Page 66 - Electrical Equipment Handbook _ Troubleshooting and Maintenance

TRANSFORMERS

TRANSFORMERS 3.23

FIGURE 3.19 A step-down connection.
V . The current in this coil is called the series current I . The current and voltage on the
SE SE
high-voltage side of the transformer are called I and V , respectively. The corresponding
H H
parameters on the low-voltage side are called I and V , respectively. From Fig. 3.18b, the
L L
voltages and currents in the coils are related by:
V C

N C
V SE N SE
N I N I
C C SE SE
The voltages in the coils are given by
V V C
L
V V V
H C SE
The currents in the coils are given by
I I I
L C SE
I I
H SE
The ratio of low and high voltage is given by

V L
N C

V N N
H SE C
The ratio of low to high current is given by

N SE N C
I L

I H N C
THREE-PHASE TRANSFORMERS

Most of the transformers used in power generation and distribution systems are three-phase.
These transformers can be constructed from three single-phase transformers connected in a
three-phase bank (Fig. 3.20). They can also be constructed by wrapping three sets of

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