Figure. 5.4.4. - 22. Angle tracking of DEF function in Petersen coil grounded network model.
When Petersen coil grounded (compensated) network mode is chosen the IED expects the fault
current to be opposite direction to zero sequence voltage. Healthy phases of healthy feeders and faulty
feeder produce capacitive current similar to unearthed network. Inductance of Petersen coil is
compensating the capacitive current and therefore residual current in fault location is close to zero.
Size of the inductance is chosen according the prospective earth fault current of the network. Desired
compensation grade is achieved when k-factor is close to 1.0 and network is fully compensated.
Network is over compensated when k-factor is greater than one and under compensated if the factor is
smaller than one.
Inductance connected to the star point of incoming transformer or like in most cases to grounding
transformer compensates the capacitance of the network but due this the capacitive fault current
cannot be measured anymore. Fault detection is handled by connecting a resistance on parallel with
the inductance. This resistance de nes the amplitude of fault current. In under –or over compensated
situations the resistive component during the fault does not change, therefore selective tripping is
ensured even if the network is bit under –or over compensated.
Directly or small impedance grounded network
Figure. 5.4.4. - 23. Angle tracking of DEF function in grounded network model.
AQ-T257
Instruction manual
Version: 2.00
61
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