I was inspired to ask this question after watching ten or so episodes of Ninja Steel and Dino Thunder while round a friend's house, and in particular, after reading this question which claims that failing to protect their secret identity could result in a Power Ranger being stripped of their powers.
In the episodes I watched, the Rangers don't seem to care at all about their secret identities. They invariably run up to the Monster of the Week in their civilian guises, say a pithy one-liner, and then transform right in front of it. As a result, the villains know exactly who the Rangers are, and at least half the episodes I saw (especially of Ninja Steel) involved one or more of the Rangers getting kidnapped, brainwashed, or tricked by the villains.
Not protecting their identities is clearly more trouble to the Rangers than it's worth, and seems to defeat the purpose of wearing a face-concealing helmet. But I only watched parts of two series, and there have been twenty-five according to Wikipedia, so maybe at some point, there were a set of Rangers that were more careful not to reveal themselves.
Have any Power Rangers, across any of the show's incarnations, ever managed to successfully conceal their secret identity from the villains for the entire series? Or do they all just transform right in front of the villains like that?