Distribution transformers have good all day efficiency due to

all day efficiency is ratio of energy output to the energy input taken over 24 hour period.
design distribution transformer for max. efficiency occurring at the avarage output power.
performance of distribution transformer represent by all day efficiency.

The purpose of skewing of rotor slots in an induction motor is

For reduction of harmonics method is given below:
Chording
integral slot winding
skewing
larger air gap
Sr≠Ss : avoid cogging
Sr-Ss≠P,2P,5P avoid synchronous cusps
Sr -Ss ≠ 3P avoid magnetic locking
here, Sr is number of rotor slot, Ss is number of stator slot

Reactance relay is used to protect against

The transient stability of a power system can be improved by

The positive and negative sequence impedance of a transmission line are

Ybus used in power flow studies and Zbus used in short circuit studies