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 62. 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 62: Example of traditional time selective transformer overcurrent
protection
The operating times of the main and back-up 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 63 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
1MRS756378 D Section 5
Protection functions
REF615 135
Application Manual
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