Is there ever a good time to use passive voice in nonfiction?
You can effectively use passive voice when the action is more important than the actor.
For example, if you were writing about the effect of pollutants on a river, you might introduce the facts in the passive voice, because in terms of the effects, it doesn't MATTER who did the action:
"The pollutants were released into the lake in 1979. Significant amounts were detected in the last test of the area, in 2010."
If this was an expose on the SOURCE of the pollutants, maybe you would want to avoid the passive voice, and say:
"Company X, Company Z, and Farmer B released the pollutants into the lake in 1979. While the govnerment said that it was all cleaned up immediately, independent labs detected significant amounts in the area in 2010."
Or there's the old "John Smith found the body on the courthouse steps," vs. "The body was found on the courthouse steps." If John Smith is just a passerby, he's not really important to the story, and it's distracting to have him in there.
I think there's also a tradition of using passive voice in lab reports, to make things seem less personal. "I turned the burner on" vs "The burner was turned on." Again, I guess it's trying to de-emphasize the importance of the actor. It doesn't matter WHO turned the burner on; the important thing is the act.
I've struggled with this in my technical writing. When I read a technical article entirely in the passive voice, it feels like the writers are either: a) just doing what everyone else does, or b) trying to sound more authoritative than they are. In either case, it certainly comes across as dry.
On the other hand, if I'm writing a paper on something I alone did, it sounds egotistical to say, "I examined the data and found...". I much prefer "We examined the data and found..." if it's a team effort, or "When we look at the data..." using "we" to refer to myself and my readers.
Still wrestling with that one...
I agree with Kate, it is best used when you are trying to illustrate something where the actors are less important than the action being performed.
The best analogy I can come up with is "cold and impersonal". So it's probably best used when writing reports.
But, like Kate said, if you need to place emphasis on the actor(s) involved, then it is better to use the active voice. Then it becomes more a case of "They did it" instead of "Something happened".