Application - ABB REF615 Applications Manual

1MRS756378 D
7.2.3
REF615
Application Manual
currents (I_A, I_B and I_C) and it is to be externally summated, that is, outside the
IED.
CCRDIF detects a fault in the measurement circuit and issues an alarm or blocks the
protection functions to avoid unwanted tripping.
It must be remembered that the blocking of protection functions at an occurring open
CT circuit means that the situation will remain and extremely high voltages will stress
the secondary circuit.

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 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 operate time in order 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 will be activated
when unbalance occur in the phase currents even though there is
nothing wrong with the measurement circuit
Reference current measured with core balanced current transformer
The function compares the sum of phase currents with the current measured with the
core balanced CT.
Section 7
Supervision functions
195