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1MAC050144-MB C
Section 4
Protection functions
615 series ANSI
337
Technical Manual
4.6.2
Restrained (low stage) and unrestrained (high stage) differential
protection for 2W-transformers, 87T
4.6.2.1
Identification
4.6.2.2
Function block
Figure 160:
Function block
4.6.2.3
Functionality
The transformer differential protection 87T is designed to protect two-winding
transformers and generator-transformer blocks. 87T includes restrained and unrestrained
stages.
The restrained differential is designed to sensitively clear internal faults while remaining
secure with through current and presence of harmonics due to inrush or overexcitation or
CT saturation. The second harmonic restraint, together with the waveform based
algorithms, ensures that the restrained differential does not trip due to the transformer
inrush currents.
The fifth harmonic restraint ensures that the restrained differential does not trip on
apparent differential current caused by a harmless transformer over-excitation.
The unrestrained differential provides a very fast clearance of severe internal faults with a
high differential current regardless of their harmonics.
The setting characteristic can be set more sensitive with the aid of tap changer position
compensation. The correction of transformation ratio due to the changes in tap position is
done automatically based on the tap changer status information.
4.6.2.4
Operation principle
The function can be enabled and disabled with the
Operation
setting. The corresponding
parameter values are Enable and Disable.
The operation of transformer differential protection can be described by using a module
diagram. All the modules in the diagram are explained in the next sections.
Function description
IEC 61850
identification
IEC 60617
identification
ANSI/IEEE C37.2
device number
Transformer differential protection for two
winding transformers
TR2PTDF
3dI>T
87T
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