Current on neutral of 480-277V 3 phase delta -wye transformer

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Shells

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Hi,
I would like some suggestions. I am getting about 20A on my neutral of a 480-277V delta -wye transformer. All the loads are balanced. When I switch on one load at a time, the current on the neutral increases about 2 A. Any suggestions on possible problem?

Thanks
Shelly
 
Hi,
I would like some suggestions. I am getting about 20A on my neutral of a 480-277V delta -wye transformer. All the loads are balanced. When I switch on one load at a time, the current on the neutral increases about 2 A. Any suggestions on possible problem?

Thanks
Shelly
If you are indeed getting 20 amps on the neutral, they are not balanced if all loads are on and running. They may be balanced on paper, but if the calc's. on the drawings aren't exactly what the amprobe is, then you could and probably would see an inbalance. 20 amps on a 200 amp service is 10% and a little high I would think. Good luck.
 
1. If indeed the loads are balanced you could have harmonics.
2. If indeed the loads are balanced you could have circulating current from another transformer with neutrals tied downstream.

What are the phase currents?
 
Thanks for everyone's responses. The loads are balanced when all loads are on. The phase currents are approx 50A each, and the neutral current is 26A. What is very confusing to me is that there is only a current on the neutral X0 to ground within the transformer. There is no current flow on the transformer groundind conductor back to earth i.e. it appears to be a circulating current within the transformer. There is only lighting on these circuits so there should not be any harmonics. Additionally, we looked at a few other 3 phase 480-277V 3 transformers and found the same thing... with close to 50% of the phase current on the neutral. The transformer wiring seems to be ok. We did discover that the neutral was not bonded to the ground in the panel but this was corrected and the only difference was that the current was now split. I cant understand why there would be no current going back to ground through the EGC. Please help
 
I cant understand why there would be no current going back to ground through the EGC. Please help

Current does not go to ground, it goes back to the transformer.

Is your transformer X0 bonded to ground at the transformer?
Is your neutral bar bonded to ground at the panelboard?

Can you place a single clamp-on ammeter around the 3 hots and neutral at one time?

Fluorescent ballasts may produce harmonics. Incandescent dimmers produce harmonics.
 
I would do a little more investigation. You need to know not only the amount of current in the neutral, but its frequency.(harmonics)
Also, do you have the necessary equipment to measure the power factor on a phase by phase basis? Although your load may be all lighting, is all the lighting of the same type?
Different types of lighting will exhibit different power factors. If this is the case, it would be possible to measure exactly 50 amperes in each phase at the transformer, and yet have significant current flow in the neutral due to the difference in power factors (or current angles) in each phase.
Sounds like some detailed investigation is in order.
 
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Shells,

The readings you are getting sound very normal to me.

The Transformer with 50 Amp loads, has 26 Amps flowing on the Grounded Conductor, results in 52% Neutral load.

Normal is anything from 50%, through 70% - and at times, even 100%.

The only loads which will _COMPLETELY_ balance out (across a 4 wire network / multiwire circuit) will be Pure Resistance loads.

Looks like the 50 Amp circuits have an average power factor of 70%

Scott
 
Thanks for everyone's responses. The loads are balanced when all loads are on. The phase currents are approx 50A each, and the neutral current is 26A. What is very confusing to me is that there is only a current on the neutral X0 to ground within the transformer. There is no current flow on the transformer groundind conductor back to earth i.e. it appears to be a circulating current within the transformer. There is only lighting on these circuits so there should not be any harmonics. Additionally, we looked at a few other 3 phase 480-277V 3 transformers and found the same thing... with close to 50% of the phase current on the neutral. The transformer wiring seems to be ok. We did discover that the neutral was not bonded to the ground in the panel but this was corrected and the only difference was that the current was now split. I cant understand why there would be no current going back to ground through the EGC. Please help

If I read this correctly you now have two neutral to ground bonds, one at the transformer and one in the panel. You should have only one. Either in the panel or in the transformer, not both.
 
Scott

Harmonics aside, if the three fundamental phase currents are separated by 120 degrees and their amplitudes are the same, there will be 0 fundamental current in the neutral.
Leading or lagging power factor to any degree will not matter.
 
Shells,

The readings you are getting sound very normal to me.

The Transformer with 50 Amp loads, has 26 Amps flowing on the Grounded Conductor, results in 52% Neutral load.

Normal is anything from 50%, through 70% - and at times, even 100%.

The only loads which will _COMPLETELY_ balance out (across a 4 wire network / multiwire circuit) will be Pure Resistance loads.

Looks like the 50 Amp circuits have an average power factor of 70%

Scott


Thanks Scott,

Can you provide some background? What is the basis for a normal neutral current between 50-70%? Are you saying that incandescent lighting circuits can be non-linear and is it because of a phase shift that the inbalance occurs also resulting in the neutral current? Does this also imply I have triplen harmonics? Also, if this is normal, then my transformer which is currently overheating is actually undersized? One transformer has already failed, this replacement is running hot. It seems the transformer is running at rated current, but if this is normal shouldnt the transformer have been rated to handle this neutral current and not overheat?
 
is the transformer neutral bounded to building steelQUOTE=Shells;945739]Thanks for everyone's responses. The loads are balanced when all loads are on. The phase currents are approx 50A each, and the neutral current is 26A. What is very confusing to me is that there is only a current on the neutral X0 to ground within the transformer. There is no current flow on the transformer groundind conductor back to earth i.e. it appears to be a circulating current within the transformer. There is only lighting on these circuits so there should not be any harmonics. Additionally, we looked at a few other 3 phase 480-277V 3 transformers and found the same thing... with close to 50% of the phase current on the neutral. The transformer wiring seems to be ok. We did discover that the neutral was not bonded to the ground in the panel but this was corrected and the only difference was that the current was now split. I cant understand why there would be no current going back to ground through the EGC. Please help[/QUOTE]
 
Thanks Scott,

Can you provide some background? What is the basis for a normal neutral current between 50-70%? Are you saying that incandescent lighting circuits can be non-linear and is it because of a phase shift that the inbalance occurs also resulting in the neutral current? Does this also imply I have triplen harmonics? Also, if this is normal, then my transformer which is currently overheating is actually undersized? One transformer has already failed, this replacement is running hot. It seems the transformer is running at rated current, but if this is normal shouldnt the transformer have been rated to handle this neutral current and not overheat?

What kind of meter are you using to take these current readings? If you have a tx that has already failed and the replacement is running hot you could have some high harmonic currents that an average-RMS ammeter would not be able to measure.

So even in a perfectly balanced system the harmonic loads will be added and the neutral becomes a current carrying condutor. De-rating the kva of the tx and taking the K-factor into consideration are ways to handle the added harmonic load.
 
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