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Secure supply

At TenneT we have a clear and critical task: to keep the lights on for 41 million people across the Netherlands and Germany. This is what we call ‘security of supply’ and it is at the heart of our business.

The heart of our business

It’s a mission that underpins everything we do, as we deliver electricity to users across 22,774 kilometres of high-voltage cables, above and below the ground, across land and sea.

In the age of renewable – and intermittent– energy supply, it’s a task that we must deliver, regardless of varying weather conditions.

As electricity is generated to meet consumer needs, we have to make sure we can fill the gaps in supply or deal with surplus when the weather changes. So, if the wind doesn’t blow in one area, we must divert electricity from another source to meet demand. This can be energy from wind, but also from fossil fuels or sun.


For us, security of supply is not simply a question of building more lines or laying more cables. It is about integrating all types of energy into the system – and making sure that society is served with electricity 24/7.

Although occasional outages will always occur, TenneT is one of the most reliable grid operators in the world with an overall security of supply of 99.9986% in 2017.

Grid availability99.9986%99.9999%99.9975%
Energy not transported (MWh)1,072593,824
Grid availability97.80%92.00%92.82%

Our onshore security of supply fell slightly to 99.9986% last year, compared to 99.9999% in 2016. This was mainly due to a power outage in January, caused by a component failure at the Amsterdam Hemweg substation and amongst others disrupted train travel and hospitals for hours. This outage had also the largest impact on the energy-not-transported figure. The failure illustrates how carefully we need to monitor and maintain our grid.

As a reliable grid operator, we have to weigh up the costs and benefits of spending time and money to detect these potentially damaging flaws. Searching for shortcomings in a fully-operational system could itself cause an outage. Because reliance on electricity is so high, and tolerance of outages so low, we only make non-essential grid adjustments if we are already doing critical work on our assets. In 2017, we took the opportunity of upgrading our grid during critical work to our new 380 kV Vijfhuizen substation in the Netherlands.

With respect to a large power outage in 2015 in Diemen, our regulator judged that we did not meet the legal requirement of proper redundancy during maintenance. Making adjustments for this across our grid would require an investment of approx. EUR 7 billion, which we believe will have minimal impact on our already high security of supply performance.

Offshore – in an even more challenging and unpredictable environment – our grid availability has reached 97.8%, a considerable achievement after 10 years of pioneering work in the North Sea. This is all the more notable considering that, unlike with onshore connections, we do not have a redundant cable that can take over electricity transport in case of a failure.

Next to technical failures also cyber crime is a potential risk to our high level of security of supply. TenneT is very well aware of this risk and is taking all necessary measures to prevent this from happening. We are preparing ourselves for ISO 27001 certification and are in close contact with national authorities to carry out security contingency plans.

In 2017, an important milestone on the Randstad 380 kV high-voltage connection was reached, securing supply in this densely populated and industrially important part of the Netherlands. In the future, the Randstad 380 kV connection will also help us to connect offshore wind farms to the onshore grid. New 380 kV substations, such as the one at Vijfhuizen, will help us convert electricity from 380 kV to 150 kV for regional distribution, monitoring and safeguarding grid stability.

In Germany, 2017 also saw us start operations on the new 380 kV line between Audorf/Süd and Hamburg/Nord with a length of 69 km. An essential artery of the energy transition, the 380 kV connection transports wind power generated in the North Sea to consumers in the south of the country.

Total circuit length (km)22,77422,55422,245
Overhead lines (km)18,97418,83018,893
Underground cabling (km)3,8003,7243,352
Number of substations462458454
Number of HVDC stations151513

* The majority of the underground cabling is in our 110/150 kV grid. For detailed information about our infrastructure per voltage level and country, click here.

Investments in our infrastructure are vital to the flexibility and resilience of our transport network and to manage the increase of renewable energy. Based on the forecasted need of transport capacity we are making long term and short term investment plans to strengthen our grid. These plans can be found online, for the Netherlands and for Germany.

SDG 7 - Affordable and clean energy

The United Nations Sustainable Development Goal 7 – Affordable and clean energy – addresses one of the basic needs of society around the world; access to affordable, reliable, sustainable and new energy sources for all. Without doubt, this is a goal where TenneT plays a major role. As a major European TSO, we embrace the challenge of integrating sustainable and new energy sources without compromising the reliability of supply. In doing so, we take societal costs into account and support the development of lower prices by installing cross-border transport capacity.

Next to investing in our grid, we work on a day-to-day basis to guarantee a reliable electricity supply. We have to make sure our grid is in good shape and therefore carry out regular inspections and maintenance work, which we plan in consultation with the regional grid operators and our connected customers. Operating our system becomes more and more challenging, due to ‘network stabilising emergency’ actions, which are caused by the transmission grid being placed under more supply pressure. The expenses associated with both actions are societal costs and are reimbursed via regulatory tariffs (see the financial section of this report). We see that the total grid expenses to operate our grid are over EUR 2 billion.
For more specific information about our revenue, costs and profit, click here.

