Maintenance of operational reliability
Maintaining the operational reliability of the power system means that the power system is dimensioned and operated so that the impacts of disturbance situations are minimised, that there are provisions for restoring the power system to the normal state, and that the disturbance incidents are cleared as quickly as possible.
Grid dimensioning
The power system in Finland has been dimensioned in accordance with principles agreed upon jointly between the Nordic countries. The main dimensioning principle is that the power system has to withstand any single fault at all times in such a way that the influence area of the fault does not expand and lead, in a worst case scenario, to a major disturbance in Finland. Such faults include a permanent disconnection of any generator, transmission line or power transformer. The worst possible fault in the power system is referred to as a dimensioning fault. A dimensioning fault varies on the basis of the operational situation of the grid, but is often the tripping of the largest production unit or an extensive busbar fault.
Transmission limits
Electricity transmissions are kept within the limits of the prevailing operational situation by means of the control of electricity transmissions in the main grid. These limits are determined by operational reliability calculations, which take into account potential faults in the power system. The transmission limits vary in different operational situations, and issues such as planned service outages in the grid may have a great impact on the transmission limits.
The transmission limits are calculated individually for each case. The principle is that a dimensioning fault must not lead to loss of synchronised operation, voltage collapse, disconnection of loads, too large voltage or frequency deviations, overloading of grid segments, or self-sustained electromechanical fluctuations.
A point which limits electricity transmission in the grid is called a bottleneck or congestion. Short-term bottleneck problems in the main grid are managed commercially through counter trade, and long-term bottlenecks are managed by applying price areas or by strengthening the grid.
The maintenance and modifications of the various parts of the grid require planned outages, during which parts of the grid have to be temporarily taken out of operation. Careful planning and timing of the outages guarantees as disturbance-free and commercially purposeful operation of the grid as possible, which benefits all parties of electricity trade.
Disturbance management
The purpose of disturbance management is to prevent disturbance situations in the power system from expanding and to restore the normal state of the power system as quickly as possible. For disturbance management purposes, both power and transmission reserves are maintained in the power system. Fingrid is responsible for the maintenance of reserves that are needed in the Finnish power system. For this, Fingrid uses its own resources and also purchases reserve maintenance from other resource owners. Restoration of the power system from severe disturbance incidents is headed by Fingrid Oyj's Power System Control Centre.
Frequency controlled and fast disturbance reserve, black start reserves
Disturbance reserves are used as a contingency measure for disturbance situations in the power system. Such disturbance situations include disconnection of a generator from the grid (consumption greater than production) and disconnection of load (production greater than consumption).
The frequency controlled disturbance reserve consists of active power reserves which are activated automatically as a result of frequency changes. The frequency controlled disturbance reserve begins to activate when frequency goes below 49.9 Hz, and the full reserve has been activated at a frequency of 49.5 Hz. The frequency controlled disturbance reserve used includes both active power reserves of power plants and disconnectable loads. More information on the frequency controlled disturbance reserve is available under item Maintenance of frequency.
The fast disturbance reserve consists of active and reactive power reserves that can be activated manually within 15 minutes. After activating this reserve, the power system has been restored to such a state that it can withstand another potential disturbance. In the Nordel grid (the inter-Nordic grid), each country must have a volume of fast disturbance reserve that equals the country's dimensioning fault. In Finland, this volume is normally 865 MW.
Machinery with a black start feature can be started to a dead grid without external power supply. This feature enables voltage to be restored to the grid in conjunction with a major disturbance. Some of Fingrid's gas turbine plants have the black start feature, but if there is no voltage available in Finland, the goal is to obtain it primarily from a 400 kV substation in Sweden.
