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Line differential protection (DIFL)
Chapter 11
Verifying settings by secondary
injection
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.
Note!
When a current is injected into phase L1 in the terminal, it will appear as an IDiff in
phase L1 and L2 with 50% of the value as an IBias and in phase L3 with 25% of the
value as an IBias. When the current is sufficiently high, the protection will operate in
phase L1 and L2 in the terminal in test mode, but it will not operate in the remote ter-
minal. For the actual reading of IDiff and IBias, see step 6 in the test procedure. Also
see figure 16 and equations 5, 6 and 7.
Figure 16: Stabilisation characteristic
85
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