Example n – 3-Winding Transformer
[dw_7SR5_function87TStructureOfTheFunction4, 1, en_US]
Figure 5-57 Transformer Application
Step 1 – Selection of Line CT Ratios
CTs with a secondary rating of 1 A are preferred as the burden imposed on the CT by the secondary wiring is
reduced in comparison with a 5 A rated secondary.
[fo_function87TExample1, 1, en_US]
CT ratio = 400/1 A is selected.
[fo_function87TExample2, 1, en_US]
CT ratio = 1250/1 A is selected.
[fo_function87TExample3, 1, en_US]
CT ratio = 2000/1 A is selected.
Step 2 – Selection of W1 ICT, W2 ICT and W3 ICT Multiplier Settings
The differential protection must process comparable currents and so the currents of each winding are referred
to a ‘reference’ power rating, the largest winding power rating is used i.e. 90 MVA.
The outputs of the interposing CTs (ICTOUT) for each winding are balanced for system healthy conditions and
transformer mid-tap position (the 1.05 factor applied to the LV windings relates to the tap changer at mid-tap
position).
[fo_function87TExample4, 1, en_US]
W1 ICT Multiplier = 400/394 = 1.02
[fo_function87TExample5, 1, en_US]
Protection and Automation Functions
5.22 87T Transformer Differential Protection
320 Reyrolle 7SR5, Transformer Protection Device, Device Manual
C53000-G7040-C015-1, Edition 11.2019
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