The pick-up activation of the function is not directly equal to start-signal generation of the function.
Start signal is allowed if blocking condition is not active.
Unearthed network
Figure. 5.4.5. - 39. Angle tracking of DEF function in unearthed network model.
When unearthed (capacitive) network mode is chosen IED expects the fault current to be lagging zero
sequence voltage by 90 degrees. Healthy phases of healthy feeders produce capacitive current during
ground fault just like faulty feeder but current is flowing towards busbar and trough incoming
transformer or grounding transformer to faulty feeder. Healthy feeders do not trip since capacitive
current is floating to opposite direction and selective tripping can be ensured.
Amplitude of fault current depends of the capacitance of the network. The sources of capacitive
currents are the outgoing feeders. The bigger the network the greater the capacitive current during
fault is. Each outgoing feeder produces capacitance according the zero sequence capacitive
reactance of the line (ohms per kilometer). It is normal that in cable networks fault currents are higher
compared to overhead lines.
Resistance of the fault affects to the amount of voltage drop during the fault. In direct earth fault the
zero sequence voltage amplitude is equal to system line to earth voltage. In direct earth fault the
voltage of faulty phase drops close to zero and healthy phase voltages increase to amplitude of line to
line voltages.
Petersen coil grounded (compensated) network
There are many bene ts with Petersen coil grounded network. Amount of automatic reclosing is highly
decreased and therefore maintenance of breakers is diminished. Arc faults die on their own and cables
and equipment suffer less damage. In emergency situations line with earth fault can be used for certain
time.
AQ-F255
Instruction manual
Version: 2.00
77
© Arcteq Relays Ltd
