Maybe I can shed a little light on this, but hold on tight because it's not really all that straightforward (surprised, I know.) You see, it's not so much that English is full of exceptions for every rule as it is that English is taught using oversimplified rules which work reasonably well, and then the 'exceptions' are memorized one at a time. The problem is English is about as much of an inbred-mutt language as has ever existed, and some of the subtleties of grammar -- both syntax (form) and semantics (meaning) -- were borrowed as much as words and spellings. There are indeed fully qualified rules without exceptions, but they often start with prerequisites like "When a Norman French conjugation occurs in a subjunctive dependent clause..." Needless to say, this isn't terribly useful for kids in English class.
That said, there are some useful guidelines for compounds, names and noun-phrases. "Headache" and "power grid," for instance, are a compound and a noun phrase, respectively. Part of what separates the two is the relationship between the nouns (semantics.) In the case of headache, there is a difference in modern parlance between a head ache and a headache. The former refers to a general pain in the head area and the latter refers to a more specific type of pain due to intracranial pressure and inflammation (which is why we treat "headaches" with NSAIDs like Ibuprofen -- non-steroidal anti-inflammatories.) Now, why is the noun-phrase "head ache" not hyphenated like "noun-phrase" itself is? Because noun phrases (like right here) usually (technical jargon follows similar but different rules -- like medicine names) only require hyphenation when appearing in certain syntactical positions, and most specifically when being treated like a single noun, such as in lists (like the start of this paragraph.) So we could say, "NSAIDs are a common headache treatment," or "NSAIDs can be used for head-ache treatment." The latter has three nouns in a row, and we have to choose where to hyphenate. Since this isn't a title or name, we have to allow some of the nouns to act like adjectives, describing the more nouny noun(s). Highly technical, I know
So our options are "head-ache treatment" and "head ache-treatment." Which you use is based on which aspect of the idea you're trying to emphasize; the treatment or the locality of the ache.
As for "power grid", there isn't a more specific common usage of the idea which requires compounding for a neologism, thus the only compounds we see are of the type "PowerGrid", which serve as names/titles. However, were we to throw a couple more nouns in there, we'd have to hyphenate; e.g., "power-grid activation-test procedures." This is a fairly awkward construction, which is why we usually only encounter long sequences of nouns in titles and names. "Power Grid Activation Test Procedures" looks less cumbersome. Nonetheless, the former structure is technically correct when used in a sentence. It also allows for some flexibility in emphasis; "power grid-activation test-procedures" would obviously shift the focus toward the initialization procedures used in testing activation.
So, tl;dr version is that there Are rules and conventions which are rather clear cut, but they're also like extremely surgical Q's on an EQ; their range of application is very narrow and tightly defined. My best advice for you is to try to study common usage in recent publications and hope you get wise to the subtle patterns underlying the usages. But if you don't, no worries. Most native speakers are awful at it, because of how layered and mixed this language is
uke:
