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
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 107)
U
kn
Knee point voltage
U
s
Stabilizing voltage
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 with the formula
R
U
I
s
s
rs
=
(Equation 108)
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 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 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 109)
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 110)
Protection functions
1MRS757644 H
602
620 series
Technical Manual
