7.2.2.203 Duration of test
The test can be stopped when both of the following conditions are met:
– the test duration is at least equal to three times the current transformer thermal time constant;
– the rate of temperature rise of the windings (and of the top oil of oil-immersed current transformers) does not exceed 1 K per hour during three consecutive temperature rise readings.
The manufacturer shall estimate the thermal time constant by one of the following methods:
– before the test, based on the results of previous tests on a similar design. The thermal time constant shall be confirmed during the temperature rise test.
– during the test, from the temperature rise curve(s) or temperature decrease curve(s) recorded during the course of the test and calculated according to Annex 2D.
– during the test, as the point of intersection between the tangent to the temperature rise curve originating at 0 and the maximum estimated temperature rise.
– during the test, as the time elapsed until 63 % of maximum estimated temperature rise.
7.2.2.204 Temperatures and temperature rises
The purpose of the test is to determine the average temperature rise of the windings and, for oil-immersed transformers, the temperature rise of the top oil, in steady state when the losses resulting from the specified service conditions are generated in the current transformer.
The average temperature of the windings shall, when practicable, be determined by the resistance variation method, but for windings of very low resistance, thermometers, thermocouples or other appropriate temperature sensors may be employed.
Thermometers or thermocouples shall measure the temperature rise of parts other than windings. The top-oil temperature shall be measured by sensors applied to the top of metallic head directly in contact with the oil.
The temperature rises shall be determined by the difference with respect to the ambient temperature measured as indicated in 7.2.2.202.
7.2.2.205 Test modalities for current transformers
The test shall be performed by applying the rated continuous thermal current to the primary winding.
NOTE Subject to an agreement between manufacturer and purchaser the test current may also be applied by energizing one or more secondary windings, if the voltages at the secondary terminals of the energizing cores are
at least as high as if connected to rated burden, with the primary winding short-circuited and the non-supplied secondary winding(s) connected to the rated burden(s).
7.2.2.206 Test modalities for oil-immersed current transformers
The test shall be performed by simultaneously applying the following to the current transformer:
the rated continuous thermal current to the primary winding;
The test current may also be applied by energizing one or more secondary windings, if the voltages at the secondary terminals of the energizing cores are at least as high as if connected to rated burden, with the primary winding short-circuited and the non-supplied secondary winding(s) connected to the rated burden(s).
the highest voltage of the equipment divided by 3 between the primary winding and earth. One terminal of each secondary winding shall be connected to earth.
7.2.3 Impulse voltage withstand test on primary terminals
7.2.3.1 General
IEC 61869-1:2007, 7.2.3.1 is applicable with the addition of the following:
The test voltage shall be applied between the terminals of the primary winding (connected together) and earth. The frame, case (if any), and core (if intended to be earthed) and all terminals of the secondary winding(s) shall be connected to earth.
For three-phase current transformers for gas insulated substations, each phase shall be tested, one by one. During the test on each phase, the other phases shall be earthed.
For the acceptance criteria of gas-insulated metal enclosed transformers, refer to IEC 62271-203:2011, Clause 6.2.4.
7.2.6 Tests for accuracy
7.2.6.201 Test for ratio error and phase displacement of measuring current transformers
To prove compliance with 5.6.201.3, 5.6.201.4 and 5.6.201.5, accuracy measurements shall be made at each value of current given in Table 201, Table 202 and Table 203 respectively, at the highest and at the lowest value of the specified burden range.
Transformers having an extended current rating shall be tested at the rated extended primary current instead of 120 % of rated current.
7.2.6.202 Determination of the instrument security factor (FS) of measuring current transformers
This test may be performed using the following indirect test method: .
With the primary winding open-circuited, the secondary winding is energized at rated frequency by a substantially sinusoidal voltage. The voltage shall be increased until the exciting current.
