43 amps on the ground wire

[Delcobob]

I have a 30KVA 3 phase dry type transformer that is wired with the 208 as the primary and 480V secondary. It serves a 50A 3 phase load. No nuetral is required. I have 3 #3's and a #6 ground feeding it, and 3 #6's with a #8 ground on the secondary to the load. The XO is bonded to the transformer frame with a #6 at the same point the other 2 grounds are, and finally I have a #4 from the transformer frame to building steel. My problem is, I have 43 amps on the #6 ground wire back to my 208V panel. This was discovered during an infrared inspection of the panel. I was told the transformer was bad, so I replaced it with a new one, but the same situation is occuring, even with the disconnect to the load turned off. Can anyone shed some light on this for me?

 

[infinity]

How is the transformer wired, is a 208Y primary-480 Delta secondary? (some would call this a reverse wired Delta/Wye) If so the X0 should have nothing connected to it.

 

[texie]

I have a 30KVA 3 phase dry type transformer that is wired with the 208 as the primary and 480V secondary. It serves a 50A 3 phase load. No nuetral is required. I have 3 #3's and a #6 ground feeding it, and 3 #6's with a #8 ground on the secondary to the load. The XO is bonded to the transformer frame with a #6 at the same point the other 2 grounds are, and finally I have a #4 from the transformer frame to building steel. My problem is, I have 43 amps on the #6 ground wire back to my 208V panel. This was discovered during an infrared inspection of the panel. I was told the transformer was bad, so I replaced it with a new one, but the same situation is occuring, even with the disconnect to the load turned off. Can anyone shed some light on this for me?

If I understand correctly, you have a 208 Y primary with a 480 delta secondary. If this is the case the neutral point on the primary should not be connected to anything. Doing so as you described can lead to serious problems. This type of setup also begs another question, such as how is the 480 delta secondary grounded? It either has to be corner grounded or if operating as an ungrounded system you need ground detectors. In general I think setups like this are a poor practice as corner grounding or ungrounded systems bring on other issues that are not desirable for much equipment today.

 

[texie]

I might also add that IMHO a much better alternative is to use a 208 delta primary X 480Y secondary transformer. While not as readily available as the other, they are available for a little more cost. This brings a wealth of benefits, even if you don't need a neutral or grounded conductor on the 480 output side for your load.

 

[roger]

How is the transformer wired, is a 208Y primary-480 Delta secondary? (some would call this a reverse wired Delta/Wye) If so the X0 should have nothing connected to it.

If I understand correctly, you have a 208 Y primary with a 480 delta secondary. If this is the case the neutral point on the primary should not be connected to anything.

I agree with both Infinity and Texie, lift the bond from the XO terminal.

Roger

 

[templdl]

The OP was somewhat vague about the rating of the transformer. Taking a guess he is using a common 480D-208Y/120 transformer and using it to step up from 208 3ph to 480v delta.
If so he may have may a common mistake when he ran 4 wires to the 208Y side of the transformer by attaching a neutral from the source to the X0 which is a big NO NO!! He should have just brought out the L1, L2, and L3 to the X1, X2, and X3 and make sure that the X0 was not grounded or bonded to the enclosure in any way.
He should have brought out an EGC and bonded it to the enclosure only.
By either bringing out a neutral from the source and terminating it on the X0 or if he did not he may have left the X0 bonded to the enclosure and then brought an EGC from his source and bounded it to the transformer enclosure in both cases it will result in a large amount of current on either the neutral or the EGC as I believe he has described.
The X0 does not need a neutral conductor and the X0 does not need to be bonded to the enclosure.

 

[GoldDigger]

The X0 does not need a neutral conductor and the X0 does not need to be bonded to the enclosure.

