A071142 V1 EN
Figure 193:
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
). 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 operate with
the maximum load current. The settings for all three stages are as in
For the application point of view, the suitable setting for instantaneous stage (I>>>) in
this example is 3 500 A (5.83 × I
2n
). I
2n
is the 1.2 multiple with nominal primary
current of the CT. For the CT characteristics point of view, the criteria given by the
current transformer selection formula is fulfilled and also the protection relay 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 4
1MRS758272 B
Requirements for measurement transformers
136
REG615
Application Manual