Last year, we took the lead in initiating a thorough analysis of grid expansion scenarios beyond 2030 in Germany. It revealed that, with a high degree of likelihood, the current grid projects defined by law are necessary to achieve renewable energy goals in Germany. Beyond 2030, it was concluded that the growing supply of renewable energy and the resulting increase in transportation requirements can be accommodated with a significantly lower further grid expansion when utilising and developing innovative technologies. During the course of 2017 additional stakeholders, from peers to governments, embraced this view. We will continue the dialogue on this innovative way of looking at grid expansion in the coming years, since we feel such a debate is necessary to be able to solve the security of supply issues in the future.


Secure supply
1We have a challenging job as we work in a fast-changing sector. Where we used transport electricity from a fixed number of fossil-fuel plants on land, we are now juggling multiple onshore and offshore energy sources and a complex, cross-border energy market. Some of the consumers are now also producers – ‘pro-sumers’- feeding energy from their solar panels or cars back into the system.We need to make sure we keep the lights on at all times, while facilitating the current and new market players. Staying in close contact with the government and the market ensures we know what changes we can expect. While all these changes have a big impact on the market we work in, our task remains the same.
2In a fast-changing market it can be hard to plan for the long term. Although our onshore assets can last for 40 years, the pace of change makes it difficult to look so far ahead in the future.When we invest, we need to ensure we are not providing society with expensive assets that could soon become obsolete. We work hard to avoid this, by weighing flexibility against cost, as well as taking into account the risk of not building. We have to weigh our decisions ever more carefully.


Looking to the future, we expect to rely increasingly on technological advances and IT for our security of supply. Simply continuing to build assets which may or may not be necessary for the long term in the fast-changing energy market is not sustainable. The old ways of working and investing are changing. That’s why we are looking beyond our own capabilities to future solutions which we cannot yet fully gauge.

For example, we can already see that cutting-edge technology can make us use our grid far more efficiently, by steering electricity to where it is needed after making an accurate forecast. This would allow us to use our existing double lines much more efficiently, for day-to-day business, without the necessity to build expensive new assets. Other examples of technological breakthroughs securing our supply may come from new storage solutions and the involvement of other energy carriers, like power to gas.

In this way, security of electricity supply will increasingly involve looking beyond what is possible now towards new capabilities and opportunities in the future.

10 years of offshore expertise


This is an anniversary year for TenneT: it is ten years since we first started our offshore operations. We started in 2007 with only six employees, growing to 400 employees within a decade, realising 5.3 GW transmission capacity.

As offshore wind energy plays a key role in the Energiewende, the German government is planning to realise its ambitious targets with a significant expansion of capacity in offshore wind production. TenneT is playing its part in connecting wind farms to the grid by 2020. To that end, technically complex AC and DC projects in the North Sea, as well as other major projects – both onshore and offshore – are being accelerated.

Wilfried Breuer, Member of the Executive Board TenneT: "We are the leading grid operator in the North Sea for interconnectors as well as offshore wind integration. Our unique and longstanding experience and skills make us the frontrunner in a complex and demanding environment."

Meanwhile, we can draw on years of experience and expertise. Our very first offshore project was Alpha Ventus, an AC connection of 62 MW. TenneT’s early days in offshore work in Germany were a rollercoaster ride. Everything was new: the technology, the contractors, the risks, the enormous financial investments as well as the political pressure. Our organisation grew rapidly to keep up, almost bursting at the seams. There was a lot at stake and it was not always easy.

Following Alpha Ventus, we realised BorWin1 in 2010, with a transport capacity of 400 MW and then a further nine offshore connections, with a total transport capacity of 5.3 GW. In 2017, these connections transported 16 TWh of wind energy from the North Sea to shore, with a high grid availability and a significant contribution to CO2 savings, in total 8.4 million tonnes. The energy flow from the North Sea has now reached a significant share of the overall wind energy generation in Germany.

We are currently planning and constructing four additional offshore connections that by 2019, will bring our total transmission capacity to 6.5 GW for electricity from renewables.

Torben Glar Nielsen, Executive Vice President "As a fellow TSO we really appreciate the excellent job TenneT has done in connecting enormous amounts of offshore wind in a short period of time."

Our experience with connecting offshore wind in Germany has helped us to increase the efficiency in our offshore connections from DolWin3 to BorWin3 and now DolWin6, which has resulted in an average contract price reduction of around 15 per cent in 2017. With our offshore expertise in Germany we have taken the lead in mapping the future development of this critical infrastructure. For example, we conducted a study that determined 66 kV voltage levels for Dutch wind farms. This can now pave the way for further innovations, and become an industry standard.