Figure 769: 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
is further evaluated. For a genuine CT secondary
failure, the magnitude of I
2
changes only on the side where zero current has
been detected. The change in the magnitude of I
2
(ΔI
2
) on the other sets of the
current transformer (other than where zero current is detected) is calculated. If
Supervision functions
1MRS759142 F
1354
REX640
Technical Manual