The mutual inductance between two unity coupled coils of 9 H and 4 H is

A Mho relay is a

Which type of back up protection is widely used for the protection of transmission lines?

Transposition of transmission line is done to

Due to skin effect

Skin Effect:
tendency of the current to crowd near the conductor surface.

Skin effect is proportional to frequency, conductor diameter and permiability.
Skin effect increases effective resistance of conductor.
In DC, no skin effect.

The efficiency of a transformer at full load, 0.8 p.f. lag is 90%. Its efficiency at full load, 0.8 p.f. lead will be

Efficiency= output power/input power
efficiency = (v*i*cosφ)/((v*i*cosφ)+(x² Pc)+(Pi)) x=fractional part of full load