brian john
Senior Member
- Location
- Leesburg, VA
A sure way to avoid so called "Nusiance Trips"
300kva transformer tripping GFI in 2000 amp switch gear
- Thread starter Fordean
- Start date
- Status
beanland
Senior Member
- Location
- Vancouver, WA
Any progress?
Any progress?
Curious to know what you found. Was the Xo bushing grounded on the 208V side of the transformer?
Any progress?
Curious to know what you found. Was the Xo bushing grounded on the 208V side of the transformer?
Ya!! We're waiting here with baited breath. What did you find out?
Fordean
Senior Member
- Location
- New Jersey
- Occupation
- Electrical Contractor
Ok i am going to go reopen it. It tripped again. Not sure if a neutral was run. Also. the Trans is five feet away from switchgear. Did not run a Grounded electrode to steel since it was so close. Ran from switch gear. Can you
explain grounding when so close. Usually if its far away would ground a beam there but since it was so close. And beam is part of GEC system. It was closer ground to switch gear. then 20 feet away to closes beam. I think the XO is giving us issues. Going to repost after i reopen. Utility is shutting down in middle of night. And when the reconnect it trips GFCI. 2000 amp 208y/120 system. Need to shut down the Individual switches before it will let us reset. If you reset with all breakers open it trips. I never reset anything with full load . Sorry for delay guys. Was very busy last couple months. Now slowing up. (which stinks)
Last edited:
Fordean
Senior Member
- Location
- New Jersey
- Occupation
- Electrical Contractor
I will repost it. Sorry have not been up there. Need to get it resolved. Problem is replicating to see if it still does it.
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
Have you ever had your complete GF system tested? In particular I would be looking for neutral-ground bonds in downstream equipment.
How do you know it is the transformer causing the problem and not some other circuit?
Have you had someone look at the Time-Current curves for the GF device, the transformer inrush, and the other branches?
While an X0 connection on the 208V side of your step up transformer is not desirable, it should have no affect on your GF tripping.
The inrush current of unloaded transformers can be extremely non-sinusoidal, combined with a 'soft' utility supply (like you might have during a storm). Your GF device may not like these harmonics.
Why do you have GF protection on a 208V service?
How do you know it is the transformer causing the problem and not some other circuit?
Have you had someone look at the Time-Current curves for the GF device, the transformer inrush, and the other branches?
While an X0 connection on the 208V side of your step up transformer is not desirable, it should have no affect on your GF tripping.
The inrush current of unloaded transformers can be extremely non-sinusoidal, combined with a 'soft' utility supply (like you might have during a storm). Your GF device may not like these harmonics.
Why do you have GF protection on a 208V service?
Fordean
Senior Member
- Location
- New Jersey
- Occupation
- Electrical Contractor
Reason for me to Think it is transformer. Is when it was first Initially Energized. It tripped immediately. Which scared the crap out of me. I am not sure. But I always do shut down every switch then tried again.
But the first time. It was only the Transformer being engage if you understand what i've written. I know clarity is a key to people answering correctly.
rcwilson
Senior Member
- Location
- Redmond, WA
If the 208 to 480V step up transformer is a 208/120V wye to 480V delta and the X0 lead on the 208V side is connected to ground, that winding can put currents in the 208 V system that cause false trips on ground fault relays. Float the X0 on the 120/208V side, no connection to neutral or ground. Just tie the ground wire from the service to the transformer frame ground.
But if the transformer is a 208V DELTA to 480/277 WYE, then you need to ground the 480V neutral.
As already suggested, check the complete system neutral grounding. Ground at only one point on the line side of the main breaker's neutral ground fault CT.
But if the transformer is a 208V DELTA to 480/277 WYE, then you need to ground the 480V neutral.
As already suggested, check the complete system neutral grounding. Ground at only one point on the line side of the main breaker's neutral ground fault CT.
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
I understand the first time you turned just the transformer on the GF tripped. As I said, energizing an unloaded transformer can really cause problems, especially if the GF is set to minimum values.
Other than that I cannot see where you have provided enough information for us to narrow down your situation. Although we have offered plenty of things to look at.
The most common problem in electrical systems is ground faults. Multiple small ground faults can 'pre-load' your GF relay such that is on the edge of tripping.
