Siemens Argus 7SR21 Technical Manual page 586

7SR210 & 7SR220 Commissioning & Maintenance Guide
The design of a Load Blinder 3Ph is based on the theory that protection blocking should be issued only during
definable allowable system power flow conditions i.e. an overcurrent blocking signal can only be issued when:
§ Positive Sequence Voltage Vpps is not significantly changed, when the Vpps is above the 21LB-3P VPPS
Setting.
AND
§ Negative Sequence Current Inps is lower than 21LB-3P INPS Setting
AND
§ Distributed generators feed in PPS current into the system. In case of high load in system due to
distributed generation hence resulting in change of Zpps.
Zpps is calculated from Vpps and Ipps. The magnitude and angle of Zpps is compared with 21LB-3P
Impedance and 21LB-3P Angle to determine if the impedance is in operate or block and/or FWD or
REV zone.
Point 8
Point 7
Load
Impedance
Point 10
Point 9
Page 74 of 82
Nominal
Impedance
Z = 1p.u. =
V / (√3 x I)
L
Block
Zone
a
b
-R
c
Figure 3.5-2 Load Blinder (21) Boundary Points
+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
© 2013 Siemens Protection Devices Limited
Point 1
Point 2
Block
Trip Level
Zone
I = Vs / Z
Load
Point 6
Impedance Point 5
Point 3
Point 4