1. What is a dimensioning fault?
In an electric power system, electricity consumption and production must be balanced at all times. However, there may be a sudden failure of the electrical system, resulting in significantly less electricity, for example.
In this case, the electricity consumption and production must be brought into balance quickly so that the power system can continue to operate smoothly. Therefore, it is necessary to determine the maximum permissible power change due to a single fault and prepare for it in advance.
In practice, a dimensioning fault refers to the power change that a system can withstand without compromising system security if the largest single power plant fails and suddenly disconnects from the grid. The largest permitted stepwise power change in the Finnish power system is 1,300 megawatts.
2. How can Fingrid prepare for dimensioning faults?
Fingrid prepares for dimensioning faults in two ways. Firstly, we purchase reserves, both nationally and from Nordic countries, to balance electricity consumption and production. In practice, reserves are sources that can adjust their power quickly for the need of the power system. For example, hydro power plants, batteries and wind power, as well as electricity consumption, can produce reserves.
Another important thing to prepare for is transmission management. For example, if a large power plant in Southern Finland disconnects from the grid, most of the replacement power for the first few minutes flows from other Nordic countries via AC connections. As Finland is connected to the other Nordic countries by two AC power lines to Northern Sweden, we need to ensure that these lines have sufficient transmission capacity available to supply the replacement power.
We also need to consider power transmission within Finland to compensate for a dimensioning fault, especially between Northern and Southern Finland.
3. Can a dimensioning fault occur in the power system?
The power system could suffer a fault leading to a power change corresponding to the dimensioning fault at any time. In Finland, such a dimensioning fault – i.e., 1,300 megawatts – could only be caused by a sudden failure at Olkiluoto 3. In the future, dimensioning faults could also be caused by the failure of a cable connecting an offshore wind farm to the main grid, for example.
We take care to prepare ourselves for a dimensioning fault at all times, so no nasty surprises happen as a consequence of it. The grid operating situation and reserves are designed in such a way that consumers and our customers do not need to worry about it.
