Delta-Wye Solid Grounded step up transformer with Delta side artificial neutral

[tricard]

Hi,

I have a question regarding line-ground currents and artificial neutrals.

I am modeling a solar power plant. The PV arrays' generated power is inverted and transformed onto a 27.6kV collector system which is then stepped up to 115kV to the grid. The main transformer is a 20MVA, 27.6kv-115kv Delta-Wye(solid-grounded) configuration. The Delta side has an artificial neutral provided by a zig-zag transformer, rated 2000A (2 seconds).

For a L-G fault (phase A to ground) on the 27.6kV side, I have total phase A fault current of 0.93pu (27.6kV base, 100MVA base). 0.22pu of that is contributed by the solar power plant; the remaining is from the utility. 0.93pu = 1.95kA, which would be expected with the 2000A zig/zag transformer.

For a L-G fault on the 115kV side, I have fault current of 18.16pu of which 1.14pu (115kV base, 100MVA base) is contributed from the solar farm. Therefore the solar plant's fault current contribution is greater on the 115kV side than on the 27.6kV delta side with the artificial neutral, even though the solar plants contribution will be reduced due to the transformer impedance.

I have a colleague who is curious to know why the 115kV side has greater contribution than the 27.6kV side. I believe it is because the artificial neutral on the delta side, which provides substantial zero sequence impedance, results in lower fault current on the 27.6kV side. The 115kV side is isolated from that artificial neutral, so there is far less zero sequence impedance in the primary side fault (being that the fault location was at the transformer terminals). Where I lose confidence in that statement has to do with the zig-zag transformer. Does it necessarily have to be isolated from the 115kV wye point? If it is not, will it in effect be like a Wye(solid-ground)-Wye(solid ground) connection? I have searched through IEEE 142 (green book) and IEEE 242 (buff book) to find out about systems utilizing zig-zags but they only have a little discussion (essentially stating that they can be used to create a source for zero sequence currents). Am I seeing this properly or have I missed something? Possibly it is a short coming of the analysis software...

Thanks in advance for the always informative feedback!

 

[mls1]

The zero sequence current on the 27.6kV system is entirely dependent on the zig zag but on the 115kV side you can completely disconnect the delta terminals on your transformer and still see your transformer as a source for ground current on the 115kV system. That is because your delta/wye grounded transformer is to the 115kV system what your zig zag is to your 27.6kV system. The delta allows zero sequence currents to circulate in the delta which makes it a grounding transformer. An ungrounded system can derive a neutral with a zig zag OR a wye grounded/delta transformer. They both provide a path for zero sequence current.

 

[odlanor]

Assume your solar powerplant is connected to a big system.
If ground fault current of 18.16pu occurs at connection point it means that contribution current of powerplant is only 1.14pu and (18.16-1.14)=17.02pu is the contribution of the system.
1.14pu is the current contribution provenient from solar powerplant. This current contribution is biphase at 27.6kV side and singlephase at 115kV side.

 

[waross]

That connection has the ability to transfer energy from the healthy phases to a missing phase.
A ground fault on one phase of the grounded wye side will be fed not only by the faulted phase, but also by a backfeed from the healthy phases. The contribution of the healthy phases will be limited by three times the transformer impedance.
The grounded wye-delta connection will attempt to correct any voltage unbalance and or phase angle error on the grounded wye side.
Again the current will be limited by three times the transformer impedance. The phantom delta effect of a three legged core may increase reduce the effective impedance of the transformer.
To visualize the effect, consider two transformers connected in open delta on A phase and B phase. The primaries are connected from line to neutral. This transformer bank will develop a virtual transformer across C phase. Now connect the third transformer secondary into the circuit forming a closed delta with the primary open. The third transformer will back feed and develop a voltage on the primary equal in magnitude and phase angle to the line voltage of C phase. If the actual C phase voltage is high or low, or if there is a phase angle error, the C phase transformer will try to make a correction. If a phase is lost on the 115 kV side, the grounded wye-delta transformer will back feed and pick up any other loads on the 115 kV side.
I have seen this connection act as a stabilizer on long, unevenly loaded distribution lines.
These effects are independent of the grounding transformer on the delta side.


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

 

[odlanor]

ERRATA!
Assume your solar powerplant is connected to a big system.
If ground fault current of 18.16pu occurs at connection point it means that contribution current of powerplant is only 1.14pu and (18.16-1.14)=17.02pu is the contribution of the system.
1.14pu is the current contribution provenient from solar powerplant. This current contribution is singlephase at 115kV side and biphase at 27.6kV side with a value reduce of zero sequence component. In this short circuit , grounding transformer on the delta side does not exist.
I did not know generator of 27.6kV neither generator of solar powerplant.