12kv transformer isolated ground
12kv transformer isolated ground
(OP)
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.eng ineering.c om/getfile .aspx?fold er=56f0e26 d-5571-4b7 6-b466-d5a 7c63700ac& amp;file=a _12kv_iso_ Ground.pdf
Thank you.
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:/
Thank you.






RE: 12kv transformer isolated ground
RE: 12kv transformer isolated ground
Welcome to Eng-Tips.
Keith Cress
kcress - http://www.flaminsystems.com
RE: 12kv transformer isolated ground
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: 12kv transformer isolated ground
An excerpt of what I asked/pointed out to the engineer of record, with no answer is (quoted below):
Part of what I am referencing is at NEC 250.184 Solidly Grounded Neutral Systems
(B) Single-Point Grounded Neutral System. Where a single-point grounded neutral system is used, the following shall apply:
(1) A single-point grounded neutral system shall be permitted to be supplied from (a) or (b):
a. A separately derived system
b. A multigrounded neutral system with an equipment grounding conductor connected to the multigrounded neutral conductor at the source of the single-point grounded neutral system
(2) A grounding electrode shall be provided for the system.
(3) A grounding electrode conductor shall connect the grounding electrode to the system neutral conductor.
(4) A bonding jumper shall connect the equipment grounding conductor to the grounding electrode conductor.
(5) An equipment grounding conductor shall be provided to each building, structure, and equipment enclosure.
(6) A neutral conductor shall only be required where phase-to-neutral loads are supplied.
(7) The neutral conductor, where provided, shall be insulated and isolated from earth except at one location.
(8) An equipment grounding conductor shall be run with the phase conductors and shall comply with (a), (b), and (c):
a. Shall not carry continuous load
b. May be bare or insulated
c. Shall have sufficient ampacity for fault current duty
By not bonding the equipment grounding conductor (4) and not running that with each group of conductors (5) & (8) the installation seems non-compliant. The lack of a bond actually appears to make the system a low-impedance grounded system, if there is such a thing? (Taken from NEC handbook). If there is going to be a deliberately introduced impedance between the equipment ground and the insulated neutral conductor it appears it must be a high impedance, I am not sure the 2' earth separation is such, but I also do not think the earth unto itself is an actual bond as provided by proper materials and connections.
END QUOTE
gare
RE: 12kv transformer isolated ground
Your problem may be more a people problem than a technical problem. Watch your liability and cover your assets.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: 12kv transformer isolated ground
You could use a low resistance ground if you need to reduce available ground current, although this is not common at 480V. Using the earth as the low resistance is not acceptable. A large voltage can develop across the resistor during a fault, so the resistor has to be located where it someone cannot get shocked. This would be difficult if the resistor were part of the earth. Designing for a particular fault current would also be difficult if the earth is the resistor.
I suspect the reason for the original design is not meant to reduce ground fault currents, but a misguided attempt to isolate the grounds for power quality reasons.
RE: 12kv transformer isolated ground
gare
RE: 12kv transformer isolated ground
Keith Cress
kcress - http://www.flaminsystems.com
RE: 12kv transformer isolated ground
Unless the transformer was being used for some kind of well-defined research/temporary load {one that comes to mind for me years ago was for large-area soil heating for environmental remediation—but it was quite the opposite—three graphite ground electrodes were used connected to their respective bushings} I can't envision any reasonable justification for a floating system.
This might be of some help, and it is a little bit dated but this NIST document may give you some ammunition. ww
RE: 12kv transformer isolated ground
That is a great article! Especially page 30 diagram 4. I appreciate the comments with relative content toward a solution.
Thanks!
gary
gare
RE: 12kv transformer isolated ground
The magic triangle by itself is not code compliant due to the issues stated here. A large (often 4/0 AWG) jumper between the magic triangle and the power ground grid assures compliance and is common.
You will have made a safe compliant installation and if the jumper is then removed, responsibility will devolve elsewhere.
Size the jumper from the code as part of the equipment grounding grid.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: 12kv transformer isolated ground
Is bonding a leg of this delta 12kv even necessary? Should an equipment grounding conductor be run with the feeders to the step-down transformer 1,700 feet away?
gare
RE: 12kv transformer isolated ground
RE: 12kv transformer isolated ground
The drawing notes say that there is no ground run with the three 12kV conductors between the MV switchgear and the utility yard padmounted transformer. The cable 12 kV will be shielded. If the shield is grounded at both ends, then the shield acts as a ground wire. If there is a ground fault on the 12 kV cable at the utility yard padmounted transformer, fault current will flow through the cable shield and it has to be sized for the current.
