HVDC transmission for renewable energy

Power lines in Vancouver

One limitation of renewable sources of energy is that they are often best captured in places far from where energy is used: remote bays with large tides, desert areas with bright and constant sun, and windswept ridges. In these cases, losses associated with transmitting the power over standard alternating current (AC) power lines can lead to very significant losses.

This is where high voltage direct current (HVDC) transmission lines come in. Originally developed in the 1930s, HVDC technology is only really suited to long-range transmission. This is because of the static inverters that must be used to convert the energy to DC for transmission. These are expensive devices, both in terms of capital cost and energy losses. With contemporary HVDC technology, energy losses can be kept to about 3% per 1000km. This makes the connection of remote generating centres much more feasible.

HVDC has another advantage: it can be used as a link between AC systems that are out of sync with each other. This could be different national grids running on different frequencies; it could be different grids on the same frequency with different timing; finally, it could be the multiple unsynchronized AC currents produced by something like a field of wind turbines.

Building national and international HVDC backbones is probably necessary to achieve the full potential of renewable energy. Because of their ability to stem losses, they can play a vital role in load balancing. With truly comprehensive systems, wind power from the west coast of Vancouver Island could compensate when the sun in Arizona isn’t shining. Likewise, offshore turbines in Scotland could complement solar panels in Italy and hydroelectric dams in Norway. With some storage capacity and a sufficient diversity of sources, renewables could provide all the electricity we use – including quantities sufficient for electric vehicles, which could be charged at times when demand for other things is low.

With further technological improvements, the cost of static inverters can probably be reduced. So too, perhaps, the per-kilometre energy losses. All told, investing in research on such renewable-facilitating technologies seems a lot more sensible than gambling on the eventual existence of ‘clean’ coal.

Author: Milan

In the spring of 2005, I graduated from the University of British Columbia with a degree in International Relations and a general focus in the area of environmental politics. In the fall of 2005, I began reading for an M.Phil in IR at Wadham College, Oxford. Outside school, I am very interested in photography, writing, and the outdoors. I am writing this blog to keep in touch with friends and family around the world, provide a more personal view of graduate student life in Oxford, and pass on some lessons I've learned here.

25 thoughts on “HVDC transmission for renewable energy”

  1. At what range does it become more energy efficient to use HVDC?

    Also, at what range does it make more sense to use electricity to produce hydrogen and then ship that? Does it make a difference if the energy source is located offshore?

  2. The phenomenon which makes AC transmission inefficient is particularly interesting. I can find wikipedia articles on inductance, and on AC transmission loses, but no proper discussion of how inductance operates in an AC circuit.

  3. Green.view
    Mission: Transmission

    Apr 28th 2008
    From Economist.com
    Harvesting the breeze is trickier than it sounds

    Transmission is expensive and often an afterthought, at least for consumers. Even within windy areas the generators are often scattered across wide expanses, which makes gathering it and bringing it to market difficult. Rob Gramlich of the American Wind Energy Association calls transmission the industry’s “biggest long-term barrier”.

  4. [P]eople do not necessarily live where the wind blows. Indeed, they often avoid living in such places. Solving these problems, though, is a task not for the mechanical engineers who build the turbines but for the electrical engineers who link them to places where power is wanted. That means electricity grids are about to become bigger and smarter.

    Bigger means transcontinental, at least for people like Vinod Khosla. His analogy is America’s interstate highway system, built after the second world war. The new grids would use direct, rather than alternating, current. AC was adopted as standard over a century ago, when the electrical world was rather different. But DC is better suited to transporting power over long distances. Less power is lost, even on land. And DC cables can also be laid on the seabed (the presence of all that water would dissipate an AC current very quickly). In the right geographical circumstances that eliminates both the difficulty of obtaining wayleaves to cross private land and the not-in-my-backyard objections that power lines are ugly. Indeed, there is already a plan to use underwater cables to ship wind power from Maine to Boston in this way.

  5. Scarcity of energy is a myth that persists in society, because our fixation remains on fossil fuels. Yet the resource potentials of solar, wind, hydro, geothermal, biomass and ocean energies are abundant far beyond our needs. The winds of the American plains are sufficient to power all the electrical demand of the United States, and solar radiation from just 3% of the world’s deserts could power all global demand. There is no shortage of renewable energy on our planet! While annual growth rates of 20-40% for geothermal, wind and solar are promising, their share of the energy pie remains less than 3%.

