I have valid h1b from sept 2015 till sept 2018, due to some reasons i had to travel to US on H4B to join my family. Currently i’m in US on dependant status inspite of having valid H1B. Am i eligible for applying H1B cap excempt as i never traveled to US on my H1B visa?

Yes, you are still eligible for cap-exempt petition. An employer can apply for it along w/ COS and once approved, you can work on H-1.