Connection of Dy generator unit transformer

[Rodmcm]

A client has a 6600kV generator and an 11kV/6600kV Dyn step up transformer. The generator has 3 neutral CTs and neutral is hard connected to earth. The 11kV synch breaker has the other set of differential CTs. Should the star point of the LV of the transformer be connected to anything, or left floating?

 

[burnt2x]

Woohoo! Never met an animal like that! What do you call that High side - Extra Ultra High Voltage? 6600 kV!
Are you sure there's no typo in there? If a typo, then the high side must be 11 kV delta and the low side is 6.6 kV wye! IMO, you don't have to ground the trafo neutral since you already have the grounded neutral of your generator (assuming this is a one-on-one setup). You do have a problem with the 11 kV side (delta) since you don't have a reference ground!

 

[waross]

???????????? How about a cup of coffee and then tell us what you have without abreviations or acronyms.
I read this as a 6,600,000 Volt generator feeding a 11,000 Volt Delta to 6,600,000 Volt Wye transformer. This will probably saturate when the 6,600,000 Volts generator output hits the 11, 000 Volt primary winding..
Three CTs on the generator neutral seems like a bit of over kill.
An 11kV/6600kV Dyn step up transformer wouldn't have a neutral on the Delta LV (11 kV) side.
Differential protection doesn't use CTs in the neutral.
Where shall I start assuming?

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

 

[Rodmcm]

Sorry folks
Dyslexia Rules KO!
Generator 6600V
Transformer primary 11000V
Transformer secondary 6600V
Transformer Dyn ( D on 11000V)

 

[waross]

Normally you would have a delta on the generator side and a wye on the outgoing side.

Consider changing the generator grounding to high impedance to limit stator damage in the event of a ground fault.
A wye delta transformer may experience switching surges if the neutral is floated. No issues if it is not switched.
A wye delta transformer with the neutral connected will develop severe circulating currents if the primary voltages are unbalanced. This may be a good thing for a generator transformer. We sometimes have issues with unbalanced loads on a generator causing badly unbalanced voltages. A wye delta transformer with the neutral connected will use the circulating currents to balance the voltages. With a connected neutral you will get less voltage rise on the unfaulted phases during a line to line fault.
For distribution I recomend wye:delta transformers and banks not be used and if they must be used, that the neutral be floated. For a Generator Step Up transformer there may be merit to connecting the neutral.
You may connect the neutral directly from the generator to the transformer and then impedance ground the neutral. (Take note, those who use the terms ground and neutral interchangably.)
Re the CTs. If your CTs are for differential protection, those three CTs will not be on the neutrals. The CTs will be on the winding leads just before they are connected together to become the neutral point. Or you may have three CTs on the neutral.


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

 

[Rodmcm]

This system generates into another power distribution system and they did not want another earth point on the 11kV hence the delta connection. Your thoughts are the same as mine that connecting the LV wye neutral to the generator neutral would rectify any misbalance and limit surge problems As to CTs, yes, one in each winding