Siemens Argus 7SR21 Technical Manual page 638

7SR210 & 7SR220 Applications Guide
5.10 Load Blinder Design
5.10.1 Application view
It is important to observe the convention regarding Forward and Reverse directions of current.
Load Blinders are designed to block forward overcurrent elements from tripping during defined load conditions.
User settings 21LB-1/3P Impedance, 21LB-1/3P Angle +ve and 21LB-1/3P Angle -ve ('a', 'b' and 'c' in Figure
5.10.1-1 Impedance Setting and Angle Setting) allow the user to define the Blinder region depending on the
network configuration - specifically the line loading and fault levels.
The measured impedance:
a. Is used to confirm the forward or reverse flow of current i.e. load blinder operated.
b. Is used to confirm that current flow is outside the defined limits of load flow i.e. load blinder not operated.
The transition between operated and non-operated is subject to a 5 degree hysteresis adjustment.
Point 8
Point 7
Load
Impedance
Point 10
Point 9
Page 44 of 48
Nominal
Impedance
Z = 1p.u. =
V / (√3 x I)
L
Block
Zone
a
b
-R
c
Figure 5.10.1-1 Impedance Setting and Angle Setting
+X
Operate
Vpps value
Zone
Phase
b
Fault
0
360
+R
c
a
a = 21LB-3P Impedance
setting
b = 21LB-3P Angle +ve
setting
c = 21LB-3P Angle -ve
setting
-X
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Point 1
Point 2
Block
Trip Level
Zone
I = Vs / Z
Load
Point 6
Impedance Point 5
Point 3
Point 4