DELIVERING RENEWABLE ENERGY TO WHERE IT IS NEEDED
Governments around the world are facing a monumental challenge: a massive increase in renewable power production capacity will be needed to meet legally binding climate targets –how to deliver this huge wave of primarily variable electricity to where it is needed in a timely, efficient and reliable way?
The answer lies within the world’s power networks. To leap forward in reducing the carbon dioxide intensity of the global energy system, we need to go above and beyond to access the best green energy resources, which are often located in remote places.
Advances in electricity transmission technology will allow us to connect the highest quality renewable energy sources with end consumers – from desert solar to Arctic winds, and powerful rivers to faraway oceans.
Connecting from west to east shifts peak demand and solar production times, while linking north and south will enable us to balance the varying electricity demand profiles of hot and cold or dark and light seasons.
Expanding the capacity of existing interconnections and adding new connections to link isolated electric power systems and remote renewables to deliver a broad range of social, environmental and economic values.
SO HOW WILL THIS ‘BRIDGING’ BE POSSIBLE?
Two or more power systems can be interconnected and operate synchronously – at the same frequency, using HVAC (high-voltage alternating current) transmission lines or asynchronously, keeping their own frequencies, using HVDC (high-voltage direct current) converter stations and lines.
The advantage of synchronously interconnected power systems is that they can remain stable while integrating gigawatt-size power plants. Scaling up and pooling generating units results in lower generation cost. Sharing reserves within a synchronous area reduces the system operating cost.
HVAC transmission lines are typically a preferred choice for shorter distances of a few hundred kilometers and when interconnecting power grids with compatible frequencies. Longer HVAC transmission distances are possible; however, they require special reactive power compensation which makes it more costly.
Enlarging the size of synchronously interconnected system increases complexity, vulnerability and cost. That is where DC technology comes into the spotlight and helps to overcome the fundamental limits of utilizing AC technology for:
- long-range power transmission via overhead lines;
- short- and mid-range subsea connections between countries, islands and offshore energy sources; and
- connection of regions with incompatible frequencies.
MAXIMIZING THE POWER OF INTERCONNECTION
HVDC transmission technology has been used for decades for efficient bulk electricity delivery over long distances and the interconnection of asynchronous grids running at different frequencies via back-to-back stations or subsea cables. It has benefited millions of consumers around the globe.
Technological innovation in valves and processors for control systems has unlocked almost unlimited possibilities for managing electricity networks with a high share of renewable sources, with lower inertia and greater feed-in variations than ever before.
The true power of HVDC interconnectors is demonstrated when providing various dynamic grid support services to enhance system stability and resilience.
In addition, fully controllable and flexible HVDC interconnectors can effectively limit short circuit currents and help to restore power supply following system-wide blackouts.
One great example of an effective combination of power trading in the expansive region of southern Africa and the stabilization of weak AC power networks is the Caprivi Link interconnector, which joins Namibia and Zambia’s electricity grids. The 950-kilometer overhead line ensures reliable power transfer capability between the eastern and western regions of the Southern African Power Pool and Voltage Source Converter-based HVDC technology helps to stabilize these weak networks and prevent blackouts under critical contingencies.
Enabled by fast and independent control of active and reactive power output of HVDC converter stations, the three main benefits include:
- Frequency regulation performed by emergency power control via droop characteristics and emulation of inertial response to compensate large active power imbalances usually caused by unexpected loss of large generating units;
- Enhanced voltage control by fast modulation of reactive power during and after faults to stabilize the system and help to prevent voltage collapse by limiting active power transfer to increase reactive power supply; and
- Damping of electro-mechanical oscillations of the rotors in the synchronous generators to maintain a safe power transfer limit in the AC system.
THE CONNECTED DREAM: MORE UNIFIED ELECTRICITY NETWORKS
Making optimal use of the world’s green energy resources by extending and linking together regional interconnections makes the dream of more unified electricity networks a reality. Efficiently flowing electrons generated in windy, wet or sunny places to urban hotspots saves money on fuel costs, on building peaking capacities and on sharing reserves. It also reduces the cost of carbon emissions and other fossil fuel waste. Yes, the initial investments are high, but our analysis has shown that the long-term benefits far outweigh the capital requirements.
Europe is a stand-out example on this journey and the European Union has, since its inception, worked towards the creation of an integrated European power network. It started in the early 1950s with the main objective being to optimize necessary reserve capacity, connect hydro resources in the Alps and facilitate cross-border electricity exchange.
