Tertiary Winding Current Calculation

[BINGMD]

I have a transformer with the following data:

50MVA, 115kV-34.5kV-13.8KV, groundWYE/groundWYE/Delta

Using Aspen software, I apply a single line to ground fault at 34.5kV side.(Note: there is no low side or other fault contributions).

Zero sequence fault current at 34.5kV side is 5850Amp.
Zero sequence fault current at 115kV side is 217Amp.

I wish to get the teriary fault current to do the settings of a relay used at the teriary. I need your wisdom and smartness to confirm if I did right.

Zero Sequence Short Circuit MVAsc at 34.5kV side = 1.732*34.5kV*5.850kA=349.56MVA

Zero Sequence Short Circuit MVAsc at 115kV side =
1.732*115kV*0.217kA=43.22MVA.

The difference is 349.56MVA-43.22MVA=306.34MVA and it must come from tertiary. So, the tertiary current is
306.34MVA/(1.732*13.8kV)=12,817Amp.

Thank you for your help.
Bing

 

[stevenal]

Don't think your approach is valid. Only source is on the 115? Then there will be no line current on the 13.8, just circulating current through the delta. Aspen does a fine job of modeling three winding transformers. Why not let Aspen do the heavy lifting?

 

[jghrist]

The 115 kV and 34.5 kV currents will be in a ratio equal to the inverse of the voltage ratio. Your ratio of 217/5850 doesn't make sense.

 

[BINGMD]

Stevenal,

Yes, only source is on the 115kV.
Although Aspen could model three winding transformers, but it could not give us tertiary current. We have need set the relay at tertiary winding.

Thanks,
Bing


Jghrist,
There is portion of 34.5kV fault currents from the circulating current in tertiary winding. Therefore, ratio inverse does not apply here.

Thanks,
Bing

 

[stevenal]

Put a relay group on the transformer next to your tertiary bus. Put a ground relay in the group, and choose "tertiary delta" next to "CT at". Look at the curves window or the phaser diagram to see the currents. Are you new at this? A training course is worthwhile. (And the Chinese banquet in SF was wonderful.)

 

[BINGMD]

Stevenal,

I got the current. I seldom use tertiary and "return" the knowledge back to trainer.

Thank you for your help.
Best Regards,
Bing