High Voltage Direct Currentelectricity – technical information IntroductionHigh voltage direct current (HVDC) technology isone of the technical options National Grid canconsider for the future development of thetransmission system in Great Britain.

Although HVDC has some disadvantages, as itsintegration within an AC system has to be carefullyconsidered and its cost can be higher than theequivalent AC solution, the advantages of HVDCtransmission are principally the following:

� The ability to interconnect networks that areasynchronous or that operate at differentfrequencies.

� The ability to transmit power over longdistances without technical limitations.

� The ability to control power flows on the HVDCconnection for all system backgrounds.

� The ability to transmit power in either directionas desired by the network operator.

� In certain cases the ability to improve ACsystem stability.

This document provides a technical overview ofHVDC technology and the situations in which it isapplied as an alternative to alternating current (AC)transmission. It describes the various types ofHVDC technology and the components that makeup a HVDC connection. It also considersoperation and maintenance issues and the costsassociated with HVDC transmission.

National Grid transmission system – an overview National Grid owns the high voltage electricitytransmission system in England and Wales andoperates the system throughout Great Britain at275,000 and 400,000 volts (275kV and 400kV).The National Grid system is made up ofapproximately 22,000 pylons with an overhead lineroute length of 4,500 miles, 420 miles ofunderground cable and 340 substations.

National Grid has a statutory obligation under theElectricity Act 1989 to develop and maintain anefficient, coordinated and economical system ofelectricity transmission, and to facilitatecompetition in the supply and generation ofelectricity. National Grid also has a statutoryobligation to have regard to the preservation ofamenity in developing the transmission system.

Electric power is normally generated, transmittedand distributed as alternating current (AC). ACpower is well suited to efficient transmission anddistribution, as the voltage can be increased orreduced by transformers. HVDC transmission ofelectricity offers some advantages overconventional AC transmission that lead to itsselection in particular applications.

A typical application of HVDC is to transmit powerbetween two independent AC networks that arenot synchronised. Examples of this are the2000MW England–France interconnector linkingthe British and French transmission systems andthe 1000MW BritNed interconnector being builtbetween Britain and The Netherlands.

Unlike AC, there is no technical limit on the lengthof cable or overhead line that can be used inHVDC connections, so HVDC has advantages forlong transmission distances.

factsheet

“Electric poweris normallygenerated,transmitted anddistributed asalternatingcurrent (AC).”

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HVDC – how it worksA typical HVDC system is shown in simplified formin Figure 1. A converter at the sending terminalacts as a rectifier and converts the AC power intoDC. A converter at the receiving terminal acts asan inverter and converts the DC power into AC.The connection between the converters may beby overhead line, cable, or both.

Power electronic valves (essentially high powered,electronic switches) within the converters allowthe power flow to be controlled. The HVDCsystem is usually designed so that the converterat either terminal can be operated as either arectifier or inverter and so the direction of thepower flow can be reversed as required.

Two main converter technologies have beendeveloped, current source converter (CSC) andvoltage source converter (VSC).

HVDC converter technologiesCurrent source convertersCurrent source converters have been incommercial use since the 1950s. Most HVDCsystems now in service are of the CSC type andthe technology is well established. A typicalcurrent source converter (CSC) electricaldiagram is shown below in Figure 2.

As a consequence of the AC-DC conversiontechnique utilised in CSC technology, theconverter absorbs reactive power from thesurrounding AC network and generatesharmonic currents, both of which impact on thequality of electrical power. AC harmonic filters

and reactive compensation must therefore beprovided to maintain the quality of the electricalpower. A further consequence is that theconverter is dependent on the AC systemvoltage for its correct operation.

The current flow in the DC circuit is unidirectional.In order to reverse the power flow on a CSCHVDC connection, it is necessary to reverse thepolarity of the DC voltage. This is performed bythe DC converter control system.

AC harmonic filters are used both to limit ACharmonic currents and to compensate thereactive power absorption of the converter. Thefilters are switched in and out automatically asrequired to meet harmonic and reactive powerperformance limits.

