Micom P721, Technical Manual

Page: 283 / 334

Commissioning and Maintenance P72x/EN CM/C21
MiCOM P721/P723 Page 13/28
Checks:
Alarm message on the LCD.
Alarm LED flashes.
Trip LED on
[87] tDiff LED on (if programmed).
Trip output closes.
[87] tDiff output closes (if programmed).
[87] Fault Timer tdiff check:
Apply a current onto phase A and measure the time delay [87] tDiff by presetting the current
above the 2
× √
2
×
[87] Threshold I Diff (I injected > 2
×
2
× √
2
×
[87] Threshold I Diff). Repeat the same test for phases B and C.
4.4 Earth differential protection ([87N] Threshold I Diff)
Set the Trip [87N] tDiff to the Trip Commands. Note that [87N] tDiff is affected by the timer,
the bus wire supervision and any blocking logic. On the other hand, [87N] Diff is
instantaneous and it is not affected by any blocking logic or the bus wire supervision. To
monitor the [87N] Diff operation, assign it to any of the relay outputs available (RL2 to RL8).
The logic scheme of the single-phase high impedance differential protection is shown in
Figure 3.
FIGURE 3: [87N] I DIFF LOGIC
4.4.1 Test wiring diagram
The test wiring diagram shown in Figure 4 makes it possible to conduct tests related to the
[87N] Threshold I Diff.
The diagram describes current injection onto the 5 Amp phase current inputs (terminals
47-48), as well as current injection onto the 1 Amp phase inputs (terminals 55-56).