Uncle representing the girl

I would like to know is it possible for a girl to take her uncle or any other guy relative as her wali while her father is alive? People in my place where I live tell their daughters when the mufti ask you who is your wali then tell him your uncle, etc. So during nikah the girl takes someone else as her wali instead of her father. And that also when the father is alive living with the girl and happy with the marriege? Is this kind of nikah valid? In Islam is this permissible or is it just a cultural thing? I am really confused. I heard in proper Islamic nikah, the wali must be the girl's father unless he is dead.