The
Start 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 start 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.
5.2.5
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 merging unit, 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.
Reference current measured with core-balanced current transformer
CCSPVC compares the sum of phase currents to the current measured with the
core-balanced CT.
1MRS758407 C
Supervision functions
SMU615
Technical Manual
147
Summary of Contents for Relion SMU615
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