7SR210 & 7SR220 Applications Guide
Page 48 of 48 ©2018 Siemens Protection Devices Limited
5.10.4 Calculating the Impedance
21LB-3P Impedance setting (‘a’)
This is the minimum system impedance that allows operation of the load blinder; it is set in secondary Ohms. To
enable blocking of the overcurrent element for values of current up to say 1.5 times the nominal load current value
(to allow for temporary reverse overload conditions) the setting is calculated from: -
Ω
1.53
110
I
V
Z
secy
Lsecy
S
42=
´
==
Note: the equivalent primary impedance to the above: -
W=´´=´´= 88742 .
110
33000
1600
1
V
V
CT
CT
ZZ
Lsecy
Lprim
prim
Asecy
SP
21LB-3P Angle setting (‘b’ and ‘c’)
A setting of 20 degrees will allow blocking of the overcurrent element for system load power factors between 0.94
and Unity.
21LB-3P VPPS setting
The Blinder is only operational during normal load levels of reverse load flow. Where reverse current flow is
caused by a system fault the positive sequence voltage will reduce significantly. To facilitate Load Blinder
operation the system should be substantially balanced i.e. positive phase sequence voltage will be high.
A setting of 0.95 Vn is selected i.e. 21LB-VPPS setting = 0.95 x 63.5 = 60 V.
21LB-3P INPS setting
The Blinder is only operational during normal load levels of reverse load flow. The system should be substantially
balanced i.e negative phase sequence current will be low.
A 21LB-3P INPS setting = 0.05 In is selected.
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