ABB RELION 620 Series Technical Manual page 236

Section 4
Protection functions
230
The overcurrent and contact based circuit breaker failure protection CCBRBRF is
used to confirm the protection scheme in case of circuit breaker malfunction.
A070978 V1 EN
Figure 111: Example of traditional time selective transformer overcurrent
protection
The operating times of the main and backup overcurrent protection of the above
scheme become quite long, this applies especially in the busbar faults and also in the
transformer LV-terminal faults. In order to improve the performance of the above
scheme, a multiple-stage overcurrent protection with reverse blocking is proposed.
Figure 112 shows this arrangement.
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
112 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
1MRS757644 F
Figure
620 series
Technical Manual