The irs said I get zero for claiming an unclaimable dependent, never did that but now my federal refund says $1,500- why?

what did you refund say before?  please provide more information. 

i guess I am not following... as long as you enter the amount you ACTUALLY received for the first two rebate payments, Line 30 will be correct.  Line 30 can not be less than zero. 

If your dependent is already claimed on anther return, or the dependent filed " no one can claim me", line 30 won't help you.

Your return will be REJECTED.