ABB RELION REF615R, Техническое руководство

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The operating times of the main and backup overcurrent protection of the above scheme
become quite long, this applies especially in the busbar faults and also in the transformer
LV-terminal faults. In order to improve the performance of the above scheme, a multiple-
stage overcurrent protection with reverse blocking is proposed.
Figure 57 shows this
arrangement.
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 57
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 70: 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/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
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
1MRS240050-IB C
Section 4
Protection functions
REF615R 121
Technical Manual