I think that a confirmation of that as the cause of the problem is that the OP stated that the primary neutral current remained, even when the load on the secondary side was disconnected.
If the voltages to ground of the three 208 lines were not absolutely equal, the transformer primary and secondary will work together to try to balance that voltage difference by sending current through the neutral. If the secondary windings were not there or were open, there would be no problem connecting the inductances of the primary windings to the neutral, but the voltages of the phase lines to ground would then remain unequal and different currents would flow in the three primary windings. But when the secondary delta was completed, those three phase currents on the primary were suddenly forced into a different relationship.

One way of looking at it that may help is that with unequal voltages on the three phase lines, a disconnected X0 will float to a point where the scalar voltages from X0 to the three lines are equal (and the vectors are in a 120 degree phase relationship). Because of the asymmetry of L1, L2 and L3, the voltage of that derived neutral point will be different from the ground voltage/neutral voltage. That difference of a few volts will cause currents to flow that will bring that point down toward ground and in the process pull the phase voltages closer to a balanced condition. The detailed current values will depend on the resistances of the phase and EGC wires.

 

[templdl]

I think that a confirmation of that as the cause of the problem is that the OP stated that the primary neutral current remained, even when the load on the secondary side was disconnected.
If the voltages to ground of the three 208 lines were not absolutely equal, the transformer primary and secondary will work together to try to balance that voltage difference by sending current through the neutral. If the secondary windings were not there or were open, there would be no problem connecting the inductances of the primary windings to the neutral, but the voltages of the phase lines to ground would then remain unequal and different currents would flow in the three primary windings. But when the secondary delta was completed, those three phase currents on the primary were suddenly forced into a different relationship.

One way of looking at it that may help is that with unequal voltages on the three phase lines, a disconnected X0 will float to a point where the scalar voltages from X0 to the three lines are equal (and the vectors are in a 120 degree phase relationship). Because of the asymmetry of L1, L2 and L3, the voltage of that derived neutral point will be different from the ground voltage/neutral voltage. That difference of a few volts will cause currents to flow that will bring that point down toward ground and in the process pull the phase voltages closer to a balanced condition. The detailed current values will depend on the resistances of the phase and EGC wires.

Regardless, you don't use the neutral from the source period. You make sure that any connection from the X0 is removed, that nothing is connected to the X0. Only bring the L1, L2, L3, and an EGC from the source connecting them to the X1,X2,X3 terminating the EGC to the transformer enclosure. This is a very simple thing to do. This is not a Y-Y transformer application
What I think is the more important issue is the secondary which you will end up with a 480v delta. Being that most are familiar with a 3ph4w 480y/277 what do you do with a 3ph3w 480v delta? Personally I would recommend corner grounding it and as such sometime it is hard to convince someone that you can ground a line conductor. And then to convince them what do you do with the EGC?

 

[MarineTech]

I have a 30KVA 3 phase dry type transformer that is wired with the 208 as the primary and 480V secondary. It serves a 50A 3 phase load. No nuetral is required. I have 3 #3's and a #6 ground feeding it, and 3 #6's with a #8 ground on the secondary to the load. The XO is bonded to the transformer frame with a #6 at the same point the other 2 grounds are, and finally I have a #4 from the transformer frame to building steel. My problem is, I have 43 amps on the #6 ground wire back to my 208V panel. This was discovered during an infrared inspection of the panel. I was told the transformer was bad, so I replaced it with a new one, but the same situation is occuring, even with the disconnect to the load turned off. Can anyone shed some light on this for me?

Before removing primary XO from the case. With the load off as before, and confirming 43A in the EGC as stated, could you please measure primary line current. Measure currents with a true RMS clamp meter if possible.

If I do a quick calc on the primary for 30kVA I get a balanced primary line current of 83A.

30000/(208*1.732)=83A. An imbalance of 43A seems very large to me.

Thanks,

MarineTech

 

[GoldDigger]

30000/(208*1.732)=83A. An imbalance of 43A seems very large to me.

Thanks,

MarineTech

Please measure the phase to ground voltages on the primary side with the transformer disconnected.
It does not take much voltage difference to cause 43A to flow in a low resistance circuit.

