The r.m.s. value of the obtained terminal voltage shall be less than the secondary limiting (see 3.4.206).
The exciting voltage shall be measured with an instrument which has a response proportional to the average of the rectified signal, but calibrated in r.m.s.. The exciting current shall be measured using an r.m.s measuring instrument having a minimum crest factor of 3.
If the measurement result should be put to question, a further measurement shall be performed with the direct test (see 2A.5, 2A.6). Then the result of the direct test is the reference.
NOTE The great advantage of the indirect test is that high currents are not necessary (for instance 30000A at a primary rated current 3000 A and an instrument security factor 10) and also that no burdens have to be made available for 50 A. The effect of the return primary conductors is not physically effective during the indirect test. Under service conditions the effect can only increase the composite error, which is desirable for the safety of the apparatus supplied by the
measuring current transformer.
7.2.6.203 Test for composite error of class P and PR protective current transformers
The following two test procedures are given:
a) Compliance with the limits of composite error given in Table 205 shall be demonstrated by a direct test in which a substantially sinusoidal current equal to the rated accuracy limit primary current is passed through the primary winding with the secondary winding connected to a burden of magnitude equal to the rated burden but having, at the discretion of the manufacturer, a power factor between 0,8 inductive and unity see 2A.4, 2A.5, 2A.6, 2A.7.
The test may be carried out on a transformer similar to the one being supplied, except that reduced insulation may be used, provided that the same geometrical arrangement is retained.
As far as very high primary currents and single-bar primary winding current transformers are concerned, the distance between the return primary conductor and the current transformer should be taken into account from the point of view of reproducing service conditions.
b) For low-leakage reactance current transformers according to Annex 2C, the direct test may be replaced by the following indirect test.
With the primary winding open-circuited, the secondary winding is energized at rated frequency by a substantially sinusoidal voltage having an r.m.s. value equal to the secondary limiting.
The resulting exciting current, expressed as a percentage of it shall not exceed the composite error limit given in Table 205.
The exciting voltage shall be measured with an instrument which has a response proportional to the average of the rectified signal, but calibrated in r.m.s.. The exciting current shall be measured using an r.m.s measuring instrument having a minimum crest factor of 3.
In determining the composite error by the indirect method, a possible correction of the turns ratio need not be taken into account.
7.2.6.204 Test for error at limiting conditions for class TPX, TPY and TPZ protective current transformers
The purpose of the type test is to prove the compliance with the requirements at limiting conditions. For test methods refer to Annex 2B.
If the current transformer is a low-leakage reactance type according to Annex 2C, an indirect type test may be performed according to 2B.2, otherwise a direct test shall be performed according to 2B.3.
The test can be performed on a full-scale model of the active part of the current transformer assembly inclusive of all metal housings but without insulation.
7.2.6.205 Test of low-leakage reactance type for class PX and PXR protective current transformers
The proof of low-leakage reactance shall be made according to Annex 2C.
7.2.6.206 Determination of the remanence factor class PR, TPY, and PXR protective current transformers
To prove compliance with
5.6.202.3.5 for class PR,
5.6.202.5.2 for class TPY,
5.6.202.4 for class PXR,
the remanence factor shall be determined. For test methods, refer to 2B.2.
7.2.201 Short-time current tests
To verify the requirements of rated short-time thermal current and of rated dynamic current given in 5.204, the two following tests are specified.
The thermal test shall be made with the secondary winding(s) short-circuited, and at a current I’ for a time t’, so that
where t is the specified duration of the short-time thermal current.
t' shall have a value between 0.5 s and 5 s.
The dynamic test shall be made with the secondary winding(s) short-circuited, and with a primary current the peak value of which is not less than the rated dynamic current for at least one peak.
The dynamic test may be combined with the thermal test above, provided the first major peak current of that test is not less than the rated dynamic current.
The transformer shall be deemed to have passed these tests if, after cooling to ambient temperature (between 10 °C and 40 °C), it satisfies the following requirements:
a) it is not visibly damaged;
b) its errors after demagnetization do not differ from those recorded before the tests by more than half the limits of error appropriate to its accuracy class;
c) it withstands the dielectric tests specified in 7.3.1, 7.3.2, 7.3.3 and 7.3.4 but with the test voltages or currents reduced to 90 % of those given;
d) on examination, the insulation next to the surface of the conductor does not show significant deterioration (e.g. carbonization).
