Generator Step-up Transformer

[genhead]

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

 

[davidbeach]

What do you want your grounding system to look (electrically) like? Does that YnYn transformer have a delta tertiary buried in it? If not, is it a 3 leg core or is it a 4 or 5 leg core? Is it one 3-phase transformer or is it three 1-phase transformers? What codes or standards might apply? In other words, It Depends.

 

[genhead]

David,
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

 

[davidbeach]

If it is a 4/5 leg core transformer you won't present as an effectively grounded system on the 11kV side unless your 400V neutral is solidly grounded. If it is a 3 leg core you probably won't either, but you will get some phantom delta effect. Can the protection on the 11kV side deal with ground fault protection however the transformer is connected? Best to connect in such a manner as to allow the high side system to perform satisfactory.

 

[Marmite]

Are there any phase to neutral connected loads on the 11kV side, or are they all either phase-phase or 3 phase?

Regards
Marmite

 

[davidbeach]

For the purposes of Marmite's question, "phase to neutral connected loads" should be read to include phase to neutral connected arrestors.

 

[linkyway]

The 400 V transformer neutral should be left ungrounded. You are grounded on the generator neutral but you may have a problem here too. Typically each generator should have a ground switch to connect the generator neutral to ground. When paralleling generators only one ground switch is closed. That way circulating currents are prevented. You will also need auxilliary swithes on the ground switches to alarm should all switches be open. Judging by your voltage of 400 V you might be in New Zealand? I left there 20 years ago. If so I recall (then) that the Wiring Code requires separate ground switches

 

[davidbeach]

The whole idea of generator grounding switches gives me the heebee geebees. I know, I know the IEC folks will say it is done all the time, but it just seems like a disaster waiting to happen. Far better would be high resistance ground each unit, leaving the neutral connect at all times, and then connect to a delta transformer winding, with a grounded wye on the system side. No circulating currents to worry about, no worries about tripping the one grounded generator and leaving the whole lot ungrounded, no worries about too many switches closed. It is also possible to parallel multiple solidly grounded generators without objectionable circulating currents.

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.

 

[genhead]

So, assuming that there is always a solid generator neutral-ground connection, but the transformer LV neutral is left unconnected, is the 11kv side effectively grounded?
This is my main concern.

 

[waross]

This has been discussed. A wye primary transformer is not happy unless it has the neutral connected. Nothing to do with grounding and everything to do with voltage balance. With the primary neutral floating, a phase to phase load may cause the voltage to rise on the unloaded phase. It may also cause phase shifts on the secondary voltages with unequal loading.
How and where the neutral is grounded is another issue.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[trosepe]

genhead,

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.

 

[jghrist]

So, assuming that there is always a solid generator neutral-ground connection, but the transformer LV neutral is left unconnected, is the 11kv side effectively grounded?
This is my main concern.

No. As davidbeach said, the 11 kV will not be grounded if the LV neutral is left unconnected.

 

[waross]

If there are separate grounding links from the HV neutral and the LV neutral, the LV link may be removed and the HV side will still be able t be grounded.
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