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Line differential protection, phase
Chapter 13
segregated (DIFL)
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 coefficients) 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 ter-
minal in test mode, but it will not operate in the remote terminal. For the actual reading of IDiff
and IBias, see step
6
in the test procedure. Also see figure
30
and equations 5,
6
and 7.
Figure 30:
Stabilisation characteristic
I
=
I
+
I
Diff
Local
Remote
(Equation 5)
126
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