Load Flow Voltage Distortion Across Transformers

[voltagevortex]

I am performing a load flow study for a low voltage distribution facility (480V) with step down transformers to 208V using SKM. Can anyone explain why the % V.D across a transformer differs when I change taps on the high side vs on the low side. In other words, -5.00% on the primary side does not equal to +5.00% on the secondary. Some transformers have over 3% voltage distortion when I set the taps on the primary but if I were to change the taps on the secondary it is less than 3%.

 

[waross]

Changing the taps puts more or less turns in the primary winding.
That has a small effect on the regulation and on the impedance voltage (and ASCC).
Depending on the power factor of the load, the actual voltage drop% may not be exactly equal to the regulation% but there is a close relationship.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[bacon4life]

Going through a delta/wye transformer does two things:
1)suppresses triplen harmonics
2)Changes the relative magnitudes of each phase. For example a large dip in a single phase on the low side will show up as a smaller dip on two phases on the high side.

 

[wcaseyharman]

If I were approaching this problem I would looks at
- load current between the two cases - do vars change much.
- possible saturation modeling in SKM. By lowering the HS taps the transformer would be operating farther up on the BH curve.

Just some thoughts that might help.

 

[cranky108]

bacon: three things, 3)changes the phase angle by 30 degrees. It may not be important unless one is connecting two utility sources, but I would not leave it out, just in case.

 

[waross]

Regulation is the transformer voltage drop caused by internal impedance.
If changing the taps puts more windings into service you will have increased the internal impedance and the voltage drop will increase.
Assume that the incoming voltage is too high.
To correct the secondary voltage, the transformer ration must be changed to match the ratio of actual supply voltage versus required secondary voltage.
If the taps are on the high side, you must add winding turns to achieve the desired ratio.
More turns means greater internal impedance and thus greater voltage drop.
If the taps are on the low side, you must remove winding turns to achieve the desired ratio.
Less turns means less internal impedance and thus less voltage drop.
Your findings are to be expected.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[voltagevortex]

Thanks for the responses, everyone. I am more informed and appreciate the knowledge shared.