Rule for converting voice of past perfect tense – 

Active VoicePassive Voice
Subject + had + verb[past participle] + object Object + had + been + verb [past participle] + by/to/with + object. 

Examples

Active VoicePassive Voice
I had cancelled my admission to the college.My admission to the college had been cancelled by me.
She had divorced him.He was divorced by her.
I had dispatched the letter.The letter had been dispatched by me.
His behaviour had shocked everyone.Everyone was shocked at his behaviour.
The man had devoured all the food in an instant.All the food had been devoured in an instant by the man.
My sister had always looked after me.I had always been looked after by my sister.
The principal had scolded her.She had been scolded by the principal.
I had taken the bitter medicine.The bitter medicine had been taken by me.
He had bought all the vegetables from the market.All the vegetables from market had been bought by him.
Susmita had forgotten me.I had been forgotten by Susmita.