ABB 615 series Technical Manual page 224

Section 4
Protection functions
218
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 to several hundred times I
n
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
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
Figure 107. The low-set stage 51P operates time-selectively both in transformer and LV-
side busbar faults. The high-set stage 50P 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 pickup value setting in each particular protection relay
where the inrush current can occur. The overcurrent and contact based circuit breaker
failure protection 50BF is used to confirm the protection scheme in case of circuit breaker
malfunction.
, depending on the impedance of the transformer and
n
depending on the fault location and transformer impedance.
n
1MAC059074-MB A
615 series ANSI
Technical Manual