Single-Phase, Three Wire System

[Humble2000]

If a single phase transformer primary is connected to L-L voltage, only one half of the three-phase short circuit MVA( GE SC calculation Manual, Page 33).Does anybody have reference to this calculation?

Thank you,

 

[DanDel]

"If a single phase transformer primary is connected to L-L voltage, only one half of the three-phase short circuit MVA"

What?

 

[Humble2000]

I apologize for this. I copied the text from word and half of it didn’t copy.
If a single phase transformer primary is connected to L-L voltage, only one half of the three-phase short circuit MVA will contribute to short circuit amperage on the secondary side.
What I need to know is if the transformer primary is connected to H1-G , what portion of three-phase short circuit MVA will contribute to short circuit from utility.
SKM needs both values for single phase calculation, however I think only L-G MVA is needed .
Regards,

 

[JJHorak]

You are likely right that optimized code could be made that only uses the SLG effective impedance for your specific calc, but you are dealing with SKM being set up for the generic three phase fault calc and hence single phase calcs need to be shoe-horned in. In the three phase world, you break every impedance into sequence impedances. You cannot separate single phase MVA into Z+, Z-, and Z0 unless you also have the three phase MVA. For this analysis, I think you will need to put in the full three phase MVA, not 1/2 or 1/3 of it. You will need to contact SKM to get a definitive answer.

 

[waross]

If it helps any, the load capacity of a delta transformer bank on single phase is 2/3 of the three phase capacity.
The in-phase transformer contributes its full capacity. The other two transformers may be considered as an open delta transformer bank. They will create a "virtual" transformer equal to the third transformer.
For example a transformer bank has three 100 KVA transformers. A single phase load is connected to "A" transformer. The transformer will supply 100 KVA. The "B" and "C" transformers together also contribute to the three phase load. If you add the vectors, voltage, current, voltage drop, etc. you will see that they create a "Virtual" transformer identical in phase angle and capacity to the "A" transformer. Together the "B" and "C" transformers will contribute 100 KVA to the load. Hence the three phase 300 KVA transformer bank will supply 200 KVA to a single phase load.
These numbers are familiar to any one who has used the double delta connection to convert a three phase generator to single phase.
respectfully