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“What's
Special About Current Transformers?" With
trilingual summary
A
current transformer (CT) operates on the same principle as any other
transformer. An a-c voltage applied to one (primary) winding causes
another voltage to appear at the terminals of the other (secondary)
winding. The voltages are inversely proportional to the numbers of
turns in the respective windings.
The difference is
that a CT's primary winding is in series with a circuit carrying a
known current. The CT secondary is connected to a circuit impedance
that will draw a smaller current (depending upon the turns ratio) for
operation of a meter or protective relay. Often the primary is a
single turn, created by a bar or cable conductor passing through an
opening in the transformer core on which the secondary winding is
wound. Thus, in contrast to the conventional power transformer,
primary current is independent of the secondary circuit.
Because of its iron core, and winding resistance, CT secondary
current will not be in the exact proportion or phase angle
established by the turns ratio. Both ratio error and phase angle
error will be present. An American National Standard defines the
limits of error for two kinds of transformer usage. In "metering"
(where transformer secondary current supplies ammeter or wattmeter
circuits) high accuracy is required over a narrow current range. For
relaying service, where primary current can be quite high and
variable, less accuracy is required over a wider operating range.
Most CTs are of the window type, particularly those used as
accessories in large motors for differential protection, load
control, or ground current monitoring. Such transformers, mounted in
motor terminal boxes, may be subject to temperatures above the
standard 30 degrees C ambient. That will necessitate transformer
derating. Also, such applications often require passing a number of
motor lead cables through CT windows as primary conductors. Some
users have expressed concern about the position of conductors within
a CT window and its effect on CT accuracy and conductor insulation.
Neither has proven to be a problem with fully insulated conductors.
From “What's
Special About Current Transformers?", Electrical Apparatus
June 2006
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copy. © 2006
Barks Publications, Inc. All Rights Reserved.
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