How our electricity flows - TenneT Link

Who is TenneT? TenneT is a leading European transmission system operator. We plan, build, maintain and operate the high-voltage and extrahigh-voltage grid in the Netherlands and in large parts of Germany, enabling the European energy market - safely, efficiently and with a minimum of environmental impact. Come along on the journey of electricity.

1 Wind farm of the wind farm operator

Offshore wind farm

More than 100 kilometres off the coast, the powerful rotors of the wind turbines rotate. They are firmly anchored in the ground and produce large amounts of renewable energy. They are often invisible from land because of the great distance.

Why offshore?

At sea, wind speeds are high and also constant, which is why offshore wind farms can produce more than twice as much electricity as corresponding plants on land. But how does the electricity gets into the grid? This is exactly where TenneT comes into play.

As a green TSO and the reliable expert in the North Sea, we are in a strong position to help Europe achieve its climate targets. That is our obligation. That is our expertise. That is the responsibility we take.

North Sea Wind Power Hub (NSWPH)

The NSWPH-consortium supports the goals of the Paris Climate Agreement and the associated greenhouse gas reduction commitments of the EU and North Sea countries.

Cooperation and integration is the way forward!

The international consortium, consisting of TenneT, Energinet and Gasunie is evaluating and developing technical concepts and solutions for providing the large-scale capacity needed to generate energy from renewable sources while minimising environmental impact and costs.

Innovative projects by combining our offshore grid development know-how with an unique transnational approach

In the future (after 2030), following the designation of large wind regions at a great distance from the coast, several wind energy distribution hubs can be developed to serve as central platforms to support the infrastructure needed to transport the energy, e.g. for power-to-gas conversion (especially 'green' hydrogen), instead of the currently used offshore converter platforms.

Europe's energy transition requires a:

consistent
integrated,
and solid political and legal framework.

Active climate protection

Using renewable energies from offshore wind farms is applied environmental protection. Renewable electricity production does not produce any harmful CO2, as is the case with the combustion of fossil raw materials such as coal, oil and gas.

As sustainable as possible

Sustainable materials are developed for offshore structures in order to make the construction work as environmentally friendly as possible. TenneT complies with all legal requirements for the protection of the marine environment and marine life on the high seas.

Wind and weather

Both offshore wind farm operators and TenneT are dependent on wind and weather during construction in the North Sea which places high demands on people and equipment. For example, work can often only be carried out up to a wave height of 1.5 metres.

2 TenneT converter platform at sea

This is where TenneT's business area begins

Converter platform on the high seas

Similar to a multiple socket, several wind farms are connected to the platform. The conversion from alternating to direct current takes place on the converter platform.

Direct current for long distances

Smaller wind farms located near the coast are directly connected to the grid on the mainland (onshore). In contrast, TenneT connects more remote wind farms with high output using direct current transmission technology which allows large amounts of wind power to be transmitted onshore with low losses.

2GW program: a new dimension

TenneT is planning at least six offshore grid connections with a transmission capacity of two gigawatts each – three in Germany and three in the Netherlands by 2030. The added six gigawatts in each country correspond to over 50% of the Dutch offshore expansion target and about one third of the German target for 2030.

Due to the innovative design and bringing the harmonisation of our offshore grid connection systems to a new level, the following can be achieved:

higher transmission capacity
less consumption of resource
reduced environmental and natural impact

Before, up to three offshore grid connection systems were required for the transmission of 2 gigawatts. Now, one strong connector has the capacity to transmit this power.

Improved marine biodiversity - fish hotels

Shelter for small and juvenile fish, allowing them to forage for food – resulting in more adult fish.

Artificial reefs

Depending on the shape, benthic reef species, lobster crab, but also fish as cod and possibly shark species, benefit.

Eco-friendly scour protection

Part of the installation of the scour protection layer. Depending on the shape, benthic reef species, lobster, crab, and reef associated species, benefit.

3 TenneT submarine cable
from sea to land

Subsea cable

Between 1.5 and 3 metres below the seabed, two direct current cables (positive and negative pole) run on one route towards the mainland. They are laid in one piece or in a few sections with the use of special technical equipment such as flushing sledges and a cable-laying ship.

Three-phase current (AC cable) for short distances

Three-phase current refers to a three-phase alternating current for large electrical outputs. Three-phase current technology is used when connecting offshore wind farms close to shore with lower transmission capacities, as it is the most efficient way of transmitting wind energy over short distances, both, technically and economically.

