Application - ABB Relion SMU615 Technical Manual

Section 5
Supervision functions
5.2.5
136
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.

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 B
SMU615
Technical Manual