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2 Advantages of HVDC over AC Transmission 3 Rectifying and Inverting 4 Polarity 5 Applications 6 References |
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, however for the bulk of situations AC transmission remains in a dominant position.
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.
The ability to transform voltages is an important economic and technical consideration as, whilst high voltages are harder to manipulate and 'handle', the lower currents required with high voltage transmission for any given level power lead to high voltage transmission requiring smaller cabless, and less loss of power in the form of heat. 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 with direct current; the manipulation of DC voltages is considerably more complex.
Despite alternating-current being the dominant mode for electric power transmission, in a number of applications HVDC is often the preferred option.
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 wires and ways to be used to carry more power into an area of high power consumption, which can lower costs.
Using thyristor technology, only synchronized AC networks can be interconnected: those which oscillate at the same rate and in phase. Many areas which wish to share power have unsynchronized networks. The UK and continental Europe operate at 50Hz, Japan has 50Hz and 60Hz networks, continental North America, whilst operating at 60Hz throughout, is divided into regions in which power is unsynchronised, East, West, Texas and Quebec. Brazil and Paraguay which share the massive Itaipu hydroelectric plant, operate on 60Hz and 50Hz respectively. DC links allow such unsynchronised systems to be interconnected.
However, IGBT-based HVDC systems makes it possible to interconnect unsynchronized AC networks, and also adds the possibilty of controlling AC voltage and reactive power flow. Even a black net can be interconnected.
Power generation systems such as photovoltaic cells generate direct current. Simply-engineered wind and water turbines generate alternating current at a frequency which depends on the speed of the driving fluid. In the former situation, we have high-voltage direct current which may be used directly for power transmission. In the latter case we have, in effect, an unsynchronised AC system, which suggests a DC interconnect. In each of these situations, it may be indicated to use HVDC transmission direct from the generating plant, particularly if located in inhospitable locations.
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.
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.
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.
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
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.
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 particulateate 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.
The controllability of current-flow through HVDC rectifiers and inverters, their applciation in connecting unsynchronized networks, and their applications in efficient submarine cables mean that HVDC cables are often used at national boundaries for the excahnge of power. Off-shore 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 environtmental concern.
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 terminalss 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]
The rise of AC transmission
Advantages of HVDC over AC Transmission
Long undersea cables have a high capacitance. This causes AC power to be lost extremely quickly in reactive losses, even on cables of a modest length.Rectifying and Inverting
Rectifying and Inverting Components
Rectifying and Inverting Systems
Polarity
Polarity and Earth Return
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.Polarity and Corona Discharge
Applications
Overview
System Configurations
References