ABB RED615 Technical Manual page 152

Section 4
Protection functions
146
possible taking into account the selectivity requirements, switching-in currents, and
the thermal and mechanical withstand of the transformer and outgoing feeders.
Traditionally, overcurrent protection of the transformer has been arranged as
shown in Figure 72. The low-set stage PHLPTOC operates time-selectively both in
transformer and LV-side busbar faults. The high-set stage PHHPTOC operates
instantaneously making use of current selectivity only in transformer HV-side
faults. If there is a possibility, that the fault current can also be fed from the LV-
side up to the HV-side, the transformer must also be equipped with LV-side
overcurrent protection. Inrush current detectors are used in start-up situations to
multiply the current start value setting in each particular IED where the inrush
current can occur. The overcurrent and contact based circuit breaker failure
protection CCBRBRF is used to confirm the protection scheme in case of circuit
breaker malfunction.
A070978 V1 EN
Figure 72: 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. Figure 73 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
1YHT530004D05 D
615 series
Technical Manual