Micom P721, Техническое руководство

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Application Guide P72x/EN AP/C21
MiCOM P721/P723 Page 23/52
3.3 Buswire supervision
For a high impedance differential scheme to function correctly, the CT and secondary AC
buswires must be in good condition. Any short circuit failure in these areas bypasses the
differential elements, leaving the equipment (transformer, busbar etc.) unprotected.
However, open circuit failures of the CTs or buswires cause a spill current to flow through the
differential element and operation occurs if this current is greater than the setting.
Two methods are used to prevent nuisance tripping due to open circuit conditions:
•
Duplicate differential elements (discriminating and check zones) fed from separate
CTs. Tripping is only permitted when both differential elements operate. The
advantage of this scheme is that the differential settings can remain very sensitive
(below full load current values) but this requires a total duplication of the scheme. The
P72x used to protect the discriminating zone should have an input set as [87CZ] Chk
Zone. The P72x used as the Check Zone should have an output contact configured
as [87N] Diff or [87] Diff. This output contact should be wired to the input [87CZ] Chk
Zone in the P72x used to protect the discriminating zone. Only output contacts
configured as [87N] tDiff or [87] tDiff should be configured for tripping purposes
because the [87N] tDiff and [87] tDiff take into consideration the Check Zone. [87N]
Diff and [87] Diff do not consider the Check Zone, as shown in FIGURE 18.
•
Buswire supervision elements are provided to monitor for a continuous low level of
spill current. After a time delay, this function would usually short circuit the AC
buswires to ensure stability and give an alarm. The disadvantage of this scheme is
that the differential elements must be set higher than the buswire supervision
elements (and usually above maximum load conditions) to avoid nuisance tripping.
This may create sensitivity issues. The supervision element should be time delayed to
prevent spurious operation during through fault conditions.
The above two methods are usually used in conjunction with each other, although to reduce
costs either scheme could be used independently. Phase segregated outputs are available
from the buswire supervision so that the protection can be “stabilized” on a per phase basis
by short circuiting the buswires as required. The [87N] tDiff and [87] tDiff are internally
blocked by the buswire supervision elements [95N] Sup and [95] Sup.
Even though the differential protection is prevented from operating, it is important to
short circuit the AC wires to prevent thermal damage to the scheme. This is
especially important when using the P79x to help with energy dissipation.
When using the P721 in busbar applications (three P721 would be required), configure the
buswire shorting contact as follows:
•
Set one of the logic equations (in this example, equation A is being used) as:
EQUATION A
EQUATION A.00 operator = =
EQUATION A.00 operand = Buswire shorting
T Operate = 0.00 s
T Reset = 0.00 s
•
Latch equation A
LATCH FUNCTIONS
Latch tEqu. A = 1
•
Assign tEqu.A to a normally closed contact:
OUTPUT RELAYS
tEqu.A = 0000000000000001
Logic output relay 2 = 1