Behind the shutdown was a technical failure on a specific piece of equipment at the converter station in Harku. As Elering had a backup located on site, the company immediately began to work on switching the affected equipment out, Elering said.

According to an announcement issued on the Nord Pool power exchange, repairs are slated to be completed by midnight on December 4.

The average market price of electricity in Estonia and Finland has been equally high in recent days. Estlink 1 going off the grid reduces the transmission capacity available between the two regions, which may contribute to an emerging difference in Finnish and Estonian electricity prices.

The Estlink 1 and Estlink 2 power transmission connections it co-manages with Finnish TSO Fingrid are among the most reliable in the region, Elering said, noting that not a single failure or event occurred with Estlink 1 last year that would have affected its transmission capacity. Together with planned maintenance, Estlink 1 reported reliability of 98.12 percent last year.

Kallas: We may indeed see short-term power outages this winter

"The Estlink 1 cable is down — there was a failure at the Harku converter station that will take 12 days to repair," Prime Minister Kaja Kallas (Reform) said at Wednesday's Question Time in the Riigikogu. "This means that we have 350 megawatts less electricity on our market. Our peak use is 1,270 megawatts and we produce 900 ourselves, and the rest is coming from Estlink 2."

Why is this important, and why is she talking to everyone about this? she continued. "We have a hard winter ahead of us, and we really have to think about how we can conserve electricity, because there is in fact less electricity than we have previously covered from our own buffers. Please think things through. We may indeed see short-term power outages this winter. Let's hope that doesn't happen, but we need to be prepared for that, and we can reduce this anxiety by preparing for it."

Elering and Fingrid gained ownership of Estlink 1 at the end of 2013; previously, they rented the undersea HVDC power transmission connection from owners Nordic Energy Link and N.E.L. Finland Oy.

Estlink 1 went off grid at 10:42 a.m. local time on Tuesday.

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