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12kv transformer isolated ground
6

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.engineering.com/getfile.aspx?folder=56f0e26d-5571-4b76-b466-d5a7c63700ac&file=a_12kv_iso_Ground.pdf

Thank you.
 

RE: 12kv transformer isolated ground

Safety violation, code violation, common sense violation; I could go on and on.  You should stay far, far, away.

RE: 12kv transformer isolated ground

Boy, if a brass monkey was standing with one foot near the ground rod and the other foot near the steel, a good ground fault may emasculate him. A star for picking that up.

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

RE: 12kv transformer isolated ground

(OP)
I am not looking for any commendation for noting this potential issue (thank you), I was hoping to get some input on the proper method to ground this, or does it even need one? It just gets stepped down to 480 at the end of its run.

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

There are times when authority and hubris overwhelm knowledge and common sence.
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

The simplest solution would be to bond the EGC to the local electrode and transformer neutral point.

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

(OP)
Thanks JG. The ground is on the 12kv side and I am merely installing - not designing. I do think the bond needs to be made from the EGC to the transformer neutral point. You are also correct in that the engineer has stated that they put it there for power quality. Without the bond you have suggested, the the distance between the two rods becomes an impedance path and is not safe. Therefore, I have refused to complete the install, a tough thing for an electrician versus approved drawings.

gare

RE: 12kv transformer isolated ground

You are a better person for it.  Plus! You won't be defending yourself in a death through negligence trial.

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.  www.everyspec.com/NIST/NIST+-+FIPS/FIPS_PUB_94_3307/
  
 

RE: 12kv transformer isolated ground

(OP)
Busbar,
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

A less confrontational solution may be to just install an appropriately sized jumper from the ground rod to the adjacent steel or ground grid. Then the installation may be code compliant. I see this quite often with the "Magic Triangle" (Three ground rods in a 10 foot triangle for a dedicated 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

(OP)
I whined enough they have asked us as the installer to present how we think it should be done, an electrician second guessing a PE..... not a good feeling. This is the first residence I have ever done with a 12kv primary. I will look up the "magic triangle". I will try and incorporate what seems valid from everyone's input and let you know what happens.
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

Talk to the utility in order to understand their supply system.  They may not want a center-tap 12kV ground on their system and you may not need it.   

RE: 12kv transformer isolated ground

I just noticed that the ground is on the 12 kV winding of the transformer.  I've never heard of center tapping a 12 kV delta winding.  That gives you a 10.4 kV high leg.  

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

(OP)
jghirst: You are absolutely correct. Even if the shield is landed at both ends, could it handle a fault? It is landed in every vault along the way, to a ground rod, but there is no detail for the ends.

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

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.

RE: 12kv transformer isolated ground

(OP)
QUOTE:
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

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 tranformer.

RE: 12kv transformer isolated ground

(OP)
QUOOTE:
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

There is no detail of T1 where the diagram shows a corner ground.  The T2 detail shows loadbreak elbows on the 12 kV side.  If T1 is the same, I'd be interested to know how one of the bushings would be grounded.  Use a feed-thru insert and a grounding elbow?  You would have to use 25 kV class elbows on the ungrounded phases.  15 kV class elbows are only rated 8.3 kV to ground.
 

RE: 12kv transformer isolated ground

(OP)
I have NEVER installed a 12 kv system, so the ratings of bushings and elbows are all a new learning experience. We have subbed this portion of work out to someone who does medium voltage work, I am watching everything to have a better grasp if it ever arises on another project. It is an interesting item to have in a residential application.

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

 

Quote (jghrist):

You would have to use 25 kV class elbows on the ungrounded phases.

If I follow this correctly, wouldn't 25kV cables be required too?
    

RE: 12kv transformer isolated ground

No, it wouldn't necessarily require 25kV cables, but it would require more than 100% insulation on the 15kV cables.  Which higher insulation level would be determined by the maximum duration of the overvoltage.  For the wild leg that duration would be continuous.  I believe there is a 175% insulation level (or something close) that would work.  25kV cable might be a more practical solution though.

RE: 12kv transformer isolated ground

T2 says 3ph-3w however it appears to be wired like 3ph-4w according to how I read the conductor call-outs.  If you change T1 to 12kv 3ph 3w wye you don't have a circuit path for ground faults (see previous threads regarding ungrounded services).  Since this seems to be someone's fix for running a 1700' line without significant voltage drop why not take a 12kV primary feed 4w from the utility and primary meter the entire site?  With all the up and down transformations you're going to wind up with so much voltage drop and such a weak system due to all the series impedance load service will probably be an issue.  Coordination will be difficult at best too.  I agree with other posts that the 12kV ungrounded system would require higher levels of insulation, a ground-fault detection system (voltage sensing) and other items not shown on the diagrams provided.

