YDY Contribution to Primary (Line) SLG Faults

[rogerj1]

Recently had a tibreaker operation on a YDY transformer due to a SLG fault on the transmission line feeding this sub (the "Y"s are grounded, feeding radial load). The upstream line breaker and the local tiebreaker at the YDY primary both operated as expected. I know the primary grounded wye is a zero seq source to the fault, but I can't seem to find a good resource of info detailing this type of event. I would like to see a good pictorial representation (sequence network connections, 3PH diagram, etc.) of this type of fault. This event often seems to be "overlooked, or forgotten", that in a radial setup like this, the YDY can still contribute to primary SLG faults.

Thanks for the help and discussion!

 

[davidbeach]

Try either/both of the Blackburn books. No sure how it could be "overlooked or forgotten" if you are doing your short circuit calculations from an accurate system model.

 

[rogerj1]

By "overlooked or forgotten" I mean those who came investigating why there was "miscoordination" between the line breaker and the tiebreaker. Initial thought was the linebreaker miscoordinated with the tiebreaker for a through-fault on the transformer.

thanks for the reccomendations!

 

[SMB1]

you should have directional overcurrent phase and earth in the Xfmr HV side. and if you don't deactivate the earth overcurrent element, this is the usual practice.

 

[waross]

Actually it is the neutral connection rather than the ground that is important.
With a four wire connected primary and a delta secondary, the transformer will try to maintain balanced voltages. It will transfer power from the healthy phases to maintain the voltage on the phase with lower voltage.
Visualized it this way, consider two phases, "A and "B", and a neutral feeding an open delta transformer bank.
"C" phase voltage will be developed across the open delta. Now if a third 7200:240 Volt transformer has its secondary connected so as to complete the delta and be fed from the open delta formed by the two energized transformers, it will generate a voltage on its primary that is equal to and in phase with "C" phase.
For the sake of example, two 7200:240 Volt transformers may feed another 7200:240 Volt transformer with the open side of the delta and this transformer is capable of supplying 7200 Volt loads.
Now suppose that this third transformer primary is connected to "C" phase. There will be an adjustment of current as the loads of the three transformers are now fed from all three phases instead of just "A" and "B" phases.
Issues
Case one, the voltage on "C" phase drops a little. There will be a circulating current in the delta as the bank transfers power to "C" phase trying to balance the voltages. The limit is the transformer impedances. With a voltage difference of greater percentage than the percent impedance of the transformers the transformers will be overloaded by the circulating current. I have seen transformer burn-outs when the voltage regulator on one phase was inoperative. This caused enough voltage unbalance on a four wire primary, wye/delta bank to burn up two transformers in a matter of weeks.
Case 2, there is a short circuit on one phase. Again, the delta winding will transfer power from the two healthy phases and back feed the fault. The current will be limited by the transformer impedances. A transformer with a three legged core forms a phantom delta winding which will also contribute to this back feed effect.
Note, in practice, wye delta distribution transformer banks may have four fused cutouts, one in the neutral. When the transformer is to be energized by a field crew the neutral connection is closed first. Then the three phases are closed in turn. This avoids switching transient overvoltages. Then the neutral link is removed and taken away in the service truck to avoid feed back issues. Four wire primary wye/delta transformer banks are not recommended for distribution systems.

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