Symmetrical Components

This is a good paper giving nice explanation to the symmetrical components. It can be found here.

Overvoltage and Single phasing protection

This is a text regarding overvoltage and single phasing protection. It can be found here

Online courses

These are a few online courses available on Electrical engineering :

Courses by various IITs can be found here

Courses by MIT can be found here

Active and reactive power

I have found nice paper that explains the difference between active and reactive power.
You can get it here