Відключення електроенергії відбулося ввечері.

Because електроенергії is in the genitive case.

The noun відключення means disconnection / shutting off, and Ukrainian often uses the genitive after this kind of noun to show what was disconnected:

• відключення електроенергії = disconnection of electricity
• more naturally in English: power outage / power cut

So:

• електроенергія = electricity
• електроенергії = of electricity

This is very common in Ukrainian with abstract nouns:

• ремонт дороги = repair of the road
• початок уроку = beginning of the lesson

Because the subject відключення is a neuter singular noun.

In the sentence:

• Відключення = the outage / the disconnection
• відбулося = occurred / took place

Past-tense verbs in Ukrainian agree with the subject in gender and number:

• masculine: відбувся
• feminine: відбулася
• neuter: відбулося
• plural: відбулися

Since відключення is neuter singular, the verb must be відбулося.

Відбулося means took place, occurred, or happened.

In this sentence, відбулося sounds a bit more formal or neutral than сталося.

Compare:

• Відключення електроенергії відбулося ввечері.
  = The power outage occurred in the evening.
  Slightly more formal/report-like.

• Відключення електроенергії сталося ввечері.
  = The power outage happened in the evening.
  Also correct, a bit more general.

So yes, they are similar, but відбулося often sounds more like occurred / took place.

Ввечері means in the evening.

It functions as an adverb of time in this sentence. You do not need a separate word for in the way English does.

So:

• ввечері = in the evening
• вранці = in the morning
• вдень = during the day / in the daytime
• вночі = at night

You may also see увечері.
ввечері and увечері mean the same thing; the choice often depends on pronunciation flow and style.

Yes, it is a normal and understandable way to say it, especially in more formal or neutral language.

Literally, it means:

• відключення = disconnection / shutoff
• електроенергії = of electricity

So the full phrase is disconnection of electricity, which in natural English is often power outage, power cut, or blackout depending on context.

Other Ukrainian expressions you may also hear:

• перебої з електроенергією = interruptions in the power supply
• немає світла = there is no electricity / the power is out
• відключили світло = they turned off the power / the power was cut

Yes, Ukrainian word order is fairly flexible.

The given sentence is a neutral, natural order:

• Відключення електроенергії = subject
• відбулося = verb
• ввечері = time expression

You can move parts around for emphasis:

• Ввечері відбулося відключення електроенергії.
  Emphasizes when it happened.

• Відбулося ввечері відключення електроенергії.
  Grammatically possible, but less neutral and less common in everyday use.

For learners, the original version is a very good standard pattern.

The dictionary form is відбутися.

This is a perfective reflexive verb, meaning to take place / to occur.

Important forms:

• infinitive: відбутися
• past neuter singular: відбулося
• past masculine singular: відбувся
• past feminine singular: відбулася
• past plural: відбулися

Because the sentence describes a completed event, the perfective verb fits well.

Because Ukrainian has no articles.

English says:

• the power outage occurred in the evening

Ukrainian simply says:

• Відключення електроенергії відбулося ввечері.

Whether something is the, a, or general is understood from context.

This is one of the biggest differences from English, and it is completely normal that there is no word corresponding to the.

By itself, it most naturally refers to a specific outage that the speaker and listener can identify from context.

So it usually means something like:

• The power outage happened in the evening
• That power cut occurred in the evening

If you wanted to speak more generally about power outages as a phenomenon, you would usually phrase it differently, for example:

• Відключення електроенергії часто відбуваються ввечері.
  = Power outages often occur in the evening.

So the singular form here strongly suggests one particular event.

You can understand it word-for-word, but the natural English version is a little different.

Word-for-word:

• Відключення = disconnection
• електроенергії = of electricity
• відбулося = occurred / took place
• ввечері = in the evening

Literal translation:

• The disconnection of electricity occurred in the evening.

Natural English:

• The power outage occurred in the evening.
• The power went out in the evening.
• There was a power cut in the evening.

So the Ukrainian is grammatically straightforward, but the most natural English translation is usually less literal.