    Critics state that renewable energies are intermittent—the sun isn’t always shining and the winds don’t always blow—and we need reliable electricity every second. The critical infrastructure that solves this is high-voltage transmission. The interconnected grid acts as the freeway for electricity from generator to user, and it is already built throughout the developed world. Today, bulk transmission can deliver power far beyond political boundaries, with over 100 nations trading electricity for mutual benefit. Interconnected grids enable load levelling, economic exchange of power, system reliability and emergency back-up options. Long-distance transmission allows us to tap remote renewable energy resources, sometimes located in neighbouring nations, and to feed clean electricity throughout the network.

  6. Lifeline for Renewable Power

    Without a radically expanded and smarter electrical grid, wind and solar will remain niche power sources.

    By David Talbot

    “Push through a bulletproof revolving door in a nondescript building in a dreary patch of the former East Berlin and you enter the control center for Vattenfall Europe Transmission, the company that controls northeastern Germany’s electrical grid. A monitor displaying a diagram of that grid takes up most of one wall. A series of smaller screens show the real-time output of regional wind turbines and the output that had been predicted the previous day. Germany is the world’s largest user of wind energy, with enough turbines to produce 22,250 megawatts of electricity. That’s roughly the equivalent of the output from 22 coal plants–enough to meet about 6 percent of Germany’s needs. And because Vattenfall’s service area produces 41 percent of German wind energy, the control room is a critical proving ground for the grid’s ability to handle renewable power.

    Like all electrical grids, the one that Vattenfall manages must continually match power production to demand from homes, offices, and factories. The challenge is to maintain a stable power supply while incorporating elec­tricity from a source as erratic as wind. If there’s too little wind-generated power, the company’s engineers might have to start up fossil-fueled power plants on short notice, an inefficient process. If there’s too much, it could overload the system, causing blackouts or forcing plants to shut down.”

  7. Transmission lies
    Against the so-called ‘need’ for new long-distance, high-voltage transmission lines
    Posted by Guest author (Guest Contributor) at 1:31 PM on 03 Feb 2009

    Planning for “peak load” is a transmission lie. Utilities have incentive to overstate “need” when they build for peaks. The higher the peak they build for (with peak occurring only several times annually), the deeper the off-peak valley and the more electricity they can sell on the market when generation is available but not “needed.” Conservation and peak-shaving is against their interest because it lowers peak and lessens the valley of market sales.

    “It’s for renewable generation” is a lie. The massive transmission infrastructure expansion proposed is not “for renewables” because transmission may not discriminate by generation type. Federal regulations prohibit discrimination among generators — it’s first come, first ready, first served. There are tens of thousands of megawatts of coal projects, with transmission studies complete or in progress, waiting for interconnection, and whatever generation is ready will be connected. Another side of this lie is when wind advocates support transmission, claiming “it’s for renewables,” and ignore the impacts of transmission on the communities it traverses. Rather than make this convoluted “it’s for renewables” claim, there’s a better way: if renewable energy mandates were directly linked with shut down of fossil generation, and if renewable generators were thoughtfully sited, both the electricity market and transmission infrastructure would be open and available.

    “Long distance transmission” is a lie. Transmission is inherently inefficient over long distances. Transmission physics entails high levels of line loss, and the longer the line, the higher the line loss. To avoid this fact of physics, the electric industry has shifted its line loss analysis for new projects to a “system wide” loss, so the numbers look low. But consider actual numbers of megawatts of line loss, and look at “coal plant equivalents” to make up that loss — for every 500-600 MW of line loss, a coal plant or more would have to be built! Line losses are charged in Federal Energy Regulatory Commission rates, but this is not considered directly in the market transactions. Line loss is an afterthought add-on to the customer’s bill after transmission service is provided. Consider too the capital cost of transmission, starting at about $1.5 million per mile for 345kV lines and upward from there

  8. Spreading electricity
    A gust of progress

    Apr 30th 2009 | CHICAGO
    From The Economist print edition
    Creating windpower transmission in the Midwest

    FRANKLIN ROOSEVELT helped bring electricity beyond America’s cities to its most distant farms. Barack Obama hopes the countryside will return the favour. Much of this challenge rests in the gusty upper Midwest. In recent years Interstates 29 and 80, highways of America’s heartland, have teemed with lorries bringing wind blades to new plants. Efforts to build transmission have moved more slowly. There are 300,000 megawatts of proposed wind projects waiting to connect to the electricity grid, says the American Wind Energy Association. Of these, 70,000 megawatts are in the upper Midwest.

