4.1.1.7
Application
51P/50P 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
51P/50P 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 pickup level for the function to trip. When the number of
pickup-phase settings is set to "1 out of 3", the operation of 51P/50P 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 pickup levels and time
delays. 51P/50P consists of three protection stages:
•
Low 51P
•
High 50P-1/2
•
Instantaneous 50P-3
51P is used for overcurrent protection. The function contains several types of time-delay
characteristics. 50P-1/2 and 50P-3 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
1MRS240050-IB C
Section 4
Protection functions
REF615R
119
Technical Manual
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