High-voltage direct current

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HVDC or high-voltage, direct current electric power transmission systems contrast with the more common alternating-current systems as a means for the bulk transmission of electrical power. First developed in the 1930s in Sweden at ASEA, early commercial installations included the USSR in 1951 between Moscow and Kashira, and a 10-20 MW system in Gotland, Sweden in 1954. [1]

The rise of AC transmission

DC transmission remains the exception, rather than the rule, in power transmission. There are environments where HVDC is the conventional solution, such as in submarine cables and in interconnecting unsynchronized AC systems, but for the bulk of situations AC transmission remains dominant.

Early attempts at transmission used direct current. However, early in the development of electrical power, AC came to dominate as a means of interconnection between generation plants and machinery. The principal advantage of AC is the possibility of using transformers to efficiently transform voltage used in power transmission. With the development of efficient AC machines, such as the induction motor, AC transmission became the norm (see War of Currents).

The ability to transform voltages is an important economic and technical consideration as, whilst high voltages are harder to manipulate and more dangerous, the lower currents required with high voltage transmission for any given level power lead to high voltage transmission requiring smaller cables, and less loss of power in the form of heat. Transmission can also be limited by peak voltage - a DC line can operate at the same peak voltage as an AC line while carrying significantly more power under this limit. Therefore, with high voltages being optimal for bulk transmission, and lower voltages for industrial and domestic utilisation, the ability of AC to be effectively transformed in voltage a number of times during transmission led it to become, and remain, the dominant means of electrical power transmission.

No equivalent of the transformer exists for direct current, so the manipulation of DC voltages is considerably more complex.

Advantages of HVDC over AC Transmission

Despite alternating-current being the dominant mode for electric power transmission, in a number of applications HVDC is often the preferred option.

• Undersea cables. (eg. 250km Baltic Cable between Sweden and Germany [3]).
• Endpoint-to-endpoint long-haul bulk power transmission without intermediate 'taps', for example, in remote areas.
• Increasing the capacity of an existing power-grid in situations where additional wires are difficult or expensive to install.
• Allowing power transmission between unsynchronised AC distribution systems.
• Reducing the profile of wiring and pylons for a given power transmission capacity. HVDC can carry more power per conductor, because for a given power rating the constant voltage in a DC line is lower than the peak voltage in an AC line. This voltage determines the insulation thickness and conductor spacing.
• Connection of certain generating plant to the distribution grid.
• Stabilising a predominantly AC power-grid.

Long undersea cables have a high capacitance. This causes AC power to be lost extremely quickly in reactive and dielectric losses, even on cables of a modest length. HVDC can carry more power per conductor, because for a given power rating the constant voltage in a DC line is lower than the peak voltage in an AC line. This voltage determines the insulation thickness and conductor spacing. This allows existing transmission line corridors to be used to carry more power into an area of high power consumption, which can lower costs.

Possible health advantages of HVDC over AC Transmission

For some time it has been suspected that there is a connection between the inductive field of alternating currents (especially at the common line frequencies of 50 or 60 hertz) and certain diseases, although statistical studies of certain working populations with greater exposure have been either inconclusive or disputed by various parties of economic interest. One property of direct current transmission is a lack of this alternating magnetic field. Such alternating fields have recently (2003) been shown in laboratory studies to increase the saturation of free radicals in the blood mass of animals (this increase may be combated with doses of anti-oxidants). Free radicals are implicated as possible causes or contributors to a number of diseases. The benefits would extend only to those near the transmission lines, as the suspected magnetic field problems are also associated with high current AC transmission and also the associated transformers, motors, and generators, and even common household appliances such as corded electric shavers and (especially) inductively charged cordless electric toothbrushes.

AC networks interconnections

Using ac transmission lines, only synchronized AC networks can be interconnected: those which oscillate at the same frequency and in phase. Many areas which wish to share power have unsynchronized networks. The UK and continental Europe both operate at 50Hz but are not synchronized. Japan has 50Hz and 60Hz networks. Continental North America, whilst operating at 60Hz throughout, is divided into regions which are unsynchronised, East, West, Texas and Quebec. Brazil and Paraguay which share the massive Itaipu hydroelectric plant, operate on 60Hz and 50Hz respectively. However, HVDC systems makes it possible to interconnect unsynchronized AC networks, and also adds the possibility of controlling AC voltage and reactive power flow. Even a black net can be interconnected.

A generator connected to a long ac transmission line may become unstable and fall out of synchronization with a distant ac power system. A HVDC transmission link may make it economically feasible to use remote generation sites. Wind farms located off-shore may use HVDC systems to collect power from multiple unsynchronized generators for transmission to the shore by an underwater cable.

