Micom P721, Техническое руководство

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Application Guide P72x/EN AP/C21
MiCOM P721/P723 Page 17/52
3. APPLYING THE MICOM P72X
To allocate the chosen protection elements (e.g. Trip [87] tDiff or Trip [87N] tDiff) to the trip
relay RL1, select
AUTMAT.CTRL > Trip Commands . Any elements allocated in the trip
commands menu will cause RL1 to pulse for 100 ms. To latch the trip output following a
protection operation, select
AUTOMAT.CTRL > Latch Functions then select the
appropriate protection element (e.g. Trip [87] tDiff or Trip [87N] tDiff).
The setting range of the P72x is as follows:
High impedance phase differential Input board rating = 0.01 – 8In
Threshold I Diff 0.02 – 2 In
High impedance neutral differential Input board rating = 0.004 – 2In
Threshold IDiff 0.01 – 1 In
3.1 Single phase applications: restricted earth fault and balanced earth fault protection
The Measurement Filter under [87N] EARTH DIFF. PROTECTION can be set to Fourier,
Sample and Fast Sample mode. In general, it is recommended to set the Measurement filter
to Fourier as the K factors are low and the operating times are adequate. Typically less than
40ms in REF/BEF applications at 5In and less than 18ms in busbar applications at 5In (three
phase fault).
The restricted earth fault relay is a high impedance differential scheme which balances zero
sequence current flowing in the transformer neutral against zero sequence current flowing in
the transformer phase windings. Any unbalance for an in-zone fault results in an increasing
voltage on the CT secondary and thus will activate the REF protection.
The high impedance differential technique ensures that the impedance of the circuit is
sufficiently so that the differential voltage which may occur under external fault conditions is
lower than the voltage required to drive setting current through the relay. This ensures
stability against external fault conditions so the relay will operate only for faults occurring
inside the protected zone.
This scheme is very sensitive and can then protect against low levels of fault current in
resistance grounded systems where the earthing impedance and the fault voltage limit the
fault current. In this application, the
Threshold IDiff setting should be chosen to provide a
primary operating current less than 30% of the minimum earth fault level.
This scheme is very sensitive so can protect against low levels of fault current in resistance
grounded systems where the earthing impedance and the fault voltage limit the fault current.
In this application, the
Threshold IDiff setting should be chosen to provide a primary
operating current less than 10-25% of the minimum earth fault level.
This scheme can also be used in a solidly grounded system. It provides a more sensitive
protection, even though the overall differential scheme provides a protection for faults over
most of the windings. In this application, the
Threshold IDiff setting should be chosen to
provide a primary operating current between 10 and 60 % of the rated current.
FIGURE 13 shows the general diagram when the P72x is used in restricted earth
fault/balanced earth fault (REF/BEF) applications: