The power system in the baltic states was originally designed as part of the former USSR power system and, therefore, the Baltic States system is interconnected to that system. These interconnections are power corridors for bulk-energy exports and imports to the Baltic power system. As there were no interconnectors with other member states of the European Union (EU), the Baltic States have been electrically islanded and solely dependent on the Russian interconnected power system.

Two of the main driving forces for an interconnector between Estonia and Finland were to reduce dependence on the Russian system and generally increase security of supply in the region. An interconnection between the Estonian and Finnish electricity networks was first considered in the early 1990s, so it has been in the development stage for a long time. However, the project started to progress more rapidly after the 2002/2003 winter, when low precipitation levels and an extremely cold winter drove up electricity prices dramatically in the Nordic electricity market.

The interconnector shown in Fig. 1 is an important step toward creating an open and competitive electricity market in the Baltic Sea area. Competitiveness of energy production prices from the Baltic energy producers in Estonia, Latvia and Lithuania, and improved security of the Baltic and Nordic power systems, has prompted the construction of a high-voltage direct current (HVDC) transmission link, known as Estlink, between Estonia and Finland.


With a nominal capacity of 350 MW, Estlink is owned and operated by AS Nordic Energy Link, a company established by the main power utilities of the Baltic countries and Finland. AS Nordic Energy Link is owned by Eesti Energia from Estonia (39.9%), Latvenergo from Latvia (25%), Lietuvos Energija from Lithuania (25%) and Finestlink from Finland (10.1%). This makes Estlink a somewhat exceptional interconnector project, because it is owned and operated by five different companies from four different countries. Normally, interconnectors of this type are developed and owned by two transmission system operators (TSOs).

The operation of Estlink was planned in two phases: a merchant project from 2006-2009 (or at the latest through 2013) and an infrastructure project (earliest 2010). To operate Estlink as a merchant project required an exemption from EU Regulation 1228/2003, “Regulation on conditions for access to the network for cross-border exchange in electricity.” Exemption from the regulation is only granted if the following conditions are fulfilled:

  • Interconnector must enhance competition in electricity supply

  • Investment risk must be so high that without exemption, the interconnector would not be constructed

  • Interconnector must be owned by an independent body, separate from system operators

  • Charges must be levied on users of the interconnector

  • No part of the costs can be recovered from the charges imposed for the use of transmission or distribution networks

  • Exemption must not be detrimental to competition or the effective functioning of the internal market.

AS Nordic Energy Link was able to satisfy the regulators of Estonia and Finland and the EU authorities that all these requirements would be fulfilled, and the EU granted an exemption that is valid through 2013. AS Nordic Energy Link investors have agreed to sell the interconnector to the TSOs of Finland, Estonia, Latvia and Lithuania before 2013.

With Estonia part of the Baltic power system and Finland part of the Nordic power system, the connection of the electricity markets in the Baltic and Nordic countries creates a reference price for the Baltic electricity market, and accelerates power trade between the Baltic and Nordic markets. Currently, the Baltic power system has a surplus of energy, which could be exported via Estlink to Finland. It is estimated that approximately 2 TWh of energy will be delivered to the Nordic electricity market annually.

Total consumption of the Finnish power system is approximately 85 TWh, with a peak load of approximately 15,000 MW. The three Baltic countries together consume approximately 21 TWh per year, with a peak load of approximately 5000 MW. Energy transmitted through the new interconnection can be supplied from renovated oil-shale-fired production units in Estonia or from the nuclear power plant in Lithuania. During the flood season, excess hydropower can be supplied from Latvia. These versatile sources of energy in the Baltic States will increase the security of energy supply on the Finnish system.

The benefits arising from Estlink for the transmission grids of OÜ Põhivõrk in Estonia and Fingrid OYJ in Finland cannot be underestimated, as the link offers a variety of system services such as emergency power control, damping control, voltage or reactive power support, and frequency control. A special feature of the Estonian power system will be a black-start capability, to help restore the Estonian power system in case of total blackout. Long term, if required, Estlink will allow energy to be imported from the Nordic countries.


Estlink is designed as a bidirectional ±150-kV, 350-MW HVDC system, which consists of two voltage-source ac-dc converter (VSC) stations and two 105-km (65-mile) HVDC cross-linked polyethylene (XLPE)-insulated cables, of which 74 km (46 miles) is submarine cable that was buried in the seabed. The 150-kV cable for the land section, 22 km (14 miles) in Finland and 9 km (6 miles) in Estonia, has aluminum conductors (2000 mm2 [3.10 in2]), and the submarine cable section uses copper conductors (1000 mm2 [1.55 in2]). Figure 2 shows the construction of the 150-kV land and submarine cables. Figure 3 shows the installation of the cable on the land section in Finland. ABB used subcontractor Global Marine Systems to lay and bury the submarine cable across the Gulf of Finland using the cable-laying vessel Sovereign. (Fig. 4)

The Estlink project was the subject of a public open tender published at the end of 2004. Because there was considerable competition between conventional and VSC HVDC technologies, in the subsequent feasibility studies and tender evaluation, the following parameters were considered: investment cost, overload capability, electrical losses, circuit availability, maintenance cost and construction time.

As a result of the pretender studies, the VSC technology was selected as the preferred choice. VSC technology is a modular design, which allows for comprehensive factory testing and shorter field-testing and commissioning time. The Estlink investors' confidence in the VSC technology was partially a result of proven similar projects operating in the United States and Australia. ABB was awarded a turnkey contract for its HVDC Light technology, which was installed completely by December 2006.

