The Design of Invocations in D&D 5th edition

I been working on a supplement for D&D 5th edition using the 5e System Reference Document as foundation.

Has Wizards are one of authors of D&D 5e system posted about how Warlock Invocations are designed.

Invocations often have a prerequisite. Sometime is a requirement that the Warlock have a specific pact, other times it is a level requirement. There are hints of a pattern but nothing conclusive.

For example some invocations allow you to cast a 1st level spell without expending spell slot. Then there are invocations that allow you to cast a 2nd level spell without a spell slot but they have a level requirement.

So I am wondering if there been any posts put out by Wizards that talk about how they designed the invocations.