ABB RELION REX640 Technical Manual page 322

Section 4
Protection functions
316
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
179 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.
Table 342: Proposed functionality of numerical transformer and busbar overcurrent protection.
DT = definite time, IDMT = inverse definite minimum time
O/C-stage Operating char.
HV/3I> DT/IDMT
HV/3I>> DT
HV/3I>>> DT
LV/3I> DT/IDMT
LV/3I>> DT
LV/3I>>> DT
In case the bus-tie breaker is open, the operating time of the blockable overcurrent
protection is approximately 100 ms (relaying time). When the bus-tie breaker is
closed, that is, the fault current flows to the faulted section of the busbar from two
directions, the operation time becomes as follows: first the bus-tie relay unit trips the
tie breaker in the above 100 ms, which reduces the fault current to a half. After this the
incoming feeder relay unit of the faulted bus section trips the breaker in approximately
250 ms (relaying time), which becomes the total fault clearing time in this case.
Selectivity mode Operation speed
time selective low
blockable/time high/low
selective
current selective very high
time selective low
time selective low
blockable high
1MRS759142 C
Figure
Sensitivity
very high
high
low
very high
high
high
REX640
Technical Manual