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 back-up 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
63 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 59: Proposed functionality of numerical transformer and busbar over current 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 - + +
HV/3I>> DT blockable/time
selective
+/- +
HV/3I>>> DT current selective + + -
LV/3I> DT/IDMT time selective - + +
LV/3I>> DT time selective - +
LV/3I>>> DT blockable + +
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 in 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 5 1MRS756378 D
Protection functions
136 REF615
Application Manual
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