ABB RELION 670 Series Applications Manual page 134

Section 6
Differential protection
Calculation:
>
UR
EQUATION1210 V1 EN-US
Select a setting of
The current transformer saturation voltage must be at least, twice the set operating voltage
U>Trip .
U CT Saturation _ _
EQUATION1211 V2 EN-US
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
which gives a total IED current of 40 mA.
To calculate the sensitivity at operating voltage, refer to equation
acceptable value, ignoring the current drawn by the non-linear resistor.
1200
=
IP
1
EQUATION1212 V2 EN-US
where:
40 mA
20 mA
20 mA
The magnetizing current is taken from the magnetizing curve for the current transformer
cores which should be available. The current value at
dependent resistor current the peak value of voltage 100 ˣ √2 is used. Then the RMS current is
calculated by dividing obtained current value from the metrosil curve with√2. Use the
maximum value from the metrosil curve given in Figure 48.
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 auto-transformers there can be a tertiary system for local distribution and/or shunt
compensation. The 1Ph High impedance differential protection HZPDIF function can be used
to protect the tertiary busbar, normally having 10-33 kV level and with relatively few feeders.
128
1150
( )
× × + =
15 3.8 1.6
1200
U>Trip =100 V
> ( 20 3 6 + . ) × 20 472 = V
U>Trip
(
× 40 0 ° + 20 0 ° + × 3 20 60
is the current drawn by the IED circuit
is the current drawn by each CT just at pickup
is current drawn by metrosil at pickup
77.625 V
− 3
)
− ° × 10 ≤ 125
approx
U>Trip is taken. For the voltage
(Equation 24)
(Equation 25)
SeriesResistor = 2500 ohm
26 which gives an
A
1MRK 504 152-UEN B
(Equation 26)
M16850-80 v9
Application manual