Gnd Wye Transformer connected to Gnd Wye Generator
Gnd Wye Transformer connected to Gnd Wye Generator
(OP)
I have been asked to consult on a new generator installation that is having startup problems. They have already fried two DC displays.
The system is installed at a primary metered facility that is fed 12470 from the utility. The feed has been re-routed to feed thru a 1500 kVA, 12470 GY to 480 GY transformer with the secondary connected to the output of the generator.
The problem they are having is this. The transformer secondary was bonded to ground, and the electrician also bonded the generator neutral inside the generator. The two ground grids were bonded together. When they started the generator, the circulating current in the ground-neutral loop was enough to fry the electronics in the generator.
After frying two units they called me. My suggestion was to remove the bonding strap in the genset and leave the bond in the transformer.They decided to follow the advice of another engineer with more experience, and removed the bonds at both ends and the neutral connection between the transformer and the generator.
My concern is the single-phase load at the facility now creating current on the downstream transformers(8 of them) with only the concentric on the underground cable as a return to the utility ground. Each downstream transformer is bonded to ground.
When the generator is in island mode, neutral current will only have the generator ground(through earth), which is not bonded to the neutral.
I have read some of the GY-GY connection threads on this site, and am still not sure what to suggest.
The system is installed at a primary metered facility that is fed 12470 from the utility. The feed has been re-routed to feed thru a 1500 kVA, 12470 GY to 480 GY transformer with the secondary connected to the output of the generator.
The problem they are having is this. The transformer secondary was bonded to ground, and the electrician also bonded the generator neutral inside the generator. The two ground grids were bonded together. When they started the generator, the circulating current in the ground-neutral loop was enough to fry the electronics in the generator.
After frying two units they called me. My suggestion was to remove the bonding strap in the genset and leave the bond in the transformer.They decided to follow the advice of another engineer with more experience, and removed the bonds at both ends and the neutral connection between the transformer and the generator.
My concern is the single-phase load at the facility now creating current on the downstream transformers(8 of them) with only the concentric on the underground cable as a return to the utility ground. Each downstream transformer is bonded to ground.
When the generator is in island mode, neutral current will only have the generator ground(through earth), which is not bonded to the neutral.
I have read some of the GY-GY connection threads on this site, and am still not sure what to suggest.
Happiness is a way of travel, not a destination.






RE: Gnd Wye Transformer connected to Gnd Wye Generator
RE: Gnd Wye Transformer connected to Gnd Wye Generator
Here is a basic One line diagram.
Happiness is a way of travel, not a destination.
RE: Gnd Wye Transformer connected to Gnd Wye Generator
"The more the universe seems comprehensible, the more it also seems pointless." -- Steven Weinberg
RE: Gnd Wye Transformer connected to Gnd Wye Generator
You need to have the neutral bonded to ground at one point. Otherwise, there is no metallic path for ground fault current. This will not provide reliable fault clearing and is a violation of the NEC.
RE: Gnd Wye Transformer connected to Gnd Wye Generator
There is a breaker in the utility feed to the high side of the first transformer controlled by the generator for utility isolation.
Here is a new drawing.
Happiness is a way of travel, not a destination.
RE: Gnd Wye Transformer connected to Gnd Wye Generator
I do not see any need to connect the neutral of the xfmr to the N of the gen. In a Y-Y xfmrs, generally the both neutrals are internally bonded and secondary side (load side) neutral is bonded to earth.
You may want to impedance ground the generator to limit the circulating current as long as there are no single phase loads on the gen bus (480V).
RE: Gnd Wye Transformer connected to Gnd Wye Generator
RE: Gnd Wye Transformer connected to Gnd Wye Generator
I do not see any need to connect the neutral of the xfmr to the N of the gen. In a Y-Y xfmrs, generally the both neutrals are internally bonded and secondary side (load side) neutral is bonded to earth.
If the xfmr neutral is not connected to the generator neutral, then 12 kV ground fault current and 12 kV load unbalance current will have to flow through the earth when the generator is the source. This is not acceptable.
That is the reason for this thread. The engineer that was consulted said that the downstream transformers would create their own neutrals, or would be able to "absorb" any neutral current that exists. He stated that we may see some heating in the first transformer because of this, but it should be ok.
I guess I am a sceptic. I would be more comfortable not relying on the ability of a transformer to "absorb" anything.
Happiness is a way of travel, not a destination.
RE: Gnd Wye Transformer connected to Gnd Wye Generator
If there are no loads on the 480V system produced by the generator, the transformer ought to have a delta winding on the 480V side, and the generator should be grounded through a resistor that limits ground fault current to 5-10A. A grounded wye winding on the 12470V side would produce a quite nice neutral.
RE: Gnd Wye Transformer connected to Gnd Wye Generator
RE: Gnd Wye Transformer connected to Gnd Wye Generator
The wye-wye transformer is making it hard for me to visualize, but if it was a delta wye transformer like davidbeach suggested then would we need to worry about grounding the neutral generator together with the transformer neutral since the transformer would be a seperate source and all fault current would flow back to it?
What if the generator neutral was grounded and not tied to the transformer, but a seperate EGC was run from the generator out to the downstream transformers. Wouldn't fault current then return to the generator source? Again the whole wye-wye things is throwing me off, so I may be overlooking something in regards to this connection.
RE: Gnd Wye Transformer connected to Gnd Wye Generator
Consider the single phase analogy. One 120:120 Volt transformer is feeding a 100 W load. Another 120:120 Volt transformer is feeding a 1000 W load.
The primaries of the transformers are connected in series and fed from a 240 volt circuit.
What happens?
The simple calculation would see 21.8 Volts across the 1000 W load and 218.2 Volts across the 100 W load.
In the real world, the over voltage on the 100 W loaded transformer would probably lead to saturation which would increase the IZ Voltage drop across the lightly loaded transformer so the voltage shifts may not be as severe as the calculations may indicate.
The calculation gets a little more complicated with a floating neutral on a wye primary transformer. You may also have a phantom delta effect that may stabilize the neutral point somewhat, but this effect is only present in some transformer designs, particularly three legged cores.
If the grounding/neutral/bonding confusion has caused burnout of DC devices, I suspect that you may be using a ground connection to replace a neutral conductor and the ground connections or path are compromised.
DON'T confuse a ground with a neutral even though they may be the same potential. Don't use grounding to replace a neutral in a circuit that needs a neutral.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Gnd Wye Transformer connected to Gnd Wye Generator
Happiness is a way of travel, not a destination.
RE: Gnd Wye Transformer connected to Gnd Wye Generator
There may be a problem with the wye-wye connection during load management mode with a solidly grounded generator. Generators have low zero-sequence impedance compared to the utility source. Unbalanced load on the utility system may be fed from the generator, causing neutral overcurrent protection trips.
RE: Gnd Wye Transformer connected to Gnd Wye Generator
He is a brilliant man! I wonder which one of the electricians I have been telling this to for the past two weeks mentioned it to him.
They have not bonded the neutral in the trnsformer or the generator. I would prefer if they bonded in the transformer, at least that would satisfy the NEC. But this is a utility installation and just because the NEC makes good common sense some times, that doesn't mean they will follow it.
Happiness is a way of travel, not a destination.
RE: Gnd Wye Transformer connected to Gnd Wye Generator
Happiness is a way of travel, not a destination.
RE: Gnd Wye Transformer connected to Gnd Wye Generator
Bill
--------------------
"Why not the best?"
Jimmy Carter