Fordean
Senior Member
- Location
- New Jersey
- Occupation
- Electrical Contractor
Ok, Opened up both transformers.
Both Xo are not in use. Just feeding for 208 Serivce Wye to 208 side Then Boost to 480 Its a wye-delta corner grounded. The Xo terminals are not used. Their are parallel grounds 3 conduits with #2/0 grounds. coming from service Terminated at Casing Ground on trans. This in turn is Bonded to X0 by means of Grounding strap. Trans is not Grounded to Neasrest beam. Since Service is closer. Let me know what else you need. Service tripps when Public service does switch overs in middle of night. Then Entire 2000 amp service powers up simaltaneously. When they turn grid area back on. (this is my guess). On this 2000 amp switch gear. 1- 300 kva Transformer and 1 - 250kva transformers is derived from.
zog
Senior Member
- Location
- Charlotte, NC
Were the added loads single phase?
rcwilson
Senior Member
- Location
- Redmond, WA
Let's see if I understand your system:
2000 Amp 208/120v, 3-phase, 4-wire service from wye transformer bank.
Main Service disconnect is 2,000 A bolted Pressure Switch (BPS) with Ground Fault Protection of unknown type.
Service has several feeders of a size and type unknown (fused switch or CB?)
One feeder supplies a 300 kVA, 208/120V-480V step up transformer.
Transformer FLA = 832 A. Inrush is probalby over 10 kA (12 X FLA).
Transformer Feeder (1000A? size and type unknown) to transformer has GF protection.
Step up Transformer's 208/120V winding X0 terminal is floating, not connected to the neutral, ground wire or the case. (verify?)
Grounding:
Utility transformer X0 bushing is grounded via casing ground strap.
Step Up transformer is grounded back to the servcie via 3-#2/0 EGC.
Service neutral is grounded at the X0 bushing and somewhere else. (Where?)
Relative locations of neutral bonding jumper, neutral disconnect link and ground fault CT's are unknown.
Unknown if the neutral is bonded to ground someplace downstream. (To check, kill power, open disconnect link and check continuity neutral to ground on the load side. Should be infinite, no continuity.).
What Happened:
Feeder to transformer tripped when first energized. The trips appear to be from ground fault unit. (Is feeder a fused switch also?) Changing setting appeared to fix problem for awhile.
If utility trips and comes back, Main trips unless all feeders are opened then energized one at a time.
Sometimes the Main trips and sometimes just the transformer feeder trips.
If all this above is correct, and your neutral wiring is good, the transformer inrush current may be saturating a sensing CT giving a false ground fault signal.
Fill in the blanks above and maybe we can help figure it out.
2000 Amp 208/120v, 3-phase, 4-wire service from wye transformer bank.
Main Service disconnect is 2,000 A bolted Pressure Switch (BPS) with Ground Fault Protection of unknown type.
Service has several feeders of a size and type unknown (fused switch or CB?)
One feeder supplies a 300 kVA, 208/120V-480V step up transformer.
Transformer FLA = 832 A. Inrush is probalby over 10 kA (12 X FLA).
Transformer Feeder (1000A? size and type unknown) to transformer has GF protection.
Step up Transformer's 208/120V winding X0 terminal is floating, not connected to the neutral, ground wire or the case. (verify?)
Grounding:
Utility transformer X0 bushing is grounded via casing ground strap.
Step Up transformer is grounded back to the servcie via 3-#2/0 EGC.
Service neutral is grounded at the X0 bushing and somewhere else. (Where?)
Relative locations of neutral bonding jumper, neutral disconnect link and ground fault CT's are unknown.
Unknown if the neutral is bonded to ground someplace downstream. (To check, kill power, open disconnect link and check continuity neutral to ground on the load side. Should be infinite, no continuity.).
What Happened:
Feeder to transformer tripped when first energized. The trips appear to be from ground fault unit. (Is feeder a fused switch also?) Changing setting appeared to fix problem for awhile.
If utility trips and comes back, Main trips unless all feeders are opened then energized one at a time.
Sometimes the Main trips and sometimes just the transformer feeder trips.
If all this above is correct, and your neutral wiring is good, the transformer inrush current may be saturating a sensing CT giving a false ground fault signal.
Fill in the blanks above and maybe we can help figure it out.
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