ABB RELION 620 Series Technical Manual page 887

1MAC504801-IB E
620 series ANSI
Technical Manual
Requirements for measurement transformers
Recommended pickup current settings
If I is the lowest primary current at which the highest set overcurrent stage is to trip,
kmin
the pickup current should be set using the formula:
Current pickup value < 0.7 x (I
I is the nominal primary current of the CT.
1n
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 trip time delay caused by the CT saturation is
typically small enough when the overcurrent setting is noticeably lower than F
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 pickup 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 trip 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 the constant of the DC component of the fault current, when the current
is only slightly higher than the pickup current. This depends on the accuracy limit factor
of the CT, on the remanence flux of the core of the CT, and on the trip 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 pickup
current. Otherwise, the inverse operation time can be further prolonged. Therefore, the
accuracy limit factor F should be chosen using the formula:
a
F > 20*Current pickup value / I
a
The Current pickup value is the primary pickup current setting of the protection relay.
/ I )
kmin 1n
1n
Section 13
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a
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