    Now action is at last replacing talk. Firms are proposing ambitious transmission lines across the plains. The region’s governors and regulators are mulling ways to help them. The federal government is playing its part. In February the stimulus package allotted $11 billion to modernise the grid. Since then members of Congress have proposed an array of bills to develop transmission. Jeff Bingaman, chairman of the Senate energy committee, intends to start marking up transmission plans next week—though debate over other parts of the energy bill may delay progress.

    America’s grid is complex: 3,000 utilities, 500 transmission owners and 164,000 miles (264,000km) of high-voltage transmission lines are stretched across three “interconnections” in the east, west and Texas. If wind is to generate 20% of electricity by 2030, as in one scenario from the Department of Energy, about $60 billion must be spent on new transmission. Just as important, regulations must change.

  9. High-Temp Superconductors To Connect Power Grids

    physburn writes “Somewhere in a triangle between Roswell (UFO) NM, Albuquerque (Left Turn) NM, and Amarillo (Do you know the way?) TX, a 22.5 square mile triangle of High Temperature Superconductor pipeline is to be built. Each leg of the triangle can carry 5GW of electricity. The purpose to load-balance and sell electricity between America’s three power grids. Previously the Eastern Grid, Western Grid and Texan Grid have been separate, preventing cheap electricity being sold from one end of America to the other. The Tres Amiga Superstation, as it is to be called, will finally connect the three grids. The superstation is also designed to link renewable solar and wind power in the grids, and is to use HTS wire from American Superconductor. Some 23 years after its invention, today HTS comes of age. “

  10. Toronto firm behind big underwater power transmission project

    Matthew L. Wald

    The New York Times News Service Published on Wednesday, Mar. 17, 2010 7:11AM EDT Last updated on Wednesday, Mar. 17, 2010 10:52AM EDT

    Generating 20 per cent of America’s electricity with wind, as recent studies proposed, would require building up to 22,000 miles of new high-voltage transmission lines. But the huge towers and unsightly tree-cutting that these projects require have provoked intense public opposition.

    Recently, though, some companies are finding a remarkably simple answer to that political problem. They are putting power lines under water, in a string of projects that has so far provoked only token opposition from environmentalists and virtually no reaction from the larger public.

    “The fish don’t vote,” said Edward M. Stern, president of PowerBridge, a company that built a 65-mile offshore cable from New Jersey to Long Island and is working on two more.

    The projects have even drawn cautious enthusiasm from some environmental groups, who say the new power lines serve their goal of getting the United States to use more renewable power.

    Nearly all the submarine cables use direct current, a form of transmission favored by Thomas Edison but mostly rejected in the late 1800s in favor of alternating current, the kind of electricity now used to run most appliances. But alternating-current lines are hard to bury, because an interaction between the current and the cable casing drives up voltage to unwanted levels.

    Direct-current transmission is also undergoing a modest revival on land, because over long distances, its line losses are smaller, and flows are easier to control. Two recent proposals for a centrally planned overhaul of the North American electric grid called for heavy use of direct current.

    New technology offered by two European companies, Siemens and ABB, has lowered the cost for some direct current projects, and shrunk the size of the terminals where alternating current is converted to direct current and back, a crucial consideration in urban projects.

    Developers and power companies are recognizing that the expense of underwater lines may be worth it if it helps them overcome political opposition.

  11. TenneT’s pylons should help allay that fear. Carrying all the cables in a “stack” between the poles, rather than hanging them separately on outward-facing arms, allows them to be arranged in a way that causes the individual fields generated by each cable to cancel each other out, weakening the overall field around the pylons. The result is far less low-frequency radiation. The combination of being less of an eyesore and producing less electrical smog should, TenneT hopes, soften objections to the construction of new overhead power lines.

    That is important for two reasons. First, the alternative—burying high-tension lines—is expensive and largely futile. The cost of putting a cable underground is between four and ten times as much as that of carrying it on a pylon. On top of that, the field generated by an alternating current interacts with the ground more strongly than it does with the air. This creates losses 40 times higher in a buried cable than in an aerial one. Unless the long-distance-transmission system were converted to direct current (which reduces transmission losses, but brings problems of its own), burial of transmission lines is not a serious option.

    The second reason TenneT’s pylons may be important is that despite these problems, a lot of new long-distance-transmission lines are going to have to be constructed, and soon. Wind power from the North Sea and the Atlantic Ocean will require that. So, more speculatively, will the idea of generating solar power in north Africa and transmitting it to Europe.

