Electricity transmission from the 330 kV Russian system to the 400 kV Finnish system started in 1981. As the Russian and Finnish power systems are asynchronous, a direct AC connection was not possible and a HVDC-back-to-back station was needed. It was decided to build the substation near Vyborg in the Leningrad region. The first unit of the substation was commissioned in December 1981 followed by the second unit in the autumn 1982 and the third unit in the summer 1984. The system was manufactured by the Ministry for Electrotechnical Industry of the USSR. A fourth unit was added in 2001 and the control and protection equipment of the three original poles has subsequently been modernised.
Technical description
The Vyborg substation is located east of Vyborg at. It is connected with the Russian and Finnish high-voltage power systems. The substation is connected to the Russian power system with two 330 kV lines to Vostochnaya substation and with one line to Kamenogorskaya substation. It is connected with Finland with three 400 kV cross-border connections to Yllikkälä and Kymi substations. The substation consist of four independent, parallel symmetrical monopole back-to-back units, each rated 355 MVA and operating with a voltage of ± 85 kV. In contrast to most other HVDC plants, its converters do not allow bidirectional energy to transfer, but only from Russia to the power grid of Finland. Each pole consists of a single Twelve-pulse bridge at each end and was built using thyristors from the outset. The first three poles were originally built using small diameter thyristors with three in parallel at each level, but these were later replaced by larger 80mm thyristors which did not require parallel connection. The converter transformers are of the relatively unusual single-phase, four-winding arrangement, with a 38.5 kV filter winding on which the tuned filters are connected. Other high-pass filters are connected directly to the 330 kV or 400 kV AC busbars. With a total transmission rating of 1000 MW, Vyborg was the world's largest HVDC-back-to-back facility until the 1800 MW Al-Fadhili facility was completed in Saudi Arabia in 2009. It is also the only back-to-back HVDC system in Russia and the only fully operational HVDC system in Russia, the others having either been shut down, never completed or are operated only at reduced capacity.
