ABB RELION REX640 Technical Manual page 733

1MRS759142 C
REX640
Technical Manual
U ≥ ⋅ 2 U
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
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R The resistance of the stabilizing resistor
s
U The stabilizing voltage of the protection relay
s
I The value of Operate value setting in secondary amperes
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
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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
voltage dependent resistor. The value of the primary current I
protection relay operates at certain setting can be calculated using
Section 4
Protection functions
(Equation 167)
Equation 168.
(Equation 168)
Equation 169.
(Equation 169)
at which the
prim
Equation 170.
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