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Instruction manual
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AQ G3x7 Generator protection IED
105 (211
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Earth fault compensation factor equation shows that the formula containing the complex
earth fault compensation factor yields the correct impedance value in case of phase-to-
earth faults only; the other formula can be applied in case of phase-to-phase faults without
ground. In case of other kinds of faults (three-phase (-to-earth), phase-to-phase-to-earth)
both formulas give the correct impedance value if the appropriate voltages and currents are
applied.
The separation of the two types of equation is based on the presence or absence of the earth
(zero sequence) current. In case of a fault involving the earth (on a solidly grounded
network), and if the earth current is over a certain level, the formula containing the complex
earth fault compensation factor will be applied to calculate the correct impedance, which is
proportional to the impedance-to-fault.
It can be proven that if the setting value of the complex earth fault compensation factor is
correct, the appropriate application of the formulas in equation above will always yield the
positive sequence impedance between the fault location and the relay location.
General method of calculation of the impedances of the fault loops
The numerical processes apply the following simple model.
Figure 3-63: Equivalent circuit of the fault loop.