals must be sized to carry the neutral load, which is determined by the setting of the impedance.

 

[coulter]

ryan -
Most of your questions/comments are covered in the first post. In order of your post:

167ohms

I did, 48A (incorrectly listed as 47A in the first post)

#2 in each parallel cable.

... Although I don't agree, the NEC calls this conductor a neutral. ...

This one eludes me. Can you give me a reference on this? All of the references I am finding call the neutral conductor the connection between Xo and the grounding resistor. The one I am asking about is the grounding conductor from the ground end of the grounding resistor out to the OCP end ground grid.

... Neutrals must be sized to carry the neutral load, which is determined by the setting of the impedance. ...

That would be my understanding. Although I didn't mention it in the the OP, the neutral, as I defined above, is a 4/0 or so. Should be okay.

carl

 

[coulter]

Here is what I a specifically after: What code section(s) determine(s) the size of the grounding conductor from the xfm to the OCP?

I did my home work. See the first post for the detailed reasoning and references.

The answer I'm coming up with is: The NEC says there has to be a grounding conductor, but does not specify the size. See the original post for the conclusions.

I'm somewhat suprised the NEC doesn't specify a size. It's okay they don't, I'm thinking the code panels realize they are not in the design business and 13.8kv isn't cookie cutter stuff. I just want to either verify my reasoning and references, or learn to right way to look at it.

carl

 

[coulter]

My inspector came back and and said he agreed with my reasoning. He feels there is a hole in the code and the grounding conductor should default to 250.66. We are okay there the conductor fits that.

Too bad no one here was up on this. I could have used some feedback on my reasoning.

carl