ABB RELION Series Technical Manual page 214

Section 4
Protection functions
208
(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
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
Figure 116. The low-set stage (F)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.
to several hundred times I
n
depending on the fault location and transformer
n
1MRS758755 A
, depending on the
n
REC615 and RER615
Technical Manual