ABB Relion 611 Series, Technical Manual

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A071142 V1 EN
Figure 250: 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 250 ). 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
Figure 250 .
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.
10.1.2 Current transformer requirements for differential protection
The sensitivity and the reliability of the protection depends on the characteristics of
the current transformers. The CTs must have an identical transformation ratio. It is
recommended that all the CTs have an identical constructions, that is, they have an
equal burden and characteristics and are of the same type, preferably from the same
manufacturing batch. If the CT characteristics and burden values are not equal,
calculations for each branch in the scheme should be performed separately and the
worst-case results should be used. In
Figure 251 , the CT winding resistance and the
burden of the branches are not equal, and hence, the maximum burden equal to 3.2 Ω
should be used for calculating the stabilized voltage.
Section 10
1MRS757454 D
Requirements for measurement transformers
494 611 series
Technical Manual