Transformer Grounding

[blues]

I have a 12.47/480-277v transformer I plan to ground the 15kv shld. cable on the pri. side to a ground rod. On the 480 side I will ground the neutral "xo" and all grounds to another ground rod on the secondary side. I will then tie the ground rods together. Would anybody have problem with that?

Thanks,
Dan

 

[roger]

I have a 12.47/480-277v transformer I plan to ground the 15kv shld. cable on the pri. side to a ground rod. On the 480 side I will ground the neutral "xo" and all grounds to another ground rod on the secondary side. I will then tie the ground rods together. Would anybody have problem with that?

Thanks,
Dan

You are fine although you really don't need rods on both sides, I would use one rod.

Roger

 

[raider1]

I agree with Roger that there is no need to have a ground rod on the primary side.

Also is this installation in a building? If so then you would need to connect to the buildings grounding electrode system.

If this is an outside transformer then using a ground rod would be fine but also remember that a single ground rod must have a resistance of not more than 25 Ohms or the ground rod must be supplemented by an additional electrode. (See 250.56 and 250.30(a) for grounding of a separately derived system)

Chris

 

[iceworm]

I have a 12.47/480-277v transformer I plan to ground the 15kv shld. cable on the pri. side to a ground rod. On the 480 side I will ground the neutral "xo" and all grounds to another ground rod on the secondary side. I will then tie the ground rods together. Would anybody have problem with that?

Thanks,
Dan

Blues -
roger and chris have covered the 480 side, so looking at the 12,460V:

12,470V, is generally 12,470/7200 Y. I've seen some impedance grounded and some solidly grounded. I have not seen any that were ungrounded. And we don't know which you have. That limits any definitive answer. So generally speaking:

The basic regulatory requirement is 250.4, in particular 250.4.A.5 (for a grounded system). Next look at 250.190.C.1 and 2. The he transformeris are going to need an equipment grounding conductor with the feeder. This is not a cookbook or NEC prescriptive design.

Here is an example: Say the system is impedance grounded, limited to a 50A ground fault current. A normal protective setting is 2 seconds at 30A. Will the shields stand that without damage?

Since you are the engineer of record, you get to decide how to meet 250.4.A.5

ice

 

[roger]

Blues -
roger and chris have covered the 480 side, so looking at the 12,460V:

Actually we were addressing the primary and secondary, they can both use a common electrode whether one or both are HRG or one or both are solidly grounded.

Roger

 

[iceworm]

Actually we were addressing the primary and secondary, they can both use a common electrode whether one or both are HRG or one or both are solidly grounded.

Roger

I don't know what you mean by, " ... they ("the primary and secondary") can both use a common electrode whether one or both are HRG or one or both are solidly grounded." I'm struggling to understand your point.

If by "primary" you mean the "transformer primary winding" - it does not have any connection to a grounding electrode

If however you meant the "primary feeder" - that does not have any connection to an electrode at the transformer either.

Color me lost.

ice

 

[roger]

I don't know what you mean by, " ... they ("the primary and secondary") can both use a common electrode whether one or both are HRG or one or both are solidly grounded." I'm struggling to understand your point.

The point is, even if the transformer is Wye/Wye a common electrode is all that is needed, and in any case, the MV cable shields can also use the same GE as the transformer secondary

If by "primary" you mean the "transformer primary winding" - it does not have any connection to a grounding electrode

Once again it does if it's Wye/Wye

If however you meant the "primary feeder" - that does not have any connection to an electrode at the transformer either.
Color me lost.

ice

Read the OP again

Roger

 

[iceworm]

The point is, even if the transformer is Wye/Wye a common electrode is all that is needed, and in any case, the MV cable shields can also use the same GE as the transformer secondary xxx

Let me see if I am translating you correctly: In post 2, you were specifically refering to a Y-Y transformer, that has the three phases and a neutral connected to the 12,470 Y primary winding, and then you connected that neutral point at the transformer to a grounding electrode (a driven rod) And then connected the phase conductor shields to this grounding electrode. Additionally there are no concerns with the issues of art 250.190.

If this is the case, perhaps you could point me to a design spec - cause I have never seen any thing like that for premisis wiring (as opposed to a utility fed transformer - which isn't anything I am discussing)

xxx Read the OP again

Okay ................................reading..........................
...................reading again...............................................

