Hola Thea,

in the passive voice, the use of the preterite or the imperfect tense will depend on the action being seen as a completed action (preterite) or seen as a descriptive action (imperfect), so, for example:

1. Los ladrones fueron arrestados.

2. Los ladrones eran arrestados.

In 1., this is seen as a completed action, i.e. it has an end. In 2., we see it as a description of what was happening, as a process, not as a completed action. 

I hope this clarified it.

Saludos,

Inma