ABB 615 Series ANSI, Technical Manual

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Section 6
1MAC050144-MB C
Supervision functions
450 615 series ANSI
Technical Manual
detected. The change in the magnitude of I2 ( ∆ I2) on the other sets of the current
transformer (other than where zero current is detected) is calculated. If the change is
detected on the healthy sets of CT, it is an indication of system failure.
• A zero current due to the CT failure does not result in a phase angle difference
between the healthy phases.
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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
6.4.4.3 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 alarm current setting. The magnitude of phase current is calculated from the
peak-to-peak value.
• Magnitude of the negative-sequence current I2 on the healthy set of current
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.
6.4.5 Application
Open or short-circuited current transformer secondary can cause unwanted operation in
many protection functions, such as ground-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 in the absence of a
zero-sequence voltage is an indication of the current transformer secondary failure.
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.