Direct current (DC) cables for long distances

High-voltage direct current transmission is used for offshore wind farms located far from the coast. Its advantage: compared to three-phase current, significantly larger amounts of electricity can be transmitted with only one cable pair per system, and losses are also significantly lower.

Submarine cable fact check

Service life:	30 years
Direct current submarine cable diameter:	between 10 and 13,5 cm
Three-phase submarine cable diameter:	between 18,5 and 20,5 cm
Direct current submarine cable weight:	between 30 and 45 kg/m
Three-phase submarine cable weight:	between 53 and 76 kg/m

Land cable

When reaching the mainland, the sea/ Wadden Sea cable is replaced by a land cable. Instead of copper, they contain an aluminium conductor for easier transport. On land, they are laid in individual pieces in sections of about 750 to 1,000 metres and then connected with joints.

Land cable fact check

Service life:	40 years
Direct current landcable diameter:	between 10 and 12,5 cm
Three-phase land cable diameter:	between 8 and 9 cm
Direct current landcable weight:	between 11 and 15 kg/m
Three-phase landcable weight:	between 7 and 8 kg/m

Cable laying at sea

Different techniques are used depending on water depth and soil conditions. On the high seas, for example, the flushing sledge is used, in coastal areas the flushing sword, and in tidal flats the soil-friendly vibrating sword is used to lay cables.

Vibrating sword

Cable laying on land

There is a suitable laying method for each area of the cable route. On land, cables are laid using the open method, while crossings, dykes and protective dunes are underpassed using horizontal drilling.

Route investigation

Before a cable is laid - whether on land or at sea - the routes are examined for contaminated sites such as explosive ordnance or archaeologically interesting finds.

On the mudflats: vibrating sword

In the tidal flats, for example in the Wadden Sea nature reserve, the even gentler vibrating sword is used. Here, the cables are gently jogged into the ground.

4 TenneT converter station on land

Converter station on land

The electricity is transported to the onshore converter station via underground cable. In contrast to the station at sea, the direct current is converted back into three-phase current and transformed to the appropriate voltage. In this way, it can finally be fed into TenneT's 380 kV grid.

Noise protection

There are specified guideline values for noise immissions of converter stations and the associated transformer stations, which TenneT naturally complies with. Noise in the transformer station is primarily caused by the transformer. Thanks to noise protection bonnets or noise protection walls around the transformer and modern construction technology, there is significantly less noise than in the past. For comparison: In the vicinity of residential areas, transformer stations must not be louder than a normal conversation.

Immission guide values in decibels

	During the day (6:00 - 22:00)	at night (22:00 - 6:00)
Industrial area:	70	70
Commercial area:	65	50
Core, village and mixed areas:	60	45
General residential areas:	55	40
Pure residential areas:	50	35
Spa areas, hospitals and nursing homes:	45	35

For comparison:

30 decibels correspond to the ticking of a quiet clock or fine rain
40 decibels is roughly equivalent to a nearby whisper or a quiet residential street
50 decibels correspond to normal conversational speech
60 decibels correspond to office noise

(Source: Ministry for the Environment)

Sustainable landscape planning

The integration of a converter station or transformer station into the landscape is an explicit goal of TenneT´s landscape planning. An accompanying plan documents the impact on the landscape which is kept as minimal as possible and determines which compensation and replacement areas must be created for the area to be built on. This means specifically that if a hedge has to be cleared, for example, it will be replanted elsewhere.

Green spaces are also created on the transformer station site

Only a minimum part of the total area of a transformer station is sealed by service roads, foundations or the service building.

5 TenneT substation

Transformer station at the converter station

Starting from the transformer stations, the electricity is transmitted several hundred kilometres further to the centres of consumption via extra-high voltage lines. This is why the 380 and 220 kV lines are also called the "transmission grid". With its transformers and switchgear, the transformer station converts the electricity to lower voltage levels (110 kilovolts) from where it is distributed.

Plenty of space for voltage

A transformer station often appears very large and confusing. But the large distance between the individual elements has a purpose: the air around the equipment isolates the live parts. All live parts are therefore mounted far above the ground and stand on scaffolding - also in order to avoid mutual interference (e.g. by induction).

Electric and magnetic fields

TenneT tests the electromagnetic environmental compatibility of each transformer station. TenneT is well below the legal limits. Even at maximum capacity, only about 50 percent of the limits are reached.

