Massive Power Outage

　Early in the morning, a massive power outage was occurred in my prefecture. This power outage cut electricity to about 380 thousand homes and stopped about 2700 traffic lights. Furthermore, 15 bullet trains stopped and this influenced about 6600 people. It was reported that the reason of the power outage was that transmission lines had been damaged, and it lasted from 5:30 a.m. to 9:00 a.m. However, actually I couldn't be aware of this power outage, because I woke up around 8:30 a.m. and I didn't use electricity for a while. I just noticed the power outage when I went to my university and watched my PC. Although I put my PC in sleep mode, it was shut down.

　Anyway, this accident was lucky it wasn't worse. I mean, despite the massive power outage, there were no large-scale traffic accidents or injuries.

• Early in the morning, a massive power outage was occurred in my prefecture.
• Early in the morning, a massive power outage was occurred in my prefecture.

  Also good: "Early this morning..."

• This power outage cut electricity to about 380 thousand homes and stopped about 2700 traffic lights.
• This sentence is perfect! No correction needed!

• Furthermore, 15 bullet trains stopped and this influenced about 6600 people.
• Furthermore, 15 bullet trains stopped, and this affected about 6600 people.

  "affected" is a more appropriate word to use here... "influenced" is a little less direct in meaning than "affected."

• It was reported that the reason of the power outage was that transmission lines had been damaged, and it lasted from 5:30 a.m. to 9:00 a.m. However, actually I couldn't be aware of this power outage, because I woke up around 8:30 a.m. and I didn't use electricity for a while.
• It was reported that the reason for the power outage was (that transmission lines had been damaged) -> damage to transmission lines, and it lasted from 5:30 a.m. to 9:00 a.m. However, actually I wasn't aware of this power outage, because I woke up around 8:30 a.m. and I didn't use any electricity for a while.

  A more conversational way to say "transmission lines" is "power cables." But if your intention is not to be casual, "transmission lines" is totally fine!

• I just noticed the power outage when I went to my university and watched my PC.
• I only noticed the power outage when I went to my university and used my PC.

• Although I put my PC in sleep mode, it was shut down.
• Although I had put my PC in sleep mode, it was shut down (when I checked it today).

  "had put" uses an English construction called the "past perfect" tense. Basically, this tense is used when you are talking about something that happened even earlier than what you re discussing using the past tense.
  
  In this case, you had put your PC to sleep a day before you came in today to check it. So to put that little logical space between your action today and your action yesterday, you can use the "past perfect" (had put) to express your action yesterday and the "simple past" to express your action today (checked).
  
  I hope this is not too confusing!

• Anyway, this accident was lucky it wasn't worse.
• Anyway, it was lucky that this accident wasn't worse.

• I mean, despite the massive power outage, there were no large-scale traffic accidents or injuries.
• (I mean,) Despite the massive power outage, there were no large-scale traffic accidents or injuries.

Nice post! すみません、I went into some detail with the corrections. I hope they are helpful.

• This power outage cut electricity to about 380 thousand homes and stopped about 2700 traffic lights.
• This power outage cut electricity of about 380 thousand homes and stopped about 2700 traffic lights.

  どちでもだいじょぶ。

• I just noticed the power outage when I went to my university and watched my PC.
• I just noticed the power outage when I went to my university and got on my PC.

  You can also say it this way.

• Although I put my PC in sleep mode, it was shut down.
• Even though I had my PC in sleep mode, it died.

  When someone says their electronic device died, it usually means that the battery ran out.

• Anyway, this accident was lucky it wasn't worse.
• Anyway, I am glad it wasn't worse.

  Shows the same amount of gratitude that it wasn't worse.

• I mean, despite the massive power outage, there were no large-scale traffic accidents or injuries.
• This sentence is perfect! No correction needed!

よかった

• This power outage cut electricity to about 380 thousand homes and stopped about 2700 traffic lights.
• This power outage the cut of electricity to about 380 thousand homes and stopped about 2700 traffic lights.

• It was reported that the reason of the power outage was that transmission lines had been damaged, and it lasted from 5:30 a.m. to 9:00 a.m. However, actually I couldn't be aware of this power outage, because I woke up around 8:30 a.m. and I didn't use electricity for a while.
• It was reported that the reason of the power outage was that transmission lines had been damaged, and it lasted from 5:30 a.m. to 9:00 a.m. However, I actually wasn't aware of this power outage, because I woke up around 8:30 a.m. and I didn't use electricity for a while.

• I mean, despite the massive power outage, there were no large-scale traffic accidents or injuries.
• This sentence is perfect! No correction needed!