Why not star star generator transformer?

[spat154]

Hi There,

I am reviewing this Transformer spec prepared by one of the consultants for connecting a 50 MW wind farm to the 132kV grid. Basically it is a 12/132kV connection. They have specified star-star connection with both sides solidly earthed (Yn0Yn0). I thought the best way to connect generator transformer is star delta (Ynd11 - where Delta is 12kV) for all the reasons such as it reduces third harmonic and reduces costs (less copper in delta winding) etc...But what I am not sure is that if we go with star delta then how do u limit the fault current on the LV side? Is it necessary to always put neutral earth resistors on LV? Any reason to not connect star star?

Regards.

 

[edison123]

A star/star connection would result in swinging or unstable neutral. A tertiary winding would stabilize the neutral potential. A delta/star (12/132 KV) would be preferable because of large currents on the LV side and lower dielectric stress on the HV side.

Did you ask your "consultant" why he chose star/star ?

 

[spat154]

Thanks Edison123.

Can you please explain that if we will have delta on the LV side then we would require earthing transformer, won't it make it more expensive? In that case isn't it just better to have tertiary connection (As u suggested)?

Also, consultant suggested that the star star is chosen so that both sides could be earthed via NER because he considers that this calls for maximum protection.

 

[edison123]

An earthing trafo (which will be single phase connected to the generator neutral) is anyway required to protect the generator (unless of course, you choose solid grouding / resistance grounding). I would estimate such a trafo would be around 25-50 KVA, which is peanuts, when compared to adding a tertiary winding in a 60 MVA trafo.

 

[jghrist]

See

http://www.beckwithelectric.com/powerlines/powerlines-37.html#4

for a comparison of transformer connections on IPP (distributed) generation.

Wye (grounded)-Wye (grounded)
Pros:
· Does not cause an overvoltage for ground fault at F1.
· Supplies a ground source for IPP facility.
Cons:
· Provides an unwanted ground current for supply circuit faults at F1 and F2.
· Supplies ground current from Breaker 1 for faults at F3.

F1 is on the utility line.
F2 is on a different utility feeder connected to the same substation.
F3 is at the generator bus.
Breaker 1 is on the utility feeder at the substation.

 

[davidbeach]

The 12kV is the result of generator step-up transformers, the wind generators don't directly produce 12kV. The transformer in question needs to be grounded on both sides and will probably have a delta tertiary that may or may not be brought out of the transformer. The transformer in question is not a generator transformer, it is a transformer between two transmission voltages; for this particular application the 12kV functions as a transmission system.

 

[cranky108]

Normally the larger wind units will have a terminal voltage of 500 to 600 vac. They will have a step up transformer which would normally be a wye-delta.
What you are looking at is the stepup from the collector system to the transmission system.

On the other hand, you may not have been given very much information on the wind farm, where you would not have known much about the collector system.
Some of these people are not very forth coming with information.

 

[edison123]

My bad. I saw 50 MW and immediately assumed GSU Trafo.

The neutral still swings though.

 

[jghrist]

Looks like a distribution substation. Why not use a 132000-12470GrdY/7200 volt delta-wye power transformer. This is what would typically be used for a distribution substation. The 132 kV system will have other ground sources.

 

[waross]

As edison123 points out;
You must be aware of neutral swings on a wye primary.
This may be avoided by;
A> Connecting a solid neutral back to the GSUs if the GSUs have a wye secondary.
B> Using a transformer with a delta tertiary winding.
C> Using a transformer with a three legged core.

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

 

[apowerengr]

jghrist - the 132kV system has other ground sources UNTIL the line faults and trips, then the wind generation can supply the fault through your proposed connection having 132kV delta without a ground reference. Without supplemental voltage detection of ground faults (27&59 simultaneously), replacing arresters on this line segment connected line-to-neutral for potential line-to-line exposure and other issue management (mid-line ground source) the system would have performance issues.

davidbeach - there are turbine mfgs. supplying directly at 12,000 volts.

Y-Y with delta tert may be the best configuration for this application. Impedance grounding would not normally be recommended for such a connection without additional protection for faults on the 132kV system and backfeed isolation.

 

[davidbeach]

OK, I'll take your word for 12kV turbines. I haven't run into any on the wind systems I've seen one-lines for.

 

[DTR2011]

"I haven't run into any on the wind systems I've seen one-lines for. "

I'd second that thought. Perhaps the GSU is located withing the tower as part of the package. Most Wind Turbines I've seen and installed have a 1500 - 3000 kVA oil filled transformer installed on a pad next to the bas of the tower.

I can't understand why the expense, and lost space in the nacille to have a 12kV rated gen set.