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. The ALARM output is deactivated after a fixed 3 s delay, and the
FAIL is deactivated.
The deactivation happens only when the highest phase current is
more than 5 percent of the nominal current (0.05 xIn).
When the line is de-energized, the deactivation of the ALARM output 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, earth-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 operation time to
prevent unwanted tripping from fast-acting, sensitive numerical protections in case
of faulty CT secondary circuits.
Open CT circuits create extremely high voltages in the circuits,
which may damage the insulation and cause further problems. This
must be taken into consideration especially when the protection
functions are blocked.
When the reference current is not connected to the IED, the
function should be turned off. Otherwise, the FAIL output is
activated when unbalance occurs in the phase currents even if there
was nothing wrong with the measurement circuit.
1YHT530004D05 D Section 6
Supervision functions
615 series 583
Technical Manual
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