Passive voice is usually tested in Part One of the Writing test in PET, so below are some links to help you with it.
First, click
here for a review in Spanish of this structure.
If you prefer to read it in English, click
here.
Then, test your knowledge with these quizzes:
UsingEnglish.com
About.com
a4esl
OM Grammar
http://www.ompersonal.com.ar/omgrammar/vozpasiva.htm
UsingEnglish.com
http://www.usingenglish.com/quizzes/46.html
About.com
http://esl.about.com/library/quiz/blgrquiz_passive2.htm
a4esl
http://esl.about.com/library/quiz/blgrquiz_passive2.htm