RE: 12kv transformer isolated ground

Quote:

We have subbed this portion of work out to someone who does medium voltage work, I am watching everything to have a better grasp if it ever arises on another project. It is an interesting item to have in a residential application.
This is a residential application?  I'd hate to be the homeowner who is stuck trying to find someone to maintain a corner grounded 12 kV system.  I suspect that your MV subcontractor will think someone is out of their mind if they are asked to put a grounding elbow on a 12 kV bushing of an energized transormer.  So will any maintenance contractor the homeowner hires.  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.  I haven't.  Is there anyone out there who has?
 

RE: 12kv transformer isolated ground

(OP)
QUOTE apwrengr: "why not take a 12kV primary feed 4w from the utility and primary meter the entire site? " END QUOTE

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

I don't think there is any problem referencing a thread.  Nothing on a public forum with anonymous contributors should be taken as a definitive reference, but it can be helpful.
 

RE: 12kv transformer isolated ground

(OP)
Thanks jghrist.

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

A couple of suggestions;
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

I guess I'd missed one little detail until reading Bill's post above.  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.

RE: 12kv transformer isolated ground

(OP)
QUOTE davidbreach:
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

Three-phase interrupting device, aka a circuit breaker, with voltage and current input to a relay that could trip the breaker on reverse overcurrent, reverse zero sequence overcurrent, phase overvoltage, phase undervoltage, zero sequence overvoltage.  Maybe more; hopefully obnoxious enough to make customers never want to install a grounded wye-delta transformer to serve load.  They shouldn't even be considered for load, though they are entire appropriate for generation and as generation would include all of the appropriate protection anyway.

RE: 12kv transformer isolated ground

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

gare

RE: 12kv transformer isolated ground

The next change should  be to connect all ground systems in the area together. That will be a big step towards code compliant and safety.
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

(OP)
Thanks waross, we are waiting to see what the final changes will be before anything else is said. I hope they have accepted the invitation to at least view this thread.

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

With out a ground source on the 480V side, or a delta tertiary, the 12,000:480 grdY-grdY won't provide much of a system ground for the 12kV circuit.

RE: 12kv transformer isolated ground

And, a wye:wye transformer needs a connection to the system neutral, (NOT GROUND) to provide balanced and stable voltages on the secondary. If you are feeding this from a delta;delta you do not have a system neutral to connect to. The secondary voltages get a little weird.
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

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

RE: 12kv transformer isolated ground

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

gare

RE: 12kv transformer isolated ground

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.

RE: 12kv transformer isolated ground

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

RE: 12kv transformer isolated ground

(OP)
Sorry:
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

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.    

RE: 12kv transformer isolated ground

(OP)
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

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.

RE: 12kv transformer isolated ground

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

RE: 12kv transformer isolated ground

(OP)
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 sad

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

(OP)
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

RE: 12kv transformer isolated ground

(OP)
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

RE: 12kv transformer isolated ground

Quote:

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?

Quote:

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.

RE: 12kv transformer isolated ground

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

RE: 12kv transformer isolated ground

(OP)
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

RE: 12kv transformer isolated ground

Quote:

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.
 

RE: 12kv transformer isolated ground

(OP)
ALL is well? smile
This would be wonderful.

gare

RE: 12kv transformer isolated ground

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:

Quote:

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.
lightsaberflush2

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

RE: 12kv transformer isolated ground

(OP)
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

RE: 12kv transformer isolated ground

Yep and all due to your perseverance.  Well done!

You should pursue an EE degree in power..

Keith Cress
kcress - http://www.flaminsystems.com

RE: 12kv transformer isolated ground

(OP)
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

RE: 12kv transformer isolated ground

(OP)
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

RE: 12kv transformer isolated ground

(OP)
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

RE: 12kv transformer isolated ground

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.

 

RE: 12kv transformer isolated ground

(OP)
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

RE: 12kv transformer isolated ground

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.
 

RE: 12kv transformer isolated ground

(OP)
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

RE: 12kv transformer isolated ground

Quote:

the mv disconnect on this project does have Fuselogic, which will open the circuit on phase loss.
This should eliminate ferroresonance as a concern.
 

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