Micom P721, Technical Manual

Page: 282 / 334

P72x/EN CM/C21 Commissioning and Maintenance
Page 12/28 MiCOM P721/P723
4.3.4 [87] Threshold I Diff with Sample mode
The sample mode compares the peak-to-peak value of the differential current divided by
2
× √
2 against the [87] Threshold I Diff.
If Diff I Threshold [87]
2 2
Idiff
peak peak
>
×
−
, then the relay trips.
Values to be recorded:
[87] Threshold I Diff for each phase
Time delay tDiff> for each phase.
[87] Threshold I Diff check:
If the time delay [87] Fault Timer tdiff is short, gradually increase the injection current up to
the value of the [87] Threshold I Diff.
If the time delay [87] Fault Timer tdiff is long, inject 0.95 x [87] Threshold I Diff and check that
there is no tripping. Then inject 1.1 x [87] Threshold I Diff and check the trip.
Gradually decrease the injected current and record the value of the drop out.
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 [87] Threshold I Diff (I injected > 2 x [87] Threshold I Diff). Repeat the same test
for phases B and C.
4.3.5 [87] Threshold I Diff with Fast mode
The Fast mode detects a number of samples above the threshold during one AC half cycle.
The Fast mode threshold is 2
× √
2
×
[87] Threshold I Diff. If four consecutive samples are
above 2
× √
2
×
[87] Threshold I Diff, then the relay trips.
Values to be recorded:
[87] Threshold I Diff for each phase
Time delay tDiff> for each phase.
[87] Threshold I Diff check:
If the time delay [87] Fault Timer tdiff is short, gradually increase the injection current up to
the value of 2
× √
2
×
[87] Threshold I Diff.
If the time delay [87] Fault Timer tdiff is long, inject 0.95
×
2
× √
2
×
[87] Threshold I Diff and
check that there is no tripping. Then inject 1.1
×
2
× √
2
×
[87] Threshold I Diff and check the
trip.
Gradually decrease the injected current and record the value of the drop out.