Siemens Argus 7SR21 Technical Manual page 642

7SR210 & 7SR220 Applications Guide
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: -
Note: the equivalent primary impedance to the above: -
=
Z
P
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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V
Lsecy
=
Z
S
I
secy
CT V
Asecy Lprim
´ ´
Z
S
CT V
prim Lsecy
110
= =
42 Ω
´
3 1.5
1 33000
= ´ ´
42
1600 110
©2018 Siemens Protection Devices Limited
= W
7 . 88