Background Information

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1 HVDC smart solution for a wide range of power transmission applications Background Information Erlangen, April 16, 2015 HVDC transmission technology is the method of choice for transmitting large amounts of electrical power over long distances with minimal losses. In developing regions, HVDC transmission systems now transmit several gigawatts (GW) of electrical power across thousands of kilometers. Power from remote offshore wind farms on the open sea can be efficiently fed into the power supply grid on-shore via HVDC technology. HVDC transmission systems with compact, voltage-sourced converters open up additional fields of application for HVDC technology, for example international grid connections (cross-border transfer points) for the coordinated exchange of power between two countries in the interconnected European system. A further important application lies in combining non-synchronous power grids using HVDC back-to-back links. These enable grids that use different frequencies, for example, to be connected together reliably. A high-voltage direct current transmission system (HVDC) essentially consists of a converter station in which the AC voltage from a conventional power grid is converted into DC voltage, a power transmission cable and a further converter station at the other end, where the DC voltage is converted back to AC. The electrical energy can be transmitted in both directions. The cables can be overhead lines or underground cables on land or submarine cables under water. A mixed arrangement is also possible, for example cables and overhead lines or submarine and underground cables. Transmission losses are lower with HVDC transmission than with comparable threephase transmission. The DC transmission voltage is several hundred kilovolts (kv), and can be as high as 800 kv in major systems today. The higher the voltage, the lower the transmission losses and the greater the amount of electrical power that can be transmitted via the cable. In late 2009, Siemens in China put the world s first HVDC Siemens AG Communications and Government Affairs Leitung: Stephan Heimbach Wittelsbacherplatz München Deutschland Hintergrund-Information: BI e Page 1/7

2 transmission system with a DC transmission voltage of 800 kv into operation, establishing itself as a global technology leader. Between England and Scotland, Siemens is constructing the world s first HVDC submarine cable connection with a DC transmission voltage of 600 kv. Previously, the highest DC voltage that could be transmitted by HVDC submarine cable was 500 kv. More dynamics in the system and a space-saver: HVDC Plus Alongside the traditional HVDC technology, mainly for the low-loss transmission of large amounts of electrical power across long distances, Siemens offers its HVDC system HVDC Plus, with converters based on self-commutating voltage-sourced converter (VSC) technology in a modular multilevel converter (MMC) configuration. Unlike traditional HVDC technology, in which the commutation processes in the converter are determined by the mains voltage, VSC converters with an intermediate voltage circuit have no driving grid voltage. VSC converters create this using the DC voltage itself. Because the VSC converters need no leading mains voltage, they can act as an electronic generator and build up the grid themselves, which enables them to perform black starts. Thyristors, which are used in converters in traditional HVDC technology, are gridcommutated and are therefore able only to energize the current flow. Deenergizing is achieved by the driving mains voltage at current zero. Selfcommutating power electronics, IGBT in the case of HVDC Plus, can energize and de-energize as required, which enhances the dynamics of the system. Connected AC grids benefit from the fact that reactive and active power can be dynamically controlled independently of each other. The design of the HVDC Plus systems also saves more space, since less space, or in some cases none at all, is used for compensation filters. This enables HVDC Plus systems to also be used on offshore platforms, to provide the grid connection for offshore wind farms located far from the coast, or as the HVDC solution for densely built-up urban environments. Siemens created the first HVDC Plus system to transmit 400 MW across San Francisco Bay from Pittsburg, California, to San Francisco to meet the city s growing demand for electricity. The Transbay HVDC Plus project also diminished the need Hintergrund-Information: BI e Page 2/7

