I
b2
I
d1
I
d2
I
b3
I
d3
100
200
300
100 200 300 400 500
End section 2End section 1
Low Trip value
Section 1 Section 2 Section 3
I
d
[%Ir]
I
b
[%Ir]
GUID-EAAB6851-B6A9-4A69-B962-1725A4928D54-ANSI V1 EN
Figure 253: Operation characteristic for biased operation of 87T
The Low trip value of the biased stage of the differential function is determined according
to the operation characteristic:
Low trip value = Id1
Slope section 2 and Slope section 3 are determined correspondingly:
Slope section
I
I
d
b
2 100
2
2
= ⋅ %
GUID-D1C2CAED-3D58-4405-A79D-17B203A8D3A9 V4 EN (Equation 87)
Slope section
I
I
d
b
3 100
3
3
= ⋅ %
GUID-72224800-6EE1-48E7-9B57-4ABE89DB350C V1 EN (Equation 88)
The second turning point End section 2 can be set in the range of 100 percent to 500
percent.
The slope of the differential function's operating characteristic curve varies in the different
sections of the range.
• In section 1, where 0 percent Ir < Ib < End section 1, End section 1 being fixed to 50
percent Ir, the differential current required for tripping is constant. The value of the
differential current is the same as the Low trip value selected for the function. Low trip
1MAC059074-MB A Section 4
Protection functions
615 series ANSI 511
Technical Manual
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