Maintenance of frequency

The frequency of the power system describes the balance between electricity production and consumption. The better the balance, the smaller the frequency variation in the grid and the better the electricity quality. The Nordic electricity grid is synchronously interconnected, and thus the entire grid has the same frequency. In the inter-Nordic grid, the frequency is allowed to vary between 49.9 and 50.1 Hz.

The grid frequency

If the grid frequency is below the nominal value 50 Hz, consumption is greater than production. Similarly, production is greater than consumption if the grid frequency is above the nominal value.

The power balance is maintained by means of frequency controlled reserves and manual regulations carried out in the balancing power market. Maintenance of frequency requires a sufficient volume of so-called spinning reserve in the power system. The frequency controlled normal operation reserve and frequency controlled disturbance reserve are active power reserves which are activated automatically by frequency changes. If it is not possible to keep the frequency within the permitted limits using the frequency controlled reserves alone, manual up-regulation or down-regulation must be carried out in the balancing power market.

The maintenance obligations concerning the frequency controlled reserves have been agreed through an operation agreement signed between the Nordic transmission system operators. Within the inter-Nordic power system, it has been agreed that the Nordel countries maintain continuously a total frequency controlled normal operation reserve of 600 MW for frequency control in a normal state. The joint reserve is shared annually between the countries involved in the inter-Nordic system in proportion to the annual energies used by them.

The volume of the frequency controlled disturbance reserve maintained in the inter-Nordic power system is such that the power system can withstand for instance the disconnection of a large production unit from the grid without it causing a permanent frequency deviation greater than 0.5 Hz. The reserve required by the entire system is defined weekly to correspond to the volume of production disconnected in conjunction with the largest individual fault in the system, deducted by the natural regulation capacity of the system. During a normal operational situation, the interconnected Nordic system is required to have approx. 1,000 MW of frequency controlled disturbance reserves, of which Finland's obligation is approx. 240 MW.

Procurement of reserves

Each Nordic transmission system operator procures its share of the reserves as it deems appropriate. Fingrid procures some of the relevant obligation from the annual market, from power plants located in Finland. In addition to the annual market, Fingrid procures reserves from the direct current transmission links from Russia and Estonia as well as through daily purchases from the hourly market in Finland and the other Nordic countries. Disconnectable loads can also serve as frequency controlled disturbance reserve.

Resource owners participate in reserve maintenance on a fully voluntary basis. A resource owner can use his power plant capacity to participate in the maintenance of frequency controlled reserves either through the annual and/or hourly market. Fingrid arranges open competitive bidding concerning the participation of power plant capacity in the annual market. The competitive bidding is arranged once a year, and it is not possible to start participating in the maintenance of reserves in the annual market in the middle of the agreement period.

The hourly market is used for supplementary procurement once a day whenever necessary. A party can participate in the hourly market by signing a separate agreement. This does not require the signing of the annual agreement. The technical requirements are the same as in the annual market.

Against a compensation paid by Fingrid, the resource owners maintain the measured regulation properties at their power plants at the agreed terms. As far as the agreements concerning the maintenance of frequency controlled normal operation reserve and frequency controlled disturbance reserve are concerned, the terms, contents and compensations specified in the agreements are identical for all parties.

The disconnectable loads serving as frequency controlled and fast disturbance reserve have been agreed upon with companies in the wood processing, chemical, and metal industries. The agreements will be valid from 2005 to 2010.

Maintenance of reserves in Finland

  • Fingrid's reserves in 2011
Reserve Available capacity Need
Frequency controlled
normal operation reserve
(50,1-49,9 Hz)

- Annually contracted, power plants
  73 MW
- Hourly market, power plants 
  58 MW 
- Vyborg DC-link 100 MW
- Estlink 50 MW

 ~140 MW *)
Frequency controlled
disturbance reserve
(49,9-49,5 Hz)		 - Annually contracted, power plants 
  347 MW
- Hourly market, power plants 
  269 MW
- Disconnectable loads 40 MW		 ~260 MW **)
Fast disturbance reserve
(manually activated)		 - Fingrid's own gas turbines 615 MW 
- Contracted capacity, gas turbines 213 MW
- Disconnectable loads 405 MW		 ~880 MW ***)


*) In the Nordic system total 600 MW, which is divided between the Nordic countries annually in proportion to the annual consumption.
**) The volume corresponding the largest dimensioning fault in the Nordic system is divided between the Nordic countries weekly in proportion to the dimensioning faults.
***) Volume corresponding to a dimensioning fault.

As far as the agreements concerning the maintenance of frequency controlled normal operation reserve and frequency controlled disturbance reserve are concerned, the terms, contents and compensations specified in the agreements are identical for all parties.

The disconnectable loads serving as frequency control and fast disturbance reserve have been agreed upon with companies in the wood processing, chemical, and metal industries. The agreements provide for a total power of around 1000 megawatts and will be in effect from 2005 to 2015.