2.4 Differential overcurrent
protection ΔI> (87)
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Equation 2: Winding currents in star side, Dy11 connection
33
22
11
''
''
''
LWL
LWL
LWL
II
II
II
Equation 3: Bias current
Equation 4: Differential current
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.
If transformer is earthed, e.g. connection group Dyn11, then
zero current must be compensated before differential and bias
current calculation. Zero current compensation can be selected
individually for IL and I‟L side.
Table 2.4-1 describe connection group and zero current
compensation for different connection groups. If protection
area is only generator then connection group setting is
always Yy0, see
Table 2.4-2. Also the settings of Un and U‟n are set to be the
same, e.g. generator nominal voltage.
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