SPECIAL
TRANSFORMERS
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The primary winding of a CT is connected in series with the
line whose current needs to be measured. The primary
winding has less number of turns and so the voltage drop
across it is also less. The secondary winding, on the other
hand, has large number of turns.
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In a potential transformer (PT), the primary is
connected across the line whose voltage needs to
be measured and the voltage circuit is connected
across the secondary winding. Potential
transformer uses large core and conductor sizes
in comparison to conventional
power
transformer. Shell-type configuration is used for
low-voltage and core-type configuration is used
for high-voltage transformers.
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• Capacitor voltage transformers are used
for metering and protection in high-voltage
network systems.
• A capacitive voltage transformer works on
the principle of voltage division, by using
two capacitances, one many times larger
than the other.
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The main purpose of an earthing
transformer is to pass the faulty current
during earth fault condition. It provides a
neutral point for grounding purpose and
carries no useful load. Low-impedance
ground path can be obtained with the help
of three pairs of concentric coils. There
coils are connected in the manner so that
they oppose the mmf or ampere turns.
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A pulse transformer is a special transformer
that is designed for producing rectangular
electrical pulses. Pulses are mainly used in
electronic circuits. Special high-voltage pulse
transformers are also used to generate highpower pulses for radar, particle accelerators, or
other high-energy pulsed power applications. A
pulse transformer generally has low values of
leakage
inductance
and
distributed
capacitance, and a high open-circuit
inductance.
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•Furnace transformers are used to step
down from voltages between 11 kV and 33
kV to levels of several hundred volts only.
This results in massive secondary currents.
•Due to the high secondary currents and
resistive losses, the furnace layout is such
as to limit the bus-bar length.
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• Rectifier transformers are used to supply
rectifier bridges for DC applications such as
DC arc furnaces. These transformers step
down the distribution voltages to the level
used by between 500 V and 3300 V
depending on the configuration of the
power electronics.
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