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The
Type of reset curve setting does not apply to PHIPTOC or when the
DT operation is selected. The reset is purely defined by the
Reset delay
time setting.
4.1.1.7
Application
(F)PHxPTOC is used in several applications in the power system. The applications
include but are not limited to:
• Selective overcurrent and short-circuit protection of feeders in distribution and
subtransmission systems
• Backup 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 backup protection
(F)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 (F)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. (F)PHxPTOC consists of three protection stages.
• Low (F)PHLPTOC
• High PHHPTOC
• Instantaneous PHIPTOC
(F)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
1MRS758755 C
Protection functions
REC615 & RER615
Technical Manual
221
Содержание Relion REC615
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