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
. The low-set stage (F)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 protection relay 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.
Figure 115: 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.
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
Protection functions
1MRS758755 C
222
REC615 & RER615
Technical Manual
