Fault current 1

[Mbrooke]

Is there anyway to calculate by how much fault current increases with a Yd11 vs a Dyn11 132-33kv transformer on the primary side duding a 132kv fault?

 

[cranky108]

I'm sure there is. Just substitute the other transformer in the fault calculations.

 

[krisys]

3-phase fault current should not change due to the change in transformer vector group (i.e. winding connection) for the given voltage ratio. However, Earth (ground) fault current would change depending upon the method of neutral grounding. This would need a detailed analysis based on the complexity of network and the available Zero Sequence sources.

 

[Mbrooke]

This is a solid ground. My concern once the change over occurs there will be increased fault current in the substation on the 132kv side, as well as the line outside it.

 

[LDKGR]

Hi Mbrooke,
Is there any special reason to change a Dyn11 with a Yd11 ?

 

[waross]

I was under the understanding that fault current calculations were based on phase to phase to phase faults and that phase to ground current and grounding or lack of grounding did not enter the equation.
Have I been mistaken?

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

 

[Kiribanda]

Mbrooke,
I think you are looking for the increase in ground fault current happens when the transformer is YNd11 (solidly grounded WYE on 132kV Utility side). During a ground fault on 132kV side, in addition to the ground fault contribution from the Utility side, there will be a a zero seq current contribution from the primary of the transformer too. Therefore the actual ground fault current at the fault location is higher. But this will not happen if the transformer is Dyn11 because the DELTA winding is not a zero seq source. One thing important here is that if the transformer is YNd11 then during such a ground fault on 138kV side the Utility current contribution becomes little lower. (If you model and calculate you will find this) Depending on the ground fault pick-up setting this lower fault current may not trip the Utility upstream bkr. Therefore, Utility has to adjust 50/51G accordingly. Is this the answer you are looking for or otherwise?

 

[waross]

On a grounded wye primary, the unfaulted delta secondary windings act as an open delta bank in parallel with the shorted winding. This will cause a back feed into the faulted phase limited by three times the impedance rating of the transformer.


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

 

[Mbrooke]

Mbrooke,
I think you are looking for the increase in ground fault current happens when the transformer is YNd11 (solidly grounded WYE on 132kV Utility side). During a ground fault on 132kV side, in addition to the ground fault contribution from the Utility side, there will be a a zero seq current contribution from the primary of the transformer too. Therefore the actual ground fault current at the fault location is higher. But this will not happen if the transformer is Dyn11 because the DELTA winding is not a zero seq source. One thing important here is that if the transformer is YNd11 then during such a ground fault on 138kV side the Utility current contribution becomes little lower. (If you model and calculate you will find this) Depending on the ground fault pick-up setting this lower fault current may not trip the Utility upstream bkr. Therefore, Utility has to adjust 50/51G accordingly. Is this the answer you are looking for or otherwise?

This is exactly my concern. The transformer being considered is wye grounded on the primary, and being a zero sequnce source I am not sure how this will effect a single phase to ground fault on the 132kv side or on the 132kv line going into the substation.

Also, what do you mean regarding the utility fault current contribution being lower?

 

[waross]

If the wye point of the wye/delta transformer is left floating, there will be no contribution to the fault current.
The wye point will shift as the voltage on one phase collapses due to the line to ground fault.

If the wye point of the wye/delta transformer is grounded, there will be a contribution or a back feed into a line to ground fault.
The steady state current will be limited by an impedance equal to three times the transformer bank impedance.

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

 

[Mbrooke]

In a floating wye, how will this collapse change the voltage on the 33kv delta secondary?

#2 make sense as a good rule of thumb, I will look in to that.


In regard to this, can has anyone heard of on-board tap changers on a delta-delta transformer? If fault currents are to great of a concern that might be considered.

 

[Kiribanda]

Mbrooke,
As mentioned before, when there is a ground fault on the 132kV WYE-GND side, there are two zero seq current contributions. One from Utility side and the other from the YNd11 transformer.
If you do a simple model, you will find that Utility ground fault current contribution is bit lower comparing to the ground fault current contribution, if the same transformer is connected as Dyn11 mode. You have to do a calculation or a small modeling to prove this.