The Finnish and Estonian power systems operate in different synchronous areas. In Estonia, Estlink is connected to the Harku Substation, which will be linked to Balti Substation by a 330-kV ac overhead transmission line routed through the eastern part of Estonia, where large oil-shale power plants (2000-MW installed capacity) are located. This 235-km (146-mile) overhead transmission line, which other than 19 km (12 miles) will be single-circuit construction, is the subject of a separate Estonia TSO contract to meet the growing demand for energy in the Tallinn area. This new transmission line, as shown in Fig. 5, was commissioned in the autumn of 2006. The Estonian transmission system is currently being refurbished with several projects underway for additional new high-voltage substations and transmission lines.

In Finland, Estlink will be connected to Espoo Substation, which has strong 400-kV ac overhead transmission line connections with the rest of the system. The Finnish power system also has planned future investment for the transmission system to accommodate a new nuclear power plant (1600 MW), scheduled to be in commission by 2010.


Two identical converter stations are connected to the networks in Harku and Espoo via 380-MVA 3-phase power transformers at each substation. The converter station locations were selected for their short-distance location from the two countries' capitals, Tallinn (Estonia) and Helsinki (Finland). The converter equipment is housed in a building to provide protection from the weather, and serves as an electromagnetic and audible noise shield. The ±150-kV XLPE-insulated cable system connecting the converters is an integral part of the HVDC Light technology. Figure 6 shows the start of the construction on the building for the Harku Converter Station.

The design of each converter station is based on a six-valve converter bridge, equipped with semiconductor valves consisting of several series-connected insulated gate bipolar transistor (IGBT) units. Each unit has 24 IGBT and 12 diode chips connected in parallel. A capacitor bank on the dc side of the converter bridge provides a low-inductance path for the turnoff current and energy storage. The midpoint-grounded capacitor is built up of capacitor units of a dry, self-healing metallized-film design. The converter's two-level topology means that, by turning the valve transistors on and off, the ac output voltage is switched between +150 kV and -150 kV. Estlink uses pulse width modulation (PWM) with special functions for harmonic cancellation, a valve switching method called Optimal PWM. Each valve is switched 23 times per 50-Hz cycle, thus the pulse number is 23.

The ac side of the converter bridge is connected to a series reactor, the converter reactor, providing low-pass filtering of the PWM pattern to give the desired fundamental frequency voltage. The power flow between the ac and dc side is defined by the fundamental voltage across the reactor, and by using the phase shift and amplitude of this voltage to vary the PWM pattern, the active and reactive power can be controlled independently. The converter reactor, one for each phase, is a large air-cooled, air-core reactor with a magnetic shield around it to eliminate magnetic fields outside the reactor.

To ensure power quality, the harmonics created by the VSC converter are on the ac side filtered by two ac shunt filters, connected on the 195-kV bus between the converter reactor and the power transformer. These filters are tuned to the 32nd and 60th harmonics, thus short circuiting the major ac harmonics created with Optimal PWM switching at pulse number 23. The Optimal PWM switching method cancels characteristics harmonics lower than the 31st harmonic. On the dc side, the dc capacitor and smoothing reactors, in series with each pole cable, suppress dc-side harmonics.


The control and protection system is the fully computerized MACH 2 system that, since the late 1990s, has been installed in some 400 installations in ac and HVDC substations including, for example, the Cross Sound Cable HVDC Light project in the United States. The control system is duplicated for full redundancy, and each control system consists of pole, converter and valve control units, as well as some auxiliary system controls. It also includes transient fault and event recorders and remote control facilities. The converter protection systems are also integrated in the MACH 2 system, but operate independently from the control systems.

The control system rapidly controls the active and reactive power flows independently of each other. In ac voltage-control mode, the converter supplies reactive power automatically to try to keep the voltage stable during network disturbances. Runbacks and other emergency power schemes, as well as frequency control and power-system damping control, are included features that will automatically vary the active power flow to support the networks during disturbances. The black-start feature will make it possible to start up sections of the Estonian network in minutes after a total blackout.

The control and protection system is designed so that the national control centers for the Estonian and Finnish power systems will be operating Estlink remotely.


The Estlink project, which is the largest in the world to employ HVDC Light technology, is a unique project not only in the area of the Baltic Sea, but also a landmark in the field of electrical power transmission. The project has had a long history and is the first step to connect the Baltic power system with the rest of the European power system, having developed during a period of economic growth in the Baltic countries.

Raine Pajo is currently Eesti Energia's technical director and a member of the management board. Prior to that he was responsible for the grid planning, investment planning and realization of the Estonian transmission grid. Pajo earned his PhD of Technology in electrical power engineering from Tallinn Technical University (Tallinn, Estonia) and is currently chairman of the National Committee of CIGRÉ in Estonia.

Indrek Aarna is CEO of AS Nordic Energy Link and is responsible for managing the construction and preparation for operation of the Estlink submarine cable. Prior to joining AS Nordic Energy Link in 2005, Aarna worked at Eesti Energia AS on business development, electricity market forecasting, analysis and strategy development. He earned a BECE degree from Tallinn Technical University (Tallinn, Estonia) and a PhD from Brown University (Providence, Rhode Island, U.S).

Matti Lahtinen earned his MSEE degree from Helsinki University of Technology. He worked in the laboratory department at Imatran Voima Oy (IVO), where he was promoted to electrical laboratory manager. He joined the IVO power system technology department as a chief specialist in 1990, and moved to IVO Transmission Services Ltd. in 1992 as R&D coordinator. When Fingrid Oyj was established in 1997, Lahtinen was appointed manager of power quality and is currently the leading expert of transmission technology. Lahtinen serves on the CIGRÉ Study Committee C4 (System Performance) and he has participated in standardization work on the International Electrotechnical Commission and CENELEC, the European Committee for Electrotechnical Standardization.