 

[templdl]

Please measure the phase to ground voltages on the primary side with the transformer disconnected.
It does not take much voltage difference to cause 43A to flow in a low resistance circuit.

If the X0 is not to be bonded to the enclosure and the EGC is bonded to the transformer enclosure no current will flow on the EGC even though you will measure from the lines to the EGC which would be the L-N voltage as it would be at the source. A voltage measure from L-G at the transformer does not imply that you will have current on a nuetral because there is none if the transformer is connected correctly. There should be no current on the EGC.
As such I may be missing your point.

 

[GoldDigger]

If the X0 is not to be bonded to the enclosure and the EGC is bonded to the transformer enclosure no current will flow on the EGC even though you will measure from the lines to the EGC which would be the L-N voltage as it would be at the source. A voltage measure from L-G at the transformer does not imply that you will have current on a nuetral because there is none if the transformer is connected correctly. There should be no current on the EGC.
As such I may be missing your point.

You are, in fact, missing my point. If the X0 is neither grounded not connected to the neutral (i.e. is connected correctly) there should be no current. We agree on that. But if there is a connection from X0 to EGC, one of the things that would drive current through that connection would be an imbalance in the phase voltages with respect to the EGC (with the transformer primary disconnected.) The amount of current, which MarineTech called larger than expected, is not the result of any load imbalance in either the secondary or primary of this transformer, but strictly driven by the low impedance the transformer primary presents to any phase voltage imbalance when the delta secondary is present. The phase voltage imbalance may be coming from POCO or may be created by imbalanced loads elsewhere in the system.

A useful simpler system to analyze would be two 120-to-120 volt isolation transformers with their primaries connected in series between L1 and L2 of a split phase system. The unconnected common point between the two transformers will be at approximately the neutral voltage when they have no load on their secondaries. If L1 and L2 are 120 and 125 volts respectively, the unconnected common point will be at roughly 2.5 volts from the neutral, with polarity the same as that of L2.
If you now connect the common point to the neutral, not much neutral current will flow, because it will only be driven by the difference in the magnetizing reactive current of the two transformers. But if you also connect the secondaries of the two transformers in parallel, (observing polarity) they will try to make the L1-N and L2-N voltages equal and a very large amount of current may end up flowing in the neutral wire.

If instead of two isolation transformers you put one 240 volt center-tapped primary connected to L1, N and L2 you will have the same effect. The current flowing in the neutral will no longer be simply the difference in current in the two primary windings we saw earlier but instead a high enough current to make L1-N and N-L2 equal (limited by the resistance of the windings and the circuit wires.)
You can do the full equivalent circuit analysis to calculate what that current will be in any particular situation, but it can be quite large. And, as stated earlier, it has no relationship at all to any load imbalance on the secondary side.

 

[templdl]

My intent is to address the correct connection of a 480d-208y/120 transformer when used as a step up transformer. How should we provide a simple response to his post? Yes, there will be current flowing as you have described no doubt. If the 208y side of the transformer were to be connected correctly there would be no current on the EGC an no neutral current because the is no neutral conductor. Is you response directed toward what happens when you connect you 208y of a transformer incorrectly or correctly?
Is your reply based upon what happens should the transformer be connected correctly or when it isn't? My objective has been to simply reply that the transformer is connected incorrectly and the correct way to do so and not the results if you don't.
Connecting the 208y side should be a very basic and simple matter. What I have always founld that there had been a lack of knowledge regarding what to when they have a 480 delta secondary as I stated in a previous post.

 

[MarineTech]

Please measure the phase to ground voltages on the primary side with the transformer disconnected.
It does not take much voltage difference to cause 43A to flow in a low resistance circuit.

Gold,

Thanks for the input.

