Application - ABB RELION REX640 Technical Manual

1MRS759142 C
6.9.6
REX640
Technical Manual
• A zero current due to the CT failure does not result in a phase angle difference
between the healthy phases.
If a system faults happens on the phase A, it results in a change in the phase angle
difference between phase B and phase C. This change in the phase angle
difference between the healthy phases is evaluated in all three sets of current
transformer, and if the change is detected in any set of CT, it is an indication of
the system failure.
If both conditions are satisfied at zero current, the FAIL output is activated
immediately. The ALARM output is activated after a fixed 200 ms delay. FAIL needs
to be active during the delay. The outputs FAIL, CTGRP1, FAIL_CTGRP2 and
FAIL_CTGRP3 are activated according to the CT group where the secondary failure
is detected.
Activation of the BLOCK input deactivates the FAIL and ALARM outputs.
It is not possible to detect the CT secondary failure happening
simultaneously with the system faults or failures or two simultaneous
failures in the secondary circuit. The function resets if the zero current
does not exist longer than 200 ms.
Internal blocking
This module blocks the function internally under specific condition to avoid any false
operation during a system fault situation. When any of the following condition is
satisfied, the function is internally blocked and the FAIL output is deactivated
immediately.
• Magnitude of any phase current for any group of current transformers exceeds the
Max operate current setting. The magnitude of phase current is calculated from
the peak-to-peak value.
• Magnitude of the negative-sequence current I
transformer exceeds the Max Nq Seq current setting.
The INT_BLKD output is activated when FAIL is deactivated if any of the above
conditions is satisfied. The ALARM output is also deactivated after a fixed three-
second delay after the FAIL output is deactivated.

Application

Open or short-circuited current transformer secondary can cause unwanted operation
in many protection functions, such as earth-fault current and differential. The simplest
method for detecting the current transformer secondary failure is by comparing
currents from two independent three-phase sets of CTs or the CT cores measuring the
same primary currents. Another widely used method is the detection of a zero-
sequence current and zero-sequence voltage. The detection of a zero-sequence current
Section 6
Supervision functions
on the healthy set of current
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