Unbalanced Transformer Loads
Unbalanced Transformer Loads
(OP)
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.






RE: Unbalanced Transformer Loads
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