High-voltage direct current

High-voltage direct current transmission (HVDC) a process of the electric power transmission with high voltage.

• 5.1 converter installations
• 5.2 line systems and earthing

Technical Background

Electrical power is almost always produced in power plants by synchronous generators as a three -phase alternating current of frequency 50 Hz or 60 Hz. The transmission of large power, from about 1000 MW upwards, over a distance of some 100 km above financially and technically feasible cable diameter enforces very high voltages of about 400 kV, so that the current can remain below about 2.5 kA. Traditionally, this high voltage is generated by power transformers for three-phase in the power plant with very good efficiency. At the end of the transmission line it is transformed down in substations to lower 3- phase AC voltages such as 110 kV or 30 kV.

In the alternating-current transmission is one of the basic requirements that the capacitance between the wires and grounding connection will remain small to keep the reactive power low. For overhead lines can this be done by sufficient distance submarine cables over a few 10 km in length allows the capacitive load is not economical operation. In this case, the transfer brings with DC benefits because they find no reactive power.

Disadvantages

The disadvantage of - compared with a transformer - very high technical complexity for high-voltage suitable, consuming power converters, which are housed in the so-called converter building or converter station.

In addition, power flows can be meshed in DC systems only control bad. In meshed AC networks like the power grid to control the load flow is well controlled in individual lines through specific phase shifts and the control of reactive power. This possibility is absent in the DC transmission, which in principle can only transfer active power. Therefore, the HVDC transmission is limited to a few exceptions, with simple branches to direct connections between two points. Methods and techniques for the realization of meshed DC systems are currently being studied ( CIGRE WG B4.52, and others).

History

The first attempt at a telecommunication DC power took place in 1882 by Mies Bach to Munich. Smaller and more likely attributable to the medium-voltage HVDC systems originated from the 1890s, particularly in Italy and Switzerland, for example, St- Maurice -Lausanne (22 kV, 3.7 MW, 60 km, 1897). The first HVDC system was the Lyon- Moutiers system with 150 kV bipolar voltage, 14.7 MW power transmission and 200 km in length. The plant was in operation from 1906 to 1936 and worked without Umrichtwerke. The electrical energy was generated by series-connected DC generators directly in a hydroelectric power plant at Moutier and implemented by DC machines in Lyon.

The first German HVDC system was from 1941 started, but never went into operation in bipolar cable transmission of the same project between the lignite-fired power plant Vockerode ( near Dessau ) and Berlin (symmetrical voltage of 200 kV to ground, maximum transmission power 60 MW). This facility was dismantled by the Soviet occupation forces in 1950 and used to build a 100 -kilometer-long, monopolar high voltage direct current transmission line with a capacity of 30 MW and an operating voltage of 200 kV between Moscow and Kashira. This line is now closed.

1954 HVDC system between the Swedish island of Gotland and the Swedish mainland was put into operation. The oldest existing HVDC system is the Kontiskan 1 between Denmark and Sweden. Significant work to improve the HVDC technology have been made in the 1960s by Uno Lamm. According to him, the award Uno Lamm Award is named, which is awarded annually since 1981 by the IEEE Power Engineering Society for essential work in the field of HVDC technology.

In 1972, the first HVDC system has been in the Canadian Eel River taken with thyristors in operation and 1975 in England HVDC Kingsnorth power station between the North and King of the City of London with mercury vapor rectifiers. On March 15, 1979, a HVDC link between Cahora Bassa and went Johannesburg (1410 km) with ± 533 kV and 1920 MW. This line was built by AEG, BBC and Siemens. The Fenno -Skan between Sweden and Finland was put into operation in 1989.

In Germany 1991-1993 was the first HVDC system in the form of HVDC short coupling in Etzenricht. In 1994, the 262 km long DC line Baltic Cable between Lübeck- Herrenwyk and Kruse mountain in Sweden in operation, the 170 -kilometer fully wired Kontek between Bentwisch followed in Rostock and Bjæverskov in Denmark in 1995.

With 580 kilometers of the end of September 2008, inaugurated NorNed is called connection between Feda in Norway and Eemshaven in the Netherlands at present ( 2008) longest subsea connection of this kind, the operators Statnett and TenneT. The plant with the highest current transmission voltage of ± 800 kV and a transmission capacity of 5000 MW over 1500 km HVDC is the Yunnan - Guangdong between the Chinese provinces of Guangdong and Yunnan. The commercial operations began in June 2010.

One of the largest manufacturers of HVDC systems include Alstom, Siemens and Asea Brown Boveri (ABB).

