ABB Relion 611 Series, Technical Manual

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clearing two and three-phase short circuits. Therefore, the user can choose how many
phases, at minimum, must have currents above the start level for the function to
operate. When the number of start-phase settings is set to "1 out of 3", the operation
of PHxPTOC is enabled with the presence of high current in one-phase.
When the setting is "2 out of 3" or "3 out of 3", single-phase faults are
not detected. The setting "3 out of 3" requires the fault to be present in
all three phases.
Many applications require several steps using different current start levels and time
delays. PHxPTOC consists of three protection stages.
• Low PHLPTOC
• High PHHPTOC
• Instantaneous PHIPTOC
PHLPTOC is used for overcurrent protection. The function contains several types of
time-delay characteristics. PHHPTOC and PHIPTOC are used for fast clearance of
very high overcurrent situations.
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
n
to several hundred times I
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
Figure 40 . 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
1MRS757454 D
Section 4
Protection functions
611 series 95
Technical Manual