ABB REF615 L User Manual

4.3.4.7 Recommendations for current transformers
The sensitivity and reliability of the protection depends a lot on the characteristics
of the current transformers. The CTs must have an identical transformation ratio. It
is 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
U
I
n
R R
s
k
in m
= +
max
( )
GUID-6A4C58E7-3D26-40C9-A070-0D99BA209B1A V1 EN (Equation 60)
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 IED
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
GUID-4F7F301A-1573-4736-B740-622605DB0FFB V2 EN (Equation 61)
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 IED alone is low, a stabilizing resistor is needed. The
value of the stabilizing resistor is calculated with the formula
1YHT530004D05 D Section 4
Protection functions
615 series 437
Technical Manual

ABB REF615 L Specifications

General

Type	Feeder Protection Relay
Rated Frequency	50/60 Hz
Weight	Approx. 1.5 kg
Communication Protocols	IEC 61850, Modbus RTU, DNP3
Protection Functions	Overcurrent, Earth fault, Thermal overload, Under/Over voltage, Under/Over frequency
Enclosure	IP20