Unbalanced Transformer Loads

[tryhard]

How do you calculate the neutral point voltage for a 3-phase, dy11, 33,000/415V, 100kVA, 4.75% transformer with the following loads: R-phase = 80% load; W-Phase = 5% load; B-phase = no load connected. Note the neutral point is not (or very poorly)earthed.

 

[230842]

If neutral is not earthed at all, then system is floating because it is not referenced to eart. If neutral is earthed (even poorly) neutral voltage is null, since no current can flow to earth.

If your question refers to the neutral voltage displacement due to umbalanced loads, then you can calculate it using the following formula:

DV = (V1*Y1 + V2*Y2 + V3*Y3)/(Y1 + Y2 + Y3 + Yn)

Where:
DV = Displacement voltage (Phasor)
V1, V2, V3 = No load output voltages (phasors) of phases 1, 2 an 3 (R, W, B)
Y1, Y2, Y3 = Complex admitances of Wye connected loads between correspondig phases and neutral (internal tranformer impedance shall be included)
Yn = Complex admitance between neutral and star center, if any

Note that when star center and neutral line are tightly internconected, then Yn tends to infinity and DV to zero.

Hope to help you Julian