Calculation:
1150
15 (0.3 0.1) 28.75
240
VR V> × × + =
EQUATION1760-ANSI V1 EN (Equation 39)
Select a setting of
TripPickup
=40 V
The current transformer knee point voltage can roughly be calculated from the rated values, considering
knee point voltage to be about 70% of the accuracy limit voltage.
( )
8 0.3 100 0.7 581 VVkneeANSI > + × × =
EQUATION1761-ANSI V2 EN (Equation 40)
that is, bigger than 2 ·
TripPickup
Check from the table of selected resistances the required series stabilizing resistor
value to use. As this application requires to have good sensitivity, select R Series= 400
ohm which gives a total IED current of 100 mA.
To calculate the sensitivity at operating voltage, refer to equation 41 which gives an
acceptable value, ignoring the current drawn by the non-linear resistor.
( )
1200
100 0 3.40 60 .52
5
IP approx A= × ° + - ° £
EQUATION1763-ANSI V1 EN (Equation 41)
where:
100mA is the current drawn by the IED circuit
40mA is the current drawn by each CT just at pickup
The magnetizing current is taken from the magnetizing curve for the current
transformer cores which should be available. The value at TripPickup is taken.
It can clearly be seen that the sensitivity is not so much influenced by the selected
voltage level so a sufficient margin should be used. The selection of the stabilizing
resistor and the level of the magnetizing current (mostly dependent of the number of
turns) are the most important factors.
For many transformers there can be a secondary system for local distribution and/or
shunt compensation. The 1Ph High impedance differential protection HZPDIF (87)
function can be used to protect the tertiary busbar, normally 10-33 kV level and with
relatively few feeders.
1MRK504116-UUS C Section 3
IED application
149
Application manual
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