The examination d) is not required if the current density in the primary winding, corresponding to the rated short-time thermal current, does not exceed:
– 180 A/mm2 where the winding is of copper of conductivity not less than 97 % of the value given in IEC 60028;
– 120 A/mm2 where the winding is of aluminium of conductivity not less than 97 % of the value given in IEC 60121.
NOTE Experience has shown that in service the requirements for thermal rating are generally fulfilled in the case of class A insulation, provided that the current density in the primary winding, corresponding to the rated short-time thermal current, does not exceed the above-mentioned values.
7.3 Routine tests
7.3.1 Power-frequency voltage withstand tests on primary terminals
This clause of IEC 61689-1 is applicable with the addition of the following:
The test voltage shall be applied between the short-circuited primary winding and earth. The short-circuited secondary winding(s), the frame, case (if any) and core (if there is a special earth terminal) shall be connected to earth.
7.3.5 Tests for accuracy
7.3.5.201 Tests for ratio error and phase displacement of measuring current transformers
The routine test for accuracy is in principle the same as the type test in 7.2.6.201, but routine tests at a reduced number of currents and/or burdens are permissible provided it has been shown by type tests on a similar transformer that such a reduced number of tests are sufficient to prove compliance with 5.6.201.3.
7.3.5.202 Tests for ratio error and phase displacement of class P and PR protective current transformers
Tests shall be made at rated primary current and rated burden to prove compliance with 5.6.202.2 and 5.6.202.3 respectively, with respect of ratio error and phase displacement.
7.3.5.203 Test for composite error of class P and PR protective current transformers
For low-leakage reactance current transformers (see Annex 2C), the routine test is the same as the indirect type test described in item b) of 7.2.6.203.
For other transformers, the indirect test described in item b) of 7.2.6.203 may be used, but a correction factor for the exciting current shall be applied to the results. This factor is obtained from a comparison between the results of direct and indirect tests applied to a transformer of the same type as the one under consideration, the accuracy limit factor and the conditions of loading being the same. In such cases, the manufacturer should hold test reports available.
NOTE 1 The correction factor is equal to the ratio of the composite error obtained by the direct method, and the exciting current expressed as a percentage, as determined by the indirect method.
NOTE 2 The expression “transformer of the same type” implies that the ampere turns are similar irrespective of ratio, and that the materials and the geometrical arrangements of the iron core and the secondary windings are identical.
7.3.5.204 Test for ratio error and phase displacement for class TPX, TPY and TPZ protective current transformers
The ratio error and the phase displacement shall be measured at rated current to prove compliance with 5.6.202.5.1.
The results shall correspond to a secondary winding temperature of 75 °C.
Therefore, the actual value of the secondary winding temperature shall be measured and the difference to its value corrected to 75 °C shall be determined. The error measurement shall be made with the burden increased by the above mentioned difference of winding resistance.
Alternatively, for TPY and TPZ cores the phase displacement at 75 °C may be determined by measuring at ambient temperature and calculating as follows:
where is the winding resistance at the ambient temperature. The influence of this resistance correction on the ratio error can be neglected.
For type and routine testing, a direct test method (using a primary current source and a reference current transformer) has to be applied. For low-leakage reactance current transformers, an indirect test method is given in Annex 2E. It may be applied for on-site measurements and for monitoring purposes.
7.3.5.205 Test for error at limiting conditions for class TPX, TPY and TPZ protective current transformers
The purpose of the routine test is to prove compliance with the requirements at limiting conditions.
If the current transformer is a low-leakage reactance type according to Annex 2C, an indirect test shall be performed according to 2B.2.
If compliance with the requirements of low-leakage reactance design cannot be established, but a type test report of a current transformer of the same type is available, an indirect test shall be performed according to 2B.2. In this case, a possibly available factor of construction Fc shall be considered if the factor is greater than 1,1. If such a type test is not available, one unit of the batch shall be type-tested and used as reference for the indirect testing of the remaining units.
