Specifications

7SR242 Duobias Applications Guide
Step 1 – Selection of Line CT Ratios
CTs with a secondary rating of 1A are preferred as the burden imposed on the CT by the secondary wiring is
reduced in comparison with a 5A rated secondary.
HV load current =
393.7A
132,0003
1090
6
u
u
A CT ratio of 400/1A is chosen.
LV load current =
A
33,0003
1090
6
7.149595.0 u
u
u
A CT ratio of 1600/1A is chosen.
Note, the 0.95 factor relates to the tap changer at mid-tap position.
Step 2 – Selection of ICT Multiplier Settings
The outputs of the interposing CTs (ICT
OUT
) must be balanced for system healthy conditions - where possible the
balance is set at 1.00A at transformer rated current/mid-tap position.
HV Secondary current =
A
00
393.7
98.0
4
HV ICT Multiplier = 1/0.98 = 1.02
LV Secondary current =
A
00
1495.7
93.0
16
LV ICT Multiplier = 1/0.93 = 1.07
X1.02
400/1
393.7A
0.98A
1600/1
1495.7A
X1.07
0.93A
0.98
Yy0Yd11
W1: 1A
1.00
1.00
0.93
W2: 1A
90MVA, Z = 14%, Yd11, 132/33kV +10% to -20%
W1 ICT
OUT
= 1.00
Terminals
ICT Multiplier
ICT Connection
W1
Bias
W2
Bias
Operate
W2 ICT
OUT
= 1.00
Figure 2-6 ICT Settings – Example 1
©2010 Siemens Protection Devices Limited Chapter 7 Page 17 of 56