If the shield in not grounded at both ends, then you need to run a ground wire. Otherwise, a ground fault at the utility yard end will have to flow through earth and the fault current may not be enough to blow the 65E fuse at the MV switchgear. High step- and touch-potentials will also occur during a ground fault.
Without a metallic return path, a ground fault at the utility yard on the high leg of the 12 kV system could result in touch-potentials at the padmounted transformer as high as 10 kV. This high voltage could also be transferred through the secondary EGC to the utility building.
RE: 12kv transformer isolated ground
apowrengr: This is not owned by the utility, I wish it was. The utility is taking their 12kv, running it about 75 ft to a 12kv/480 trans, then about 30 feet into the main switchgear and metering it. After that, the client is bumping it back up to get it to the next location where it is knocked down again.
Interestingly, there is a detail at the other end showing a bond between equipment without explaining how it is accomplished.
gare
RE: 12kv transformer isolated ground
I can't cite specific code references, but I think you'll find that for an ungrounded 12kV system {id est, 12kV-delta windings back-to-back} the insulation level of the interconnecting cables should be 173% at 15kV, or possibly 100% at 25kV.
RE: 12kv transformer isolated ground
RE: 12kv transformer isolated ground
apowerengr (Electrical)
15 Mar 09 21:08
If the intent of this work is to provide a ground on the separately derived 12kV system it should be done at the source as is proposed and then a suitable grounding conductor needs to be run with the circuit.
END QUOTE
Bond H1 or H2 or H3 to ground and run an EGC with the feeders?
gare
RE: 12kv transformer isolated ground
If you want a grounded system, I recommend installing a zig-zag or grdY-delta grounding tranformer.
RE: 12kv transformer isolated ground
I would not corner ground or ground the midpoint of one winding of a 12 kV transformer. I can't find offhand where the NEC prohibits this, but IEEE Std 142-1991 recommends against midpoint grounding systems over 240V. Ø-ground voltage will be 12 kV on two phases if corner grounded, 10.4 kV on one phase if mid-point grounded. Insulation, connectors, and arresters must be rated appropriately.
If you want a grounded system, I recommend installing a zig-zag or grdY-delta grounding transformer.
END QUOTE
Thanks JG, we are proceeding with recommending changing the spec on the transformer to a wye secondary so as to do this as has been suggested by you and others.
I have attached my "interpretation of what is being asked for at present, it may be a tad confusing because I have been rushing.
gary
gare
RE: 12kv transformer isolated ground
RE: 12kv transformer isolated ground
We have requested a detail for T1 as they have provided for T2. They made the comment that they should not have to detail everything for us, perhaps that is correct. We are thinking they are more prone to ordering a different transformer now, but who knows just yet.
I modified the drawing a little as the GC wanted all of my comments clouded. I will post the resolution when it is made, hopefully tomorrow.
Thank you for monitoring the thread and providing the input you have. I have forwarded on the information with no reference to you, as I want all liability in appropriate courts. I take no credit for anything, I merely state "Others I have communicated with have...."
Is this ok with you?
gare
RE: 12kv transformer isolated ground
If I follow this correctly, wouldn't 25kV cables be required too?
RE: 12kv transformer isolated ground
RE: 12kv transformer isolated ground
RE: 12kv transformer isolated ground
RE: 12kv transformer isolated ground
The utility said they will not do this because "there is not enough demand" Even we electricians thought the waste through multiple transformers was just that, a waste.
QUOTE jghirst: I've only been dealing with MV distribution for 40 some years, so maybe someone with more experience has heard of a corner grounded or midpoint grounded 12 kV system. END QUOTE
I have made use of most of the input from here and they are now reconsidering a different transformer and better grounding, Thank you all very much.
May I reference the thread to them? I have deliberately not referenced it specifically so that all responsibility rests on us as the sub, not on any comments from anyone in here. I would sure love to give them the link. If one person says no, I will not provide it.
gare
RE: 12kv transformer isolated ground
RE: 12kv transformer isolated ground
I want to honor the spirit of the forum, seeking/providing great information without the ever present liability issue in the back of someone's mind. At the same time, credit rightfully belongs to those who expense their knowledge, in this case at no cost to others.
I will wait this day out and barring no "no's" I will reference the thread to all parties involved at my end. I am convinced it is contributing to an installation that will not be as much a hazard to life as it started out. Sadly, I do not think the EOR would have stepped up and said, "oh, wait, this does not appear safe". Thanks to all of the input from here, they have backed off from crucifying me.
gare
RE: 12kv transformer isolated ground
The utility may consider primary metering if you offer to pay the cost of the metering transformer(s).
A wye:delta transformer is often problematic. If the primary neutral is left floating, (recommended) there may be transient overvoltages due to switching surges when the transformer is energized. If the primary neutral is connected, the transformer becomes intolerant of voltage unbalances and phase angle errors. It will overheat on PU unbalances of the same order of magnitude as the transformer PU impedance. If a primary phase is lost, the bank will back feed full voltage into the "lost" phase until the transformer primary fuses blow.
A wye:wye transformer is a good choice.It provides a neutral ground point.
A delta:wye step down at the other end is a good choice. If you use a wye:wye to step down, you have to run 4 wires instead of 3 wires. (Not including the controversial ground wire.)
As well as posting a link, invite your engineer to join this site.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: 12kv transformer isolated ground
RE: 12kv transformer isolated ground
As a utility we would not accept a customer installing a grounded wye (source)-delta (load) transformer without extensive protection on the wye side to trip the transformer off line should there be any ground fault on the utility system. END QUOTE
How on earth could any one person ever have a complete grasp of all of this? What would constitute "extensive" protection?
gare
RE: 12kv transformer isolated ground
RE: 12kv transformer isolated ground
gary
gare
RE: 12kv transformer isolated ground
I would suggest, (if you can) extending your main ground grid to include the area of the transformer and then connecting the transformer wye point to the main, extended grid.
One lethal touch potential can ruin the rest of your life, however short it may be.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: 12kv transformer isolated ground
gare
RE: 12kv transformer isolated ground
If I had my druthers, I'd install a 480:12,000 Δ-Δ on the "street" side, allowing the 480V overcurrent device to simultaneously protect the "street" transformer, the cables and the building transformer. Then, put a 12,000:480 grdY-grdY transformer at the building that would allow for "system" grounding of the 12kV circuit. {In the Article-240 tap rules, now NEC allows 3-wire Δ-Δ transformers to be protected by one overcurrent device.}
Also, run a buried bare copper conductor to bond all of it together.
RE: 12kv transformer isolated ground
RE: 12kv transformer isolated ground
There is another thread facing this problem. No neutral connection on a wye primary and unbalanced secondary voltages as a result.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: 12kv transformer isolated ground
RE: 12kv transformer isolated ground
gare
RE: 12kv transformer isolated ground
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.
RE: 12kv transformer isolated ground
gare
RE: 12kv transformer isolated ground
RE: 12kv transformer isolated ground
PGE = local utility, Pacific Gas and Electric. Somewhat identifies where we are at, I should have just noted that as
utility.
gare
RE: 12kv transformer isolated ground
This certainly would have been the preferable solution from a maintenance perspective.
RE: 12kv transformer isolated ground
gary
gare
RE: 12kv transformer isolated ground
RE: 12kv transformer isolated ground
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
RE: 12kv transformer isolated ground
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
RE: 12kv transformer isolated ground
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
RE: 12kv transformer isolated ground
gare
RE: 12kv transformer isolated ground
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.
RE: 12kv transformer isolated ground
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
RE: 12kv transformer isolated ground
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
RE: 12kv transformer isolated ground
RE: 12kv transformer isolated ground
This would be wonderful.
gare
RE: 12kv transformer isolated ground
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:
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
RE: 12kv transformer isolated ground
gare
RE: 12kv transformer isolated ground
Thanks a LOT!!!
Changed T1 to Delta/Wye, added EGC, it is MUCH nicer
gare
RE: 12kv transformer isolated ground
You should pursue an EE degree in power..
Keith Cress
kcress - http://www.flaminsystems.com
RE: 12kv transformer isolated ground
A few of you mentioned 25kv insulation, is this no longer a need? The spec is 15kv.
gary
gare
RE: 12kv transformer isolated ground
RE: 12kv transformer isolated ground
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
RE: 12kv transformer isolated ground
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
RE: 12kv transformer isolated ground
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.
RE: 12kv transformer isolated ground
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
RE: 12kv transformer isolated ground
RE: 12kv transformer isolated ground
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
RE: 12kv transformer isolated ground