ABB RELION 620 Series Technical Manual page 486

Section 4
Protection functions
480
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 U
stabilizing voltage U .
s
The required knee point voltage U
formula:
U ≥ × 2 U
kn s
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U the knee point voltage
kn
U 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 using the formula:
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
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
a few tens of watts because of the possible CT inaccuracy which might cause some current
through the stabilizing resistor in a normal load situation.
If U is high or U is low, a resistor with a higher power rating is needed. Often the resistor
kn s
manufacturers allow 10 times the rated power for 5 seconds. Thus, the power of the
resistor can be calculated using the formula:
+ 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 using the
kn
1MAC504801-IB E
(Equation 56)
(Equation 57)
620 series ANSI
Technical Manual