For specified values of primary time constant above 80 ms, the tests can be performed in equivalent accuracy limiting conditions (by modifying duty cycle and/or burden), subjected to agreement between manufacturer and purchaser.
During the energization period, the first peak of the primary current shall be not less than the value corresponding to the specified conditions.
In laboratory practice, it may be difficult to reproduce the exact duty cycle specification. In this case, the calculated Ktd value of the applied duty cycle shall not be less than the calculated Ktd value of the specified duty cycle. To meet this requirement, the duration of the energization(s) and/or the secondary burden may be adjusted.
NOTE As the calculation of Ktd is based on worst-case formulas (formula 6 in 2B.1.2 may deliver Ktd values which are 30 % higher than necessary), the current transformer may satisfy the duty cycle without reaching the flux corresponding to the calculated Ktd value.
The error value according to 3.4.222 shall be determined.Its value shall not exceed the limit given in Table 206.
For class TPZ current transformers, the a.c. component of the error current is measured as one half of the peak-to-peak value (see Figure 2B.15). The error value according to 3.4.223 shall be determined. Its value shall not exceed the limit given in Table 206.
NOTE It is possible that the class definition does not contain a duty cycle. In this case, for test purposes, a duty cycle leading to the given Ktd value shall be agreed between manufacturer and purchaser.
If the real Ktd value of the current transformer has to be determined, the duration of the energization and/or secondary burden shall be increased so that the measured instantaneous error current reaches the limiting value for the accuracy class concerned (Table 206). For class TPZ, linear interpolation is used to determine the instant at which the limiting value of the a.c. component of the error current is reached.
The total dimensioning factor Ktd of the current transformer is the ratio to the peak value of the a.c. component under steady-state conditions. This a.c. component can be derived from a secondary linked flux measurement in the test a), which has to be related to the exact (theoretical) value of the short circuit current Kssc x Isr. The measurement shall be made using the abovementioned formula.
The error in flux measurement shall not exceed 5 %.
2B.3.3 Determination of the factor of construction
If compliance with the requirements of low-leakage reactance design cannot be established to the mutual satisfaction of the manufacturer and purchaser by reference to drawings, then the factor of construction Fc shall be determined as follows:
The secondary linked flux values in both a direct test and an indirect test have to be determined, in both cases for the magnetizing current at accuracy limiting conditions. If a transient performance class is specified by the alternative definition, the appropriate duty cycle and burden shall be chosen in order to achieve the specified Kssc x Ktd value.
The secondary linked flux, which is obtained in the direct test according to 2B.3.2, shall be determined.
In the tests, the error in flux measurement shall not exceed 5 %.
If Fc is greater than 1,1, it shall be considered when dimensioning the core.
NOTE The value of primary current required to perform direct tests on certain transformer types may be beyond the capability of facilities normally provided by manufacturers. Tests at lower levels of primary current may be agreed between the manufacturer and purchaser.
