Voltage Drop Across Xfmr

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marshavillareal

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I'm using ETAP to model a new electrical system and I am seeing a large voltage drop across my incoming transformer (~5%) The transformer is not overloaded I'm using about 16 out of my 20 MVA ONAF rating. When I increase the transformer rating the voltage drop is only a couple percent. Can anyone explain what is going on here? Is there any standard that talks about transformer loading / sizing?
 
I don't think there is a meter involved. I suspect it is a design calc program (etap)

mars (or marsha :)) -

If I understand, you are wondering why the Vd with 16 megs of load on a 5%Z, 20mva xfm is more than the Vd for 16meg load on a 5%Z, 40 mva xfm - if so:

The impedance of a 5%, 40mva xfm is half that of a 5%Z, 20mva xfm. But I'm thinking that you already knew that.

For standards, I'd probably start with the IEEE color book series.

cf
 
marshavillareal said:
I'm using ETAP to model a new electrical system and I am seeing a large voltage drop across my incoming transformer (~5%) The transformer is not overloaded I'm using about 16 out of my 20 MVA ONAF rating. When I increase the transformer rating the voltage drop is only a couple percent. Can anyone explain what is going on here? Is there any standard that talks about transformer loading / sizing?
Click to expand...

The voltage drop across the transformer is based on the transformer %Z.
If your transformer has a %Z = 6% then you will have a 6% drop at full load.
Assuming you have 6% Z, 16 mva/20mva x 6% = 4.8% drop. Are you using 16 mva
with the 40 mva sub?
 
bob said:
The voltage drop across the transformer is based on the transformer %Z.
If your transformer has a %Z = 6% then you will have a 6% drop at full load.
Assuming you have 6% Z, 16 mva/20mva x 6% = 4.8% drop. Are you using 16 mva
with the 40 mva sub?
Click to expand...
This is not correct. Both the impedance and the load current are complex values. The voltage drop is only equal to the %Z for full load if the load impedance angle equals the transformer impedance angle.
 
jghrist said:
This is not correct. Both the impedance and the load current are complex values. The voltage drop is only equal to the %Z for full load if the load impedance angle equals the transformer impedance angle.
Click to expand...

Since the OP did not specify any of the information regarding impedance angle, how can you say its incorrect?
 
bob said:
Since the OP did not specify any of the information regarding impedance angle, how can you say its incorrect?
Click to expand...
Isn't the xfm impedance mostly reactive? Generally x/r >10. And the standard industrial load is .8pf? I would think it would be a good guess that the two won't line up.

But I still would not have said you were wrong :grin::grin:

cf
 
Cold Fusion said:
Isn't the xfm impedance mostly reactive? Generally x/r >10. And the standard industrial load is .8pf? I would think it would be a good guess that the two won't line up.

But I still would not have said you were wrong :grin::grin:

cf
Click to expand...

This OP is not using actual load. He is using ETAP to model an electrical system.
 
bob said:
This OP is not using actual load. He is using ETAP to model an electrical system.
Click to expand...
Yes, I knew that. I think I mentioned that in post 3.

If I were modeling in etap, (which I occasionally do), I'd either show the initial loading as .8pf or resistive.

cf
 
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