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Old January 18th, 2008, 03:49 AM   #1
leitmotif
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Transformer wierd voltages

OK so here is what I know

Have Fed Pacific 15KVA transformer
Hi side delta with corner ground.
Hi side nominal 480.
Hi side windings are tapped.

Low side 120 208 wye 4 conductor. Volt readings taken on cord cap - 4 pole. Phases are arbitrarily numbered 1 thru 3 and ground is ground.
Phase to phase 1 to 2 211.8
1 to 3 211.9
2 to 3 211.6 all 211 plus or minus 0.1
Phase to ground
1 84.4
2 127
3 183
I have not read the high side voltages but from experience know they are pretty close in other portions of system and am ASSUMING they are here too.

I DID check continuity from ground pin of cord cap to frame of transformer. It was a good reading but I did not record it.
However all that tells me is that the ground conductor is good AND it is connected to transformer mounting bracket. I have no idea if it is tied in to the main system ground.

I have not opened it up and do not intend to do so until I have an idea of what to look for and not do some explore hunt n peck operation. Also boss says it is working do not fix what aint broke. At this time we do not intend to feed any single phase loads from this.

It supplys only a single motor load.
Seems to me that with phase to phase values all consistant that
all the tap settings are the same
the star point connection is good

Went in to Tierney transformer and asked them about this. They are puzzled but think that maybe the ground connection is bad.

IF I am lucky the contractor pulled in a ground conductor (FAT CHANCE) so all I can rely on is conduit grounding.

I feel a little better that a transformer mfr is puzzled
but,,
do you guys have any ideas of what to look for??

Dan Bentler

Last edited by leitmotif; January 18th, 2008 at 03:53 AM.
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Old January 18th, 2008, 05:15 AM   #2
chop347
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What are your current draws per phase?
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Old January 18th, 2008, 10:47 AM   #3
Lancie1
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Dan,

It does seem like a grounding problem. Because the phase-to-ground voltages are not equal, but phase-to-phase are, I guess that the secondary side WYE ground may be loose or intermittant, maybe even disconnected. That 183 high voltage reading on Phase 3 bothers me. Anything less than 130 volts might be okay if it is a loose or high-resistance ground on the neutral of the transformer. But that 183 volts makes me think you are reading the voltage back to the corner ground on the high side of the transformer. If so, it means the grounds from the primary and secondary sides of the transformer are tied together, but do not actually make a good connection to ground.

PS: You do have a ground wire connected to the WYE side of that transformer, don't you?
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Old January 18th, 2008, 02:22 PM   #4
Gil47
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Quote:
Originally Posted by leitmotif
Phase to phase 1 to 2 211.8
1 to 3 211.9
2 to 3 211.6 all 211 plus or minus 0.1
Phase to ground
1 84.4
2 127
3 183

I find it significant that if
Phase to ground
1 84.4
2 127
are added together they equal 211.4 which is what your phase to phase voltage is.


Some thing else to ponder
If you take 127 and multiply by the square root of 2 which is 1.414 then you get 180
get the 127 and divide by 1.414 you get 90
Now dont ask me to explain but it might jog someones brain for an explaination.
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Old January 18th, 2008, 02:35 PM   #5
Gil47
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Were your voltage readings taken at no load or with the motor running ?

What type of starter is on the motor VSD, Soft start, DOL, etc ?
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Old January 18th, 2008, 02:39 PM   #6
TConnolly
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Sounds like a partially grounded winding in the transformer secondary.

If you can, take the transformer out of service, disconnect all four leads and megger each winding to ground.
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Old January 18th, 2008, 02:58 PM   #7
leitmotif
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Lancy
Like I said I have not opened the thing up yet. Have only been there six months. I did not install the unit. The unit was connected by a contractor. Frankly I would be surprised if they pulled in a ground conductor and just relied on conduit grounding.

IF (boss doesn't care -- it works don't mess with it) and or when I open it up I will definitely check all connections including grounds.

The corner ground on the hi side bugs me also. IF it is connected it still should make the primary and secondary completely independant.
What has me scratching my head is with the phase to phase so good then the star point (neutral point ??) should be good. The star point is bonded to ground. I have difficulty seeing how the (assumed) single connection to ground can screw it up. I wonder if I disconnected the ground if it would do the same thing. From working on ungrounded distribution I think it would.

On the bright side we are moving to new plant. SUPPOSEDLY we will have ground conductros pulled into everything
- no conduit grounding JOY o JOY. I also have boss talked into pulling a neutral into everything. That way we can easily get rid of all that XXXX line to line control circuits.

