# Dy transformer problem

Discussion in 'Homework Help' started by PsySc0rpi0n, Sep 11, 2016.

1. ### PsySc0rpi0n Thread Starter Well-Known Member

Mar 4, 2014
1,420
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I have a problem to solve that involves a poly phase (3 phases) transformer...

The given info about is is:
Wiring setup - Dy
Sn = 50kVa
50Hz
30000V/400V -- 75
I_0 = 1%
cos(φ) = 0.2

r1 = 810Ω
x1 = 1080Ω
r2 = 48mΩ
x2 = 64mΩ

I'm calculating the approximate equivalent circuit parameters and I already found a couple of parameters:

$
m = \frac{1}{sqrt{3}}\cdot \frac{N_1}{N_2} = 129.9
$

r12 = r1 / m² = 48mΩ
x12 = x1 / m² = 64mΩ

R2 = r12 + r2 = 96mΩ
X2 = x12 + x2 = 128mΩ

Now, I was trying to find the current that I called I_0 which is 1% of In

$
S_n = I_{2n}\cdot \sqrt{3}\cdot U_c
$

$
I_{2n} = \frac{50000 V}{\sqrt{3}\cdot 400 V} = 72.17A
$

Then I_0 = 0.01 * 72.17 A = 0.7217 A

But I tried to do it using the high voltage side just to check if I would get the same result but I can't get the same result.

The Sn is the same on both sides of the transformer, knowing that one wiring is delta setup (high side) and the other is a star setup (low side).

How should I do the math to get the same result, but using the 30000 V side of the transformer?