A percentage differential protection scheme is used for a power transformer. Which quantity is primarily used as the restraining quantity in a biased differential relay?

Average of the magnitudes of currents entering and leaving the protected zone

For a transmission line protected by a distance relay, the relay fundamentally measures which electrical quantity to estimate the fault location?

Apparent impedance seen from the relay location

the measured current is reduced, so the apparent impedance V/I becomes larger than actual

zero-sequence impedance differs from positive-sequence impedance, affecting the apparent impedance seen by the relay

fault current can flow in either direction due to multiple sources

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Formula of the day

Machines · Transformer · 6 formulae in the cheatsheet

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At resonance X_L = X_C, so Z = R (minimum) and current is MAXIMUM — not zero. A common series-RLC slip.

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