7SR210 & 7SR220 Applications Guide
© 2013 Siemens Protection Devices Limited Page 45 of 48
5.10.2 Design View
5.10.2.1 Load Blinder 3Ph
Figure 5.10.2.1-1 Logic Diagram 3 Phase
The 3-phase load blinder has the following setting parameters:
21LB – 3P Angle +ve
21LB – 3P Angle -ve
21LB – 3P Impedance
21LB – 3P VPPS
21LB – 3P INPS
The design of Load Blinder 3Ph takes into consideration that the protection blocking should only be issued during
definable allowable system power flow conditions i.e. an overcurrent blocking signal can only be issued when:
a. Positive Sequence Voltage Vpps is not significantly reduced i.e. is above the 21LB-3P VPPS Setting.
AND
b. Negative Sequence Current Inps is lower than 21LB-3P INPS Setting
AND
c. The impedance measured in all phases is within the operate zone.
Zpps is calculated from Vpps and Ipps. The calculated Zpps is used to determine if the impedance is in
the operate zone i.e. greater than the 21LB-3P Impedance setting and less than the 21LB-3P Angle
settings. Suitable settings can be chosen to provide overcurrent blocking in the forward or reverse
directions.
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