In general, however, an HVDC power line will interconnect two AC regions of the power-distribution grid. Machinery to convert between AC and DC power is expensive, and a considerable cost in power transmission. The conversion from AC to DC is known as rectification, and from DC to AC as inversion. Above a certain break-even distance (about 50 km for submarine cables, and perhaps 600-800 km for overhead cables [3]), the lower cost of the HVDC cable outweighs the cost of the electronics. The conversion electronics also present an opportunity to effectively manage the power grid by means of controlling the magnitude and direction of power flow. An additional advantage of the existence of HVDC links, therefore, is potential increased stability in the transmission grid.

Rectifying and Inverting

Rectifying and Inverting Components

Early systems used mercury-arc rectifiers, which were unreliable. The thyristor valve was first used in HVDC systems in the 1960s. The thyristor is a solid-state semiconductor device similar to the diode, but with an extra control terminal that is used to switch the device on at a particular instant during the AC cycle. The insulated-gate bipolar transistor (IGBT) is now also used.

Because the voltages in HVDC systems, around 500 kV in some cases, exceed the breakdown voltages of the semiconductor devices, HVDC converters are built using large numbers of semiconductors in series.

The low-voltage control circuits used to switch the thyristors on and off need to be isolated from the high voltages present on the transmission lines. This is usually done optically. In a hybrid control system, the low-voltage control electronics sends light pulses along optical fibres to the high-side control electronics. Another system, called direct light triggering, dispenses with the high-side electronics, instead using light pulses from the control electronics to switch light-triggered thyristors (LTTs).

A complete switching element is commonly referred to as a 'valve', irrespective of its construction.

Rectifying and Inverting Systems

A simple DC to AC converter using 2 solenoids, a capacitor and a triode

Rectification and inversion use essentially the same machinery. Many substations are set up in such a way that they can act as both rectifiers and inverters. At the AC end a set of transformers, often three physically separate single-phase transformers, isolate the station from the AC supply, to provide a local earth, and to ensure the correct eventual DC voltage. The output of these transformers is then connected to a bridge rectifier of a number of valves. The basic configuration uses six valves, connecting each of the three phases to each of the DC rails. However, with a phase change only every sixty degrees, considerable harmonics remain on the DC rails.

An enhancement of this configuration uses twelve valves (often known as a twelve-pulse system). The AC is split into two separate three phase supplies before transformation. One of the sets of supplies is then configured to have a gamma secondary, the other a delta secondary, establishing a thirty degree phase difference between each of the sets of three phases. With twelve valves connecting each of the two sets of three phases to the two DC rails, there is a phase change every thirty degrees, and harmonics are considerably reduced.

In addition to the conversion transformers and valve-sets, various passive resistive and reactive components help eliminate harmonics on the DC rails.

Polarity

Polarity and Earth Return

Once the DC has been generated, a constant potential difference exists between two rails. It must then be determined which rails are earthed. In a common configuration, one of the rails is earthed, establishing it at earth potential. The other rail, at a potential high above, or below, ground, is connected to a transmission line.

The earthed rail may or may not be connected to the corresponding connection at the inverting station by means of a second transmission line conductor. This is a common source of terminological confusion. A monopole transmission line is sometimes, confusingly, used to refer to a transmission line without an accompanying earth conductor.

Without an accompanying earth, the potential is equalised by an earth current (known as telluric currents and other extremely low frequency waves) between the earth electrodes at the two stations. There are many areas and jurisdictions where such a considerable earth current is not permissible. The issues surrounding earth-return current include

• Areas which contain extended metal objects, such as pipelines, may have a considerable current induced, resulting in oxidation problems if the object is not effectively cathode protected, and sparking and shock problems if the earthing is incomplete.
• If either of the earth electrodes is near the sea, the current flowing through the body of water, where concentrated, can cause toxic chlorine emission and alkalinisation of the water body near the electrode.
• The presence of a considerable earth current can generate an anomulous and extensive DC magnetic field, which can affect navigational compasses.

These effects can be compensated for by laying a second conductor alongside the monopole, for carrying the earth current. In 2000s the issues of a metallic return remained a controversial political as well as technical issue[4], through the conflicting economic and environmental interests.

An alternative to monopolar transmission is bipolar transmission. In bipolar transmission a pair of conductors is used, each at a high potential with respect to ground, in opposite polarity. Bipolar transmission is a more expensive option than monopolar transmission owing to the increased cost of line. Although monopolar transmission with an earth return uses two conductors, the earth return is at a negligible potential with respect to its surroundings, minimising or eliminating additional insulation costs. Bipolar transmission, by contrast, requires two high-potential lines. However, there are a number of advantages to bipolar transmission which can make it the attractive option.

