Micom P721, Technical Manual

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Commissioning and Maintenance P72x/EN CM/C21
MiCOM P721/P723 Page 9/28
4.3 Phase differential proteciton ([87] Threshold I Diff])
Set the Trip [87] tDiff to the Trip Commands. Note that [87] tDiff is affected by the timer, any
blocking logic, the bus wire supervision and the check zone. On the other hand, [87] Diff is
instantaneous and it is not affected by any blocking logic, the bus wire supervision or the
check zone. To monitor the [87] Diff operation, assign it to any of the relay outputs available
(Rl2 to Rl8). The logic scheme of the three-phase high impedance differential protection is
shown in Figure 1:
FIGURE 1: [87] I DIFF LOGIC
4.3.1 Test wiring diagram
The test wiring diagram shown in Figure 2 makes it possible to conduct tests related to the
[87] Threshold I Diff.
The diagram describes current injection onto the 5 Amp phase current inputs (terminals 41-
42, 43-44, 45-46), as well as current injection onto the 1 Amp phase current inputs (terminals
49-50, 51-52, 53-54).