Regarding this part:
When I convert it to UTF-8 without bom and close file, the file is again ANSI when I reopen.
The easiest solution is to avoid the problem entirely by properly configuring Notepad++.
Try Settings -> Preferences -> New document -> Encoding -> choose UTF-8 without BOM, and check Apply to opened ANSI files.
That way all the opened ANSI files will be treated as UTF-8 without BOM.
For explanation what’s going on, read the comments below this answer.
To fully learn about Unicode and UTF-8, read this excellent article from Joel Spolsky.