ABB RELION REX640, Technical Manual, Page: 334 / 1960

Figure 201: Backup overcurrent protection for line differential applications
Transformer and busbar overcurrent protection with reverse blocking principle
By implementing a full set of overcurrent protection stages and blocking channels
between the protection stages of the incoming feeders, bus-tie and outgoing
feeders, it is possible to speed up the operation of overcurrent protection in
the busbar and transformer LV-side faults without impairing the selectivity. Also,
the security degree of busbar protection is increased, because there is now a
dedicated, selective and fast busbar protection functionality which is based on
the blockable overcurrent protection principle. The additional time selective stages
on the transformer HV and LV-sides provide increased security degree of backup
protection for the transformer, busbar and also for the outgoing feeders.
Depending on the overcurrent stage in question, the selectivity of the scheme in
Figure 202
is based on the operating current, operating time or blockings between
successive overcurrent stages. With blocking channels, the operating time of the
protection can be drastically shortened if compared to the simple time selective
protection. In addition to the busbar protection, this blocking principle is applicable
for the protection of transformer LV terminals and short lines. The functionality and
performance of the proposed overcurrent protections can be summarized as seen in
the table.
Protection functions 1MRS759142 F
334 REX640
Technical Manual