Generator Step-up Transformer
Generator Step-up Transformer
(OP)
There are several 400v generators connected to a 400/11,000v YnYn transformer. The 11kv system supplies some industrial plant. It is a separate system with no utility.
The 11kv transformer neutral is solidly grounded.
The generator neutrals are connected to an LV neutral bus, with one connection to ground.
We would normally expect a delta winding on the 400v, so I'm not sure how the 400v transformer neutral should be connected.
a) Left unconnected.
b) Connected to the LV neutral bus with the generators.
c) Other
The 11kv transformer neutral is solidly grounded.
The generator neutrals are connected to an LV neutral bus, with one connection to ground.
We would normally expect a delta winding on the 400v, so I'm not sure how the 400v transformer neutral should be connected.
a) Left unconnected.
b) Connected to the LV neutral bus with the generators.
c) Other






RE: Generator Step-up Transformer
RE: Generator Step-up Transformer
It doesn't have a tertiary winding. It is one three-phase transformer.
Rather than code issues, at this stage I was more concerned with technical issues - Is it technically sound to either connect or leave the transformer neutral unconnected? If so, then what are the advantages/disadvantages?
Having the wye-wye transformer is new to me
RE: Generator Step-up Transformer
RE: Generator Step-up Transformer
Regards
Marmite
RE: Generator Step-up Transformer
RE: Generator Step-up Transformer
RE: Generator Step-up Transformer
But, anyway...
If the 400V transformer neutral is left ungrounded, the 11kV side will not be an effectively grounded source. That may or may not matter, but my guess is that it does matter, otherwise the 11kV winding wouldn't be solidly grounded.
RE: Generator Step-up Transformer
This is my main concern.
RE: Generator Step-up Transformer
How and where the neutral is grounded is another issue.
Bill
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"Why not the best?"
Jimmy Carter
RE: Generator Step-up Transformer
The gounded wye / grounded wye is a common transformer connection. The only disadvantage is zero sequence current on 11 KV side will operate ground overcurrent protection on 400 v side; in most cases that is not really a disadvantage. I have a 10 story telecommunication building operating on two paralled 2MW, 480v gens connected to a 5MVA, 480/277v - 12,000/7200v transformer (12KV side connected to buiding service equipment)with both neutrals solidly grounded. Has been operating for 6 months with no circulating neutral current issues. My experience has been, unless zero sequence levels are an issue, solidly grounding system neutrals is the best for system protection and safety.
RE: Generator Step-up Transformer
RE: Generator Step-up Transformer
There are two issues with the LV side. One issue is safe grounding.
The other issue is a proper neutral connection, grounded or not.
The choice of a wye/wye makes this challenging.
I would suggest a solid neutral bus connecting all the generators' neutrals. a solid (unswitched) connection from this bus to the transformer LV neutral.
The bus may be solidly grounded, use a ground switch, or use high impedance grounding.
The grounding of the HV neutral should be separate from the LV neutral and treated as a separate issue.
If the LV grounding link is removed inside the transformer, there will be no connection between the primary windings and the secondary windings and the grounding may be different on each side of the transformer.
Bill
--------------------
"Why not the best?"
Jimmy Carter