3 to build new power stations in San Francisco, reduced bottlenecks in the East Bay transmission grid and improved overall power supply security and reliability. Less transmission losses with HVDC compared to three-phase systems Transmission losses with high-voltage direct current transmission are generally 8 to 15 percent less compared to transmission via a comparable three-phase system. A standard HVDC transmission system normally has two poles, each of which carry half of the power. If one pole or one cable should suffer an outage, half of the transmission capacity would still be available. HVDC connections are structured to accommodate overload operation for a limited time. Compared to a similar three-phase line the transmission losses with an HVDC link are 30 to 40 percent less. HVDC can also act as a firewall to prevent faults from spreading between connected three-phase systems and thus avoid power outages. For distances greater than 600 km, overhead line connections that use HVDC technology are also more economical than three-phase systems. With cable connections, the cost-efficiency limit is reached at about 80 km. Once this distance is exceeded, virtually no power reaches the other end with underground or submarine cables using three-phase transmission, because usable energy is lost through the uploading and downloading processes. In an HVDC submarine cable connection at 600 kv DC and 2,200 megawatts (MW), as in the cable between Scotland and England, losses are just below three percent for a 400 km connection, including line and converter losses. HVDC market valued between 5 and 7 billion p.a. for the next five years The market for energy transmission is fundamentally volatile because it is characterized by large-scale projects. The global market for energy transmission solutions (HVDC connections, offshore power links and FACTS grid stabilization systems) will be valued between 5 and 7 billion p.a. over the next five years. The HVDC market, included in this total, will virtually double within five years from its most recent figure of 2.5 billion p.a. Demand for HVDC technology is growing rapidly. Hintergrund-Information: BI e Page 3/7

4 The past 40 years have seen HVDC connections totaling 100 GW installed worldwide. This total will increase by a further 250 GW in this decade alone. Siemens is a leading supplier on the high-voltage direct current transmission market and has implemented about 50 HVDC projects world-wide, one-fourth of these in China. The total amount of electrical power that flows via these HVDC connections is comparable to the average power consumption of entire countries such as Spain or Italy. International system connections The preconditions for a common European energy market are more than just stable grids and good supply reliability: they also include properly developed international connections (cross-border transfer points). Fewer bottlenecks for international energy trade mean electricity demand in the countries in question can be covered more efficiently. HVDC connections between countries prevent the occurrence of these bottlenecks. It will also be possible to smooth out local fluctuations in power generation or consumption. Western Link, a Siemens HVDC connection between England and Scotland, and BritNed, another link connecting the UK and the Netherlands, also fit into this category. A Siemens HVDC transmission system between Estonia and Finland, with a transmission capacity of 650 MW and a DC voltage of ± 450 kv, boosts the power transmission capacity between the Baltic and Nordic countries from 350 MW to 1,000 MW. HVDC connections also improve power supply reliability. The capacity between the two stations flows along a 14 km overhead line, a 145 km submarine cable through the Gulf of Finland and a 12 km underground cable. Connecting offshore wind power plants to the grid High-voltage direct current transmission is essential once a cable exceeds a length of 80 km, because the losses with three-phase transmission become excessive at that point. In Germany, offshore wind farms are located far from the coast to protect the landscape and to benefit from greater wind exploitation. For the Sylwin 1 HVDC grid connection, for example, which links the DanTysk, Sandbank 24 and Nördlicher Grund wind farms to the power supply system, the submarine cables are about 160 km long. Because of the greater opportunities for wind exploitation and the fact that Hintergrund-Information: BI e Page 4/7

