scheider electric MiCOM P746, Technical Manual

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P746/EN OP/G31 Operation
(OP) 5-
4
MiCOM P746
1.1.2.1 Bias Characteristic and Differential current
The operation of the busbar differential protection is based on the application of an algorithm
having a biased characteristic, (Figure 2), in which a comparison is made between the
differential current and a bias or restraining current. A fault is detected if this differential
current exceeds the set slope of the bias characteristic. This characteristic is intended to
guarantee the stability of protection during external faults where the scheme has current
transformers with differing characteristics, likely to provide differing performance.
The algorithm operands are as follows:
Differential Current:
i
diff
(t) =
⏐Σ ⏐
Bias or Restraining current:
i
bias
(t) =
Σ ⏐ ⏐
i
Slope of the bias characteristic (zone x):
kx
Tripping permitted by bias element for:
i
diff
x(t) > kx × i
bias
(t)
The main differential current element of the busbar protection will only be able to operate if
the differential current reaches a threshold
I
D
x>2. In general, this setting will be adjusted
above the normal full load current.
i
OP
1.1.2.2 Scheme supervision by "Check Zone” element
The use of a "Check Zone" element is based on the principle that in the event of a fault on
one of the substation busbars, the differential current measured in the faulty zone will be
equal to that measured in the entire scheme.
One of the most frequent causes of maloperation of differential busbar protection schemes is
an error in the actual position of an isolator or CB in the substation to that replicated in the
scheme (auxiliary contacts discrepancy). This would produce a differential current in one or
more current nodes. However, if an element monitors only the currents "entering" and
"leaving" the substation, the resultant will remain negligible in the absence of a fault, and the
error will lie with the zone’s assumption of the plant position at this particular point in time.