what is Novikov self-consistency principle?

You can find reasonable answers to this and your other questions via wikipedia.

 

The principle comes from sending "billiard balls" round closed time-like curves to scatter of themselves. Novikov found that the balls almost always scattered making the system self-consistent. I think they found that with reasonable physical assumptions that to not get scattering is almost impossible.

 

This suggests that if one could time-travel then what ever you do in the past you already have done it. You were always part of the past in a consistent way.

I understand, that is, this principle says that if an event exists that could give rise to a paradox, then the probability of that event happening is zero..

 

 

Is it logic ?

I think it is logical. In essence it states that scenarios that create paradoxes are not realised. It is different question to ask if it is true.

 

As an aside, Novikov refers to Igor Dmitriyevich Novikov and not Sergei Petrovich Novikov who is one of my mathematical grandfathers.