29-08-2013, 03:00 PM
Current Transformers
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Application
Current Transformers (CT’s) are instrument transformers that are used to supply a reduced value of current to me-
ters, protective relays, and other instruments. CT’s provide isolation from the high voltage primary, permit
grounding of the secondary for safety, and step-down the magnitude of the measured current to a value that can be
safely handled by the instruments.
Ratio
The most common CT secondary full-load current is 5 amps which matches the standard 5 amp full-scale current
rating of switchboard indicating devices, power metering equipment, and protective relays. CT’s with a 1 amp
full-load value and matching instruments with a 1 amp full-range value are also available.
Many new protective
relays are programmable for either value.
CT ratios are expressed as a ratio of the rated primary current to the rated secondary current.
For example, a 300:5 CT will produce 5 amps of secondary current when 300 amps flows through the primary. As
the primary current changes the secondary current will vary accordingly. With 150 amps through the 300 amp
rated primary, the secondary current will be 2.5 amps ( 150 : 300 = 2.5 : 5 ). When the rated primary amps is ex-
ceeded, which is usually the case when a fault occurs on the system, the amount of secondary current will increase
but, depending on the magnetic saturation in the CT, the output may not be exactly proportional.
Current Flow Analysis
In analyzing the current flow in a system utilizing CT’s the following observation can be made:
When current flows in the CT primary from the H1 lead (polarity + ) to the non- polarity H2 lead, current will
be forced out the secondary X1 (polarity + ) lead, through the burden (load), and return to the secondary X2
non-polarity lead. The next half-cycle the current will reverse, but for the purpose of analysis and for con-
structing phasor diagrams, only the above indicated one-half cycle is analyzed.
Draw-Out Instrument Cases
Meters and protective relays are available in draw-out cases that auto-
matically short-circuit the CT when the instrument is removed for test-
ing and calibration. Voltage and trip-circuit contacts will be opened.
See symbol for draw-out case on Sheet 2.
Excitation Curves
The family of curves below describe the excitation characteristics for the 600/5 multi-ratio bushing current
transformer shown on the previous sheet. This is a plot of the CT secondary current against secondary voltage.
These curves illustrate how high the secondary voltage will in rise in order to force the rated secondary current
through the burden. The effect of magnetic saturation is also illustrated by the knee of the curve.