To better model the problem, I believe you are saying, the 43A EGC current (XO bonded) is due to the difference in primary currents created by unequal line voltages (and perhaps some coil mismatch?) when we look at the primary coils as individual line loads to the star point (Neutral) without any coupling (mutual inductance) from the floating (disconnected) secondary.

Is this correct?

Best Regards,

MarineTech

 

[GoldDigger]

Gold,

Thanks for the input.

To better model the problem, I believe you are saying, the 43A EGC current (XO bonded) is due to the difference in primary currents created by unequal line voltages (and perhaps some coil mismatch?) when we look at the primary coils as individual line loads to the star point (Neutral) without any coupling (mutual inductance) from the floating (disconnected) secondary.

Is this correct?

Best Regards,

MarineTech

Yes, that is exactly what I am saying. The proper practice is not to connect the X0 terminal in this situation and no further justification is needed, but I was trying to show how such a large current could be possible if the connection is made. (And that is the reason behind the rule not to connect it!)

If the secondary is not there (not just unloaded) there will be no large interaction among the primary wye windings and the neutral current will be small. But once both the mutual inductance between primary and secondary and the internal winding connections that make up the delta are in place, the situation changes dramatically.

 

[eddunnpe]

If I understand correctly, there is no neutral conductor on the primary side. If there were one, unbalanced currents on the secondary (480v) side will be reflected on the primary (208v) side as current in the neutral. Since the neutral point (X0) on the primary is bonded to the ground, it makes the ground conductor the return path for unbalanced currents.

The solutions are to remove the bonding X0 bonding jumper and operate without a primary side neutral, or add a neutral conductor also leaving off the bonding jumper. I would prefer the latter otherwise zero-sequence currents will circulate in the delta-connected windings of the secondary.

Other recommendations about ground detectors and markings on the ungrounded secondary side are required by code. Re: NEC 250.21.

 

[meternerd]

Huh?

Huh?

Unless I missed something, the OP never answered what kind of transformer. Y-Y Y-delta, delta-Y.....he just said the X0 was bonded. Didn't say which side. Is the primary X0 bonded or the secondary X0 bonded? Is there a neutral on the 208 side? Is the secondary side bonded but without any neutral run to the load?....just curious.

 

[GoldDigger]

Unless I missed something, the OP never answered what kind of transformer. Y-Y Y-delta, delta-Y.....he just said the X0 was bonded. Didn't say which side. Is the primary X0 bonded or the secondary X0 bonded? Is there a neutral on the 208 side? Is the secondary side bonded but without any neutral run to the load?....just curious.

My guess is that since he was using a step-down transformer backwards, the X0 designation makes it the 208 volt side, and the corresponding terminal on the 480V side would be HO if it has one.
And because most use of 208/120 is wye, I am pretty sure that is what he was using as the primary.
As has been repeated numerous times, when a wye-wound primary is connected to delta-wired source, the neutral point should be left unconnected, since there is no primary neutral to connect it to. If the source was a wye, with an available neutral, then I believe you would have a choice in how to wire it depending on whether you wanted to reduce imbalance at the cost of circulating currents in the primary windings.

 

[beanland]

Grounding Transformer

Grounding Transformer

A 208V wye-grounded (source) to 480V delta transformer is a grounding transformer. Any unbalance in the voltages on the 208V side will lead to current circulating in the 480V delta winding and current flowing on the neutral. See 450.5 for discussion on grounding transformers. The correct solution is to disconnect the 208V star from neutral/ground, treat the 208V as a 3-phase 3-wire delta. However, then the transformer is functionally delta-delta so the protection requirements need to be checked.

You may also need a ground detector per 250.21(B) on the 480V side.

 

[AdrianWint]

As a general rule ....

If the 'powered' side is WYE & the 'load' side is DELTA then you must NOT connect the 'powered' side star point to neutral or ground.

If the 'powered' side is WYE & the 'load' side is WYE then you MUST connect both star points to their respective neutrals

I use the terms powered & load in an attempt to remove the confusion when a transformer is used in reverse ......

Adrian