The Swedish national grid is based on alternating current (AC) technology, being the dominating technology in all stages of electricity supply. 400 kV overhead power lines are most commonly used when we reinforce the grid. This is in order to meet the requirements for a cost-effective, operationally reliable and environmentally adapted electricity transmission system - which is the backbone in a reliable and secure Swedish electricity supply.
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Svenska kraftnät reinforces the national grid on commission from the government
AC power - the dominant transmission technology
A Swedish national grid with AC power
A strong national grid for a reliable electricity supply
Therefore we do not bury AC at high voltage levels
Overhead power lines – the cost-effective alternative
High voltage ground cables are exceptional also abroad
Direct current – a transmission technology for long stretches
Direct current cannot replace alternating current
Svenska kraftnät follows the technological development
Svenska kraftnät is a public business authority, responsible for Sweden's national electricity grid. The network covers 15,000 km lines for 400 kV and 220 kV with stations and foreign links. The operations are managed under the government’s commission to the authority; the main task is to manage, operate, and develop a cost-effective, operationally reliable and environmentally adapted electricity transmission system in a businesslike manner. Based on these criteria Svenska kraftnät develops the network and builds new power lines.
Alternating current technology is presently dominating in all stages of supply. More or less all electricity is produced, transferred and consumed as AC power. Therefore, the Swedish national grid, just like all large electricity systems around the world, is based on alternating current. The Nordic countries are interconnected into one AC network that constitutes the backbone in our electricity system. The common technology is a prerequisite for the power networks to be kept interconnected and to be run as one synchronous system. This allows a common Nordic balance and backup mechanism and provides a more robust system.
A robust and operationally safe grid is a premise for a reliable and secure electricity supply. A robust system is both stable and flexible and can handle disturbances and changes, without leading to serious consequences. Such occurrences in the grid network can be faults in lines and stations or rapid changes in production and use.
The Swedish national gridis almost exclusively built with AC power lines with high voltage. Three fourths are 400 kV and one fourth is 220 kV. When building new grid lines we normally use 400 kV AC. It is the most efficient way to transfer electricity, today being an established international standard. Direct current (DC) is used only in exceptional cases, when lines are built for special purposes or when restricted by circumstances requiring alternative solutions.
A high voltage level is both effective and environmentally friendly. High voltage allows transmission of larger quantities on the line, at the same time making transfer losses relatively lower. Lower voltage requires more power lines in order to achieve the same capacity; four to eight 220 kV-lines are required to replace one 400 kV connection. For that reason Sweden, in line with most countries, uses 400 kV in the national grid.
When Svenska kraftnät builds a new line, it is normally in order to reinforce the national grid, aiming to maintain a reliable and secure electricity supply. Reasons to strengthen the network in a specific area can be to allow connection of new production sources such as wind power, to handle increased internal transfer needs or to replace ageing lines. Changes in the relation between production and consumption can also force measures in the national grid to allow electricity to be transported reliably and securely to the consumers. When we build new lines aiming to strengthen the grid network, we do it with 400 kV AC.
AC lines with voltage as high as 400 kV are not suitable in cable trenches. A major reason is that substantial phase shifts quickly arise between current and voltage, creating so-called reactive effect. This means that large parts of the electricity that is fed in become useless already after short stretches.
In order to correct the phase difference, compensating stations must be built with intervals of 20-40 kilometres. Each such station requires a surface of approximately 120 x 60 metres, depending on compensation requirement. In addition to increased land requirement and the visual impressions made by the stations, one must consider that it still is an unproven technology with large technical complexity and uncertainty.
In addition to the technical restrictions for transfer capacity and compensation needs, ground cables also offer considerably poorer operational reliability. To maintain high operational stability in the national electricity system is an important part of Svenska kraftnät's commission from the government. More components built into the power network create more potential fault sources. Apart from compensation stations ground cables require one splice per 700 metres. Each splice and each station that is built become a potential source for network fault. Thereby, the operational reliability is reduced.
A further important factor for the operational reliability is the repair times when faults arise. Ground cables take considerably longer time to troubleshoot and to repair than overhead lines. A major fault in an overhead line can be repaired within a few hours while a ground cable may take weeks to restore; 730 hours according to European statistics. Compared with overhead lines the cables' higher probability for breakdown, along with the longer repair time, make them a considerably poorer alternative from an operations reliability view.
An additional advantage with overhead power lines is that they have twice as long technical life as ground cables. An overhead line has a length of life of approximately 70 years before it needs to be replaced. A ground cable lasts for about 35 years.
