You Do Need power plug adapter while traveling from Bulgaria to Cocos (Keeling) Islands.

Here is Why You Need An Adapter

The outlet and plug type used in Cocos (Keeling) Islands is I and in Bulgaria are C and F. This is how they look like.

Bulgaria

In Bulgaria, outlets of type C and F are in use.

Type C

Type F

Cocos (Keeling) Islands

In Cocos (Keeling) Islands, outlets of type I are in use.

Type I