Current transformer selection
Non-directional overcurrent protection does not set high requirements on the accuracy class or on the actual accuracy limit factor (Fa) of the CTs. It is, however, recommended to select a CT with Fa of at least 20.
The nominal primary current I1n should be chosen in such a way that the thermal and dynamic strength of the current measuring input of the protection relay is not exceeded. This is always fulfilled when
I1n > Ikmax / 100,
Ikmax is the highest fault current.
The saturation of the CT protects the measuring circuit and the current input of the protection relay. For that reason, in practice, even a few times smaller nominal primary current can be used than given by the formula.
Recommended start current settings
If Ikmin is the lowest primary current at which the highest set overcurrent stage is to operate, the start current should be set using the formula:
Current start value < 0.7 × (Ikmin / I1n)
I1n is the nominal primary current of the CT.
The factor 0.7 takes into account the protection relay inaccuracy, current transformer errors, and imperfections of the short circuit calculations.
The adequate performance of the CT should be checked when the setting of the high set stage overcurrent protection is defined. The operate time delay caused by the CT saturation is typically small enough when the overcurrent setting is noticeably lower than Fa.
When defining the setting values for the low set stages, the saturation of the CT does not need to be taken into account and the start current setting is simply according to the formula.
Delay in operation caused by saturation of current transformers
The saturation of CT may cause a delayed protection relay operation. To ensure the time selectivity, the delay must be taken into account when setting the operate times of successive protection relays.
With definite time mode of operation, the saturation of CT may cause a delay that is as long as the time constant of the DC component of the fault current, when the current is only slightly higher than the starting current. This depends on the accuracy limit factor of the CT, on the remanence flux of the core of the CT, and on the operate time setting.
With inverse time mode of operation, the delay should always be considered as being as long as the time constant of the DC component.
With inverse time mode of operation and when the high-set stages are not used, the AC component of the fault current should not saturate the CT less than 20 times the starting current. Otherwise, the inverse operation time can be further prolonged. Therefore, the accuracy limit factor Fa should be chosen using the formula:
Fa > 20 × Current start value / I1n
The Current start value is the primary start current setting of the protection relay.