Wye neutral connection on a Delta-Wye Distributed Generation Transformer

[rockman7892]

Looking at an application where a new 24.9kV - 12.47kV transformer is being added for interconnection of wind farm to utility 24.9kV distribution line. Transformer is 15MVA and will have delta side facing generation (with grounding transformer on that side) and the wye side facing 24.9kV distribution. The wye point will be bonded to ground with a CT monitoring this connection wtih 51N for rapid detection of site generation feeding into a ground fault on distribution feeder.

The 24.9kV distribution line appears to have a neutral in the form of a multiground neutral on the distribution feeder with the neutral being used for L-N connections of 1-Ph transformers etc... on distribution system.

With the addition of the new interconnection transformer should this neutral be connected directly to neutral HO bushing on wye side of transformer? As I mentioned, this HO bushing will be bonded to local ground but trying to understand how to connect neutral coming into site (neutral is concentric cable neutral coming up with UG risers from main OH line). Should this neutral just be carried to first disconnect means (recloser) ground point?

We will have a 1-Ph station service transformer between incoming and transformer so thinking we need to at least carry the neutral in for that.

 

[jghrist]

The 24.5 kV neutral should be connected to the H0 bushing.

There will be current in the neutral during a ground fault on the 24.5 kV system even if the generator is not connected to the transformer. The delta-wye transformer is a ground source.

 

[rockman7892]

The 24.5 kV neutral should be connected to the H0 bushing.

There will be current in the neutral during a ground fault on the 24.5 kV system even if the generator is not connected to the transformer. The delta-wye transformer is a ground source.

Thanks. I suppose the best way to visualize this by drawing the symmetrical components for a fault on 24.5kV system and see that the fault current has path through zero transformer in zero sequence network regardless of generator is connected or not. At least I think this is the way I explained it to myself

What about similar interconnections on transmission systems that typically don't have a neutral?

 

[waross]

Look at the two healthy phases as an open delta, with the delta side of the faulted phase as a load on the open delta.
The wye delta transformer has the ability to transfer power from one phase to other phases.
I believe that the current back-fed into the fault will be limited by three times the impedance of the wye delta transformer.
Fault current will circulate in the delta winding.

 

[dpc]

The wye-delta transformer will act as a ground source for any faults on the wye side, even if nothing is connected to the delta side. They are often used for grounding transformer instead of the zig-zag autotransformer. Zero sequence current will flow in the delta winding.

For a core-type transformer, the fault current at the transformer terminals will be slightly greater than for a three-phase fault, but fault magnitude drops pretty quickly as the distance from the transformer increases.