Siemens 7SR242 Duobias, Technical Manual

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©2015 Siemens Protection Devices Limited Chapter 7 Page 16 of 64
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 =
393.6A
132,000 3
10 90
6
CT ratio = 400/1A is selected.
LV load current =
1500A
1.05 33,000 3
10 90
6
CT ratio = 1600/1A is selected.
Note, the 1.05 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 =
0.98A
400
393.6
HV ICT Multiplier = 1/0.98 = 1.02
LV Secondary current =
0.94A
1600
1500
LV ICT Multiplier = 1/0.94 = 1.06
Figure 2-6 ICT Settings – Example 1