Although it is required and I understand the reasoning behind grounding this bonding to ground of all systems makes me think we have opened Pandora's box. What it means is all systems 440 delta or wye, 277, 208, 120 are all tied into a common ie ground. Personally I would much rather work on ungrounded - probably because that is where I learned electrical work -- in the Navy. We had a requirement of one meg to ground for the entire electrical system. Anytime it went below that we had to chase down the ground and fix it IF you got two grounds then you could get into real trouble especially in our case where we ran port and starboard distibuition independent when at sea. We had a ground on port lighting and on starboard lighting - made the ground detector lamps look like synchronizing lights - had to kill all AC lighting fed off two circuits to fix it. I guess the nice thing about grounded distribution is that grounds become shorts which automatically forces people like these dingbats I work for to fix it - but only because the machine does not run and the Production God gets irritated.

Another poster enquired about phase current on the transformer. It supplys a single motor (SUPPOSEDLY). I ASSUME then that the currents in the transformer phases should be 1;73 less the line current. Regardless IF I open it up that would be another thing I would check.
Dan Bentler
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Old January 18th, 2008, 03:47 PM   #8
Jim Dungar
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The corner ground on the primary has nothing to do at all with your voltage readings on the secondary. Stop looking at the primary.

You need to take (10) voltage readings on the secondary. Ideally all of these readings would be done at No-load and under-load.

L1-L2
L2-L3
L3-L1
L1-N
L2-N
L3-N
L1-G
L2-G
L3-G
N-G

Based on your given voltage readings your secondary neutral-ground connection is "poor". Effectively you have a floating neutral, so depending on the load balance, the L-G voltage can look perfectly normal (with balance or zero load) or be as almost high as L-L.
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Old January 19th, 2008, 04:10 AM   #9
leitmotif
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Thank you all for the help.

There is no neutral in the cord feeding the motor just the ground. I sure wish there was a neutral.

Even on 120 single phase receptactles I check neutral to ground conductor resistance and voltage.

I am willing to bet it is grounded only with conduit grounding.

I intend to open this thing up and have a look see and check all
connections.

At least I now have a good idea of what to look for.
Will let you know what I find.
Dan Bentler
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Old January 19th, 2008, 11:02 AM   #10
Lancie1
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Quote:
The corner ground on the primary has nothing to do at all with your voltage readings on the secondary. Stop looking at the primary.
Quote:
..or be as almost high as L-L
But it should not be higher unless something is wrong! If the secondary neutral (ground point) is connected with a wire to one corner of primary delta, BUT that wire is NOT grounded or once was but is now disconnnected or broken, then it will have an effect on voltage readings on the secondary, namely the readings from line-to-ground, but maybe not the readings from line-to-line.

Dan now suspects that the secondary side is only grounded through the conduits. That may be the entire reason for the strange readings. Without a hard ground wire to the transformer neutral point, the line-to-ground voltages can jump all around, depending on the changing resistance of a loose conduit connection.

Why would anyone hard-ground the Delta high side of the transformer instead of the Wye low side? If that is the case, that ground wire should be moved, and do it soon! I see no need to have a corner ground on the primary side. It is more important to have a grounded secondary neutral.

Last edited by Lancie1; January 19th, 2008 at 11:38 AM.
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Old January 19th, 2008, 11:43 AM   #11
Jim Dungar
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The fact that the primary has a corner grounded phase conductor does not affect the voltage readings on the secondary: isolation transformers isolate. Your problem has to do with the way the transformer secondary is connected to ground. The secondary is a seperately derived system, if it is not connected to ground (i.e. floating) then your secondary L-G voltage measurements are looking at the voltage across coupling capacitance.

Quote:
If secondary neutral (ground point) is connected to one corner of primary delta, BUT that wire is NOT grounded, then it certainly will have an effect on voltage readings on the secondary, namely the readings from line-to-ground, but not the readings from line-to-line.
If the secondary neutral point is connected to the grounded primary conductor then in reality it is grounded, just poorly. If the secondary neutral point is connected to a primary ungrounded conductor then you no longer have an isolation tranformer and the secondary does not have a ground reference.

There should only be a single connection to ground (at the service or system disconnect) for any grounded conductor, be it a neutral or a phase.
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Old January 19th, 2008, 12:48 PM   #12
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I believe that the connected corner ground on the primary and the center ground on the secondary has shifted the center of the wye configuration. Feel free to correct this statement if you want. But then the solution would be to run the ground through the center tap and then to the case on the secondary, while taking the primary ground conductor to case only.
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Old January 19th, 2008, 03:01 PM   #13
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I've seen this before. If you are sure that it is a WYE secondary, then someone did not bond the neutral of the wye to the ground.

