There was a blackout in Seoul for 30 minutes.
→ 서울에 30분간 정전이 되었었다.

※blackout : 정전
(= no electricity)

[예문]
There was a blackout in Busan for an hour after the storm hit the city.
: 폭풍이 강타한 부산은 한시간동안 정전이 되었었다.