[James_H._Harlow]_Electric_Power_Transformer_Engin(BookSee.org)

113kV
26.18kV
20
BG
Y
CTR1=60 CTR2=240
3B
33.6 MVA
87
L
o
a
d
Percentage
18
16
14
12
10
8
2 nd Harmonic
4 th Harmonic
6
FIGURE 3.8.22 Transformer energization while A-phase is faulted and the transformer is loaded.
4
2
3 rd Harmonic
0
0 1 2 3 4
Cycles
FIGURE 3.8.25 When the loaded transformer is energized with reduced voltage, the fourth harmonic provides
information to restrain or block the differential element.
100
90
Percentage
80
70
FIGURE 3.8.23 Element 1 inrush currents from the high- and low-side transformer windings after relay scaling.
2
1.5
Multiples of TAP
1
0.5
0
-0.5
-1
0 1 2 3 4
Cycles
FIGURE 3.8.24 Differential current during transformer energization with the power transformer loaded.
60
50
40
0 1 2 3 4
Cycles
FIGURE 3.8.26 DC content of the differential current for Case 2.
3.8.7.2.1 Second- and Fourth-Harmonic Blocking
The second and fourth harmonics properly block the differential element. Notice that the secondharmonic
percentage must be set to 6% for independent harmonic blocking applications.
3.8.7.2.2 All-Harmonic Restraint
The harmonic-restraint relay that uses all harmonics maintains its security because of the even-harmonic
content of the signal.
© 2004 by CRC Press LLC
© 2004 by CRC Press LLC