Maintaining system security

With regard to the electricity transmission grid, system security refers to the ability of the power system to withstand sudden disturbances or failures in grid components so that the requirements imposed on the operation of the grid are fulfilled. In such a situation, electricity can be transmitted from generating units to electricity users, the thermal or mechanical limits of the grid components are not exceeded, the voltage level of the grid remains within the prescribed limits, and the stability of the power system is retained.

The system security level can be assessed on the basis of the failures occurring in the grid and on the basis of the impacts of these failures. The system security level is influenced by the dimensioning principles applied to the planning and operation of the grid. The system security criteria of the power system are conditions which must be fulfilled in order to attain a certain system security level. Finland and the other Nordic countries use the N-1 criterion, according to which the power system withstands normal individual faults and the disconnection of a faulty component in the 400 and 220 kV meshed network without an interruption in electricity production or consumption and without secondary failures.

More wide-spread consequences are permitted for rarer faults and combinations of several faults. In assessing the system security level, Fingrid also monitors the annual number and duration of outages at the customers’ connection points and the disadvantage caused by outages (KAH value). These must remain at a reasonable level of previous years.

Maintaining a good system security level as the grid evolves and as electricity consumption and production patterns change means that the fulfilment of the system security criteria must be assessed continuously in grid planning and power system operation. System security can be improved by reducing the likelihood of failures or the extent of their consequences.