Overhead power lines are a cost-effective alternative. A ground cable is eight to twelve times more expensive to build than an overhead line and has only half as long technical life. It is, however, the restrictions regarding technology and operational reliability rather than the economic aspects, which lead Svenska kraftnät to avoid ground cables in the AC network.
Svenska kraftnät is a public business authority, directed by its government commission, and not a profit maximising company. At the same time it should be emphasised that inherent in the government commission is the obligation to carry out a cost effective activity. In a scenario where ground cables were technically and operationally acceptable, the cost would become an important factor to take into consideration. Increased costs for the national grid imply higher charges that in the end lead to higher prices for the electricity customers.
Svenska kraftnät is sometimes accused of using outdated technology with reference to lines being dug down in other countries. Denmark is often mentioned as an example.
It is certainly correct that national grid lines are built with ground cables in Denmark, however, almost exclusively at lower voltages. This is considerably less complicated than at high voltage levels. There is therefore no comparison between the Danish cables and Sweden's high voltage lines. AC lines with voltage comparable with the Swedish are built as overhead lines also in Denmark. The technology is at present insufficiently developed for ground power cables with high voltage levels to become an acceptable and favourable alternative, other than possibly on short stretches with special requirements. In the entire world only approximately 250 km is ground cable for 400 kV AC power. The longest high voltage cable is a 50 km long 500 kV stretch in Japanese. AC ground cables are primarily found over shorter distances in major city centres. The high effect requirements in large cities combined with the difficulties in passing through urban environment sometimes make trenches the only option for building the power network at all.
AC power technology is dominant within the electricity supply and in the whole world electricity is being produced, transferred and consumed as AC power. In electricity's infancy, before AC power was established, DC power was the dominating technology for electricity transfer.
DC technology has properties allowing transfer from one point to another over long distances. It also has the advantage that it can be located in the ground, without the technical restrictions that AC power has.
DC power is used today on stretches where the objective is to transfer electricity over long distances between two points in a power system, to link together dissimilar systems (for example two AC power systems that are not synchronous with each other) and to make possible transfer in marine cables over longer distances. It makes the technology primarily applicable in links between countries, and in order to connect wind power from far out at sea.
DC technology can also be used to transport large quantities of electricity over long stretches to overcome transfer limitations in an AC network, so-called bottlenecks. An example on this is the SouthWest Link, partially including a DC connection that will transfer electricity from central to southern Sweden in order to compensate for the production shortage in the country's southern parts.
A frequently heard desire is that Svenska kraftnät should use DC technology instead of AC power, just because DC lines may be located in the ground. AC and DC power are, however, two entirely different transfer technologies with different functions that make them applicable for separate purposes.
The Swedish AC network can be complemented with, but not be replaced by, DC connections. When Svenska kraftnät builds a new line, it is usually to strengthen the AC network in order to achieve a reliable and secure electricity supply. If the AC power in these cases were to be replaced with DC, the objective with the line would not be met and the required reinforcement would fail to appear.
When developing the national grid, it must also be co-ordinated with the expansion and the capacity in underlying networks, in order to avoid regional networks from becoming overloaded when a fault occurs on a national grid line. If regional networks are overloaded it may lead to major regional power interruptions with very serious consequences. In a transfer system consisting of AC connections, the lines act as automatic backup for one another. When a line is taken out of service, due to a fault or for repair, the power that passed in the faulty line is automatically and directly led to other AC lines in a predictable way. If sufficient capacity is not available in the remaining national grid lines, however, the electricity is instead passed down into the regional network. This carries a risk for overload and regional power interruptions.
DC connections do not handle this transition in the same direct and automatic manner, instead requiring intervention by the DC link's control system, most often to be initiated manually. That delay increases the risk for the regional network to become overloaded and power interruption to arise. This is one reason that DC power cannot replace AC power lines; only complement them in special cases.
Svenska kraftnät carries out and supports research projects in order to meet important challenges for the national grid. When new plants are built, the authority always endeavours to apply the best possible technology in order to comply with the commission to provide an operationally reliable, environmentally friendly and cost-effective national grid. Svenska kraftnät follows closely the technical development within the area for electricity transfer technology in order to ensure that the applied technology is modern and dependable. For obvious reasons Svenska kraftnät cannot jeopardise the reliability in the electricity supply by making the national grid a test plant for unproven technology. On longer stretches AC cable trenches at the national grid's voltage levels can only become realistic when proven and acceptable technology is available that meets the requirements for operational reliability, cost effectiveness and environmental impact.