Fault Current Contribution - HV transmission

[Tinh123]

Thanks Dandel, davidbeach, and others who gave me very beneficial suggestions. I have recently worked on a problem of fault current contribution from the surrounding elements on a high voltage bus 138 kV. What I noticed is that when i changed one set of generation and transformer using completely different impedance values for both the transformer and generator- the value of the fault current contribution of the new set of generator and transformer to the bus fault is significantly less. As anticipated, If the fault contribution from one element is less then the overall bus fault should be decreased. The results I got indicated differently. The fault current contribution of the new element to the bus fault is less, however, the fault contribution from other circuits are increased significantly; hence, it increases the overall bus fault. Why would this be?

 

[DanDel]

Are you talking about fault current or fault MVA? Current will change with a change in system voltage, as I mentioned before.

 

[Tinh123]

This is with respect to fault current. No odd manipulation of the voltage or anything. I just use a different generator and different transformer. What I don't understand is that why an increase in bus fault current eventhough there is a decrease in fault current contribution from this generator? Side Note: I noticed an increaase in fault current contribution from other generators as the result of new generation.

 

[DanDel]

You say you are using a different generator and a different transformer, and you didn't change the voltages. The new generator has less contribution, but the fault on the secondary of the transformer is greater?
This would indicate that the new transformer has less impedance, is that true?

 

[stevenal]

Sounds like you might have a phasing problem. Do a check network anomalies to see if you have a source that is significantly out of phase with the others. You can also apply a very high impedance fault and see that the current flow is on the order of load rather than fault current.

 

[Tinh123]

Dandel,

The combined fault current contribution from the new set of generator and transformer is lower, given the transformer has less impedance than the original configuration. However, I don't understand why the impedance of the new transformer would affect the fault current contribution to the bus from other elements, as i observed the fault current contribution from other attached generators are increased compare with the original configuration.

 

[DanDel]

At what point or points in the system has the other element's contributions increased?

I'm now assuming that you have two or more generators connected to the primary of a transformer, and are looking at faults and contributions on the transformer secondary, primary, and generator connection buses.

You say that changing the transformer impedance only changes the fault current from the generators? Or are you changing the bus voltages at, and the impedances of, the generators also?

 

[Tinh123]

Dandel,

Given a 138 kV bus with step up transformers A, B, C, & D associate with Gen_A, ...Gen_D. If i were to increase the zero sequence impedance of transformer A, then i noticed a decrease in fault current contribution from Gen_A and A transformer to the 138 kV bus, which is understandable, but why would the increase in transformer A zero sequence impedance results in an increase in fault current contribution from B, C, D equipments ? I also tried to see what is the overall effect if i were to increase transformer A positive sequence impedance. The result is that fault current contribution from element A decreases, but fault current contribution from B, C, D are also decrease. Any thought on this ? Note: All these are with respect to Single Line to Ground fault. Thanks.

 

[DanDel]

If your transformers are delta-wye, secondary line-to-ground faults are not related to primary (generator) line-to-ground faults. I'm not sure how you are changing the Z0 for the primary delta winding unless there is a reference to ground. Typically, the generators would have a neutral resistor or reactor which limits the line-to-ground fault current if there is a ground reference at all.

 

[Tinh123]

There is no ground reference for the transformer I modelled. It's a Y-D, delta lags transformer.