While we don’t officially support automations that loop at 15 minute intervals, a common approach is to toggle a variable true/false every X minutes as your timer… then you can use that variable turning true as a trigger to periodically run your rule.
This thread covers the main concepts:
And Justin from SimplySmart also covered the concept in one of his videos:
So a basic rule that flips a variable true/false every 15 minutes might look like:
Then you could use the variable changing to true as a periodic trigger in your other rules. Just keep in mind there might be rate limits on other platforms or actions that your rule takes. 
