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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