More and more environmentally friendly insulating gases

The pioneering insulating gas mentioned here as an example is purified air and consists of 79.5% nitrogen and 20.5% oxygen. It is therefore cleaner than air found in the environment and is used in medium- and high-voltage equipment to prevent voltage flashovers. Up to now, the gas SF6 (sulphur hexafluoride) has been used as a switching and insulating gas. In future, this will be increasingly replaced by environmentally friendly alternative gases at TenneT. This is an important step towards CO2 neutrality.

Protection for the dormouse

The dormouse is an endangered species. In order to prevent it from being affected by construction work, the dormice are therefore relocated before work begins. For this purpose, a sufficient number of dormouse boxes and dormousetubes will be hung up in the vicinity. Additionally, woody plants will also be planted to maintain the habitat for the dormouse.

Flower strips and green spaces for skylark & co.

The skylark avoids electricity pylons and areas covered by overhead lines. To create new habitat for them, flowering strips and extensively used green spaces are planted outside the power line area. Flowering strips and green spaces are also well suited as habitats and shelters for partridges, brown hares and field hamsters. Soil and water bodies are protected because particles and excess nutrients are retained. The areas are also kept free of woody plants and no pesticides or fertilisers are used.

6 TenneT high-voltage grid overhead line

Up to now, electricity has been generated mainly from nuclear energy or fossil fuels, especially close to the centres of consumption. In the future, these centres of consumption are to be supplied with renewable energy. That is the energy transition! To achieve this, the wind energy generated in the north must be transported to the centres of consumption in the south and west of Germany. The electricity grid was not designed for this and therefore the energy transition requires grid expansion with new extra-high voltage connections. TenneT operates the entire high-voltage transmission grid in the Netherlands as well as a large part of the high and extra-high voltage grids in Germany.

Overhead lines - a proven technology

In an industrial nation like Germany, overhead lines are the traditionally proven technology of energy transmission and have been part of the landscape for decades. The steel lattice towers with a service life of about 80 years are the basis of a safe, effective and economic transmission network. Legislation also provides for overhead lines for the construction of new extra-high voltage three-phase power lines.

Each new pylon is precisely defined by the authorities in the planning approval procedure. Depending on the ground conditions, the foundation and the installation are planned in detail. The masts are placed at intervals of 400 metres on average and are usually 50-70 m high. Once the mast is in place, conductor cables and lightning protection cables are attached. For this purpose, the conductor cables are pulled over pulleys mounted on the cross beams by means of a cable winch. The conductor cables which are responsible for the transmission of electricity are made of conductive aluminium that winds around a so-called steel cable. The construction time per mast is up to four months.

One exception is the crossing of the Elbe between Schleswig-Holstein and Lower Saxony. To enable navigation on the Elbe, the masts have a height of 227 meters.

Cable entry Elbekreuzung

TenneT-grid in numbers

Voltage levels in the Netherlands	more than 110 kV
Voltage levels in Germany	more than 220 kV
Frequency (according to European standard)	50 Hz
Total length (Across country borders)	24.500 km

Structure of the electricity grid

Our electricity grid is similar to the road network: there are motorways for the long distances, federal and country roads for medium distances and local roads that connect our residential areas. They are connected to each other via intersections and on- and off-ramps. The motorways for the electricity grid are the extra-high voltage lines with a voltage of 220 or 380 kilovolts.

The NOVA principle

In order to minimise the impact of grid expansion measures on people and the environment, TenneT operates according to the NOVA principle: grid optimisation before reinforcement before expansion. This means that before new lines are built or expanded, attempts are first made to optimise existing lines, e.g. through overhead line monitoring. If this is not possible, it is checked whether the network can be strengthened - for example with high-temperature cables. Only if this is not sufficient new lines are built or extended.

Replacement afforestation: forest for forest

Not all forest areas can be bypassed when building the line. If clearing is necessary, the area is replaced by afforestation. In this process, between 8,000 and 10,000 trees per hectare are planted on the areas around the pipeline routes and maintained in the sense of sustainable forestry. Oak, beech, cherry, service tree, sycamore and littleleaf lime are such possible tree species.

Ecological use of a right of way

Once the construction of a new line has been completed, TenneT continues to attach importance to ecological maintenance. Instead of regularly and completely removing the vegetation, the natural balance is strengthened in a targeted manner. For example, through coppice management or stepped forest edges. In combination with young forest these are particularly wildlife-friendly. Created small water bodies or open ground areas and special structures in the route corridor can, for example, provide new habitats for endangered regional species.

