12kv transformer isolated ground 6

[garetoo]

I am an electrician being asked to install a ground that appeared on the latest reved single line. The transformer is a 480V wye step-up to 12kv delta. The ground wire is to be insulated and not be bonded to the equipment ground. Since the transformer resides in a common switchgear enclosure and is not insulated from it, I am concerned that a fault to the steel would not be a direct ground fault.

The ground rod for the center tap will reside a couple of feet away from the ground rod for the equipment ground. It appears to me that a fault to the metal enclosure would go out on the equipment ground and then through the earth to the center tap. I think there will be a voltage potential. Am I wrong to think that safety is at risk here? I would rather be wrong and be embarrassed then right and not say anything.

I uploaded a portion of the single line.

http://files.engineering.com/getfil...-b466-d5a7c63700ac&file=a_12kv_iso_Ground.pdf

Thank you.

 

[garetoo]

It is out!
Thanks a LOT!!!
Changed T1 to Delta/Wye, added EGC, it is MUCH nicer

gare

 

[itsmoked]

Yep and all due to your perseverance. Well done!

You should pursue an EE degree in power..

Keith Cress
kcress -

http://www.flaminsystems.com

 

[garetoo]

itsmoked: I am on overload as it is, I could never do what you guys do.

A few of you mentioned 25kv insulation, is this no longer a need? The spec is 15kv.

gary

gare

 

[jghrist]

I sort of hate to bring this up, but I notice on your latest one-line that the 12 kV cable is protected by fuses at the MV switchgear. One problem with feeding a delta-wye transformer with cable is the potential for ferroresonance if one phase is open. See

http://www.mikeholt.com/technical.p...erroresonance and Its Effects On Transformers

 

[garetoo]

I do not know how to do quotes in here, sorry.

jghirst"One problem with feeding a delta-wye transformer with cable is the potential for ferroresonance if one phase is open."

I will read the thread referenced, especially since I do not know what ferroresonance is nor its impact when fuses are present. I really appreciate the input. Imagine having to bring it up in a hostile environment.

gare

 

[garetoo]

JG - it appears it becomes more of an issue at voltages aboe 12kv, yes? I think I saw a thread on it in here, I will go find it before I say or ask something not necessary.
No, I will ask one, wha does "remote" mean #3 below)
QUOTE FROM MIKE HOLT FORUM
To recap, the following issues combine to increase the probability of ferroresonance:
1 Higher-voltage distribution systems (such as 34.5kV)
2. Ungrounded primary transformers
3. Single-phase switching or protective device operation remote from the transformer
4. Lightly-loaded transformers
5. Underground distribution (higher capacitive reactance)
END QUOTE

gare

 

[jghrist]

It's more of a problem at 34.5 kV, but it can be a problem at 12 kV. Remote means there is cable between the switching or fuse point and the transformer. Ferroresonance happens when cable capacitance to ground is in series with a transformer winding. The more capacitance (more cable length) there is, the worse the problem.

Are the switches in the MV switchgear 3Ø or 3-1Ø? Switching 3Ø is OK. A possible solution would be a mechanism to open all three phases when one fuse blows. Another possible solution is to use a relay controlled 3Ø vacuum interrupter instead of fuses. G&W and Elastimold make these.

 

[garetoo]

Well, I asked the question, I thought I posted it in here but I do not see it:
This is how I phrased my question:

MY QUESTION:
I believe this is my last question with respect to the now Delta/Wye transformer(s). Should there be any concern for ferroresonance? I will accept a simple no.

http://www.eng-tips.com/viewthread.cfm?qid=240418

QUOTE:
A good general reference on ferroresonance is IEEE C57.105, Guide for Application of Transformer Connections in Three-Phase Distribution Systems. §8 lists grdY-grdY and grdY-Y as some of the configurations least susceptible to ferroresonance.
END QUOTE

This is the response:
Gary,
There is no problem, the wye/delta is a typical installation.
Thanks
(Name removed)

I felt the answer is not an answer and is condescending, so I did respond with this:

Thank you, I understand the delta/wye as typical, the FR referenced is an apparent phenomenon at the higher voltages and with the integral fuses I knew it was a possibilty. I have no other issues.
gary

With respect to jghirst:
QUOTE
Are the switches in the MV switchgear 3Ø or 3-1Ø? Switching 3Ø is OK. A possible solution would be a mechanism to open all three phases when one fuse blows. Another possible solution is to use a relay controlled 3Ø vacuum interrupter instead of fuses. G&W and Elastimold make these.END QUOTE

It is a three pole disconnect but I believe the transformer has integral fuses which could apparently cause an issue if one blows.



gare

 

[jghrist]

The integral fuses at the step-down transformer are not a problem because there will be no cable capacitance between a blown fuse and the transformer. The problem is blowing a fuse at the MV switchgear on the source side of the 12kV cable.

 

[garetoo]

The somewhat condescending answer of the EOR:
Gary,
There is no problem, the wye/delta is a typical installation.
Thanks
xxxxx

So we went to the manufacturer:
Please see the response below from our Consulting Specialist. However, this question really should have been posed to the Engineer of record for this project:

Although most people feel that ferroresonance can not occur at 12 kv and can only occur at voltage classes above 25 kv, this isn't exactly true. However, the conditions that need to occur in order for your customer to experience this phenomenon are extremely rare. Anthony's concern regarding the length of feeders having sufficient capacitance is one condition. The others are a loss of phase, low losses in the circuit and low resistive load along the lines of no-load on the transformer.

All of this being said, the mv disconnect on this project does have Fuselogic, which will open the circuit on phase loss. The exposure of downstream equipment would be equal to the clearing time of the fuse even if all the required stars aligned.

AWESOME information. I wish I were an EE....no, belay that. I wish I knew what good EE's know. You all have been just that.

gare

 

[jghrist]

the mv disconnect on this project does have Fuselogic, which will open the circuit on phase loss.

This should eliminate ferroresonance as a concern.