2/2012 - Fingrid

Relative price / puto technical and economic reasons. Thehighest alternating voltage nowadaysused in overhead transmission lines forexample in Russia, Japan and China is1,000 to 1,200 kV (kilovolt, kV = 1,000volts). Cable networks have to make dowith a voltage which is about half ofthese, but the transmission connectionsemploying the highest voltages are usuallyso long that the use of cables is noteven possible in them, let alone economicallyviable.Direct current (DC) is used in verylong overhead transmission lines and insubmarine cable connections. DC connectionsare not as numerous as ACconnections, and standardisation doesnot give DC connections similar savingsas with alternating current. Thisis why the DC voltages are determinedby project-specific requirements andtechnological developments. The highesttransmission voltage in DC overheadlines is at present 800 kV, and approx.550 kV in DC cable connections.As the volume of electricity consumptionvaries, so does the magnitudeof voltage. In order to prevent excessivevoltage fluctuation, voltage mustbe controlled at the different pointsof the network. Voltage can be controlledin an interconnected network bymeans of synchronous machines andat transformer substations by changingthe number of windings on transformercoils (on-load and off-load tapchangers). In other parts of the network,voltage control employs control providedby compensation devices, in otherwords capacitors and reactors (“coils”),previously also by synchronous compensators(no-load synchronous machines).A capacitor raises while a reactordecreases the voltage.A properly connected combinationof reactors and capacitors can alsocater for matters affecting the qualityof electricity. Instead of conventionalcompensation devices controlled by acircuit breaker, there are now also compensationdevices operated by means ofpower semiconductors. These devicescan be controlled almost continuously.In addition to compensation, they canalso be used for changing the dynamicproperties of the network, such as addingattenuation after transients. Rated voltage of network (kV)14001200100080060040020010.750.50.250S=50 km525380220 28715 50 1101880 1900 1920 1940 1960 1980 2000 2020YearS=100 kmEconomically most suitabletransmission voltageS=200 kmS=400 kmS=600 km0 200 400 600 800 1000 1200 1400 1600Voltage of network U R/ kVFigure 1: Dependence of theprice of a transmission line ofa certain length on the voltage(solid line), and the optimumtransmission voltage(broken line) at differenttransmission distances in acertain situation. The priceis expressed as the specificcost, i.e. as a price scaled withrespect to the transmittedpower. The end result isshown as a relative value.Figure 2: Development of transmission voltage. In some countriesthe voltage of 380 kV has replaced 400 kV, and 300 kV or 365 kV areused instead of 287 kV.7351200From 7.8 to 400 kilovolts in 60 yearsThe first three-phase electricity transmission line of 8 kilometres and7.8 kilovolts (kV) in Finland was built in Ladoga Karelia in 1897 tofeed electricity for pumps, crushers, cableways and other equipmentused in the mining industry. The first power transmission system (15 kV and33 km) planned completely by Finns was introduced in 1900 between Säkkijärviand Vyborg in Karelia. The first 110 kV line in Finland was commissionedin 1929, transmitting the electricity generated at the Imatra rapidsin Eastern Finland via Southern Finland further to Turku on the west coast.Finland shifted to the 220 kV voltage level in the early 1950s. Swedenwas the first country in the world to introduce the 400 kV voltage in theearly 1950s, when power plants on the Luleå river were connected to theSwedish grid. Before that, the highest voltage anywhere in the world hadbeen 287 kV. Finland was also among the first countries to adopt 400 kVtransmission in the early 1960s.At the end of the 1970s, Sweden even made preparations for the useof a transmission voltage of 800 kV. However, a reduction in the growthof electricity consumption and intensified debate about the environmentaldrawbacks and potential health impacts of 800 kV transmission linesresulted in that the Swedish Government did not give a permit in 1980 tothe 800 kV line which had already been constructed between the Forsmarknuclear power plant and the Stockholm region.FINGRID 2/2012 | 33