We moved into an office once and all the surge suppressors started catching on fire. I took voltage readings and found that the neutral to ground voltage was over 180 volts! Took the system down and bonded the neutral to the ground. Problem solved.

In power systems, the neutral and the ground should only be connected at one point and that is where the system voltage is derived. Too many times I've opened panels to see the neutral bar jumped to the ground bar in all the panels in that system.
Someone probably did not think that it needed to be bonded because it is only serving a 3 phase load. If that was the case, then it should have been a delta-delta transformer. Since it is a Wye, de-energize and lock it out, then open it and bond the neutral of the transformer to the ground bar inside it.

Quote:
I believe that the connected corner ground on the primary and the center ground on the secondary has shifted the center of the wye configuration. Feel free to correct this statement if you want. But then the solution would be to run the ground through the center tap and then to the case on the secondary, while taking the primary ground conductor to case only.
Not quite right. The primary corner ground is derived at the service entrance, not at this transformer. It should not be connected to ground in the transformer. There also is no "shifting" of the ground reference due to the primary. It is either ground solidly or not. I believe it is NOT.

Quote:
Why would anyone hard-ground the Delta high side of the transformer instead of the Wye low side? If that is the case, that ground wire should be moved, and do it soon! I see no need to have a corner ground on the primary side. It is more important to have a grounded secondary neutral.
Lancie, this is a very common (albiet older) voltage system. I've worked on many of these and there is nothing wrong with having a corner grounded 480 volt system. The ground is not at this transformer, it is where the voltage is derived. One important thing to note about fusing corner grounded delta systems is that the grounded phase should not get a fuse. Too many times I've seen either fuses in the disconnect or even worse I've seen a "slug" on B phase but the grounded leg was not in the middle. At that point, you have a fuse in a grounded leg and a high leg without protection.

Last edited by brucechase; January 19th, 2008 at 03:11 PM.
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Old January 20th, 2008, 07:57 PM   #14
leitmotif
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FIRST thank you all for insights.
The only data I have from this is the drawing from Fed Pac in the enclosure. It shows delta primary with a corner ground. It also shows wye secondary with a star point and another line coming off it (neutral). The only other data I have is the readings off the cord feeding the motor with the motor unplugged. These readings again were 212 phase to phase and 83, `127 and 180 phase to ground.

The building is probably from 1920 or so and there have been who knows how many people with varying levels of comptency adding and modifying the electrcal before me.
That is all I know.

I will have to check to see if 480 is even grounded and if it is corner grounded.

I am somewhat bewildered why the ground can screw the secondary up. My reasoning is this.
1. If I hood the delta primary to 480 and leave it ungrounded
AND I hook the secondaries to a common point (I am calling it star) and have all the taps set the same and leave secondary ungrounded then I should get get (on secondary) nominal 208 phase to phase and I should get 120 phase to star point and they should all be pretty equal.
2. Now if I tie a conductor to the star point this then becomes a neutral.
3. If I bond the star point and the neutral connection to ground I have now referanced all three legs to ground potential such that I will get 120 to ground and to the neutral (since they are both at same potential)
4. IF the 480 3 phase is grounded at service (as it should be)
AND it is corner ground
AND assuming A is grounded
then to ground anywheree on 480 distrubition
A should be zero, B shoud be 480 and C should be 480 and
A to C is 480 A to B is 480 and B to C is 480.

According to NEC
the 480 must be grounded either corner or with a center tapped phase (which gives 240/480)
and the secondary must also be grounded (in this case wye) to give me 120 / 208.

IF I put a control transformer in a piece of equipment I must ground teh secondary which for sake of argument I am going to choose 24V.

Now I have the 480 AND 120 / 208 AND 24 all referanced to ground. IF everything is connected properly then there should be no problems.
However techically we now have all three systems tied together to the ground. So if there is a wrong connection things could get all screwed up.

Seems to me we have created Pandora's box with all this grounding. If all three systems here were completely independent it seems like it would be a lot simpler.

I suppose there is no magic answer because if all were ungrounded as it was on the submarine then if you get two grounds that could get kind of interesting also.

Dan Bentler
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Old January 20th, 2008, 08:08 PM   #15
Jim Dungar
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The primary side grounding method, of an isolation transformer, absolutely positively cannot affect the secondary line to ground readings. It makes no difference if the primary is grounded or ungrounded nor if the primary is a wye or a delta connection.

If your transformer has a wye connected secondary, your strange L-G voltages come only from an improper (probably a non-existent one)neutral to ground bonding on the secondary.
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