A071142 V1 EN
Figure 459: 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 IED (not visible in the figure above). 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 IED operates with the minimum fault current and it does not operate with
the maximum load current. The settings for all three stages are as in the figure above.
For the application point of view, the suitable setting for instantaneous stage (I>>>)
in this example is 3 500 A (5.83 x I
2n
). For the CT characteristics point of view, the
criteria given by the current transformer selection formula is fulfilled and also the
IED setting is considerably below the F
a
. In this application, the CT rated burden
could have been selected much lower than 10 VA for economical reasons.
Section 12 1YHT530004D05 D
Requirements for measurement transformers
846 615 series
Technical Manual
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