Mr C is right that there's no problem with the passive voice here; Ms A is right that there is a different problem with your example sentence and your 'where' should be 'when'.
That said, it's worth saying that the reason Microsoft Word always dings the passive voice is because English teachers generally ding the passive voice.
In the first five Google results for "passive voice", you've got Hamilton College's English Department sniping
The First Deadly Sin
[The] Passive voice produces a sentence in which the subject receives an action. In contrast, [the] active voice produces a sentence in which the subject performs an action. Passive voice often produces unclear, wordy sentences, whereas active voice produces generally clearer, more concise sentences. To change a sentence from passive to active voice, determine who or what performs the action, and use that person or thing as the subject of the sentence.
Also in the first five, you have Purdue University's English Department opining with bolded and underlined text
Active voice is used for most non-scientific writing. Using active voice for the majority of your sentences makes your meaning clear for readers, and keeps the sentences from becoming too complicated or wordy. Even in scientific writing, too much use of passive voice can cloud the meaning of your sentences.
And it takes until you get to UNC's Writing Corner to read
1. Myth: Use of the passive voice constitutes a grammatical error.
Use of the passive voice is not a grammatical error. It’s a stylistic issue that pertains to clarity—that is, there are times when using the passive voice can prevent a reader from understanding what you mean...
4. Myth: You should never use the passive voice.
While the passive voice can weaken the clarity of your writing, there are times when the passive voice is OK and even preferable.
5. Myth: I can rely on my grammar checker to catch the passive voice.
See Myth #1. Since the passive voice isn’t a grammar error, it’s not always caught. Typically, grammar checkers catch only a fraction of passive voice usage.
Do any of these misunderstandings sound familiar? If so, you’re not alone. That’s why we wrote this handout. It discusses how to recognize the passive voice, when you should avoid it, and when it’s OK.
So English teachers (a) generally learn at some point during their tertiary education that wordy Jamesian prose lost and manly terse Hemingway won and (b) quickly realize that the passive voice lets students reach their word counts much too quickly and (c) often fall back on blanket proscription instead of explaining when and how to use the passive voice for best effect.
In your [corrected] example,
Our current electric grid was conceived more than one hundred years ago when energy needs were simple.
the focus is on the grid itself, which is where it should be. Starting in on a subject
General Electric and its progeny and clones established the backbone of our current electric grid more than one hundred years ago, when energy needs were simple.
teaches us a little history but involves needless advertising and verbiage.
Old white men established our current electric grid more than one hundred years ago, when energy needs were simple.
meanwhile, has its own host of problems, if you weren't writing this for a women or cultural studies credit.
You don't have to just sit there and take misguided abuse from that paperclip, though. You can follow the instructions in this article in reverse and just tell the program to stop bothering you about the passive voice at all.