ABB Relion 611 Series, Technical Manual

Page: 502 / 536

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
kn
must 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
GUID-4F7F301A-1573-4736-B740-622605DB0FFB V2 EN
(Equation 68)
U
kn
the knee point voltage
U
s
the stabilizing voltage
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:
R
U
I
s
s
rs
=
GUID-EA4FE2BC-4E93-4093-BD14-F20A4F33AEF2 V1 EN
(Equation 69)
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, 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
kn
is high or U
s
is low, a resistor with a higher power rating is needed. Often the
resistor manufacturers allow 10 times the rated power for 5 seconds. Thus, the power
of the resistor can be calculated using the formula:
U
R
kn
s
2
10
×
GUID-8C4CAB10-997F-4C14-9F30-5DB6F4CEC54E V1 EN
(Equation 70)
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
Section 10
1MRS757454 D
Requirements for measurement transformers
496 611 series
Technical Manual