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 116
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 225: Proposed functionality of numerical transformer and busbar overcur-
rent protection. DT = definite time, IDMT = inverse definite minimum time
O/C-stage Operating char. Selectivity mode Operation speed Sensitivity
HV/3I> DT/IDMT time selective low very high
HV/3I>> DT blockable/time
selective
high/low high
HV/3I>>> DT current selective very high low
LV/3I> DT/IDMT time selective low very high
LV/3I>> DT time selective low high
LV/3I>>> DT blockable high high
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.
1MRS758755 C
Protection functions
REC615 & RER615
Technical Manual
223
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