5 the areas closer to the coast had already been allocated in the first two tender rounds, future wind power plants in the UK will be constructed further from the coast. For Round 3, involving a capacity of 32 GW, areas located between 40 and 200 km off the coast have been identified for wind farms. HVDC submarine cables will be used to transmit the capacity generated there to the coast. Siemens recently commissioned the second offshore wind farm grid link, HelWin1, and delivered it to the ordering party. The offshore platform containing the HVDC Plus converter system lies about 85 km off the coast to the north-west of the island of Heligoland, which gave the platform its name. All in all, this grid connection will allow up to 576 megawatts of green power to be transmitted, which will be enough to supply more than 700,000 households in Germany. HVDC Plus keeps the transmission losses per connection, including cable losses for this type of wind farm grid connection, down to below four percent. HVDC: the right choice if a new power station cannot be built Mallorca is connected to the Spanish mainland grid via a 400 MW HVDC connection using a 244 km cable through the Mediterranean Sea. The benefits are twofold: the reliability of the electricity supply to Mallorca has been enhanced, and it was possible to avoid building a further power station on the island. The need to build new power stations in San Francisco to accommodate increasing electricity demand was tapered by the creation of a 400 MW HVDC connection to the city from Pittsburg (California) through San Francisco Bay. The HVDC connection also reduced bottlenecks in the transmission grid in the East Bay and improved overall power supply safety and reliability. Close couplers back-to-back converters HVDC back-to-back links that connect two non-synchronous AC networks and act as a firewall to prevent faults from spreading to the adjacent grid are a further application for HVDC technology. Unlike long-distance HVDC transmission, which may involve distances of 1,000 kilometers or more between the two converter stations, with an HVDC back-to-back link the connection between both converters is essentially back-to-back. The three-phase current from one grid is converted to DC in the first converter, and is directly transmitted to the other via an intermediate DC Hintergrund-Information: BI e Page 5/7

6 circuit. There, the DC current is converted back to three-phase with simultaneous adjustment to the parameters (i.e. frequency, phase angle) of the grid into which it will be fed, to ensure that both grids are linked without problems. In the country of Georgia, for example, Siemens created two turnkey HVDC back-to-back links to connect the Georgian power supply system to the Turkish grid. The two back-toback systems each transmit 350 MW of controlled capacity from various hydropower stations in Georgia to Turkey. In the US, Siemens has connected New Jersey s power supply system with New York s using an HVDC back-to-back link, to enable New York to receive a further 660 MW. Here, however, the overall arrangement also includes, besides the back-to-back solution with the two converters, a 12 km 345 kv high-voltage three-phase cable that runs via the Hudson River. The HVDC back-toback link makes it possible to direct the power flow and can help stabilize the connected grids since it can be quickly adjusted if a fault or an outage occurs and also prevents bottlenecks in New York s power supply. Low-loss, long-distance transmission of bulk power Powerful HVDC connections are in demand in large countries with strongly growing electricity needs such as China and India, in particular, to bridge long distances with bulk power as part of renewable energy projects. In China, for example, Siemens constructed the 800 kv Yunnan-Guangdong HVDC connection with a transmission capacity of 5,000 MW. This 1,400 km connection transmits the electrical power generated by numerous hydropower stations from the province of Yunnan to the industrial region in the Pearl River Delta in Guangdong province, home to the cities of Guangzhou and Shenzhen, with minimal losses. In India, too, Siemens set up a 780 km high-voltage direct current transmission system linking the provinces of Uttar Pradesh and Rajasthan. The 500 kv HVDC system, with a transmission capacity of 2,500 MW, bolsters the power supply for the growing region around the megacity of New Delhi, without having to construct additional power stations there. This background paper is available at For further information on HVDC transmission technology, please see Hintergrund-Information: BI e Page 6/7

7 Contact for journalists Dietrich Biester Phone: Follow us on Twitter at: Siemens AG (Berlin and Munich) is a global technology powerhouse that has stood for engineering excellence, innovation, quality, reliability and internationality for more than 165 years. The company is active in more than 200 countries, focusing on the areas of electrification, automation and digitalization. One of the world s largest producers of energy-efficient, resource-saving technologies, Siemens is No. 1 in offshore wind turbine construction, a leading supplier of combined cycle turbines for power generation, a major provider of power transmission solutions and a pioneer in infrastructure solutions as well as automation, drive and software solutions for industry. The company is also a leading provider of medical imaging equipment such as computed tomography and magnetic resonance imaging systems and a leader in laboratory diagnostics as well as clinical IT. In fiscal 2014, which ended on September 30, 2014, Siemens generated revenue from continuing operations of 71.9 billion and net income of 5.5 billion. At the end of September 2014, the company had around 343,000 employees worldwide on a continuing basis. Further information is available on the Internet at Hintergrund-Information: BI e Page 7/7