The rapid increase of new renewable energy sources and new demand loads are transforming the electric grid.
Electricity generation and consumption need to be balanced and power generated at remote locations due to the nature of renewables being transmitted across distances as long as 3,000 km to busy consumption centers.
The transportation of electricity by interconnections between renewable generation in locations with the best wind and solar conditions, where local demand cannot meet the peak generation is the most economical way to use and balance peak generation. High voltage direct current (HVDC) is the preferred means of transmission due to very low losses and less space requirements compared to traditional high voltage alternating current (HVAC) transmission.
With HVDC, controllable power electronic-powered substations are used to convert alternating current to direct current, and vice versa. These transmission assets also provide the voltage and frequency support needed to strengthen grids with large amount of rapidly changing renewable but fluctuating energy generation. The HVDC interconnections enable the integration of renewables and power grid markets.
HVDC transmission, a technology innovation by ABB, has been used commercially since 1954, initially connecting the island of Gotland to the mainland of Sweden over a distance of more than 100 km. This enabled the island to use remote hydropower instead of local fossil-fueled power plants. This milestone was recently awarded by IEEE as one of key landmarks in grid development.
More than 200 HVDC systems have been installed across continents since this landmark. The ever increasing pace of HVDC deployment has been spurred by successful grid operation experience, rapid development of power electronics performance and a growing demand for renewable energy generated electricity.
HVDC links are being increasingly deployed to connect regions, countries and even continents, enabling power trading, balancing loads and maximizing the use of renewables. An HVDC interconnector is composed of two or more converter stations connected by an overhead line, a cable systems or combination of both. Transmission distance varies from about 100 km to more than 3,000 km. A special purpose solution is the ‘back-to-back’ converter stations, where both stations are located next to each other. Here the transmission distance is only a few meters but still interconnect two non-synchronized power grids. Kriegers Flak is a good example of this kind of interconnection, enabling the integration of wind to the Danish and German grids.
A main advantage with HVDC interconnectors is the transmission loss reduction: an HVDC system can reduce transmission losses by around 50% compared with HVAC. HVDC is the only viable method to transport large amount of electricity in cable system, to overcome the challenge of capacitive charging of the insulation caused by alternating current operation, which consequently leads to large losses of active power in long distance HVAC cable systems. This means that the amount of power being transmitted by HVDC links has been rising constantly.
An example is the ultra-high voltage direct current system Changji-Guquan ±1,100kV project in China. Once construction is completed in 2019, it will carry more than 12,000 MW of power in one set of power towers. That is equal to approximately 12 large power plants or close to 2,000 large-scale wind turbines. The distance stretch from North West to South East of China is equivalent to that from Lisbon to Berlin.
The introduction of voltage sourced converters (VSC) that can control the AC grid led to the development of HVDC Light® by ABB in the 1990s. This made it possible to support and even start up weak grids in case of outages. Today, these converters are dominating new projects under construction in Europe. They are also being deployed to connect remote offshore wind.
For instance, the Dolwin 1 and 2 links connect 1,700 MW of wind power from North Sea to the German mainland grid. VSC-HVDC interconnects in construction, such as NordLink and North Sea Link are connecting Norway with Germany and United Kingdom, respectively. These two 1,400 MW HVDC interconnectors will be used to trade wind power from Germany and UK with abundant large scale hydropower stored in Norwegian reservoirs. The load and demand can be balanced by adapting the hydropower generators to renewable capacity and market demand in Germany or UK. The nature of electricity makes it possible to operate and adapt power flows in real time with todays advanced control systems.
The 20th anniversary of HVDC Light in 2017 marked the next chapter in ABB’s development of this technology. The latest generation of HVDC Light can more than double existing power capacity to over 3,000 MW and by enhancing voltage levels further to 640 kilovolts also double the distance capability to 2000 km without compromising on losses. The solution is also much more compact – with a potential to deliver 350 percent more power per square meter of space used – a huge benefit in applications like offshore wind transmission, city center in-feeds or interconnections. The latest ABB Ability-based MACH control and protection system provides greater efficiency and reliability. All this can bring substantial cost benefits, facilitate more interconnections and greater integration of renewables, lowering environmental impact.
While energy storage is a good method to bridge gaps between renewable generation and local loads, transmission is significantly cheaper. It brings further advantages such as grid resilience, trade of electricity to reach healthy price levels, avoids excessive construction and enables renewable power plants to be constructed at sites with the best conditions for solar and wind. HVDC interconnectors will increasingly be one of the most valuable tools for grid operators to build the greener, stronger and smarter grid, today and tomorrow.