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.

 

[jghrist]

The second diagram posted of the system shows a 4-wire 480/277Y feed to T1, so a grdY-grdY connection should be OK.

 

[garetoo]

It is a 4wire feed as jghirst mentions. I have passed along the suggestion of the wye/wye.

gare

 

[davidbeach]

Not if T1 were delta-delta as suggested by busbar. I'd try really, really, really hard for primary metering, has to be less expensive than the step up transformer, although there may be minimum load requirements for primary metering. Then for each one of the transformers I'd go with delta-grdY. You'd need fuses on the secondary of T1 (assuming T1 is the first transformer, I'm illequiped to deal with DWG files at the moment) since the low side can't protect the high side.

Better yet might be single phase service - primary metered if at all possible. In that case you could protect all the way through and have system grounds.

 

[garetoo]

I uploaded a pdf of the single line and the details referenced from other dwgs. Clouds are my comments/questions to the EOR.

gare

 

[davidbeach]

On your drawing, what does "PGE" stand for? Where is this to be installed?

 

[garetoo]

Sorry:
PGE = local utility, Pacific Gas and Electric. Somewhat identifies where we are at, I should have just noted that as
utility.


gare

 

[jghrist]

Are there any loads served from the 480/277Y ahead of T1? I suppose there is a good reason that the 480/277Y service from PGE was not at T2, which would eliminate the need for the customer owned 12 kV system and transformers altogether.

This certainly would have been the preferable solution from a maintenance perspective.

 

[garetoo]

No loads. I have only heard that the local utility provider felt the demand was too low for them to provide the 12 kv as a metered feed, therefore they mandated the installation of a step-down xfmr to 480. After the meter, they do not seem to care what the end user does with it.
gary

gare

 

[davidbeach]

Oh, good, it wasn't Portland General Electric. Pacific Gas and Electric is generally PG&E. PGE would not accept a 480V grdY-12kV delta transformer connected to a 480V service. I don't know if PG&E will or not. If the PG&E service transformer that provides the 480V is a grdY-grdY transformer you risk significant damage to you installation for certain utility faults unless you have the protection package mentioned above.

 

[waross]

A couple of comments;
Generator; The sketch appears to show the generator neutral bonded to ground at the generator. If this is done you must use a 4 pole transfer switch and switch the neutral. The When running on the generator the ground at the generator becomes the system ground. Only one connection is allowed between the neutral and the ground grid and the switched neutral is required to comply with this. You still need 5 conductors. 3 phase, 1 neutral and 1 ground. This becomes the system ground and if your code treats system grounds differently than equipment grounds you must use the system ground rules.
Alternate:
Remove the neutral grounding jumper at the generator. Use a three pole transfer switch and leave all neutrals solidly connected. The generator may now be treated as a piece of equipment and be grounded from the panel with an equipment grounding conductor as if it was a large motor.
Shield grounding;
The normal treatment of an MV cable is to ground the sheath at the supply end and insulate it at the load end. The sheath conductors are generally cut off and taped over when the stress cone is installed. In each man hole, the incoming cables would have the sheath insulated and the out going would have the sheath connection brought out and connected to the equipment grounding conductor. Connection to a local ground rod is optional. An exception is concentric neutral cable. But the cable may have to be derated if the concentric neutral is connected at both ends.
Below about 100 amps underground and about 200 amps above ground it may be used without derating.
Ground conductors in conduit;
Anecdote time, many generations ago our local Electrical Contractors Association did some testing. They found that 10 feet of steel conduit would act as a choke and limit the ground current to about 100 amps. They then bonded the cable to the conduit at each end and found that about 100 amps flowed through the conductor and the rest flowed through the conduit.
Comment, this is a magnetic effect, so aluminum conduit or PVC is electrically preferable to steel conduit for ground conductors.
If you must use steel conduit, remember that the grounding bushings may carry the major part of any fault current and exercise appropriate workmanship.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[garetoo]

QUOTE:
Generator; The sketch appears to show the generator neutral bonded to ground at the generator. If this is done you must use a 4 pole transfer switch and switch the neutral. END QUOTE

We are using an ASCO 7000 4-pole transfer switch per the EOR.

QUOTE: Then, when running on the generator the ground at the generator becomes the system ground. Only one connection is allowed between the neutral and the ground grid and the switched neutral is required to comply with this. You still need 5 conductors. 3 phase, 1 neutral and 1 ground. This becomes the system ground and if your code treats system grounds differently than equipment grounds you must use the system ground rules. END QUOTE

The EOR has these conductors. Code is NEC, we are still waiting for what year is applicable for this portion of the installation.

