ABB RELION REX640, Technical Manual, Page: 765 / 1960

U
I
n
R R
s
k
in m
= +
max
( )
(Equation 181)
I
kmax
the highest through-fault current in primary amps. The highest earth-fault or
short circuit current during the out-of-zone fault.
n the turns ratio of the CT
R
in
the secondary internal resistance of the CT in ohms
R
m
the resistance (maximum of R
in
+ R
m
) of the CT secondary circuit in ohms
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
kn
should be at least two
times higher than the stabilizing voltage U
s
.
The required knee point voltage U
kn
of the current transformer is calculated using
the formula
U U
kn s
≥ ×
2
(Equation 182)
U
kn
the knee point voltage
U
s
the stabilizing voltage
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
U
I
s
s
rs
=
(Equation 183)
R
s
the resistance of the stabilizing resistor
U
s
the stabilizing voltage of the protection relay
I
rs
the value of the
Operate value
setting in secondary amps.
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
kn
is high or the stabilizing voltage is low, a resistor with a higher power rating
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
R
kn
s
2
10
×
(Equation 184)
1MRS759142 F Protection functions
REX640
Technical Manual
765