ABB RELION REX640 Technical Manual page 1354

Protection and control

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Supervision functions

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

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

• 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

been detected. The change in the magnitude of I

current transformer (other than where zero current is detected) is calculated. If

is further evaluated. For a genuine CT secondary

changes only on the side where zero current has

1MRS759142 F

Min operate current . This

) on the other sets of the

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