Transformer Question

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radiopet

radiopet

Senior Member
Location
Spotsylvania, VA
Hey Guys,

I had a call to me today from someone asking me a question on a specific transformer setup and I told him to install ground detectors but i made me think ...and I figured you all may have some additional info.

1.) He has a 27 KVA step up transformer which has 208V primary Wye and a 480V secondary Delta config.

2.) The secondary conductors are leaving the transformer and feeding an elevator control panel.....he has only run (3) conductors for the 480V naturally as well as an EGC.

He informed me he did connect to the water pipe ground from the transformer and to the case of the transformer....

He has a primary overcurrent device ( fuses ) and has a grounded conductor run to the OCPD...but then into the transformer no grounded conductor as it is not needed in his setup.

My question is ( and I told him to install ground detectors )...he wanted to make sure that the fuses would blow and protect the transformer...he asked if he should install a system bonding jumper connection from the HO in the transformer ( thats right..no XO exists ) and the case and I said no.

Any suggestions on this......would anyone have additional info to add.
 
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I may have missed something in your post (I most likely did..) did the installer run the grounded conductor on the primary side to the transformer? If this is the case, I would say that a system bonding jumper would definitely not be wanted as this would bond the grounded conductor of the building service to the EGC again. I believe that the primary OCPD would open under a ground fault condition due to the EGC that was routed with the primary to the transformer. I'm still scratching my head as to why the grounded conductor would even be needed.

Pete
 
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He did not run any Grounded conductors to the transformer. Are you saying he does not need ground fault detectors on the secondary..he did run EGC's in both the primary to the transformer and to the secondary as well...to the best of my knowledge.
 
In general, unless you are a utility, on a wye-delta step up transformer you never connect the wye neutral point to anything. Connection of this wye point requires special engineering considerations.
 
Paul,

I agree with you and I think ground detectors would need to be installed on the secondary side of this system in accordance with 250.21(b).

Pete
 
jim dungar said:
In general, unless you are a utility, on a wye-delta step up transformer you never connect the wye neutral point to anything. Connection of this wye point requires special engineering considerations.
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This may be a stupid questions but since I deal with so many classes teaching about the XO termination.....How come the HO does not appear in most graphics in terms of this type of issue. Maybe I just am not clear on the HO versus the XO....as we teach the XO all the time....

How about some XO and HO ...101....;)
 
radiopet said:
This may be a stupid questions but since I deal with so many classes teaching about the XO termination.....How come the HO does not appear in most graphics in terms of this type of issue. Maybe I just am not clear on the HO versus the XO....as we teach the XO all the time....

How about some XO and HO ...101....;)
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According to ANSI, H normally means the high voltage windings, and X means the low voltage, however some people and step up transformer manufacturers use H as the primary and X as the secondary regardless of voltages. This is why I did not refer to the primary wye point as either X0 or H0.

But to answer your question, the vast majority of non-utility transformers are delta-wye step down so they do not have an H0 bushing which is why you never talk about one.
 
So in regards to the Secondary on a step up transformer the HO is simply not used. Easy to know the XO's function on a normal Transformer but like in my original example.....the load from the transformer is to an elevator controller...which is 480V So the concern was how does he clear a fault on the controllers OCPD on the secondary conductors without a system bonding jumper in the transformer which is why the ground detectors were mentioned.

Guess i wanted to know if the HO , which is something I dont deal with alot could have been a corner ground on the Delta secondary....and if bonding it to the case would achieve what h desired.......any thoughts jim.
 
Guess I take for granted that a load from a fault on the secondary will make the primary OCPD trip IF the load exceeds the rating of the primary. I still think he needs ground detectors but it is not my call...only advice.
 
First fault on the secondary (high voltage side is free), which is why a ground detector should be used to detect this first ungrounded conductor short to ground.

The terminations are marked on the transformer trying to rename them in a class and then have the men go in the field to find something else would only be confusing.

I have stated here before that a 208 delta to 480/277 WYE is IMO the best set up, your post proves this. Most electricians are totally confused by using a 480 delta/208/120 wye "backwards". 208 delta/490/277 wye are readily available from the manufactures.
 
I would understand that the transformer being used is a common 480D-208Y/120 transformer beibg used as a step-up and I would bet that the taps are on the 480v side.

If availablity is an issue this is a commonly available transformer.
I would recommend that the 480v delta be corner grounded as a ground detector scheme for an ungrounded delta is an excellent idea whoat happens if a ground is detected? In a manufacturing facitlity there should be a qualified person present to address ground fault alarms and their cause.
But wye would you feed an elevator with an ungrounded delta? Is there a safey reason?
If not have you considered corner grounding the delta, i.e. the 'B' phase, replacing the 'B' phase fuse with a solid link? At lease in one phase goes to ground the fuse will clear it. Arcing faults in an ungrounded delta are extremely unlpleasent and dangerous.

It is of my opinion that even though you didn't have any L-N loads if your were to have specified a 208D-480Y/277 transfomer you could have simply grounded the H0, carrying out trhe H1,H2,and H3 through your fuses ignoring the neutral. Of course the EC is connected to the H0 and in grounded as required by the NEC. Of course this may be redundant and 20-20 hind sight.
 
Temp...Are you saying the electrician should put in a system bonding jumper from the HO Temination to the case......this setup from what im told is a 208 Wye to 480 Delta setup.....

As for the Elevator.....I asked him about that ans he said the elevator runs off 480 and their was none in the bulding so the transformer was needed and it not feeding a panel....only the elevator control module.

