ABB RELION 620 Series Technical Manual page 888

Section 13
Requirements for measurement transformers
13.1.1.3
13.1.2
882
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.
A071142-ANSI V2 EN
Figure 458: 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 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 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 the figure above.
For the application point of view, the suitable setting for instantaneous stage (50P-3) in
this example is 3 500 A (5.83 x I
given by the current transformer selection formula is fulfilled and also the protection relay
setting is considerably below the F
been selected much lower than 10 VA for economical reasons.
Current transformer requirements for differential protections
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
). For the CT characteristics point of view, the criteria
2n
. In this application, the CT rated burden could have
a
1MAC504801-IB E
620 series ANSI
Technical Manual