Applications

High-voltage direct current transmission is used for electrical power transmission in various and shown in the following fields of application. In the list of HVDC systems there is a table listing various realized and proposed facilities.

DC Kurzkupplungen

If the transfer length of the direct current only a few meters and both converters in the same building or in immediately adjacent buildings are housed, one speaks of an HVDC close-coupling ( DC short coupling, GKK, English Back to back converter ). This form, technically an intermediate circuit, provides a direct electrical energy exchange between the three-phase alternating current systems, which are not mutually operated with synchronous grid frequency and different control ranges are assigned. Examples are the 1993 to 1995 operated in Germany GKK Etzenricht or in Canada the Châteauguay - DC is a close coupling of the Hydro-Québec. In Japan, caused to be transmitted by two different network frequencies between the two frequency systems performance only by means of HVDC short couplings. One example is the plant in Shizuoka.

Power transmission over long distances

The HVDC technology is the transfer of energy by means of direct current over long distances - these are distances of around 750 km upwards - since the HVDC despite the additional converter losses in total has a lower transmission losses than the transmission with three-phase alternating current from certain distances. Examples are unfinished, with final 2400 km long HVDC Ekibastus center in Siberia, 1700 km long HVDC Inga - Shaba in the Congo and the 1000 -km-long HVDC Quebec- New England between Canada and the United States. In Europe exist due to the comparatively narrow spatial relationships so far no HVDC systems in this length range.

The essential at large distances conduction losses without converter losses are realized at facilities such as the NorNed at a transmitted power of 600 MW ( 85 % of nominal power) and a line length of 580 km around 3.7 %, which is about 6.4% relative losses 1000 km line length corresponds.

Underground and submarine

The high-voltage direct -current transmission is also used for power transmission over short distances comparable by some 10 km up to several 100 km, when the electric transmission cable by design has a very high capacitive pad. This is typically in submarine cables or underground cables in the case, which is why there are HVDC systems in Europe almost exclusively in this application. European examples are the submarine cable NorNed between Norway and the Netherlands, the submarine cable Baltic Cable between Sweden and Germany or BritNed between Britain and the Netherlands.

Special Applications

In addition, the technology of HVDC is applied to a smaller extent also for special solutions such as:

• In the context of Flexible AC Transmission System (FACTS ) to make by means of transverse and longitudinal control using the technique Unified Power Flow Controller ( UPFC ) on individual lines in the three-phase AC networks targeted load flow control.
• For de-icing of overhead lines as in the Lévis - icer in Canada.

Execution

Converter installations

There is a converter station, also called converter station at both ends of a high voltage direct current transmission system. In addition to the control systems essentially contains the converter and usually in the outdoor area adjacent to the hall, the converter transformers, smoothing reactors and harmonic filters. The power converters used to work both as a direct and as an inverter, thus allowing both directions in the load flow normally in both directions. There are also special HVDC Pacific DC Intertie as on the west coast of the U.S. or the HVDC Inter- Iceland in New Zealand, which transmit the electric power in normal operation only in one direction.

The interior of a HVDC converter hall with the inverter is completely shielded usually due to the electromagnetic compatibility metallic from the outside area and can not be accessed during operation. For the converters connected in twelve-pulse circuit thyristors and IGBTs are now also used in modern systems. In old plants mercury arc rectifiers came with a very large construction used. In order to ensure the required blocking voltage of about 500 kV, several dozen thyristors / IGBT are connected in series, since the reverse voltage per thyristor / IGBT technological reasons, is only a few kV, respectively. All thyristors connected in series have almost the same trigger within a microsecond to avoid damage due to unequal distribution of stress on the inverter.

The thyristors or IGBTs are controlled because of the strong electromagnetic interference in the interior of the hall is not directly electrically by means of copper cables, but with fiber optic light guides. The disturbances are due to the high rate of change of voltage at the same potential between the driving unit and is at high- voltage potential thyristors is achieved. In no longer located in the regular operating systems with mercury arc rectifiers, the transmission of the ignition pulses by means of high frequency was performed.

For removing the power dissipation of the thyristors liquid coolant such as pure water may be used, which is pumped into the electrically insulated pipe system by the converter hall to the individual thyristors. The heat is dissipated to the ambient air outside the hall in the form of heat exchangers.

The smoothing coil at the DC output is used to reduce the ripple of the DC current. They can be designed as air - or iron choke. Their inductance is about 0.1 H to 1 H.

