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From Equation (2.19-1), (2.19-2), and (2.19-3), the following equations are derived:
The magnitude of the fault current depends on the impedance across the system, the
difference in phase angle, and the magnitude between the power source voltages behind both
ends.
As noted earlier, the BCD element detects the presence of a fault by measuring the ratio
between the negative sequence current and the positive sequence current (I
2F
/I
1F
). The ratio is
obtained from the negative sequence impedance (Z
2
) and the zero sequence impedance (Z
0
);
hence, the equation of the ratio is expressed as follows:
A ratio value of 0.5 or higher is obtained when the zero sequence impedance is larger
than the negative sequence impedance. The ratio, thus, is close to 1.0 if the system is high-
impedance earthed or a single-end earthed system.
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