So many old ideas are becoming new again. This is a phenomenon that takes place over and over again as technology advances and matures. Think about renewable energy. Using wind power to produce electricity actually dates back to 1888. Solar, hydrogenation and geothermal have all been around for a long time, but they were not practical, feasible or economical until technological breakthroughs made them that way.
Wind turbines are a good example. There was a 1.25-MW wind turbine in the early 1940s, but the blades broke off the hub after only a few hundred hours of operation. The technology was not there yet. After countless hours of fiddling with materials and other technologies, 10-MW turbines now exist. And mega wind farms are on the drawing boards with ratings of thousands of megawatts (T&D World, March 2011).
Unfortunately, the best locations for renewable energy generation seem to be extremely remote or far offshore from load centers. Transmission resources may not be available or they are challenged by the present alternating-current (ac) technology.
Strangely, the solution to this challenge brings the electric utility industry back full circle to the beginning of its existence. The industry's founding fathers, Thomas Edison and Nikola Tesla, tussled over a direct-current (dc) grid or an ac grid, with ac proving to be more adaptable.
A hundred years later, dc has become a stabilizing influence as high-voltage direct-current (HVDC) is integrated into ac grids worldwide. It comes in two basic forms: the classic HVDC technology, known as the line-commutated converter (LCC), and the voltage source converter (VSC) technology. These advancements are proving to be a powerful force promising to reshape grids worldwide.
Combining LCC with VSC flexibility is a huge plus for integrating dc into the ac grid. These HVDC schemes are giving the old idea of a multinational electric grid, connecting countries and continents to each other, new life. These multinational grids have been called super grids, mega grids, electricity highways and even the global grid.
Regardless of what we call these transmission grid configurations, the goal is the same: to move tremendously large blocks of power over extreme distances, increasing access to electricity worldwide. R. Buckminster Fuller talked about the world having a global grid for electricity in the early 1970s. He envisioned an intercontinental electrical network, integrating all the continents into an interconnected global network.
With grid interconnections, it is sometimes difficult, if not impossible, to connect two ac networks. High-voltage ac networks have challenges such as stability, congestion and frequency shifts.
Even in highly developed parts of the world, the complexity of operating transmission systems is a challenge. Europe's huge European Network of Transmission System Operators for Electricity (ENTSO-E) system extends to Africa by a link between Gibraltar and Ceuta. Even networks with the same nominal frequency have some slight variation, normally less than ±0.1 Hz, which challenges high-voltage ac interconnections.
There are also networks with both 50-Hz and 60-Hz frequencies within close proximity (for example, South America and Japan) of each other. In the United States, the eastern and western power systems are both 60-Hz networks, but they are not synchronized with each other.
Global-Scale Power Grids
Because of difficulties such as these, there is increasing interest in HVDC bulk transmission, ac-dc grid interconnections, asynchronous connections and dc networks. VSC technology continues to advance as insulated-gate bipolar transistors (IGBTs) increase in power ratings. These higher voltage and current ratings give utilities even more options and applications for renovating their transmission grids.
Reliable bulk-power transmission plays a key role in today's power system, but high-voltage ac needs reinforcements to get the job done. China, India, Africa and South America are leading the world in this area. They are rich in renewable resources, having some of the largest hydropower resources in the world. Unfortunately, the load centers are remote, as far as thousands of kilometers from those resources or any type of transmission facilities, but these countries are doing something about it.
In China, ±500-kV and ±600-kV systems are fairly common, and now ±800-kV systems are having a big impact. China has huge renewable resources (that is, hydropower generation) located on the western side of the country. Its load centers are located on the eastern side of the country, and they are huge, too.
At the Electric Power Research Institute's 2011 HVDC conference, the State Grid Corporation of China (SGCC) made several presentations on the HVDC activity taking place on its grid. SGCC reported, “China is constructing a power highway. Ultrahigh-voltage direct-current (UHVDC) is one of the key technologies to meet these requirements.”
UHVDC projects have pushed the HVDC solution providers as well as the boundaries on voltage levels. By the end of 2010, there were two ±800-kV projects in service on the SGCC network. The 2,000-km (1,243-mile), 7,200-MW Xiangjiaba-Shanghai transmission line was the world's first UHVDC scheme in commercial operation, transmitting clean hydropower to Shanghai to about 24 million people. The 1,450-km (901-mile), 5,000-MW Yunnan-Guangdong transmission line was also placed in service that year.
