Step Up transformer Installation

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MFRichardson16

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Location
Charleston, SC
We have installed a 4160 volt Power distribution system on a airfield and are having power fluctuations once or twice a month.

Leaving the Airfield lighting vault with a 250 Amp 480 volt w/3-250MCM & #4 ground Feeder to a 3 phase (step up)150 KVA Pad Mounted 480/277Y to 4160/2400Y . The 3 phase 4160 volt system feeds 6- 25 KVA single phase pad mount transformer 4600: 240/120 single phase loads. The single phase transformers are spread out along 12,00 feet of airfield with the 3 phase 4160 system alternating two phases at each single phase transformer locations. We did not ground XO in the 150 KVA transformer due to past experience that it is a dead short across the 480v Side. Due to power fluctuations we talked to the ABB factory and the ABB factory engineer talked us into grounding XO/HO. When we turned power on did it destroyed the 250A 65KAIC breaker in the vault. So we went back to un-grounded XO/HO system. I have attached the transformer nameplates. The system has been since October but we are still having power fluctuations once or twice a month. Also in 4 or the remote sites the equipment are on UPS systems. The XO and HO are tied together inside the transformer under oil.

Why can you not ground XO/HO in the 150 KVA transformer?

What is causing the voltage fluctuations?

Will the 150 KVA Pad Mounted 480/277Y to 4160/2400Y work or should the transformer be changed to 480V Delta: 4160/2400 so we can ground HO?

Thanks in advance.

Mike
 

Attachments

  • 3-Phase Pad Mount Transformer ANSI Nameplate Rev1.pdf
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  • 25 kva nameplate.pdf
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GoldDigger

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Placerville, CA, USA
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Retired PV System Designer
With a wye to wye transformer it should be possible to connect the primary X0 to neutral with no excess current as long as the transformer does not have a tertiary delta winding.

The reason that you cannot connect the wye point in general with a wye secondary or tertiary coil (although POCO may do it on their distribution lines because they are able to control the line voltage balance) is that the source wye voltages will not necessarily transform to voltages on the delta secondary side that form a closed vector triangle. If the triangle does not naturally close, then there will be circulating currents in the secondary and the primary that can be several times the normal full load current.

You apparently saw that kind of behavior that destroyed your breaker. It makes me wonder whether your characterization of the transformers in the system is complete and correct.
 
I

Ingenieur

Senior Member
Location
Earth
Y-Y connections can cause voltage instability issues
Is the 480 from the vault feeding the 150 kva a wye?
if it is I would go delta:wye on the 150 kva

looking at the xfmr diagram that may not be possible
 
M

MFRichardson16

Member
Location
Charleston, SC
GoldDigger said:
With a wye to wye transformer it should be possible to connect the primary X0 to neutral with no excess current as long as the transformer does not have a tertiary delta winding.

The reason that you cannot connect the wye point in general with a wye secondary or tertiary coil (although POCO may do it on their distribution lines because they are able to control the line voltage balance) is that the source wye voltages will not necessarily transform to voltages on the delta secondary side that form a closed vector triangle. If the triangle does not naturally close, then there will be circulating currents in the secondary and the primary that can be several times the normal full load current.

You apparently saw that kind of behavior that destroyed your breaker. It makes me wonder whether your characterization of the transformers in the system is complete and correct.
Click to expand...

I attached the nameplates of the transformer. The 150 KVA is a Y/Y
 
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Bugman1400

Senior Member
Location
Charlotte, NC
Are the local xfmrs grounded to earth locally? The #4 ground wire may be too long and allowing the neutral point to shift around. May need a data logger to confirm.
 
P

Phil Corso

Senior Member
Location
Boca Raton, Fl, USA
Mike...

1) Do you, or POCO, have evidence, whether observed or recorded, of wildly fluctuating current/voltage-magnitudes and PF-changes between lag and lead?

2) What is distance between 480V source and step-up xfmr?

Phil
 
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J

JFletcher

Senior Member
Location
Williamsburg, VA
"480 volt w/3-250MCM & #4 ground Feeder to a 3 phase (step up)150 KVA Pad Mounted 480/277Y to 4160/2400Y..."

Wouldnt a 3 wire feed + ground (no primary neutral) require a delta-wye xfmr? If you bond HO to XO to ground, with no primary neutral, and you have any secondary imbalance (An>0), isnt this going to cause problems with circulating currents within the xfmr?
 
J

jim dungar

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Location
Wisconsin
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PE (Retired) - Power Systems
According to the transformer nameplate, there is a neutral disconnect point that would allow you to separate the X0 and H0 winding connections.

I would look into not connecting the primary wye (let it float), the secondary neutral could then be grounded. All primary side switching should be ganged operated.
 
I

Ingenieur

Senior Member
Location
Earth
jim dungar said:
According to the transformer nameplate, there is a neutral disconnect point that would allow you to separate the X0 and H0 winding connections.

I would look into not connecting the primary wye (let it float), the secondary neutral could then be grounded. All primary side switching should be ganged operated.
Click to expand...

That is what I would do plus tie the 480 N's together (feeder and xfmr prim)
 
I

Ingenieur

Senior Member
Location
Earth
JFletcher said:
There is no primary neutral feeder on the 480V side; OP has 3W + ground.
Click to expand...

Pull one
the vault is likely close to the xfmr
and the step up to 4.16 kv for the long runs

something has to change
what he has appears unstable

op
define power fluctations please
random or when switching lights on?
Under what circumstances?
 
J

jim dungar

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Location
Wisconsin
Occupation
PE (Retired) - Power Systems
Ingenieur said:
That is what I would do plus tie the 480 N's together (feeder and xfmr prim)
Click to expand...

I would never do this. I want the transformers primary windings to be able to accommodate imbalanced currents by letting the voltage 'move around'. Tying the X0 to a grounded reference point forces the voltage relationship to be fixed, in this case at 277V, which means the winding currents may need to increase beyond the load current.
 
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