The invention of HVDC technology gave engineers a powerful tool to accomplish previously impossible tasks such as connecting the asynchronous power grids of UK and Nordic countries to continental Europe, and islands like Sardinia and Mallorca to the mainland with long subsea cables. Several back-to-back HVDC converter stations enabled power exchanges with asynchronous power grids at the Eastern extremes of the European system.
The shift from fossil generation sources towards new renewables and the massive electrification of final energy use in sectors such as transportation, industrial and buildings (e.g. heating) are heavily dependent on stronger and more flexible power networks.
Interconnection goes hand-in-hand with flexible storage concepts. It enables users to take advantage of the full range of available storage technologies implemented across the total system – ranging from batteries for short-term energy storage, through to pumped hydro for bulk and longer-term storage. Interconnection especially complements pumped hydro energy storage plants, as these are location-dependent facilities that cannot be built everywhere.
Historically, interconnectors have been used to integrate remote hydro power plants, in the past decade the focus has extended towards a seamless integration of new renewables, such as wind and solar. The number of interconnectors has increased significantly over the past 20 years, mainly driven by the increasing need for renewable integration and growing power exchanges.
Today, the European electric network already handles high shares of variable renewables which on some selected hours may reach a peak of almost 50% of total generation and stay above 30% of total generation during 2,100 hours per year. (Source: ENTSO-E Data Transparency Platform). The highest intensity is from February to March. Some countries have even managed to run on pure renewable power for days, taking advantage of very favorable weather/wind conditions and an extended interconnection with the rest of the European system. However, transmission capacities and network flexibility need further investment to meet the ambitious targets, and possibly run fully on renewable power at all times.
(Source: ENTSO-E Data Transparency Platform).
The latest recommendations indicate that European member states should ensure cross-border interconnection capacity of 15 percent of peak demand or of installed variable renewable generation capacity, depending on which is higher, by 2030, to avoid any significant curtailments of wind and solar generation while providing high quality and continuity of electricity supply.
Today this percentage varies greatly between European countries. Denmark, whose cross-border transmission capacity is more or less equal to its demand, is an exceptional case, which highlights the benefits of a strongly connected power grid. When there is excess wind power generation in Denmark – it can sometimes reach 150-160% of local demand – it exports surplus electricity via subsea cables and overhead lines to its neighbors. Norwegian consumers, for example, can use Danish green electricity while Norway’s domestic production is stored in hydroelectric reserves. When Danish domestic power supplies are insufficient, consumers receive Norwegian hydroelectricity.
The recent energization of the NordLink project, a 623-km long HVDC interconnection linking German and Norwegian power markets enables the integration of renewables from both countries. The connection provides the German power grid with reliable access to hydropower resources in Norway, and Norway access to Germany’s substantial base of renewable energy, particularly wind and solar energy resources.
Between now and 2050, the whole range of connection types will be needed to establish connections within Europe and to other continents – from domestic offshore wind to remote renewables and from back-to-back stations to long distance subsea cables and overhead lines.
We need to speed up the process of making regional grids more flexible, controllable and resilient, by embedding HVDC links into existing AC networks.
When placed at strategic locations they will result in more stable and effective utilization of existing assets and optimal use of fast-growing distributed energy resources. Connecting islands, offshore O&G platforms and expanding offshore wind clusters into a highly reliable and efficient offshore HVDC grid is another important step towards sustainable energy supply.
In future, a truly integrated and interconnected European power system could use long-distance interconnectors to tap large-scale remote renewable resources – the solar energy of the Sahara and Middle East, the hydro resources of sub-Saharan Africa and the intense winds of the Arctic and Central Asian regions. And at some point, the power networks of China and India could even be connected with Europe.
The carbon-neutral energy system requires us to connect all the dots, including:
- local and remote clean energy sources;
- diurnal and seasonal energy storage;
- more electrified mobility and heating loads; and
- sites where excess renewable electricity can be converted to clean fuels in applications where direct electrification is impossible or economically not feasible.
A high level of interconnection will enable society to exchange and use clean energy at the highest levels of efficiency, resilience and reliability.
It is crucial that the foundations – specifically, the short-, medium- and long-distance interconnections – needed to accelerate us towards a carbon-neutral energy system, are laid down now. The technology is proven, but trust is also needed to build these cross-border links. Now is the time for deeper collaboration with and across regulators and policymakers. The combination of technology and trust will enable countries to reach their carbon-neutral goals.