ABB RELION 620 Series Technical Manual page 604

Section 6
Supervision functions
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Figure 306: Functional module diagram
No-load detection
No-load detection module detects the loading condition. If all the three-phase currents of
any two sets of current transformer are zero, the protected equipment is considered to be
in the no-load condition and the function is internally blocked by activating the
INT_BLKD output.
To avoid any false operation, the function is also internally blocked if any two-phase
currents of any set of current transformers are below Min operate current. This activates
INT_BLKD. The value of the Min operate current setting depends on the type of
equipment to be protected. For example, in case of transformer protection, Min operate
current depends on the no-load current rating. Typically, it can be set equal to the
transformer no-load current rating.
CT failure detection
This module detects the CT secondary failure in any sets of current transformers. The
module continuously scans the value of all the three-phase currents in all groups of current
transformers to detect any sudden drop in the current value to zero. The detection of a zero
current should not be the only criterion for considering a fault in the current transformer
secondary. Two other criteria are evaluated to confirm the CT failure:
• A zero current due to the CT failure does not result in a negative-sequence current on
healthy CT sets.
On the detection of a zero current in any phase on either group of CT, the negative-
sequence current I
2
magnitude of I changes only on the side where zero current has been detected. The
2
is further evaluated. For a genuine CT secondary failure, the
1MAC504801-IB E
620 series ANSI
Technical Manual