  12. Bipole III

    Transmission Reliability Project

    Bipole III is a proposed new high voltage direct current (HVDC) transmission project required to improve overall system reliability and dependability. The project includes the HVDC transmission line, energy conversion facilities, and system connections.

  13. HAVING decided to shut its nuclear power plants over the coming decade, Germany’s big idea for keeping the lights on is the Energiewende, or energy transition, a state-backed drive towards renewable energy. One of its most ambitious elements is to build 14 gigawatts (GW) of wind turbines off the North Sea and Baltic coasts by 2023, to provide about 9% of the country’s electricity needs by then. In the North Sea eight huge platforms will also be built, each seven storeys high and the size of a football pitch, to collect the output from the windmills, convert it into high-voltage direct current and then send it ashore through cables.

    The technology for building and operating this sort of transmission system in the howling gales of the German Bight is largely untried. And once the cables reach shore they will have to pass through populated areas on their way to connect with Germany’s central power grid. To appease NIMBYs (those who reflexively say “not in my backyard”) some of the lines may have to be buried, at further great expense: perhaps 25 times the cost of stringing the cables on pylons. As the project keeps slipping behind schedule the combined cost of the platforms and cables, most recently put at €8 billion ($10.4 billion), looks like rising further.


  14. Can parallel lines meet

    Power transmission: How to build a real supergrid by making existing electricity lines more efficient at transmitting power

    Now, however, an experiment by Amprion and TransnetBW, two German electricity-transmission firms, suggests it could be easier than engineers had feared. If true, this not only solves Germany’s local problem, it could also lead to the construction of a European supergrid to carry solar energy from the sunny south, and wind energy from the stormy west, to the continent’s industrial heartlands.

    The difficulty the transmission companies face is that they need to have their cake and eat it. They want to transmit power 400km along an existing line from North Rhine-Westphalia, which has a surplus generated by conventional means, to Baden-Württemberg, which relies on Philippsburg 2, a nuclear-power station that will shut, unless there is a change of policy by the end of 2019. Although using DC is the best way of doing so, they also have to keep some AC capacity on the line, since that is more useful for local purposes. This means having parallel cables hanging from the pylons, some carrying AC and some DC.

  15. A greener grid
    China’s embrace of a new electricity-transmission technology holds lessons for others

    The case for high-voltage direct-current connectors

    YOU cannot negotiate with nature. From the offshore wind farms of the North Sea to the solar panels glittering in the Atacama desert, renewable energy is often generated in places far from the cities and industrial centres that consume it. To boost renewables and drive down carbon-dioxide emissions, a way must be found to send energy over long distances efficiently.

    The technology already exists (see article). Most electricity is transmitted today as alternating current (AC), which works well over short and medium distances. But transmission over long distances requires very high voltages, which can be tricky for AC systems. Ultra-high-voltage direct-current (UHVDC) connectors are better suited to such spans. These high-capacity links not only make the grid greener, but also make it more stable by balancing supply. The same UHVDC links that send power from distant hydroelectric plants, say, can be run in reverse when their output is not needed, pumping water back above the turbines.

  16. Converting ac to dc was one of many jobs in which the vacuum tubes used in the first half of the 20th century were replaced by semiconductors in the second half. They offered improvements, but they still had problems—one of which was that it required the full power of an ac grid to get high-voltage dc (hvdc) lines going. When China built hvdc lines to bring solar and wind power from the north and west to the eastern seaboard in the 2010s it had to build coal-fired power plants alongside them to start them up.

    Technology which uses igbts does not have that problem. It also offers much more flexible switching, making the conversion process much easier, and takes up less space. That has proved quite the advantage. Mr Holt, the Siemens Energy board member, says that 99% of the hvdc systems now sold are based on igbts. And its attractions are also making the overall market larger. hvdc is not just a way to link far-off generators to existing grids, as in China and a number of developing countries with big, remote dams. It can also provide bridges from one part of a grid to another, thus easing congestion. And it can link together grids that could never be united into a single ac system.

    The valves, which Hitachi Energy makes using igbts from a specialist manufacturer, use components called capacitors to store small amounts of electric charge for brief periods of time. The igbts control the charging and discharging in such a way as to turn the ac input into a continuous dc output. They can also work the other way round, charging and discharging the capacitors in a way which turns incoming dc into ac.

    “Multiterminal” hvdc, only possible with the flexibility and control offered by igbt-based conversion, will allow big offshore wind farms to serve more than one grid, and to act as links between all the grids which they serve.


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