Actually, there's no real limit to ranks.
The PL does limit several things. It limits damage DC. It limits bonuses to saves, and to skills and to attributes. These limits can make further ranks pointless, but it doesn't actually outlaw them.
For example:
At PL 6, you can't have a bigger bonus to damage than +6. This doesn't mean you can only buy 6 ranks of a Blast though. I could buy 10 ranks if I wanted. It's just that I'd only be getting +6 to damage DC from those 10 ranks. The only good those extra 4 ranks would be doing me would be increasing range.
The PL does limit several things. It limits damage DC. It limits bonuses to saves, and to skills and to attributes. These limits can make further ranks pointless, but it doesn't actually outlaw them.
For example:
At PL 6, you can't have a bigger bonus to damage than +6. This doesn't mean you can only buy 6 ranks of a Blast though. I could buy 10 ranks if I wanted. It's just that I'd only be getting +6 to damage DC from those 10 ranks. The only good those extra 4 ranks would be doing me would be increasing range.