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I
DIFF
I
A
I
B
I
C
I
REF
_
_
_
_
_
=
+
+
−
GUID-9CC931FA-0637-4FF8-85D4-6F461BD996A9 V2 EN
(Equation 159)
The
Pickup value
setting is given in units of ×In of the phase current transformer. The
possible difference in the phase and reference current transformer ratios is internally
compensated by scaling
I_REF
with the value derived from the
Primary current
setting
values. These setting parameters can be found in the Basic functions section.
The activation of the
BLOCK
input deactivates the
FAIL
output immediately.
Timer
The timer is activated with the
FAIL
signal. The
ALARM
output is activated after a fixed
200 ms delay.
FAIL
needs to be active during the delay.
When the internal blocking is activated, the
FAIL
output is deactivated immediately
immediately. However, the
ALARM
output is deactivated immediately after a fixed delay
of three seconds.
The function resets when the differential current is below the pickup value and the highest
phase current is more than 5 percent of the nominal current (0.05 × In).
If the current falls to zero when the
FAIL
or
ALARM
outputs are active, the deactivation
of these outputs is prevented.
The activation of the
BLOCK
input deactivates the
ALARM
output.
6.2.5
Application
Open or short-circuited current transformer cores can cause unwanted operation in many
protection functions such as differential, ground-fault current and negative-sequence
current functions. When currents from two independent three-phase sets of CTs or CT
cores measuring the same primary currents are available, reliable current circuit
supervision can be arranged by comparing the currents from the two sets. When an error
in any CT circuit is detected, the protection functions concerned can be blocked and an
alarm given.
In case of high currents, the unequal transient saturation of CT cores with a different
remanence or saturation factor can result in differences in the secondary currents from the
two CT cores. An unwanted blocking of protection functions during the transient stage
must then be avoided.
The supervision function must be sensitive and have a short trip time to prevent unwanted
tripping from fast-acting, sensitive numerical protections in case of faulty CT secondary
circuits.
Section 6
1MAC059074-MB A
Supervision functions
848
615 series ANSI
Technical Manual
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