n
, depending on the impedance of the transformer and the source impedance of the
feeding network. From this point of view, it is clear that the operation must be both
very fast and selective, which is usually achieved by using coarse current settings.
The purpose is also to protect the transformer from short circuits occurring outside
the protection zone, that is through-faults. Transformer overcurrent protection also
provides protection for the LV-side busbars. In this case the magnitude of the fault
current is typically lower than 12xI
n
depending on the fault location and transformer
impedance. Consequently, the protection must operate as fast as 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
. 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 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 121: 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.
1MRS757644 H
Protection functions
620 series
Technical Manual
245
