ABB RELION 620 Series Technical Manual page 600

Section 4
Protection functions
594
The current transformers must be able to force enough current to operate the relay
through the differential circuit during a fault condition inside the protection zone. To
ensure this, the knee point voltage U
stabilizing voltage U .
s
The required knee point voltage U
Equation 106.
U U
≥ ⋅ 2
kn s
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U Knee point voltage
kn
U Stabilizing voltage
s
The factor two is used when delay in the operating time of the protection is not
acceptable in any situation. It is advisable to use current transformers whose
secondary winding resistance is of the same size as the resistance of the measuring
loop to prevent the knee point voltage from growing too high.
As the impedance of the protection relay is low, a stabilizing resistor is needed. The
value of the stabilizing resistor is calculated using
U
s
R
=
s
I
rs
GUID-B4B21756-7F5D-4458-AD94-15CE2B686E41 V1 EN
R The resistance of the stabilizing resistor
s
U The stabilizing voltage of the protection relay
s
Operate value setting in secondary amperes
I The value of
rs
The stabilizing resistor should be capable of dissipating high energy within a very
short time. Therefore, a wire wound type resistor should be used. The rated power
should be in class of a few tens of watts in minimum because of the possible CT
inaccuracy, which might cause some current through the stabilizing resistor in a
normal load situation.
If the U is high or the stabilizing voltage is low, a resistor with a higher power rating
kn
is needed. Often the resistor manufacturers allow ten times rated power for five
seconds. Thus the power of the resistor can be calculated using
2
U
kn
R
10
s ⋅
GUID-56E8371C-3B82-4975-9484-8C12DCD4796F V1 EN
The actual sensitivity of the protection is affected by the protection relay setting, the
magnetizing currents of the CTs connected in parallel and the shunting effect of the
should be at least twice higher than the
kn
of the current transformer is calculated using
kn
Equation
1MRS757644 F
(Equation 106)
107.
(Equation 107)
Equation 108.
(Equation 108)
620 series
Technical Manual