It is a 27KVA unit......I asked Mike about it and he said an ungrounded system is fine and agreed on the ground detector but offer nothing in the way of possibly getting the OCPD in the controller to trip.......which without a system bonding jumper on the secondary....would be difficult....you would only hope the fault on the secondary transfred over the primary side as a load in excess of the primary OCPD and it trips......but how long would this take.........I dont feel comfortable in telling him to corner ground the delta....and if he did....I am not comfortable in thinking the HO would achieve that.....
 
radiopet said:
...this setup from what im told is a 208 Wye to 480 Delta...
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If this is truly the case, there would be no H0 terminal (i.e. where "H" designates secondary terminals)
 
Temp...Are you saying the electrician should put in a system bonding jumper from the HO Termination to the case......this setup from what I'm told is a 208 Wye to 480 Delta setup.....
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Standard transformer markings Delta H1, H2, H3, Wye X1, X2, X3, X0.

In a corner grounded system you ground H1, H2 or H3. So if you grounded H1 once again the average electrician would freak out when he measured, A phase to ground "0" volts, B phase to ground 480 VAC and C phase to ground 480 VAC.

I have been on several projects where the contractor was baffled by his meter readings. And it was due to an ungrounded system or a corner grounded system.

In addition there could be controller operational issues with a corner grounded or ungrounded system.
 
As mentioned before if you had a ground detector system then what?? It detects a grounded conductor when is somebody eventually going to respond to it and what does that person going to do?

With a corner grounded delta should one of the ungrounded phases go to ground at least you will provide the opportunitiy for the OCPD to clear the fault.

Is there a reasom that an ungrounded 3ph 3w system is required? Tis question still hasn't been address.

I am strongly of the belief that a 480D-208Y/120 was easier to obtain (stock) or the person who specified it concluded that just because he was supplying the transformer with a 208Y/120 system. He simply needed to supple 480v 3ph3w that a 208Y/120-480D is what he should purchase. The tell tail would be, if the existing transformer had taps would they be on the 'H' side of 'X' side?

When we build a common step down transformer we design it with the LV windings on the inside next to the core and HV windings on the outside where the taps are.
When we build a step-up transformer we put the HV winding on the inside and the LV windings on the outside. Also, since the LV windings are now on the outside the magnetizing current inrush will be less than if that winding was on the inside.

It is very common for a 480v 3ph3m motor control center to be supplied from a transformer with a 480Y/277v secondary. You bring out the line conductors, the X0 in grounded, and you bring out an EGC to ground the MCC. No neutral conductor is required.
The great thing about that is ground faults are 277v and not 480v. In ungrounded deltas the voltage across an arcing fault can for exceed 480v by 2-3 tome as I can recall.

Personally, I don't care for ungrounded deltas unless there is a very specific reason to do so. They can be very dangerous even with a ground detector system.
What would a replacement transformer cost that would be a D-Y when compared to the grief of messing with a ground detector system, what would be done is it detects a ground, and if left unattended what happens should one line to to ground and a second line goes to ground in another location.
 
Temp..I dont believe him buying another transformer is the option here. The elevator they are installing needs 480V and they only have 208 in the building so he needs to step up 208 to 480 and anything else is not an option from what he has told me.

In regards to the HO....I asked him to tell me exactly what he see and he looked at the cover and then opened it up and said all he saw is H1 ,H2, H3 and HO as well as X1, X2 and X3.........XO was not an option as it does not exist inside the transformer.

I think the concept of the Ground Detector was an issue of the compliance with the NEC in the application...not that it would make it safer by any means.

He said the diagram on the front of the unit shows a Delta/Wye symbols and he has 208V coming in and 480V going out to feed the elevator conrolled that powers the elevator...and that the elevator is indeed 480V....in fact he said he has (2) and before he called me one is up and working with no problem....the reason he called me was wondering what the HO was for as his boss told him to NOT connect it.....and thus he has an ungrounded system and wanted to know how he could make it a grounded system....thus with the ability to clear an OCPD...

I do not feel confortable enough telling the guy to take a transformer and connect a SBJ to the H2 phase and try to corner ground it...nor do I feel comfortable enough to say anything about the HO terminal.

Now.....if it was a normal Delta/Wye or Delta/Delta I would not have a problem as we teach that all the time and it is rather straight forward...this one was not in my mind.

He did say the elevator load at 480V is only 32 amps....so primary side ampacity is 75A and secondary is 32A.....he placed his primary protection at only 60A fused.....The actual manual of the controlled he said shows no secondary bonding jumper......said it actually pictured it as a ungrounded system but did state the importance of the EBJ.
 
If the transformer has HO on the 480 VAC side then the transformer is not a 3 wire delta. It may be a wye/wye or and this is a rare unit but are available,
208/120 to 480/277 buck boost.

Had one last week baffled me for a few minutes.
 
Well this is what he is telling me Brian. I had him look right at the cover and it says 208V Primary and 480V Secondary...and it indeed has only an HO connection.

IN fact I asked him what the fron symbols showed...it was a Delta/Wye symbol but he said the building only has 120/208V in the building....and already has one elevator working and he said it says 480V right on the elevator controller so I can only assume he is telling me the facts....

I have delt with alot of transformers as well and this is why it sparked my post regarding it......just not the normal 480/208 type setups i teach about...and indeed it only had an HO terminal....no XO at all as I confirmed it with him yet again.
 
radiopet said:
...

In regards to the HO....I asked him to tell me exactly what he see and he looked at the cover and then opened it up and said all he saw is H1 ,H2, H3 and HO as well as X1, X2 and X3.........XO was not an option as it does not exist inside the transformer.

...

He said the diagram on the front of the unit shows a Delta/Wye symbols and he has 208V coming in and 480V going out to feed the elevator...
Click to expand...
Bond H0 to Ground. No need for ground detector
 
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