GE T35 UR Series Instruction Manual page 253

5 SETTINGS
The T35 percent differential element is based on a configurable dual-breakpoint / dual-slope differential restraint character-
istic. The purpose of the preset characteristic is to define the differential restraint ratio for the transformer winding currents
at different loading conditions and distinguish between external and internal faults. Differential restraint ratio variations
occur due to current unbalance between primary and secondary windings and can be caused by the following:
1. Inherent CT inaccuracies.
2. Onload tap changer operation: it adjusts the transformer ratio and consequently the winding currents.
3. CT saturation.
GE Multilin
Winding 2
Winding 1
current
current
waveform
waveform
Magnitude Magnitude
phase angle, and phase angle, and
zero sequence zero sequence
compensation compensation
(as required) (as required)
Decaying dc
Decaying dc
offset filter
offset filter
Discrete Fourier Discrete Fourier
Transform Transform
∑
Differential
phasor
Figure 5–45: PERCENT DIFFERENTIAL CALCULATIONS
T35 Transformer Protection System
5.6 GROUPED ELEMENTS
Winding 'n'
...
current
waveform
Magnitude
phase angle, and
zero sequence
compensation
(as required)
Decaying dc
offset filter
Discrete Fourier
Transform
MAX
Restraint
phasor
828714A1.CDR
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