ABB RELION REX640 Technical Manual page 684

Section 4
Protection functions
678
recommended that all current transformers have an equal burden and characteristics
and are of same type, preferably from the same manufacturing batch, that is, an
identical construction should be used. If the CT characteristics and burden values are
not equal, calculation for each branch in the scheme should be done separately and the
worst-case result is then used.
First, the stabilizing voltage, that is, the voltage appearing across the measuring
branch during the out-of-zone fault, is calculated assuming that one of the parallel
connected CT is fully saturated. The stabilizing voltage can be calculated with the
formula
I
k max
U ( R R )
= ⋅ +
s in m
n
GUID-C1BDCB31-CB0A-4B3D-A568-3CAAD4DDFBBC V1 EN
I the highest through-fault current in primary amps. The highest earth-fault or short circuit current
kmax
during the out-of-zone fault.
n the turns ratio of the CT
R the secondary internal resistance of the CT in ohms
in
R the resistance (maximum of 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
higher than the stabilizing voltage U
The required knee point voltage U
formula
U U
≥ ⋅ 2
kn s
GUID-B3B099B9-F91B-485A-ADAD-8014F3021005 V1 EN
U the knee point voltage
kn
U the stabilizing voltage
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 ) of the CT secondary circuit in ohms
in m
kn
.
s
of the current transformer is calculated using the
kn
1MRS759142 C
(Equation 128)
should be at least two times
(Equation 129)
REX640
Technical Manual