SwePol

The SwePol HVDC project is a high-voltage direct current submarine cable link connecting the power grids of Sweden and Poland across the Baltic Sea. Commissioned in 2000, it is the seventh HVDC cable link between continental Europe and the Nordic region, and the first to utilize a metallic return system.

Technical Specifications

The SwePol link has a total length of 254.05 kilometers, with 239.28 kilometers of submarine cable. It runs from the Stärnö peninsula near Karlshamn, Sweden, to Bruskowo Wielkie, near Słupsk, Poland. The link operates as a monopolar HVDC system with a power rating of 600 MW at a voltage of 450 kV DC. It has a 20% overload capacity at low ambient temperatures and a 10% overload capacity at any ambient temperature when redundant cooling is available.

The high-voltage cable has an outside diameter of 140 mm, with a central conductor made of copper segments taking up 53 mm. This segmented design improves flexibility while maintaining an effective copper cross-section exceeding 99%. The cable is constructed with multiple layers of insulation, sealant, and double crosswind armouring to withstand mechanical stresses during installation.

Cable Layout and Installation

The submarine portion of the cable was manufactured in four sections and joined on board the laying barge. It was buried approximately one meter beneath the seabed to protect against damage from anchoring or trawling. The high-voltage cable was laid separately from the two return cables, with a separation of 5 to 40 meters depending on water depth.

On the Swedish side, 2.22 kilometers of underground cable connect the Stärnö HVDC Station to the Baltic Sea shore. In Poland, the submarine cable comes ashore near Ustka and runs underground for 12.55 kilometers to the Bruskowo Wielkie HVDC Static Inverter Plant.

Metallic Return System

Unlike typical monopolar HVDC schemes, SwePol employs a metallic return system. This consists of two cables with 630 square millimeter cross-sections for the submarine portion and a single cable with a 1,100 square millimeter cross-section for the land portions. This design eliminates the need for electrode stations to transmit the return current, setting a new environmental standard for HVDC transmission.

Converter Stations

The SwePol link utilizes ABB's HVDC rectifier technology. Each converter station is equipped with static inverters switched as 12-pulse thyristor bridges, consisting of 792 thyristors arranged in three 16-meter high towers installed in a valve hall. The stations use air-core inductance smoothing rectifiers of 225 mH, weighing 27.5 tonnes each.

Both stations incorporate filters for the 11th, 13th, 24th, and 36th harmonics. Each filter comprises a coil and a capacitor switched in row, with the 11th and 13th harmonic filters being adjustable. The filters provide 95 Mvar of reactive power, supplemented by an additional 95 Mvar from a capacitor bank.

Control and Operation

The SwePol link is controlled, monitored, and supervised by an advanced MACH control system installed at each station. Telecommunication between stations is facilitated through one dedicated, leased line and a backup dial-up channel.

The link's rapid power balancing capability allows for compensation of frequency and voltage fluctuations in the event of grid disruptions. Technically, it can reverse the entire 600 MW power throughput in just 1.3 seconds, although this feature is not used in practice. A typical emergency power measure could involve ramping up 300 MW within a few seconds to prevent grid failure if the voltage in southern Sweden were to drop below 380 kV.

Environmental Impact

The SwePol link facilitates the import of approximately 1.7 TWh of electricity from Sweden to Poland annually. This is expected to reduce emissions from Polish power plants by 170,000 tonnes of sulphur dioxide and 1.7 million tonnes of carbon dioxide, contributing to Poland's efforts to meet European Community environmental standards.

Upgrade Project

An upgrade project for the SwePol link is scheduled for completion in 2024. This upgrade will include a complete replacement of the HVDC control and protection equipment, ensuring the continued efficient operation of this critical power transmission infrastructure.