GSU winding configuration for solar farms

[9wearSuave]

Hello,

We have typically been designing large scale PV plants with Y-D-Y GSU's with a NGR on X0. Why not use a Y-D transformer and use novel methods of detecting ground faults (3V0)? I understand that you would need to do a more in depth insulation study because of the larger neutral shift and possible TOV issues, but it seems it would present an overall savings on a GSU winding and smaller concentric neutrals for the MV feeders. Also, we are already having to do TOV studies since the inverter transformers are wired delta on the high side.

Is the 3V0 detection not reliable enough from a safety perspective?

 

[wcaseyharman]

I’ve asked the same (almost) question as my company is purchasing wind and solar farms. I was asking if we could use a high resistance ground method like is done on conventional generators. The answer I got was the substation equipment was often ordered before the TOV study was performed and therefore the specs of the various pieces of substation equipment didn’t necessarily meet the TOV requirements for a high resistance ground. At the time of a TOV it would be very expensive to adjust equipment specifications so the substation tends to be effectively ground to avoid that issue.

It seemed if the TOV needs were determined at the outset of the project a high resistance ground would be preferable for the reasons you mentioned. In addition it would potentially improve equipment life since the transformer would never be mechanically stressed during ground faults, which of course are the most common.

 

[FPelec]

Here in Italy (and, to my knowledge, also in many other places in Europe and North Africa), the YNd connection scheme is widely used for PV and wind farm step-up transformers, as well as inverter step-up units. The HV (or EHV) side neutral is solidly grounded, and a grounding reactor is often used on the MV side. In addition to the 3Vo protection (used for busbar or transformer faults), directional protections (67N) are often used for protecting MV feeders.

Si duri puer ingeni videtur,
preconem facias vel architectum.

 

[cranky108]

One problem with delta systems, is that it is difficult to find the location of a ground fault. A YDY transformer with a NGR on the secondary is that it limits the ground fault current, and gives you enough current that you can use relays to find the faulted feeder.