ABB RET670 Applications Manual page 155

1MRK504116-UUS C
Application manual
Calculation:
1150
> × × +
VR 15 (0.3 0.1)
240
EQUATION1760-ANSI V1 EN
TripPickup =40 V
Select a setting of
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.
( )
VkneeANSI > + ×
8 0.3 100 0.7
EQUATION1761-ANSI V2 EN
TripPickup
that is, bigger than 2 ·
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
acceptable value, ignoring the current drawn by the non-linear resistor.
1200
(
= × ° +
IP 100 0
5
EQUATION1763-ANSI V1 EN
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.
=
28.75 V
× =
581 V
)
- ° £
3.40 60 approx
Section 3
IED application
(Equation 39)
(Equation 40)
41 which gives an
.52 A
(Equation 41)
149