Sices D-PRO, Техническое руководство

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42 D-PRO Protection relay Technical Manual
Genset (87G) and Transformer (87T) differential protections
Once the type of differential protection (genset or transformer) is selected, the controller
activates a similar trigger for 87G and 87T, after analysing the different points of the curve.
First of all, the protection analyses the instantaneous value of the differential current for each
of the three phases as instantaneous difference between the currents of the upper part (A)
and the ones of the lower part (B); then it analyses the true RMS of each phase:
IdL1 = IL1(A)
– IL1(B)
IdL2 = IL2(A)
– IL2(B)
IdL3 = IL3(A)
– IL3(B)
It also analyses the load average current for each of the three phases as average between
the currents of the upper part (A) and the ones of the lower part (B); then it analyses the true
RMS of each phase:
IsL1 = (IL1(A) + IL1(B)) / 2
IsL2 = (IL2(A) + IL2(B)) / 2
IsL3 = (IL3(A) + IL3(B)) / 2
The protection doesn’t trigger if the value of the differential current is lower than the parameter
P.0371
“Minimum differential current threshold”, while it always triggers if higher than the value
set in the parameter P.0372
“Maximum differential current threshold”.
In the central part, the protection triggers according to the slope of the curve. In particular, the
constant K1 indicates the slope of the segment A-B, while the constant K2 indicates the B-C
one. They are expressed in percentage values. Usually, K2 has to be higher than K1 (in case
of wrong configuration, the controller forces the value of K2 to equal to K1).
87G and 87T
P.0371 (Id1)
P.0373 (Is2)
Is3
Load average current
Dif
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P.0372 (Id3)
Id2
Is1
A
B
C
TRIP
NO TRIP