Instruction manual
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AQ F3x0 Feeder protection IED
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The software modules of the distance protection function are as follows:
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Z_CALC calculates the impedances (R+jX) of the six measuring current loops:
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three phase-phase loops,
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three phase-ground loops.
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POLY compares the calculated impedances with the setting values of the five
polygon characteristics. The result is the decision for all six measuring loops and for
all five polygons if the impedance is within the polygon.
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SELECT is the phase selection algorithm for all five zones to decide which decision
is caused by a faulty loop and to exclude the false decisions in healthy loops.
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I_COND calculates the current conditions necessary for the phase selection logic.
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FAULT LOCATOR calculates the distance to fault after the trip command.
The following description explains the details of the individual components.
Principle of the impedance calculation
The distance protection continuously measures the impedances in the six possible fault
loops. The calculation is performed in the phase-to-phase loops based on the line-to-line
voltages and the difference of the affected phase currents, while in the phase-to-earth loops
the phase voltage is divided by the phase current compounded with the zero sequence
current. These equations are summarized in following table for different types of faults. The
result of this calculation is the positive sequence impedance of the fault loop, including the
positive sequence fault resistance at the fault location. For simplicity, the influence of the
zero sequence current of the parallel line is not considered in these equations.
