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
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 220: 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.
Protection functions
1MRS757644 H
246
620 series
Technical Manual
