Cyclades Interconnection Phase D : Santorini - Folegandros - Milos - Serifos

In February 2025, IPTO reported that the submarine high-voltage cable for the Folegandros–Thira (Santorini) interconnection had been laid as part of Cyclades Interconnection Phase D, completing the offshore cable installation between the two islands. This work was executed alongside the Milos–Folegandros link to integrate the southwestern Cyclades into the High Voltage System.

In September 2023, IPTO signed contracts for the 150/20 kV GIS high-voltage substations on Folegandros, Milos and Serifos, completing the main substation procurement for Phase D of the Cyclades interconnection and placing the entire project into the construction phase. Earlier tender results had identified TERNA as the provisional contractor for the Folegandros and Milos substations and a Nari–Electromec partnership as the provisional contractor for the Serifos substation.

In September 2023, contracts were signed for the High Voltage Substations on Folegandros, Milos and Serifos, which placed the entire Cyclades Interconnection Phase D – including the Santorini–Folegandros–Milos–Serifos links – into the construction phase and signalled the start of onshore substation works on these islands.

In February 2023, IPTO signed cable supply and installation contracts for the remaining three islands of the southwest Cyclades—Folegandros, Milos and Serifos—under Phase D of the Cyclades interconnection. Hellenic Cables was contracted for the Lavrio–Serifos and Serifos–Milos sections with a total length of about 170 km and a contract price of EUR 195 million, while Prysmian Group was contracted for the Milos–Folegandros and Folegandros–Santorini sections totalling about 127 km and priced at approximately EUR 150 million, covering turnkey design, manufacture, laying, protection and commissioning of 150 kV three-core XLPE submarine and land cables to integrate these islands into the High Voltage System.

In 2023, the European Investment Bank (EIB) and Greece’s Independent Power Transmission Operator (IPTO) signed a long-term €108 million loan agreement using Recovery and Resilience Facility (RRF) funds to co-finance construction of the western and southern Cyclades power interconnector linking Santorini (Thira), Folegandros, Milos and Serifos to mainland Greece, topping up an earlier €157 million EIB own-resources loan for the project.

IPTO announced that the project to interconnect Folegandros, Milos and Serifos with Santorini and Lavrio—forming the southwestern part of Cyclades Phase D, including the Milos–Folegandros and Folegandros–Santorini 150 kV AC submarine links—has been included for co‑financing of EUR 103.5 million from the Recovery and Resilience Fund. This EU‑backed grant support contributes to the project’s total budget of EUR 345 million and helps fund the design, supply and installation of the underground and subsea high‑voltage cables needed to integrate these islands into the mainland Electricity Transmission System.

The European Investment Bank signed an own-resources loan of about EUR 157 million with Greece’s Independent Power Transmission Operator (IPTO) on 16 December 2022 to co-finance Cyclades Interconnection Phase 4, which links Thira (Santorini), Folegandros, Milos and Serifos to the mainland grid. This development bank financing forms part of the EUR 524 million project cost and supports construction of approximately 350 km of underground and submarine cables to connect the Western and Southern Cyclades to the Greek transmission network.

In August 2022, the Cyclades Phase D interconnection project linking Santorini, Folegandros, Milos and Serifos to the Hellenic transmission system was officially designated as a project of general importance for the Greek economy. This designation was made by ministerial decision and published in Government Gazette No. 494/4.8.2022, highlighting the national economic significance of the interconnection, which is co‑financed under the Recovery and Resilience Facility 'Greece 2.0'.

ADMIE/IPTO’s June 2021 corporate presentation states that Phase IV of the Cyclades interconnection (interconnection of Santorini, Milos, Folegandros and Serifos with Naxos) has been included in the National Recovery and Resilience Plan and will receive EU Recovery and Resilience Facility funding alongside national resources. This establishes that construction is to be financed from state and EU funds, with IPTO financing the operation, i.e. the financing model for Cyclades Phase D is set out.

The Greek Ministry of Environment and Energy approved the environmental terms for Phase D of the Cyclades electrical interconnection, which links the Lavrio GIS substation on the mainland to the new Naxos GIS via the southern Cyclades islands of Serifos, Milos, Folegandros and Thira (Santorini). The approval covers the 150 kV AC submarine and underground cable system—comprising roughly 350+ km of subsea cables and about 20 km of onshore underground cables—as well as the construction of new GIS substations on Serifos, Milos, Folegandros and Thira, enabling these islands to be connected to the mainland grid within the broader Cyclades interconnection programme.