Report on cross-border disturbance in Northern Finland completed

Thunderstorm caused a cross-border disturbance in Northern Finland on August 10, which led Finland to be isolated from the Nordic frequency area. The lowest measured frequency was 49.31 hertz. Finland was connected manually back to the Nordic system after five minutes.
The Keminmaa-Pikkarala powerline was out of service due to investment and maintenance work. Finland was consequently connected with only one 400 kV line (Petäjäskoski-Letsi) to the Nordic grid. Tornio area industrial load was fed from Svartby Sweden due to outage. Thunder caused tripping of the Petäjäskoski-Letsi line at 17:50, which led Finland to disconnect from the Nordic grid to its own frequency area. In isolation the lowest measured frequency was 49.31 hertz. The frequency was recovered within three minutes to the level prior to the disturbance. The isolated network was connected manually back to the Nordic grid after five minutes. The network operation in isolation was successful. The frequency controlled reserves were able to keep frequency on an acceptable level. The disturbance didn't cause any remarkable supply disturbances or trips of power plants. Fingrid's customers have reported some relay protection actions, probably due to the frequency dip. The previous time that Finland was operating in an isolated network with its own frequency was on 27 December 1983, due to a blackout in Sweden originating from Hamra substation. For more information: Timo Kaukonen +358 9 030 395 4228