SGCC also has plans for two more ±800-kV projects and the first ±1,100-kV project. The new ±800-kV projects are the Xiluodu-Zhexi line and the Hami-Zhengzhou line. The ±1,100-kV project is the Zhundong-Chengdu line. In all, SGCC announced it will build 11 UHVDC projects in the next five years with a combined transmission capacity of roughly 88,000 MW and a total combined distance of approximately 40,000 km (24,855 miles).
China is not the only country using UHVDC technology. Powergrid Corporation of India Ltd. has begun an 8,000-MW, ±800-kV North-East Agra UHVDC project through India's famous Chicken's Neck area, the Siliguri Corridor, a narrow strip of land between Nepal and Bangladesh.
India plans to create power pooling points (multiple terminals) in the northeastern region, collecting power from several hydropower stations and using LCC technology (±800-kV UHVDC bipolar lines) to transport it to major load centers.
The first phase of that plan is the North-East Agra project. The transmission line will run about 1,728 km (1,074 miles). This will be one of the largest UHVDC transmission projects built and is expected to be in service by 2015. It is estimated this 8,000-MW project will be capable of meeting the electrical needs of approximately 90 million people.
In South America, Brazil gets roughly 95% of its generation from hydropower and is very interested in integrating high-voltage ac with HVDC to bring these remote resources to its load centers. Brazil is certainly no stranger to HVDC projects. Its Itaipu project set a few records in its day, but current plans will dwarf those.
The first stage is the Rio Madeira project, which includes three stations and the longest HVDC transmission line to date, approximately 2,500 km (1,553 miles). It is a flexible, long-haul transmission scheme combining an HVDC LCC point-to-point design with a VSC back-to-back converter element.
The LCC portion is a ±600-kV project rated at 3,150 MW, which connects the hydropower stations located near Porto Velho with the Sao Paulo load center. The VSC portion is an 800-MW back-to-back converter station considered to be a flexible connection for the high-voltage ac system in northwestern Brazil.
Africa is the second-largest continent in the world, but the majority of the population living there has little to no access to electricity; no wonder it is called the Dark Continent. But that is changing, thanks to a number of HVDC projects in planning, several under construction and a few in operation.
Africa also is the home of the Inga dams located on the Congo River at the Inga Falls, the largest waterfalls in the world. Inga I and II have an installed capacity of 1,775 MW, with plans to refurbish the dams. There are proposals to add Inga III and Grand Inga, two massive hydropower plants — 4,500 MW and 39,000 MW, respectively — to the facilities.
NamPower's 300-MW, ±350-kV Caprivi Link Interconnector project connects Namibia and Zambezi. It was commissioned in 2010, but what makes it unique is that two very weak ac networks are connected with a 950-km (590-mile) overhead line between two VSC stations. (Yes, that is correct, an overhead transmission line connecting two VSC stations!) Previously, VSC technology was limited to underground/submarine cables, but technology moves on, and this project proved VSCs can be used with both cable and overhead connections.
Moving thousands of megawatts great distances to load centers with tens of millions of customers fits well in the framework of China, India and other huge markets, but that enormity does not fit the scale of most of the world's networks. Consider the grids in North America or Europe; they are not built to handle a transmission line with that amount of power. A 5,000-MW bump on the Eastern U.S. grid would create huge problems; in the Western U.S. grid it would be overwhelming, and in the Electric Reliability Council of Texas grid, it would be catastrophic.
The European Union has plans for a different type of HVDC scheme using HVDC transmission technology (LCC and VSC) to build an interregional HVDC grid. This makes sense for the established networks of Europe, which include one grid in Western Europe, one in Eastern Europe and one in the Nordic countries. Then add the islands of Great Britain, Iceland, Ireland, Sardinia, Corsica, Crete and others; they all have their own grids and no ac connections to the European continent.
The European Union recognizes its network needs more flexibility and intelligence with the ability for long-haul transmission, too. It also recognizes the need for sharing resources, making interconnections, integrating renewables and phasing out nuclear generation in some countries.
