Is there a story line that connects all of the power rangers?

Answer 1

The Morphin Grid and Zordon can be used to connect all the different Power Rangers, but the proper "Power Rangers" storyline ended with the Countdown to Destruction in Power Rangers in Space and semi-extended into Lost Galaxy. The Morphin Grid was seen in physical form in the Dino Thunder "Lost Episode" as well as Operation Overdrive's "Once A Ranger". It has been referenced in many different seasons as well, the most recent being Jungle Fury

Answer 2

Yes there is a storyline that connects most of the Power Rangers but not all of them. The Morphin' Grid is an energy grid that powers all of the Ranger teams. Zordon's Ranger teams fight against Rita, Zedd, the Machine Empire and Divatox which goes up to and including Turbo. Dark Specter was the grand monarch of Rita, Zedd, the Machine Empire and Divatox so that connected Space to the previous seasons. Karone, the Astro Megaship, Alpha 6, D.E.C.A, Professor Phenomenus and Bulk were in Galaxy so that connected Galaxy to Space. Lightspeed and Time Force were not connected to the previous seasons except for their teamups however Wild Force's "Forever Red" teamup included all of the previous Red Rangers which would connect all of those teams to the previous season. Dino Thunder has Tommy in it which would connect Ninja Storm and S.P.D. to the previous seasons. Mystic Force does not have any connections to any of the previous seasons but Overdrive has a connection to Mystic Force due to a reference to Fire Heart, Jungle Fury and RPM have no references and neither do Samurai so Jungle Fury, RPM and Samurai have no connections to previous seasons however Megaforce does due to Gosei knowing Zordon as well as the Megaforce Rangers teaming up with most of the previous Rangers so that would connect all of the teams including RPM with one another.