GE T35 Instruction Manual page 248

5.6 GROUPED ELEMENTS
5
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.
5-114
Winding 1
current
waveform
Magnitude
phase angle, and
zero sequence
compensation
(as required)
Decaying dc
offset filter
Discrete Fourier
Transform
∑
Differential
phasor
Figure 5–42: PERCENT DIFFERENTIAL CALCULATIONS
10
8
6
4
Breakpoint 1
2
Pickup
2
0
Figure 5–43: PERCENT DIFFERENTIAL OPERATING CHARACTERISTIC
T35 Transformer Protection System
Winding 2
...
current
waveform
Magnitude
phase angle, and phase angle, and
zero sequence zero sequence
compensation compensation
(as required)
Decaying dc
offset filter
Discrete Fourier Discrete Fourier
Transform
Breakpoint 2
Transition region
(cubic spline)
4 6 8
Ir
5 SETTINGS
Winding 'n'
current
waveform
Magnitude
(as required)
Decaying dc
offset filter
Transform
MAX
Restraint
phasor
828714A1.CDR
Slope 2
region
10
828750A1.CDR
GE Multilin