Cable fault site found but Finland-Estonia electricity interconnection months away from coming back online

After the failure in the cable system forced the second high-voltage direct current (HVDC) electricity interconnection between Estonia and Finland, known as EstLink 2, offline, Estonian transmission system operator (TSO) Elering has confirmed the detection of the fault location and the duration of the out-of-service period for the link.

EstLink 2, officially launched in 2014 with a capacity of 650 MW, shut down in the first hour of January 26. The approximate location of the fault was revealed a few days later on the Estonian coast. Elering claims that several different diagnostic methods have continued to determine the exact location since then.

As a result of the investigations carried out on the evening of Thursday, February 29, the TSO confirmed the fault location in cooperation with the cable manufacturer EstLink 2, which is located in the part of the submarine cable on land at the Estonian end. The location was suspected as one of the possible failure sites during the search that began at the end of January.

Reigo Kebja, Elering’s management board member, underlined: “The area fixed last week was one of the main suspected failure areas from the beginning. Repairing the cable in this location is time-consuming, as a cable section of more than 500 meters long has to be replaced in one of the most technically difficult sections, i.e. at the border between land and sea, where the height of the ground changes sharply.”

In parallel with the work to determine the exact location of the fault, Elering underlines that it already started preparations, including design work, in February 2023 to speed up the repair of the cable at the now-identified fault location. The TSO highlights that this is “a geotechnically demanding area and a unique cable,” which is why EstLink 2 will remain out of operation until August 31, 2024, according to the updated fast market announcement (UMM).

Elering also points out that external intervention is excluded for this cable failure. The total length of the second link is around 170 km, some 14 km of which is overhead line in Finland, about 145 km of submarine cable laid on the bottom of the Gulf of Finland, and about 11 km of underground cable in Estonia.

As the cable was installed in the shore ledge using the directional drilling method, Kebja noted: “The geological conditions there are difficult, and when planning the repair work, we have to find a solution to avoid the resulting effects on the new cable section. In the sea, repair work is complicated by the fact that the ships normally used for laying the cable cannot get close to the shore, where the sea is shallow.”

Additionally, Elering elaborates that there are signs of mechanical damage in the transition joint of the sea and land cable located approximately 200 meters from the coastline. The final technical solution of both the replaceable cable and the transition sleeve will be revealed during the design work that has begun. Kebja highlights that Elering has enough marine cable in reserve and sets for making cable couplings.

“Security of electricity supply is guaranteed in Estonia despite the failure of EstLink 2. In the assessments of security of supply, Elering also takes into account scenarios where two elements important for the operation of the electricity system in the Baltic countries are out of order at the same time, for example, in addition to EstLink 2, the Lithuanian-Swedish connection is also out of order. Even in this case, it is possible to keep the system running,” said Elering.

There are currently two electricity interconnections operating on direct current between Estonia and Finland, which are EstLink 1, completed in 2006 with a capacity of 350 MW, and EstLink 2 with a capacity of 650 MW. However, a third interconnection is also in the works.

At the start of February 2023, the TSO disclosed that it had applied to the Consumer Protection and Technical Regulatory Authority to be granted a superficies license for constructing the submarine EstLink 3 cable, with the onshore connection point situated in northwestern Estonia.

This third electricity interconnection with Finland is expected to further contribute to the security of supply and the achievement of climate and energy policy goals, binding the Baltic and Nordic countries into an even more cohesive market area.

Kalle Kilk, Chairman of the Management Board of Elering, commented: “We have analyzed various potential locations for EstLink 3 and we now consider northwestern Estonia to be the most promising area. For security reasons, it will be appropriate to build the new interconnection as far as possible from the existing ones.

“In the long term, this route option will allow connecting the development areas of offshore wind farms that could potentially be built in the area of western Estonian major islands with the resilient power grid. When comparing the alternatives, northwestern Estonia shows the best assurance for the future.”

The planned EstLink 3 in Estonia consists of a direct current submarine cable, overland cable, a converter station with a potential location around Aulepa, and a new 330-kilovolt overhead transmission line on land, which will connect EstLink 3 to the existing 330-kilovolt grid in western Estonia.

Moreover, the European Commission has added the EstLink 3 project to the list of projects of common European interest, which creates the prerequisites for getting the support of the European Union for the financing of the new interconnection.

With a planned capacity of 700 MW, the investment decision on constructing EstLink 3 could be made in the second half of the decade after the planning procedure has been completed. The interconnection could be completed in the middle of the next decade.