Nope. Didn't see anything about Y-Y. Still

ice

 

[roger]

Let me see if I am translating you correctly: In post 2, you were specifically refering to a Y-Y transformer,

Nope, I was just making the point that one electrode was needed what ever the case may be, you are trying to read to deeply into my words.

that has the three phases and a neutral connected to the 12,470 Y primary winding, and then you connected that neutral point at the transformer to a grounding electrode (a driven rod) And then connected the phase conductor shields to this grounding electrode. Additionally there are no concerns with the issues of art 250.190.

Not that I see

If this is the case, perhaps you could point me to a design spec - cause I have never seen any thing like that for premisis wiring (as opposed to a utility fed transformer - which isn't anything I am discussing)

And how do you know this is not a utility or privately owned transformer under NESC rules? I work at Universities in NC that are their own POCO's and I must admitt this is partly included in my comments.

Okay ................................reading..........................
...................reading again...............................................

Nope. Didn't see anything about Y-Y. Still

ice

I guess you missed the part where the OP was grounding the sheilds to an electrode that was jumpered to the secondary electrode.

I will then tie the ground rods together.

Roger

 

[iceworm]

Nope, I was just making the point that one electrode was needed what ever the case may be, you are trying to read to deeply into my words.
That could be. I'll give it my all to stop


Not that I see
Here I think you are saying the issues of 250.190 are not a concern. (Or maybe I'm reading too deep (;-)

If so, I disagree. I'm not looking at code minimums, I'm looking at good design. I think it is important to have the relaying operate prior to the shields turn into vapor. And I think the issues of art 250.190 are an important part of that. Even the utilities are concerned with that aspect.


And how do you know this is not a utility or privately owned transformer under NESC rules?
I don't. But the OP doesn't feel like a utility. And his (or possibly "her") occupation is listed as "building engineer". I think likely not - but then again --- i don't think it matters much

I work at Universities in NC that are their own POCO's and I must admitt this is partly included in my comments.
I truly feel your pain. Twenty five years ago, I spent a bit of time as a university power plant - plant engineer. The EE department PHDs occasionally came up with bad dream designs that could only be built out of unobtanium - yuck

I guess you missed the part where the OP was grounding the sheilds to an electrode that was jumpered to the secondary electrode.
No, I did not miss that part. Considering my current understanding of medium voltage power system feeders, I think that tying the primary power feeder conductor shields to a ground rod (which includes the secondary neutral and likely the xfm case) and not considring the relaying aspects, to be poor design pratice. It wouldn't matter if it was utility or private.

As I asked earlier, you could point to a valid design spec and I would stand educated. I'd be truly happy with that.

ice

 

[roger]

As I asked earlier, you could point to a valid design spec and I would stand educated. I'd be truly happy with that.

ice

Iceworm, see page 53 of this document

Roger

 

[iceworm]

Yikes - this site is addictive.

Roger -
Your reference is a good spec. If I were building for the Great State of North Carolina, then I would install a loop fed, MV/LV, padmount exactly as shown, and described in the notes, on page 53. Of course one would also note that earlier in the document, only Y-Y transformers are speced for this application. It's a bit curious why NC State Construction office is stuck on Y-Y transformers - but that's a different issue. (I would suspect NC allows other transformer connections where good design dictates)

Okay, back on task:
For this discussion, equally important is Note 2 on page 53 - which follows exactly what I have been saying. It is important to insure there is a sufficient path for fault currents to operate the protective relaying - without vaporizing the primary cable shields. Note 2 on page 53 mandates a primary neutral connection to the shields that accomplishes this. And that is what 250.190 and 250.4.A.5 is about.

I am in no way knocking anything said in posts 2 or 3. It is that (2008) 250.30 applies to "Grounding Separately Derived AC Systems" - which is the secondary. 250.30 does not cover the grounding and bonding issues of the MV primary.

(Back off task) Left to my own, I likely would have picked a different system than specified by NC..... :grin:

ice ...(addictive or not - I really got to go back to work)

 

[roger]

(Back off task) Left to my own, I likely would have picked a different system than specified by NC..... :grin:

I have questioned NC's reason for using Wye/Wye for years.



Roger