QUOTE - Alternate:
Remove the neutral grounding jumper at the generator. Use a three pole transfer switch and leave all neutrals solidly connected. The generator may now be treated as a piece of equipment and be grounded from the panel with an equipment grounding conductor as if it was a large motor. END QUOTE

I dare not suggest anything more :-(

QUOTE:
Shield grounding;
The normal treatment of an MV cable is to ground the sheath at the supply end and insulate it at the load end. The sheath conductors are generally cut off and taped over when the stress cone is installed. In each man hole, the incoming cables would have the sheath insulated and the out going would have the sheath connection brought out and connected to the equipment grounding conductor. Connection to a local ground rod is optional. An exception is concentric neutral cable. But the cable may have to be derated if the concentric neutral is connected at both ends.
Below about 100 amps underground and about 200 amps above ground it may be used without derating. END QUOTE

I need to thoroughly read this part. I have also attached the EOR's vault detail, I do not think it is as noted above.


QUOTE - Ground conductors in conduit;
Anecdote time, many generations ago our local Electrical Contractors Association did some testing. They found that 10 feet of steel conduit would act as a choke and limit the ground current to about 100 amps. They then bonded the cable to the conduit at each end and found that about 100 amps flowed through the conductor and the rest flowed through the conduit.
Comment, this is a magnetic effect, so aluminum conduit or PVC is electrically preferable to steel conduit for ground conductors. END QUOTE

We are using PVC but all penetrations through concrete are Steel, but much less than 10 feet, often less than 2'. However, I need to look at whether these nipples are bonded, should they be? *sigh*

QUOTE: If you must use steel conduit, remember that the grounding bushings may carry the major part of any fault current and exercise appropriate workmanship. END QUOTE

Again, I hope "they" are following this because I believe they view my posting any comments in an email as being an annoyance. Well, I guess if they are following the thread they can email me and let me know? I did send the link.

gare

 

[garetoo]

QUOTE:
Oh, good, it wasn't Portland General Electric. Pacific Gas and Electric is generally PG&E. PGE would not accept a 480V grdY-12kV delta transformer connected to a 480V service. I don't know if PG&E will or not. If the PG&E service transformer that provides the 480V is a grdY-grdY transformer you risk significant damage to you installation for certain utility faults unless you have the protection package mentioned above.END QUOTE

Oh, I know they use PG&E but I rarely stick the & in. I will remember the confusion this may create. I do not know what their primary winding is. This makes it appear as though it is an important piece of information. I will ask if the details were provided by them and taken into consideration.

gare

 

[garetoo]

Does the local utility ever have an interest in what the next transformer down line from their transformer is, as well as what protective devices are being used? Might they have based their transformer design on what is being fed versus what they are providing?

gare

 

[jghrist]

No loads. I have only heard that the local utility provider felt the demand was too low for them to provide the 12 kv as a metered feed

But why not have PG&E extend their primary to the T2 location, install and own T2, and provide service at 480V?

The normal treatment of an MV cable is to ground the sheath at the supply end and insulate it at the load end.

I'll have to disagree with Bill on this. Normally, the shield is grounded at both ends and in each vault. The transformer is only 400 kVA (19A at 12 kV) so derating should not be a concern.

 

[waross]

A transformer with a neutral connected wye primary and a delta secondary is a special case. This configuration will back feed energy into a missing primary phase. Any thing that will feed energy into a de-energized line is the concern of the utility, even if the source of the energy is other phases on the same system. There are a number of serious issues that may arise If a distribution circuit loses a primary phase at the source and there is a 4 wire wye:delta transformer on the circuit. The back feed is real power limited by the impedance of the transformer. It may be at or close to system voltage.
If two primary phases are lost, the wye:delta transformer will
back feed approximately 50% voltage to the two de-energized phases. This often causes the failure of refrigerators and freezers. All of this make that connection a valid concern for the utility.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[garetoo]

jghirst: PG&E does not want to "own" a bunch of 12kv wiring on the premises and the homeowner does not want PG&E to enter any further into the property as is necessary.

I somewhat appreciate the difference of opinion on the shield landing, ONLY because it means an electrician should not be making the call, ie me.

waross: Then I would expect that PG&E has looked at the single line and proposed changes may need to be brought to their attention? I will ask, though I hate asking right now. I am not near as concerned with a refrigerator as I am some person not expecting this back feed. I am not saying that equipment is unimportant.

gare

 

[jghrist]

The first change has been made, they are changing T1 to a delta/wye. Thanks Everyone!
gary

With this change, there should be no concern about the backfeeding. T1 is 480V delta to 12kV grounded wye. T2 is 12 kV delta to 480V grounded wye. All is well.

 

[garetoo]

ALL is well?
This would be wonderful.

gare

 

[waross]

Hello garetoo;
I agree with jghrist on the delta:wye transformers being OK.
I will accept jghrist's comments concerning the shield treatment also. No problem at 19 amps.
My comments concerning back feed were in answer to your question:

Does the local utility ever have an interest in what the next transformer down line from their transformer is, as well as what protective devices are being used?

Tip; Click on Process TGML at the bottom of the rely box to see how to use Quotes and other formatting. Emoticons/smileys has the fun stuff.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[garetoo]

Thanks jg and Bill, we have been told that a complete new single line with all necessary details is coming out tomorrow. I also appreciate that I probably do not need to contact PG&E as the new transformer should take care of the potential back-feed issues.

gare