With the transformers on the AC side of not only the high voltage is generated, they suppress next with their inductance and circuit manner ( series connection of delta and star connection ) already numerous superimposed harmonics of the delivered current. The harmonic filter on the three-phase side in turn suppress other undesirable harmonics. When investing in twelve-pulse circuit they must suppress only the 11th, the 13th, the 23rd and the 25th harmonic. Sufficient for this purpose on the 12th and 24th harmonic of resonant circuits tuned.

In addition, they also serve to generate the required reactive power for commutation. In principle, an HVDC transmission can be realized even without harmonic filters, such as in the station Volgograd Volgograd -Donbass HVDC.

Line systems and earthing

The transfer can take place both monopolar and bipolar.

• Monopolar in this context means that a DC voltage of a particular denomination, such as 450 kV is present, with one pole on the two cable ends is connected to ground and, therefore, a conductor wire pair ( earth as " return line ").

• Bipolar means that in contrast to the monopolar HVDC two conductors are used, which is grounded on medium potential: a conductor, the relation to the earth has a positive voltage and a conductor, the relation to the earth has a negative voltage, for example, ± 450 kV. In this case, the DC voltage between the two conductors is double the voltage and between a conductor and earth, in this example, 900 kV.

In a bipolar system, the grounding of the center potential is used to avoid damage to the insulation due to an uneven distribution of stress between the conductors, since the isolation of the two conductors made ​​against ground potential. The earthing leads in bipolar systems operating power, but only a small transient current. In a monopolar HVDC, the operating current of the installation of some kilo ampere is passed over the earth electrode. According spacious, with an extension of several kilometers, the Erderanlage must be designed and good conductivity, such as near the coast in the sea or in the area of rivers, be anchored in the ground. As with any grounding the area and shape of the earth electrode and the electrical conductivity in the immediate vicinity of the earth electrode is primarily decisive for a low earth resistance. Due to the large cross-sectional area, the electrical conductivity of the rest of the earth material practically does not matter between the two Erderelektroden the widely separated HVDC converter plants.

Bipolar systems can also be designed so that, if required, can also be operated as two parallel monopoles is possible. This was realized in the HVDC Inga - Shaba. It also comes as it is direct current, depending on the current direction and the material used for an electrolytic decomposition at the earth electrode. In particular, the anode is subject to a decomposition process, similar to a sacrificial anode, which is why they are implemented, for example, petroleum coke, or in the form of titanium nets. The cathode may be designed as large bare copper rings. Many bipolar systems are designed so that even a monopolar operation is possible. If electrodes are to serve as both a cathode and an anode, as in these cases, they must all be designed to be corrosion resistant.

HG overhead lines usually have two conductors. Frequently monopolar lines for future bipolar expansion can be equipped with two conductor cables that as long as the bipolar expansion was not completed, be connected in parallel or one of which serves as a low- voltage conductor for grounding. Almost always, the one-level arrangement of conductor cables is applied.

The conductor for grounding electrode can also take on the role as earth wire because it is a very low resistance grounded through the grounding electrode. But he needs to in order to avoid electrochemical corrosion of the masts may be attached to this isolated. For the derivation of lightning currents spark gaps are therefore needed on the insulators.

To avoid electrochemical corrosion, the grounding electrode shall not be directly in the line route, so that a separate routing is necessary at least for the last piece of the electrode line. This may, as in the case of non-parallel transfer of the electrode line to the high -voltage line, either as an overhead line ( similar to a medium-voltage line ) underground cables or overhead lines and underground cables combination of be designed. The insulation of the electrode lead is generally designed for an operating voltage of about 10 to 20 kV ( medium voltage ).

Benefits

In the popular three-phase three-phase systems are always compounds having at least three conductor strands needed. In contrast, comes the DC transmission with two, when using the earth as the second pole even a single ladder. This saves both the pipe material as well as the overhead line system ( poles and insulators, etc.) high costs.

The HVDC allows power transmission through submarine cables over long distances. Through the Inherent structure of a cable with external shield in inner conductor has a submarine cable compared to an overhead line a high capacitance per unit length. The transhipment of this capacity generated with AC voltage reactive currents, an additional load on the cable. For three-phase lines reactive power compensation of the line is required so that the cable is burdened about the natural performance. Therefore compensating coils must be installed along the line at certain intervals. This is possible only with great technical effort for submarine cables. Therefore, the HVDC is used from 70 km transmission distance under water. The differences are shown in the picture and are illustrated by the transfer of 1500 MVA over a distance of 500 km.

