Wind and solar farm collector sub transformers 3

[wroggent]

Located in central US where we own some wind farms and have power purchase agreements with many dozens of others. It seems that at all of these locations, the transformer at the collector sub (35kV to transmission) is a GndWye-Delta-GndWye configuration. I have a few ideas of why this transformer configuration was used, but no one seems to know definitively why.

Does anyone know why this is common?

For those familiar with solar farms in the 50MW range, what transformer configuration is common there? We are considering the same delta-GndWye transformers common to our distribution substations.

 

[cranky108]

Many NUG systems require a grounded system, just as most utility transmission systems require a grounded system. Also a ground reference is needed to be able to locate phase the ground faults.
Most wind farm applications that I have seen use 35kV as a collector voltage, I assume because 35kV is the highest voltage that can be contain in switchgear. The issue maybe to avoid building an open bay yard, which requires more space, and requires more engineering time of the developer.

In inverter applications, the step-up transformer is part of the filtering circuit, and requires a good ground on the collector system to avoid GPR, which can kill some electronics.

 

[Kiribanda]

During a single phase to ground fault on the utility side, the feeder will be disconnected by the Utility substation breaker
and isolate from their end. But the WTG continues to feed to the ground fault because he is blind for any ground fault
on the utility side. That means there is a safety issue of operating a live line with a ground fault on the line which Utility
does not want to happen. Utility side always should be effectively grounded for any ground faults. That means the step-up transformer
should have WYE grounded on the high side. Collector side can also be wye grounded or even delta depending on the protection scheme you are going
to have for the collector side. Normally operating a delta collector system at 34.5kV with long collector cables give transient overvoltage
issues.

 

[xnuke]

I always recommend reading "The Whys of the Wyes," available here. It has good discussion on wye-delta-wye transformers in it.

xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[prc]

1) Cranky 108- Collector transformers for wind farms are specified in IEC 60076-16: 2011 and for solar farms by IEEE C57.159-2016. In solar and wind farms, usual connection is Dyn and the LV side neutral is never grounded (see clause 5.1.4 of C57.159) In India, where the LV neutrals were grounded hy mistake, resulted in premature failure of inverters.

2) In substation collector transformers ( connected to utility) YNyn connection without stabilizing tertiary is used in India for ( 3 phase 3limbed only). Both HV and LV neutrals solidly grounded. Where Ynd is used, a grounding transformer is provided on 33 kv to avoid problems cautioned by Kiribanda.