ABB REL 551 2.3 Series Installation And Comissioning Manual page 105

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Line differential protection (DIFL)
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When one of the protections is in test mode, the opposite terminal mode of operation is
changed. In the opposite terminal, the received current values (a and b Fourier coeffi-
cients) are echoed back to the other terminal, but transposed in the following way: the
received value for the L1 phase is returned as the L2 current, L2 is returned as L3, and
L3 as L1.
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