ABB RELION 620 Series Technical Manual page 153

1MAC504801-IB E
620 series ANSI
Technical Manual
A070978-ANSI V2 EN
Figure 63: Example of traditional time selective transformer overcurrent protection
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.
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
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
Section 4
Protection functions
Figure 64 shows this
Figure 64
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