LV terminals and short lines. The functionality and performance of the proposed
overcurrent protections can be summarized as seen in the table.
Table 210: 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/51P DT/IDMT time selective low very high
HV/50PHV/
50P-1/2
DT blockable/time
selective
high/low high
HV/50P-3 DT current selective very high low
LV/51P DT/IDMT time selective low very high
LV/50P DT time selective low high
LV/50P-3 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 1MAC059074-MB A
Protection functions
220 615 series ANSI
Technical Manual
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