Micom P721, Technical Manual

Page: 285 / 334

Commissioning and Maintenance P72x/EN CM/C21
MiCOM P721/P723 Page 15/28
4.4.3 [87N] Threshold I Diff with Fourier mode
The Fourier mode is based on the differential current fundamental module. If
, then the relay trips. Diff I Threshold [87] Idiff
>
Values to be recorded:
[87N] Threshold I Diff
Time delay tDiff>
[87] Threshold I Diff check:
If the time delay [87N] Flt Timer tdiff is short, gradually increase the injection current up to
the value of the [87N] Threshold I Diff.
If the time delay [87N] Flt Timer tdiff is long, inject 0.95 x [87N] Threshold I Diff and check
that there is no tripping. Then inject 1.1 x [87N] 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
[87N] tDiff LED on (if programmed).
Trip output closes.
[87N] tDiff output closes (if programmed).
[87N] Flt Timer tdiff check:
Apply a current onto the single phase CT input and measure the time delay [87N] tDiff by
pre-setting the current above the [87N] Threshold I Diff (I injected > 2 x [87N] Threshold I
Diff).
4.4.4 [87N] 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 [87N] Threshold I Diff. If
Diff I Threshold [87N]
2 2
Idiff
peak peak
>
×
−
, then the
relay trips.
Values to be recorded:
[87N] Threshold I Diff
Time delay tDiff>
[87] Threshold I Diff check:
If the time delay [87N] Flt Timer tdiff is short, gradually increase the injection current up to
the value of the [87N] Threshold I Diff.
If the time delay [87N] Flt Timer tdiff is long, inject 0.95 x [87N] Threshold I Diff and check
that there is no tripping. Then inject 1.1 x [87N] 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