Siemens Argus 7SR21 - Page 586

7SR210 & 7SR220 Commissioning & Maintenance Guide
Page 74 of 82 © 2013 Siemens Protection Devices Limited
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.
+R
+X
Phase
Fault
Block
Zone
Load
Impedance
b
a
Nominal
Impedance
Z = 1p.u. =
V
L
/ (√3 x I)
c
-R
-X
Vpps value
Point 1
Point 2
Point 3
Point 4
0
360
Trip Level
I = Vs / Z
Operate
Zone
a = 21LB-3P Impedance
setting
b = 21LB-3P Angle +ve
setting
c = 21LB-3P Angle -ve
setting
Point 6
Point 5
Block
Zone
Load
Impedance
b
a
c
Point 7
Point 10
Point 9
Point 8
Figure 3.5-2 Load Blinder (21) Boundary Points