• In a transmission line, the total reactive power is 500.3, 8 = 1900 MVA MVA ( inductive). This is therefore greater than the effective power, the system is still economical.
• In a cable system, the total reactive power is 500.8 MVA = 4000 MVA (capacitive). Since this exceeds the effective power several times, the system is very uneconomical.
• In an HVDC, the total reactive power 0 MVA.

In direct current the skin effect does not occur in appearance, gives rise to alternating current displacement at the edges of the pipe cross-section. Therefore, the cable cross sections can be better utilized than in a comparable AC transmission.

With DC voltage occur in the cable insulation no dielectric losses, and inhomogeneities not lead to pre-discharges. The insulation can therefore be carried out less expensive than for a three-phase cable. For overhead lines the losses by corona discharges are much lower than with an equally high AC voltage at DC voltage; they require at alternating voltage at lower voltages above about 100 kV conductor bundle to reduce the field strength at the conductor surface.

While synchronization is mandatory within an AC network, this does not apply to the direct power transmission. HVDC is also sometimes used on the intermediate compounds in a large spatially extended synchronous AC mains. An example of such a route is the HVDC Italy - Greece within the European power system synchronous between the Italian city Gala Tina and about 300 km from the place Arachthos in Greece - but this is only because of the length of the HVDC submarine cable needed.

In addition, should the DC power isolation not be interpreted to a peak value of, as with DC voltage corresponding to the peak voltage of the effective voltage.

Disadvantages

The converter stations are very expensive, technologically complex and little overload compared with three-phase transformers. The reasons of safety attached to the outside of the converter station converter transformers generate the harmonics more noise than comparable power transformers in substations normal.

With short connections are the losses incurred in the converter is greater than the reduction of losses in the line by the use of direct current, so the HVDC is not useful for short transmission distances. Exceptions are the HVDC close couplings with which each asynchronous AC networks can be associated with correspondingly high converter losses.

In the steady state arising at high DC voltages from 500 kV problems caused by pollution and wetting by rainwater ( outdoor installations ) on the insulator surfaces and conduit channels: A moist dirt pad can therefore cause a distortion of the electric field along the insulator, that of a breakdown along can lead insulator and in its failure. For this reason, much longer than insulators in alternating current used in HVDC.

View

As an alternative to conventional HVDC technology with line-commutated converters with current intermediate circuit technologies increasingly come with self -commutated converters with voltage intermediate circuit used. It can be used as switching elements, for example, IGBTs. Such systems are however only been used for small loads.

DC lines with more than two stations or even DC systems remain questionable. In theory, such systems are feasible, but so far only a few such facilities as the SACOI (HVDC Italy - Corsica - Sardinia) are practically been executed because of this a lot of effort is needed and also easily can degrade the transmission characteristics. When contemplated and previously completed projects such as Desertec or the European super grid is assumed in a 5000 -km-long HVDC transmission line 800 kV line losses by 14%. This represents approximately 2.8% relative conduction losses at 1000 km.

In September 2011, the Frankfurter Allgemeine Zeitung and Financial Times Germany reported that the German transmission system operators plan three power lines in HVDC technology in Germany, with which particular power should be transported by wind to southern Germany. plan Accordingly

• 50 Hertz Transmission a route from Magdeburg in the Rhine -Main area,
• TenneT one among others along the Rhine running route of Schleswig -Holstein to Bavaria and
• Amprion and EnBW a route between the Rhineland and Baden- Württemberg.

On October 24, 2013 Tennet and EnBW announced that they want to build a 800 km long HVDC, which is scheduled for completion in 2022. The project consists of two connections: one of Wilster (60 km north of Hamburg ) to Grafenrheinfeld near Schweinfurt (distance: about 550 kilometers by road ) and one of Brunsbüttel to Großgartach in Baden- Württemberg. The project is called " SuedLink ". In February 2014, the two operating companies suggested the course plan for the backbone of Wilster after Grafenrheinfeld. The planned as a direct current connection route is to go in 2016 in construction and will be completed in 2022.

In November 2012, the company ABB announced to have developed a direct-current circuit breakers for high voltages and currents and use in pilot projects to want. The construction of a meshed HVDC network would considerably facilitated. The circuit breaker is a combination of electronic and mechanical elements.

To avoid riparian complaints when laying new tracks, is also contemplated to use existing AC lines as HVDC transmission lines ( HVDC conversion).

Under the brand name Ultranet is a "parallel" HVDC and AC distance of 430 km between the north and south of Germany planned: they could go into operation in 2017-2019. This direct current and alternating current would fließen.Vorlage in various conductor cables on the same poles: Future / in 3 years

A 1000 km long cable is considered to flow from geothermal power plants in Iceland to the UK to lead.