ABB REF615 Applications Manual page 140

Section 5
Protection functions
134
• Selective overcurrent and short-circuit protection of feeders in distribution and
subtransmission systems
• Back-up overcurrent and short-circuit protection of power transformers and
generators
• Overcurrent and short-circuit protection of various devices connected to the
power system, for example, shunt capacitor banks, shunt reactors and motors
• General back-up protection
PHxPTOC is used for single-phase, two-phase and three-phase non-directional
overcurrent and short-circuit protection. Typically, overcurrent protection is used for
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
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
to several hundred times I
n
depending on the fault location and transformer
n
1MRS756378 D
, depending
n
REF615
Application Manual