• Under normal load, negligible earth-current flows, as in the case of monopolar transmission with a metallic earth-return; minimising environmental impact.
• When a fault develops in a line, if considerable electrodes have been installed at earth potential at each end of the line, current can continue flow under these fault conditions using the earth as a return path, operating in monopolar mode.
• As, for a given power, bipolar lines carry only half the current of monopolar lines, the cost difference of producing monopolar and bipolar cables is less than might otherwise be imagined.

Polarity and Corona Discharge

Corona discharge is the creation of ions in a fluid (such as air) by the presence of a strong electromagnetic field. Electrons are torn from unionised air, and either the positive ions or else the electrons are attracted to the conductor, whilst the charged particles drift. This effect can cause considerable power loss, create audible and radio-frequency interference, generate toxic compounds such as oxides of nitrogen and ozone, and lead to arcing.

Both AC and DC transmission lines can generate coronas, in the former case in the form of oscillating particles, in the latter a constant wind. With monopolar transmission (with or without a metallic return) the choice of polarity of the energised conductor leads to a degree of control over the corona discharge. In particular, the polarity of the ions emitted can be controlled, which may have an environmental impact on particulate condensation (particles of different polarities have a different mean-free path). Negative coronas generate considerably more ozone than positive coronas, and generate it further downwind of the power line, creating a potential health hazard at high voltages. The use of a positive voltage will reduce the ozone impacts of monopole HVDC power lines. Some suggest that negative ions have a health benefit (they are used in air ionisers) particularly in being responsible for condensing particulate matter. If these are substantiated, it would suggest a counterargument, suggesting the use of negative potential on these lines.

Applications

Overview

The controllability of current-flow through HVDC rectifiers and inverters, their application in connecting unsynchronized networks, and their applications in efficient submarine cables mean that HVDC cables are often used at national boundaries for the exchange of power. Offshore windfarms also require undersea cabling, and their turbines are unsynchronized. In very long-distance connections between just two points, for example around the remote communities of Siberia, Canada, and the Scandinavian North, the decreased line-costs of HVDC also makes it the usual choice. Other applications have been noted throughout this article.

The choice of monopolar or bipolar link, and the presence or otherwise of a metallic return, is largely an economic and environmental concern.

System Configurations

A HVDC link in which the two AC-to-DC converters are housed in the same building, the HVDC transmission existing only within the building itself, is called a back-to-back HVDC link. This is the common configuration for interconnecting two unsynchronised grids.

The most common configuration of an HVDC link is a station-to-station link, where two inverter/rectifier stations are connected by means of a dedicated HVDC link. This is also a configuration commonly used in connecting unsynchronised grids, in long-haul power transmission, and in undersea cables.

Multiterminal HVDC power transmission (using three or more stations) is rarer than the other two configurations, owing to the high cost of the inverting/rectifying stations. The configuration of multiple terminals can be series, parallel, or hybrid (a mixture of series and parallel). Parallel configuration tends to be used for large capacity stations, and series for lower capacity stations. and the other at ground potential.

Monopolar systems carry typically 1500 MW. [2]

A bipolar link uses two wires, one at a high positive voltage and the other at a high negative voltage. This system has two advantages over a monopolar link. First, it can carry twice as much power as a monopolar link, typically 3000 MW (the current is the same, but the potential difference between the wires is doubled). [2] Second, it can continue to operate despite a fault in one of the wires or in one module of the converter equipment, by using the earth as a backup return path.

Multi-terminal HVDC links, connecting more than two points, are possible but rare. An example is the 2000 MW Hydro Québec system opened in 1992. [3]

Realized HVDC Systems

• Baltic-Cable
• Kontek
• GKK Etzenricht
• Konti-Skan
• Elbe-Project (HVDC-project between Dessau and Berlin, incompleted)
• HVDC Gotland
• HVDC Wolgograd-Donbass
• HVDC Cross-Channel (HVDC-link England-France)
• HVDC Inter-Island (HVDC link between the Islands of New Zealand)
• Sakuma
• HVDC Italia-Corsica-Sardinia (SACOI)
• HVDC Vancouver-Island
• Pacific-Intertie
• Nelson River Bipole
• HVDC Kingsnorth
• Cross-Skagerak
• Cahora-Bassa link from Mocambique to Pretoria
• Inga-Shaba
• HGÜ-Kurzkupplung Dürnrohr
• GK Wien-Southeast
• Cross Sound Cable, New Haven-Long Island USA

References

[1] Narain G. Hingorani in IEEE Spectrum magazine, 1996.

[2] Siemens AG "HVDC Basics" page.

[3] ABB HVDC website

[4] Basslink project

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