Example For Non-Directional Overcurrent Protection - ABB RELION 615 Series Applications Manual

1MAC109016-MB C
4.1.1.3
REF615 ANSI
Application Manual
Requirements for measurement transformers
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 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.

Example for non-directional overcurrent protection

The following figure describes a typical medium voltage feeder. The protection is
implemented as three-stage definite time non-directional overcurrent protection.
1n
Section 4
.
a
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