ABB RELION REX640 Technical Manual page 765

1MRS759142 F
U
=
s
I
kmax
n
R
in
R
m
The current transformers must be able to force enough current to operate the
protection relay through the differential circuit during a fault condition inside the
zone of protection. To ensure this, the knee point voltage U
times higher than 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 protection relay 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
REX640
Technical Manual
I
k max
( R R )
+
in m
n
the highest through-fault current in primary amps. The highest earth-fault or
short circuit current during the out-of-zone fault.
the turns ratio of the CT
the secondary internal resistance of the CT in ohms
the resistance (maximum of R
U
2
≥ ×
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
+ R ) of the CT secondary circuit in ohms
in m
.
s
of the current transformer is calculated using
kn
Operate value setting in secondary amps.
Protection functions
(Equation 181)
should be at least two
kn
(Equation 182)
(Equation 183)
(Equation 184)
765