Tyrantrum is
Rock/Dragon, not Ground/Dragon. Under normal circumstances, Tyrantrum resists Electric due to being part-Dragon, meaning it takes half the damage Electric moves normally deal. Pure-Rock gets normal damage from Electric. The only reason for this misconception dates back to Generation I, where most of the Rock-types were part Ground (Geodude/Graveler/Golem, Onix, and Rhyhorn/Rhydon). The only other Rock types that weren't part-Ground back then-the Fossil Pokémon from that era-were actually weak to Electric due to them being either part-Water (Omanyte/Omastar, Kabuto/Kabutops) or part-Flying (Aerodactyl).
If Tyrantrum were part-Ground, then yes, it would be immune to Electric despite the secondary type getting half damage. Same for Wooper, were it not for it being Ground, would be weak to Electric (part-Water).
(Darn it, I got ninja'd as I was writing this.)
Neither Electric nor Dragon is immune to Electric, they just resist it.
Additionally, Electric/Dragon would have double resistance (quarter the normal damage), same with Electric/Grass and Grass/Dragon. Works the same way as Water/Flying being doubly weak to Electric (four times normal damage).