ABB RELION REX640 Technical Manual page 812

Protection functions
The current transformers must be able to force enough current to operate the IED
through the differential circuit during a fault condition inside the zone of protection.
To ensure this, the knee point voltage U
the stabilizing voltage U
The required knee point voltage U
the formula
U
kn
U
kn
U
s
The factor two is used when no delay in the operating time of the protection in any
situation is acceptable. To prevent the knee point voltage from growing too high, it
is advisable to use current transformers, the secondary winding resistance of which
is of the same size as the resistance of the measuring loop.
As the impedance of the IED alone is low, a stabilizing resistor is needed. The value
of the stabilizing resistor is calculated with the formula
R
=
s
R
s
U
s
I
rs
The stabilizing resistor should be capable to dissipate high energy within a very
short time; therefore, the wire wound-type resistor should be used. Because of the
possible CT inaccuracy, which might cause some current through the stabilizing
resistor in a normal load situation, the rated power should be 25 W minimum.
If U
is needed. Often resistor manufacturers allow 10 times rated power for 5 seconds.
Thus the power of the resistor can be calculated with the equation
U
kn
R
×
s
The actual sensitivity of the protection is affected by the protection relay setting,
the magnetizing currents of the parallel connected CTs and the shunting effect of
the voltage-dependent resistor ( VDR). The value of the primary current I
which the protection relay operates at a certain setting can be calculated with the
formula
I
prim
812
s
2 U
≥ ×
s
The knee point voltage
The stabilizing voltage
U
s
I
rs
The resistance of the stabilizing resistor
The stabilizing voltage of the protection relay
The value of the
is high or the stabilizing voltage is low, a resistor with a higher power rating
kn
2
10
( )
n I I m I
= ⋅ + + ⋅
rs u m
should be at least two times higher than
kn
.
of the current transformer is calculated using
kn
Operate value setting in secondary amps.
1MRS759142 F
(Equation 213)
(Equation 214)
(Equation 215)
at
prim
(Equation 216)
REX640
Technical Manual