Current transformers convert an alternating current usually high
in to a proportional lower one, depending on their use. Measurement
type CTs are required to transform the primary current, at various
classes of accuracy, as specified by the class designation, over
a current range from 1 to 120 percent of its rated primary ratio.
The design of this type of transformer requires the inclusion
of a core and winding which will when connected to its rated burden;
perform within the limits of error as indicated by the standard’s
specification. It is an advantage for a measurement type transformer
to saturate above this range, which provides a protection against
damage to instruments by limiting the secondary current when surge
currents or faults appear in the primary circuit.
What Is:
A Measuring Transformer
A Current Transformer
Rated Primary Current
Rated Secondary Current
Rated Transformation Ratio
Current Error (Ratio Error)
Phase Displacement (Phase Error)
Accuracy Class
Burden
Rated Burden (VA)
Rated Output
Highest System Voltage (HSV)
Rated Insulation Level (IL)
Rated ShortTime Thermal Current (Ith)
Rated Dynamic Current (Idyn)
Rated Continuous Current (Un)
Instrument Security Factor (ISF or Fs)
A Routine Test
A Type Test
Special Tests
A current transformer intended to supply indicating instruments integrated
meter, relay and similar apparatus.
An instrument transformer in which the secondary current, in normal
condition of use, is substantially proportional to the primary current
and differs in phase it by an angle which is approximately zero for
an appropriate direction of connections.
The value of primary current which appears in the designation of the
transformer and on which the performance of the currant transformer
is based.
 The value of secondary current which
appears in the designation of the transformer and on which the performance
of the currant transformer is based.
The ratio of the rated primary current to the rated secondary current.
The error with a transformer introduces into the measurement of a
current and which arises from the fact that actual transformation
ratio is not equal to the rated transformer ratio. The current error
expressed in percentage is given by the formula:
Current error, percent = (Ka.IsIp) x 100 / Ip
Where Ka= rated transformation ratio
Ip= actual primary current
Is= actual secondary current when Ip is flowing under the conditions
of measurement
The difference in phase between the primary and secondary current
vectors, the direction of the vectors being so chosen that the angle
is zero for the perfect transformer. The phase displacement is said
to be positive when the secondary current vector leads the primary
current vector. It is usually express in minutes.
A designation assigned to a current transformer the errors of which
remain within specified limit under prescribed conditions of use.
The impedance of the secondary circuit in ohms and power factor.
The impedance of the secondary circuit on which the accuracy requirements
are based. It is usually expressed as apparent power (in VA), at the
rated secondary current and at a specified power factor.
The value of the apparent power (in voltamperes at a specified
power (factor) which the current transformer is intended to supply
to the secondary circuit at the rated secondary current and with
rated burden connected to it.
 The highest rms line to line voltage which can be sustained under
normal operating conditions at any time and at any point on the system.
It excludes temporary voltage variations due to fault condition and
the sudden disconnection of large loads.
That combination of voltage values (power frequency and lightning
impulse, or where applicable, lightning and switching impulse) which
characterizes the insulation of a transformer with regard to its capability
to withstand by dielectric stresses. For low voltage transformer the
test voltage 4kV, at powerfrequency, applied during 1 minute.
The rms value of the primary
current which the
current transformer will withstand for a rated time, with their
secondary winding shortcircuited
without suffering harmful effects.
The peak value of the primary current which a current transformer
will withstand, without being damaged electrically for mechanically
by the resulting electromagnetic forces, the secondary winding being
shortcircuited.
The value of current which can be permitted to flow continuously in
the primary winding, the secondary windings being connected to the
rated burdens, without the temperature rise exceeding the specified
values.
The ratio of rated instrument limit primary current to the rated primary
current. The times that the primary current must be higher then the
rated value, for the composite error of a measuring current transformer
to be equal to or greater than 10%, the secondary burden being equal
to the rated burden. The lower this number is, the more protected
the connected instrument are against.
Tests carried out on each current transformer to check requirements
likely to vary during production.
Dependeing on which standard the transformer is to meet e.g. IEC or
IEEE/ANSI. In general the following tests apply to each individual
transformer:
1. Verification of terminal markings
2. Powerfrequency withstands test primary winding.
3. Partial discharge measurement.
4. Powerfrequency withstand test on secondary windings.
5. Powerfrequency withstand test, between sections.
6. Interturn over voltage test
7. Determination of errors.
The order of the tests is not standardized, but determination of error
shall be performed
after the other test.
Tests carried out to prove the general qualities and design of a given
type of current transformer in accordance with the requirements of
the applicable standers Tests may be carried out on a prototype which
may incorporate special arrangements for the measurements required
by applicable standard.
The following tests are type test:
1. short time current test
2. temperature rise test
3. lightning impulse test
4. switching impulse test
5. wet test for outdoor type transformer
6. determination of errors
7. radio interference voltage measurement (RIV) (As specified in IEC
600441)
All the dielectric type test should be carried out on the same transformer,
unless
otherwise specified.
Chargable testing which may be in the nature of type tests or routine
tests, and are carried out only by agreement between ITL & the
customerr.
