Voltage Drop - Include Transformer VD?

[W@ttson]

Hello,

I have tried to do as precise voltage drop calculations as possible. Even tried to use the "exact formula" from IEEE. What I have never included was the VD across a step down/step up transformer. I was recently doing a simulation in SKM and noticed that the VD was pretty bad but nothing was happening to correct the issue by changing conductor size, then I noticed it is because of the transformer.

Do you include transformer VD in hand calculations? I have always just transferred the voltage to the secondary based on what the VD was up to that point, but I am thinking it might be prudent to start including it in hand calculations?

 

[dkidd]

You can adjust the taps to compensate.

 

[W@ttson]

You can adjust the taps to compensate.

Yes, but that is variable with load and if I could, I'd rather be able to account for the voltage drop by calculation. I just want to see if anyone does it, or is this just something only encountered when using a simulation software like SKM?

 

[W@ttson]

Also, I guess what is interesting about this situation is that, you can't really do anything about it unless you change the transformer. Upsizing conductors doesn't get you very far, only when you change the transformer and its impedance changes do you see a drop in VD. I guess that is why it has never really been brought up.

 

[xptpcrewx]

Either way, you can account for it manually with hand calcs or do a load flow analysis in SKM.

Feeder VD calculations are only as good as the assumptions behind them; in this case, it sounds like your hand calcs only accounted for the cable VD and assumed the voltage was nominal... whereas, SKM shows the voltage at the transformer is lower than nominal due to VR...

Some factors to consider whether you do hand calcs or run a load flow simulation:

The lowest available voltage at the service,
The Thevenin equivalent impedance at the point of interest,
Effects of on-load tap changers, regulators and/or fixed tap positions,
Temperature adjustments,
Power factor of the load,