ABB RELION REX640 Technical Manual page 332

Protection functions
Transformer overcurrent protection
The purpose of transformer overcurrent protection is to operate as main protection,
when differential protection is not used. It can also be used as coarse back-up
protection for differential protection in faults inside the zone of protection, that
is, faults occurring in incoming or outgoing feeders, in the region of transformer
terminals and tank cover. This means that the magnitude range of the fault current
can be very wide. The range varies from 6xI
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
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
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 200: Example of traditional time selective transformer overcurrent protection
332
Figure 200
. The low-set stage PHLPTOC operates time-selectively both
to several hundred times I
n
depending on the fault location and transformer
n
1MRS759142 F
, depending
n
REX640
Technical Manual