A071142-ANSI V2 EN
Figure 580: 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 pickup current setting for low-set stage (51P) is selected to be about twice the nominal
current of the cable. The trip time is selected so that it is selective with the next protection
relay (not visible in Figure 580). 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 pickup current settings have to be defined so that the protection relay
operates with the minimum fault current and it does not trip with the maximum load
current. The settings for all three stages are as in
Figure 580.
For the application point of view, the suitable setting for instantaneous stage (50P-3) 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.
12.1.2 Current transformer requirements for transformer differential
protection
The more important the object to be protected, the more attention has to be paid to the
current transformers. It is not normally possible to dimension the current transformer so
that they repeat the currents with high DC components without saturating when the
residual flux of the current transformer is high. 87T operates reliably even though the
current transformers are partially saturated.
Section 12 1MAC059074-MB A
Requirements for measurement transformers
1134 615 series ANSI
Technical Manual
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