Grounding the secondary of a step-up 3 phase transformer

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Marvin_Hamon said:
I would be interested in hearing what fault would give me 960V to ground using a derived neutral.

Thanks
Click to expand...
I don't believe I ever addressed a derived neutral.

I was talking about ungrounded systems.
Arcing faults are part of it.
This is one reason that MV cable manufacturers say to use 133% if grounds will be cleared within 1 hour, and 177% insulation if grounds will persist longer.
It is also why we do not use L-N voltage insulation levels on wye systems (i.e. the issue with slash rated breakers).
 
Oh yeah, I'm still voting for the corner ground. It may be a bit less than optimum in regards to phase to ground potential, but at least you will know what your phase to ground potential will be.

And if you need to do a test, requiring an ungrounded delta supply, the bonding jumper is very easy to lift
 
Yup, when we bring in capacitive coupling to ground and arc faults in ungrounded systems all kinds of strange voltage spikes can occur. I was just considering the plain old line to ground potential that would be present under the different types of grounding.
 
Are we engineering off of the junk pile here? You got any junk induction motors there say maybe in the 25 HP and up range that have a good armature and are capable of being connected 480V wye? Don't worry about run out bearings, or damaged rotors, because we going to throw the rotor back in the scrap anyway.

We can get your ground to phase potentials all down to 277V without spending too much money, But the code nazi's are going to go nuts.
 
Start with something damaged or obsulete (like a U frame) or some strange thing that has no market value.

Throw out everything but the armature.
 

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I am surprised that none of the former Naval electricians haven't chimed in about ungrounded systems.
I don't have a problem with an ungrounded delta used this way (I didn't mean Joe's but I like it) just due to the fact that it will work just fine. The first ground if free, the second one will cost you.
A simple ground detector would be three 277volt lamps connected to ground. When a lamp lights, run..:)
 
Ungrounded systems are not that uncommon in industrial situations where there is a need to limit electrical outages.

However, user beware because the 277V unfaulted phases will rise to as much as 480V when one phase goes to ground. This requires that the equipment all be rated for line-line voltage. Probably not an electrical system issue since most LV electrical equipment is rated for 600V. However, equipment in question being tested may be another issue.

For sensing, voltage provides the best method to indicate a ground fault.
 
Joethemechanic said:
Are we engineering off of the junk pile here? You got any junk induction motors there say maybe in the 25 HP and up range that have a good armature and are capable of being connected 480V wye? Don't worry about run out bearings, or damaged rotors, because we going to throw the rotor back in the scrap anyway.

We can get your ground to phase potentials all down to 277V without spending too much money, But the code nazi's are going to go nuts.
Click to expand...


Hi Joe.

An old 220 / 440 transformer from a scrapped welding machine could be wired up to function as an auto-transformer per your attached sketch as well--attach incoming power at the junctions 6-9, 8-5, and 4-7.......then take your output from 1, 2 and 3......the junction 12, 11, 10 can then be grounded and can also serve as a neutral from which one could also obtain 277 vac single phase if desired.
 
Sam Lockwood said:
Hi Joe.

An old 220 / 440 transformer from a scrapped welding machine could be wired up to function as an auto-transformer per your attached sketch as well--attach incoming power at the junctions 6-9, 8-5, and 4-7.......then take your output from 1, 2 and 3......the junction 12, 11, 10 can then be grounded and can also serve as a neutral from which one could also obtain 277 vac single phase if desired.
Click to expand...


Hi Sam,

Yes I can see that working as well. I guess I am just engineering with what I typically see on my scrap pile. I am long on junk motors, and low on junk 3 phase welding machines lol
 
GearMan said:
Let's back up to the OP. You have 240V. You can't use that xfmr, it is rated for 208V.
Click to expand...

You are almost correct. The taps on the high voltage side did not provide the proper turns ratio to convert 240v to something under 480v. The lowest was 511.

What I did was pull out one winding and tap into it with a split bolt.

I now get 476 volts, it seems to work nicely. I may be losing a little bit of kVA, but I do not care.

i
 
ichudov said:
You are almost correct. The taps on the high voltage side did not provide the proper turns ratio to convert 240v to something under 480v. The lowest was 511.

What I did was pull out one winding and tap into it with a split bolt.

I now get 476 volts, it seems to work nicely. I may be losing a little bit of kVA, but I do not care.

i
Click to expand...

You are in violation of the MFG's published data for this xfmr, and the NEC. Sure it will work until the insulation system breaks down. Below would be the proper application and solve prior post issues.

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Search Parameters:
e-Catalog*> Transformers > Dry Type Vented >General Purpose;Phase:3;PriVoltage:240;SecVoltage:480Y/277;KVA:45;Coil Material:AL;TempRise:150
Transformers,Dry Type Vented
Product #: 9T83B3053
Short Description: AL 3P 45KVA 240-480Y/277
SPECIFICATIONS
Category General Purpose
Phase 3
PriVoltage 240
SecVoltage 480Y/277
KVA 45 KVA
Coil Material AL
TempRise 150 ?C
ElecShield N
SubType VENTED
Frequency 60 Hz
UL Yes
Impedance 5.8 %
FrameSize EE73
cUL No
CE No
Width 23.8 in
Depth 18.4 in
Height 32.2 in
TempClass 220 ?C
AmbTemp 40 ?C
InsulSys IS-19C
Taps (+2,-4 2.4%)
GSA Compliance No
GO Schedule 67A
List Price $8179


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9T83B3053
Additional Resources for General Purpose
Application and Technical
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2004 CSI Master Format
1995 CSI Master Format

GE Type QL transformers meet NEMA TP-1 efficiency standards. They are available with aluminum or copper windings and utilize a UL recognized 220?C insulation system.

-Quiet Performance
-Core and coil assemblies are mounted on rubber isolation pads to reduce noise
-Bolted coil terminations
-Single-piece front/back is easily removable for service
-NEMA 2 drip-proof enclosure is standard; weathershield kits are available for conversion to NEMA 3R outdoor configuration
-Qualified to the seismic requirements of IEEE-693-1997 and IBC-2003
-Copper or aluminum windings
-Copper ground strap
-Robust packaging with top and side protection protects against shipping damage
-Accessible mounting flanges with front/back slotted mounting holes make installation easier
-100% factory tested for shorts and coil integrity, current and loss, voltage, impedance and noise.
-Clear, comprehensive documentation and labeling
 
GearMan said:
Sure thing. Oops on all the data, the paste didn't quite come out right. Bottom line, xfmrs should not be operated in excess of the name plate data.
Click to expand...

Thanks. For my education, practically speaking, if I am operating the primary windings at 240 volts input, as opposed to 208 that the primary is rated for, just how much trouble can happen, realistically?
 
If it was me, I'd sell the transformer, and use an old wye connected electric motor stator as an auto-transformer.autotrans.JPG

It would take care of the grounding issue, and put a couple of bucks in your pocket :)


Red is high, blue is low
 
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