ABB RELION 620 Series Technical Manual page 890

Section 13
Requirements for measurement transformers
884
I
k max
V R R
( )
= +
s in m
n
GUID-FA89F7B0-661F-4FBF-ABFC-5EC3A256F8EE V1 EN
I the highest through-fault current in primary amps. The highest earth fault or short circuit current during
kmax
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 protection zone. To
ensure this, the knee point voltage V
stabilizing voltage V .
s
The required knee point voltage V
56.
V ≥ ⋅ 2 V
kn s
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V the knee point voltage
kn
V the stabilizing voltage
s
The factor two is used when a delay in the operating time of the protection is not
acceptable. To prevent the knee point voltage from rising too high, CTs with the secondary
winding resistance same as the resistance of the measuring loop should be used.
As the impedance of the protection relay is low, a stabilizing resistor is needed. The value
of the stabilizing resistor is calculated with
V
s
R
=
s
I
rs
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R the resistance of the stabilizing resistor
s
V the stabilizing voltage of the protection relay
s
Operate value setting in secondary amps.
I the value of the
rs
The stabilizing resistor should be capable to dissipate high energy within a very short time;
therefore, a wire wound-type resistor must be used. The minimum rated power should be
+ R ) of the CT secondary circuit in ohms
in m
must be at least two times higher than the
kn
of the current transformer is calculated with
kn
Equation 57.
1MAC504801-IB E
(Equation 133)
Equation
(Equation 134)
(Equation 135)
620 series ANSI
Technical Manual