7SR11 & 7SR12 Description Of Operation
©2017 Siemens Protection Devices Limited Chapter 1 Page 79 of 88
5.6 2nd Harmonic Block/Inrush Restraint (81HBL2) phase
elements only
Inrush restraint detector elements are provided, these monitor the line currents.
The inrush restraint detector can be used to block the operation of selected elements during transformer
magnetising inrush conditions.
The 81HBL2 Bias setting allows the user to select between Phase, Sum and Cross methods of measurement:
Phase Each phase is inhibited separately.
Sum With this method the square root of the sum of the squares of the second harmonic in each
phase is compared to each operate current individually.
Cross All phases are inhibited when any phase detects an inrush condition.
An output is given where the measured value of the second harmonic component is above the 81HBL2 setting.
Figure 5-8 Functional Diagram for Harmonic Block Feature (81HBL2)
81HBL2 Element
Enabled
Disabled
IL1
IL2
IL3
81HBL
2 Bias
L2 81HBL2
81HBL2 Setting
c
>
>
>
L
1 81HBL2
L3 81HBL2
81HBL2
>
1
Inhibit 81
HBL2
&
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