Section 6
1MAC050144-MB C
Supervision functions
442
615 series ANSI
Technical Manual
When the line is de-energized, the deactivation of the
ALARM
output is prevented.
The activation of the
BLOCK
input deactivates the
ALARM
output.
6.3.5
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 may result in differences in the secondary currents from the
two CT cores. 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 in order to prevent
unwanted tripping from fast-acting, sensitive numerical protections in case of faulty CT
secondary circuits.
Reference current measured with core-balanced current transformer
The function compares the sum of phase currents to the current measured with the
core-balanced CT.
The deactivation happens only when the highest phase current is more than
5 percent of the nominal current (0.05 xIn).
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.
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