Zero Sequence Current

You have to do lots of reading & if the reading still does not make you wiser on the subject, help will be at hand.

Positive, Negative and Zero sequence currents

If you can learn how to Google, You will have all your answers at your finger-tips.

As mentioned by Joshi, you need to read the topics, especially the symmetrical components and unbalanced fault chapter of any power system book. However, these links may help to understand it quickly:

http://cr4.globalspec.com/comment/692664

http://cr4.globalspec.com/comment/680874

-MS

you have to know the characteristic of each short circuit. what is the phasor diagram for 3phase fault, Line to line fault, line to ground fault etc..

This phasor had been divided into positive negative and zero sequence for the purpose of studying. later on as you go in studying the protective relay you will find the application of these components.

positive negative and zero sequence component method is mathematically proven. Line to ground fault should have zero sequence while positive component is always present in balanced or unbalanced sys. This is theoritical but important in relay application.