Autotransformer Differential Protection - ABB RELION 670 Series Applications Manual

Section 6
Differential protection
Calculation:
EQUATION1207 V2 EN-US
Select a setting of
The current transformer saturation voltage must be at least twice the set operating voltage
U>Trip .
E P 5
EQUATION1208 V1 EN-US
that is, bigger than 2 ˣ
Check from the table of selected resistances the required series stabilizing resistor value to
use. As this application requires to be so sensitive select
gives an IED operating current of 100 mA.
Calculate the primary sensitivity at operating voltage using the following equation.
2000
IP =
EQUATION1209 V2 EN-US
where
100 mA is the current drawn by the IED circuit and
10 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 200 ˣ √2 is used. Then the RMS current is
calculated by dividing obtained current value from the metrosil curve with√2. Use the value
from the maximum metrosil curve given in Figure
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.
6.2.4.4

Autotransformer differential protection

When Autotransformers are used it is possible to use the high impedance scheme covering the
Autotransformer windings, however such differential arraignment does not typically cover the
faults within the tertiary delta winding. The protection zone and connection of the High
impedance differential protection is shown in Figure 44.
126
U>Trip =200 V.
( )
> + × =
20 6.2 20 524 V
U>Trip
(
100 0 ° + 20 0 ° + × 3 10 60
1
is the current drawn by each CT just at pickup
is current drawn by metrosil at pickup
SeriesResistor = 2000 ohm, which
− 3
)
− ° × 10 ≤ approx 275
U>Trip is taken. For the voltage
48
1MRK 504 152-UEN B
(Equation 21)
(Equation 22)
A
Application manual
(Equation 23)
M16850-41 v6