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
(Equation 213)
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
R
U
I
s
s
rs
=
(Equation 214)
R
s
The resistance of the stabilizing resistor
U
s
The stabilizing voltage of the protection relay
I
rs
The value of the
Operate value
setting in secondary amps.
The stabilizing resistor should be capable to dissipate high energy within a very
short time; therefore, the wire wound-type resistor should be used. Because of the
possible CT inaccuracy, which might cause some current through the stabilizing
resistor in a normal load situation, the rated power should be 25 W minimum.
If U
kn
is high or the stabilizing voltage is low, a resistor with a higher power rating
is needed. Often resistor manufacturers allow 10 times rated power for 5 seconds.
Thus the power of the resistor can be calculated with the equation
U
R
kn
s
2
10
×
(Equation 215)
The actual sensitivity of the protection is affected by the protection relay setting,
the magnetizing currents of the parallel connected CTs and the shunting effect of
the voltage-dependent resistor ( VDR). The value of the primary current I
prim
at
which the protection relay operates at a certain setting can be calculated with the
formula
I
n I
I
m I
prim
rs
u
m
=
⋅
+
+
⋅
(
)
(Equation 216)
Protection functions
1MRS759142 F
812
REX640
Technical Manual