The factor 0.7 takes into account the protection IED 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 F
a
.
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 IED operation. To ensure the time
selectivity, the delay must be taken into account when setting the operate times of
successive IEDs.
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 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 pickup current setting of the IED.
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.
1YHT530004D05 D Section 12
Requirements for measurement transformers
615 series 845
Technical Manual
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