Application of linear analysis for stability improvements in the Nordic power transmission system

This paper presents research work on stability assessment and stabilizer design that has been carried out for the Finnish transmission system operator, Fingrid. The aim of the work has been to assess observability and controllability of low frequency inter-area modes in the Finnish transmission grid, and to design stabilizing controls to improve damping of critical modes. Challenges in system modeling are described, and main results from linear analyses are presented and compared with nonlinear dynamic simulations. The results are seen to correspond well with real system observations. Modal analysis and nonlinear simulations are applied to verify design of power system stabilizers of some important generators in Finland. Similar methods are also used for retuning and verification of stabilizers on the Fennoskan HVDC link between Sweden and Finland. The stabilizer performance is demonstrated in terms of stability improvements. A main conclusion is that linear techniques can be of great benefit when designing damping controllers for large power systems. It is also illustrated, with the use of validated models, how linear tools can be used in operation planning for determination of power transfer limits.