ABB RELION REX640 Technical Manual page 1254

Section 6
Supervision functions
1248
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Figure 680: 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
failure, the magnitude of I
detected. The change in the magnitude of I
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.
is further evaluated. For a genuine CT secondary
2
changes only on the side where zero current has been
2
(ΔI ) on the other sets of the current
2 2
1MRS759142 C
REX640
Technical Manual