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Section 4
Protection functions
615 series ANSI 115
Technical Manual
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 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 47
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 90: Proposed functionality of numerical transformer and busbar overcurrent protection. DT = definite
time, IDMT = inverse definite minimum time
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.
O/C-stage Operating char. Selectivity mode Operation speed Sensitivity
HV/51P DT/IDMT time selective low very high
HV/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-1/2 DT time selective low high
LV/50P-3 DT blockable high high