As a result, regional HVDC links are being built right now with many in service using both classic (LCC) and VSC technology. These regional applications consist of submarine HVDC cable connections between grids, such as the 2006 Estlink link between Finland and Estonia. It is a 350-MW, ±150-kV VSC underground/submarine cable interconnection bringing hydropower from Finland to Estonia. A second cable link, Estlink 2, recently started construction using LCC technology (rated at 650 MW and ±450 kV) and is scheduled for completion in 2014.
Another dc element is the Fenno-Skan link connecting Sweden and Finland through classic HVDC with an underground/submarine cable. It is comprised of two stages. Fenno-Skan 1 is a 500-MW, ±400-kV link that has been in service since 1989. The second link, Fenno-Skan 2, was completed in December 2011, rated at 800 MW and ±500 kV.
Adding to the interconnections is the NordBalt project using VSC technology with an HVDC cable connection between Sweden and Lithuania. It is rated at 700 MW and ±300 kV. This system is expected to be commissioned in 2016.
HVDC is Offshore, Too
Regional HVDC grids are not limited to cable connections between countries; they also are being used with the large offshore wind farms. There is a growing trend to build larger wind farms further out to sea. They are being developed in groups and connected to the grid in clusters.
They go by names such as BorWin, DolWin and HelWin. Bor stands for Borkum, Dol for Dollart, Hel for Helgoland and Win for wind. To keep track of the connection portion of the project, numbers are associated with these facilities, such as BorWin1 or DolWin1.
The BorWin1, consisting of 80 5-MW wind turbines, is the most remote offshore wind farm. The HVDC link was completed in 2009, and the wind turbine installation will be completed this year. BorWin1 is connected to the Germany power grid using VSC technology with a 125-km (78-mile) submarine cable and a 75-km (47-miles) underground cable. The scheme's rating is 400 MW and ±150 kV.
No sooner had the BorWin1 project been completed than the German company TenneT Offshore GmbH announced the 800-MW DolWin Alpha North Sea project. It will connect the wind farms in the DolWin1 cluster (400-MW plus future wind farm additions) to Germany's grid at the Dorpen/West connection point.
DolWin uses the latest VSC technology and takes advantage of recent extruded polymer (cross-linked polyethylene) cable advancements with the higher ±320-kV operating voltage to increase its power-transfer ratings and reduces losses. Its submarine cable length is about 75 km (47 miles) and its underground cable length is 90 km (56 miles). The project will be placed in service in 2013.
North America has not had as much HVDC action as the rest of the world. There have been several life-extension projects for older schemes, but there has not been much activity in the area of long-haul transmission for renewables or offshore wind farms.
That does not mean there has not been interest in HVDC. Some notable projects have been built and there are more on the drawing boards. The United States and Mexico were interconnected by the first large-scale open-access ties with the 150-MW Sharyland back-to-back converter station in 2007.
In Canada, Hydro-Québec built the 1,250-MW Outaouais back-to-back scheme interconnecting Ottawa and Québec in 2009. The station has two independent HVDC blocks (625 MW) for reliability and operating flexibility. This link provides Ottawa with access to Québec's vast hydroelectric generation renewable resources.
There also have been several underground/submarine cable projects using VSC technology, such as the Cross-Sound Cable, Neptune and Trans Bay Cable projects.
It is an interesting time to be working in the electricity industry. Renewable energy resources are one of the principal driving forces in the industry today. It is one of the fastest-growing sectors as dependency on fossil fuels is trimmed down, the carbon footprint is reduced and greenhouse gasses are lessened.
For utilities to be able to harvest this renewable power potential, they must be able to move it efficiently from the remote sites where it is generated to the load centers. Increased bulk transmission capacity is the solution to utility-scale renewable generation. Integrating HVDC technology into the ac grid is the key to that solution. HVDC's inherent properties make the high-voltage ac backbone more flexible, controllable and stable.
Electric Power Research Institute | www.epri.com
ENTSOE | www.entsoe.eu
Hydro-Québec | www.hydroquebec.com
NamPower | www.nampower.com.na
Powergrid Corporation of India | www.powergridindia.com
State Grid Corporation of China | www.sgcc.com.cn
TenneT Offshore GmbH | www.tennettso.de