resistance are transferred to the secondary side
The transferred primary and the second
impedances are combined together
In the next slide, the simplified circuit of Fig. 7.26
is presented
52

Short Circuit Test
Figure 7.26
Simplified circuit for
calculation of series impedance
Primary and
secondary
impedances
are combined
The combined leakage impedances and coil
resistances are calculated by repeated use of the
basic power equation and Ohm’s law
53
s
p
s
e
R
T
R
R
2
_
s
p
s
e
X
T
X
X
2
_

Short Circuit Test
The short circuit test evaluation results are
evaluated using
54
2
e_s
2
sh_s
e_s
2
2
sh_s
sh_s
sh_s
2
sh_s
sh_s
e_s
)
(R
)
(Z
X
I
V
Z
)
(I
P
R
X
R
Z

Equivalent Circuit
Equivalent circuit obtained by measurement is
Figure 7.29
Equivalent circuit for a real transformer resulting from
the open and short circuit tests
55

Equivalent Circuit
Figure 7.30
Modified equivalent circuit
The circuit can be further modified to a form
similar to the traditional equivalent circuit.
The calculated series impedance is transferred
from the secondary to the primary side and
divided by two.
56

Transformers
Example for Single-Phase
Transformer Operation
Analysis
57

Operation Analysis Example
Single-phase 150 kVA transformer.
Primary voltage is 7.2 kV and secondary voltage is
240 V.
Calculate the primary and secondary current,
required supply voltage and voltage regulation
when the load is 80% of the transformer capacity
at rated secondary voltage
and the power factor is
0.8 lagging
The equivalent circuit parameters are
R
p
= 0.12 Ω
R
s
= 0.03 Ω
R
m
= 2500 Ω
X
p
= 0.3 Ω
X
s
= 0.09 Ω
X
m
= 800 Ω
58

Operation Analysis Example
Equivalent circuit used for the calculation
S
Tr
= 150 kVA
E
p
= 7.2 kV
E
s
= 240 V
pf
L
= 0.8
59

Operation Analysis Example
With the load at 80% of the transformer capacity and a
0.8 lagging power factor, the load complex power is
The corresponding load current is
60

Operation Analysis Example
The transformer secondary voltage is calculated by the
loop equation
Transformer primary voltage
61

Operation Analysis Example
Transformer primary current is
Transformer magnetizing current is
62

Operation Analysis Example
The supply current is the sum of the primary current and
magnetizing current
Supply voltage is the sum of the primary voltage and
voltage drop on primary impedance
63

Operation Analysis Example
Finalized supply voltage is
The voltage regulation is
Unacceptably high; it
should be less than 10%
64

Operation Analysis Example
The supply complex power and real power are
Supply power factor is calculated
Transformer rating is 150 kVA, serious overload
65

Three-Phase
Transformers
Descriptions and Basic
Features
66

Three-Phase Transformers
A three-phase transformer is described by its total
apparent power capability (|
S
|) and voltage ratings
(
V
rate
).

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- Fall '08
- GORUR
- Volt, SPX Transformer Solutions, Diagram, Bushings, Fig., ThreePhase Transformer