ABB RELION REX640, Техническое руководство, Страница: 1917 / 1960

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 F
a
should be chosen using the formula:
F
a
> 20 ×
Current start value / I
1n
The
Current start value is the primary start current setting of the protection relay.
12.1.1.3 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.
Figure 1019: Example of three-stage overcurrent protection
The maximum three-phase fault current is 41.7 kA and the minimum three-phase
short circuit current is 22.8 kA. The actual accuracy limit factor of the CT is
calculated to be 59.
The start current setting for low-set stage (3I>) is selected to be about twice the
nominal current of the cable. The operate time is selected so that it is selective with
the next protection relay (not visible in Figure 1019 ). The settings for the high-set
stage and instantaneous stage are defined also so that grading is ensured with the
downstream protection. In addition, the start current settings have to be defined so
that the protection relay operates with the minimum fault current and it does not
1MRS759142 F Requirements for measurement
transformers
REX640
Technical Manual
1917