NOTE 2 The number of measurement points may be agreed between the manufacturer and the purchaser.
NOTE 3 Usually, the actual knee point e.m.f. is determined, which must be higher than the rated knee point.
7.3.204 Inter-turn overvoltage test
Tests shall be performed to demonstrate compliance with 5.3.201.
The inter-turn overvoltage test shall be performed at the full winding in accordance with one of the following procedures. If not otherwise agreed, the choice of the procedure is left to the manufacturer.
Procedure A: with the secondary windings open-circuited (or connected to a high impedance device which reads peak voltage), a substantially sinusoidal current at a frequency between 40 Hz and 60 Hz and of r.m.s. value equal to the rated primary current (or rated extended primary current if specified) shall be applied for 60 s to the primary winding.
The applied current shall be limited if the test voltage given in 5.3.201 is obtained before reaching the rated primary current (or rated extended primary current).
If the test voltage given in 5.3.201 is not reached at maximum primary current, the obtained voltage shall be regarded as the test voltage.
Procedure B: with the primary winding open-circuited, the test voltage given in 5.3.201 (at some suitable test frequency) shall be applied for 60 s to the terminals of each secondary winding.
The r.m.s. value of the secondary current shall not exceed the rated secondary current (or the appropriate extended value if specified).
The test frequency shall be chosen in order to reach the test voltage, but it shall not exceed 400 Hz.
If the test voltage given in 5.3.201 is not reached at maximum secondary current and maximum test frequency, the obtained voltage shall be regarded as the test voltage.
When the test frequency exceeds twice the rated frequency, the duration of the test shall be reduced.
NOTE The inter-turn overvoltage test is not a test carried out to verify the suitability of a current transformer to operate with the secondary winding open-circuited. Current transformers should not be operated with the secondary winding open-circuited because of the potentially dangerous overvoltage and overheating which can
occur.
7.4 Special tests
7.4.3 Measurement of capacitance and dielectric dissipation factor
This clause of IEC 61869-1:2007 is applicable with the addition of the following:
The test voltage shall be applied between the short-circuited primary winding terminals and earth. Generally, the short-circuited secondary winding(s), any screen, and the insulated metal casing shall be connected to the measuring device. If the current transformer has a special terminal suitable for this measurement, the other low-voltage terminals shall be short-circuited and connected together with the metal casing to the earth or the screen of the measuring device.
The test shall be performed with the current transformer at ambient temperature, the value of which shall be recorded.
7.4.6 Internal arc fault test
This clause of IEC 61869-1:2007 is applicable with the addition of the following note:
NOTE For top-core oil-immersed current transformers, the area in which in-service failure occurs, the incept is, in many cases, located in the upper part of the main insulation. For hair-pin oil-immersed current transformers this area is generally located in the bottom part of the main insulation.
7.5 Sample tests
7.5.1 Determination of the remanence factor
Usually, as sample test for each production series, the type test given in 7.2.6.206 is repeated.
7.5.2 Determination of the instrument security factor (FS) of measuring current transformers
Usually, as sample test for each production series, the type test given in 7.2.6.202 is repeated using the indirect method.
