I just read the Superman Blue volume 1 trade paperback (collecting Superman #122-125, Adventures of Superman #545-547, Action Comics #732-734, Superman: The Man of Steel #67-69, and Superman Annual #9, published April 1997 through July 1997). In it, Superman loses his traditional powers and gets new electricity-themed powers (and when he switches them off and becomes Clark Kent, he is as vulnerable as any human).
Throughout the comic series, Superman seems largely uninterested in why he got these powers. The only reference we get is in The Adventures of Superman #547 where Superman, having no crisis to deal with that day, asks Professor Hamilton about their origins.
Hamilton: So -- what brings you to my humble laboratory?
Superman: Well, I hoped you could help me find out where these powers came from…what triggered the change…
Hamilton: Ah! As it happens, I've already made some notes on that topic! They're on my desk somewhere…
Unfortunately, the two get distracted and we never do find out (in these collected comics anyway) what the origin of his powers were.
I tried reading up on Wikipedia and other online sources, but it didn't seem to answer this question. How and why did Superman get his electricity-themed powers in the first place?