Protection for bats and birds

Before the construction of a route begins, potential bat and bird roosting trees are marked. If a potential roosting tree has to be felled, it is thoroughly examined beforehand. In addition, bat boxes and/or nesting aids are hung up as replacement roosts for felled cavity trees. Instead of boxes, crevices or cavities can also be created in old trees.

7 TenneT underground cable

Underground cables - not a panacea

Awareness of the need to preserve the landscape has increased. Underground cables are an obvious alternative to overhead lines at the expense of much higher costs. This technology has been tried and tested for medium and low voltage levels. Underground cables are also proven and safe for high-voltage direct current transmission (HVDC). In the three-phase extra-high voltage range, on the other hand, there is no experience on the operational behaviour of underground cables in the meshed grid and the impact on grid stability. The legislator has defined pilot projects in the course of grid expansion in which subsections should be underground cabled under certain conditions - for example, a nearby residential development along the planned route.

In order to connect the overhead line with the cable, cable transition facilities are required, which can have an area requirement of 50 x 70 to 130 x 150 metres, depending on the requirements.

Underground Cable 360°

Direct current or three-phase underground cable?

With alternating current, the current changes direction 50 times a second from the positive pole to the negative pole, resulting in the well-known frequency of 50 hertz. The meshed electricity grid up to the low voltage in households in Germany is operated with alternating current. This grid has grown historically and most end-use devices also only work with alternating current. When electricity is transmitted in alternating current networks, distance-dependent electricity heat losses occur. With direct current, the current does not change direction. Direct current lines are therefore suitable as low-loss point-to-point connections over long distances. For this reason, the legislator has given priority to underground cabling in direct current technology for the transmission of electricity over long distances. Three-phase and direct current underground lines differ not only in their technology and approval procedure, but also in their different construction methods and route widths.

Three-phase underground cables in numbers:

Trench depth:	approx. 2 m
Cable depth	1,6 m in heat dissipating material
Required width of the construction site:	approx. 50 m
Protective strip:	on average 25 m (free of deep-rooted woody plants)

Preservation of soil functions

During regular operation, the three-phase power line under the ground reaches about 30 degrees Celsius. To allow normal farming above the line, the cables are laid at a depth of 1.60 metres. The cables are surrounded by a heat-dissipating bedding material and individual soil layers are excavated and lifted separately. This prevents soil mixing and supports the restoration of soil functions.

Soil science construction monitoring

Throughout the entire construction process, TenneT´s soil monitoring advises the companies carrying out the construction work and the owners on soil-conserving construction methods, appropriate subsequent management and accompanying renaturation. After only a short time, underground cable routes are virtually invisible again. Only forest aisles, for example, give a hint of the underground cable route.

8 Substation of the distribution system operators

This is where TenneT's business area ends

What does a substation do?

In Germany there are around 900 distribution system operators (DSO) who deliver electricity from the transmission system operator (TSO) to the end consumers. For this purpose, the transformer stations are the central hubs in the network of electricity motorways. This is where lines come together and are interconnected - like a road junction. Energy that arrives at the substation at the maximum voltage of 220 or 380 kilovolts is transformed there to the lower voltage level of 110 kilovolts.

Location criteria

Transformer stations should be built as close as possible to a planned transmission line route. Moorland and floodplains are out of question for construction and safety reasons. Nature conservation must also be taken into account in the selection process. In addition, TenneT always looks for sites with the greatest possible distance to residential areas in order to meet the requirements for the protection of the residential environment as far as possible.

From gravel to greenery

With hedges and trees, TenneT ensures that the transformer substation blends well into the landscape. If business operations permit, gravel areas near the substations are converted into green spaces. The unmanned stations are an excellent place for biodiversity development. A pilot project conducted by the Butterfly Foundation showed excellent results in the increase of insects and other pollinators.

Hardly any impact on the surrounding area

Once the construction work, which takes about two years, is completed, a transformer station has much less impact on the surrounding area than, for example, industrial plants or production halls. There are no particularly loud, unpleasant or frequently changing noises. TenneT also tests the electrical and electromagnetic environmental compatibility of every new transformer station. In the area accessible to the public, the corresponding values are well below the specified limits.

9 Consumers

Who are the end consumers?

The more people produce green electricity themselves, the more the boundaries between electricity producers and consumers blur: they merge into "prosumers". TenneT and some other European transmission system operators are using the "Equigy" data platform to enable owners of electric cars, home storage systems or heat pumps to offer the capacities of their systems flexibly and earn money with it. These flexible capacities help to keep the grid more stable.

From consumer to electricity producer