GE T35 Instruction Manual page 156

5.5 GROUPED ELEMENTS
5
The differential current is calculated as a vector sum of currents from all windings after magnitude and angle compensation.
The restraint current is calculated as a maximum of the same internally compensated currents.
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.
5-78
Winding 1
current
waveform
Magnitude
phase angle, and phase angle, and
zero sequence zero sequence
compensation compensation
(as required) (as required)
Decaying dc
Decaying dc
offset filter
Discrete Fourier Discrete Fourier
Transform
∑
Differential
phasor
Figure 5–40: PERCENT DIFFERENTIAL CALCULATIONS
I I
=
d 1
comp
⋅
I max (
=
r
T35 Transformer Management Relay
Winding 2
...
current
waveform
Magnitude
phase angle, and
offset filter
Transform
... I
+ +
6
comp
I , ..., I )
1 6
comp comp
5 SETTINGS
Winding 'n'
current
waveform
Magnitude
zero sequence
compensation
(as required)
Decaying dc
offset filter
Discrete Fourier
Transform
MAX
Restraint
phasor
828714A1.CDR
GE Multilin
(EQ 5.21)
(EQ 5.22)