VAMP 59 - Page 77

Technical description
2 Protection functions
2.12 Line differential protection
LdI> (87L)
VM59.EN001
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77
dI
module
Phase current amplitude
Trip region
dI> pick-up
Basic setting
Slope
Start of slope (Basic limit)
Figure 2.12-1 Tripping threshold characteristics
Bias current calculation is only used in protection stage I>.
Bias current describes the average current flow in transformer.
Bias and differential currents are calculated individually for
each phase.
Equation 2.12-1: Bias current
2
21 RELAYRELAY
b
II
I


Equation 2.12-2: Differential current
21 RELAYRELAY
d
III 
Figure 2.12-2 Example settings