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 206: Proposed functionality of numerical transformer and busbar overcurrent 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.
Section 4 1MRS758755 A
Protection functions
210 REC615 and RER615
Technical Manual
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