On the DC side of the converter, a reactor isprovided to smooth the DC current. The reactoralso reduces the peak current in the event of afault on the DC connection. DC filters may alsobe required to reduce harmonic voltages on theDC circuit, particularly when the circuit includesoverhead lines.

CSC HVDC systems are suitable for operation athigh voltages and power transfers. A 6400MWconnection between Xiangjiaba and Shanghai inChina, operating at +/- 800kV, is scheduled to gointo service in 2010. This connection is made viaDC overhead lines which are capable of carryingthese high voltage DC currents. Power capabilitiesvia DC cables are presently still limited to lowervoltages and power transfer capabilities.

“Unlike AC, there is no technicallimit on the length of cable oroverhead line that can be usedin HVDC connections, so HVDChas advantages for longtransmission distances.”

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-Converter Converter

ACsystem

ACsystem

DC connection

Figure 1: HVDC system

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A typical current source converter (CSC) electrical diagram is shown below in Figure 2.

DC SystemAC System

DC reactor

DC filter

filters

Pole 1

Pole 2

Metallicreturn

conductor

Y

Y

Y

Y

Y Y

Valve

Convertertransformer

Figure 2: Example current source converter (CSC)

Voltage source convertersVoltage source converters (VSC) have been usedin HVDC transmission systems since 1997.Comparatively few VSC HVDC systems are inservice and the technology is still developing interms of rating and capability. A typical voltagesource converter is shown in Figure 3.

VSC converters use insulated gate bipolartransistor (IGBT) valves. The device is self-commutating, meaning that the converter is notdependent on the AC system voltage for itscorrect operation.

The VSC HVDC systems in service so far havebeen limited to lower voltages and power ratingsthan CSC systems. The 500MW East-Westinterconnector between Ireland and GreatBritain, operating at +/- 200kV, is scheduled togo into service in 2012 and will represent thehighest power rating for a VSC HVDC systeminstalled at this time.

Comparison of CSC and VSCThe features of current source and voltage sourceconverters are compared in Table 1.

HVDC transmission technologiesCable typesThere are two main types of cable technologysuitable for use in HVDC applications, cross-linked polyethylene (XLPE) and massimpregnated (MI) insulation. All cables consist ofa copper or aluminium conductor surrounded bya layer of insulation (the thickness of which isdependent upon the operational voltage), ametallic sheath to protect the cable and preventmoisture ingress and a plastic outer coating. Anadditional requirement for cables intended forsubsea use is steel armouring, which consists ofsteel wires helically wound around the cable,increasing the cable’s strength and protecting itfrom the rigours of subsea installation.

XLPE insulation is made by extruding the polymerover the cable core, and is the technology nowgenerally used for AC transmission cables. XLPEcables are available at the moment for HVDCvoltages up to 300kV. XLPE cables are unsuitablefor use in CSC applications where, in order toreverse the direction of power flow, the polaritymust be reversed.

DC SystemAC System

Connection tomain ACsystem

Interfacetransformer

DCcapacitor

Phasereactor

Valve

AC filter

Current sourceconverter

Voltage sourceconverter

Maturity of technology Mature Developing

Valves Thyristor, dependenton AC system voltagefor commutation

IGBT, self commutating

Commutation failure Can occur Does not occur

Minimum DC power Typically 5-10% ofrated power

No minimum value

Reactive powerexchange with ACsystem

50% of active powertransmitted

Independent control ofactive and reactivepower

Reactivecompensation

Required Not required

AC harmonic filters Switchable filtersrequired

Less filtering required,filters need not beswitchable

Converter transformers Special designrequired

Conventionaltransformers can beused

Reversal of power flow DC voltage polarityreversal required

Controllable in bothdirections, no reversalof DC voltage polarityrequired

Converter stationfootprint (relative size)

200m x 120m x 22m(100%)

120m x 60m x 22m(~40%)

Conversion losses (per converter end)

0.5% to 1% oftransmitted power

1% to 2% oftransmitted power

DC voltage Up to 800kV available Up to 350kV available

Figure 5: MI cable

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Figure 4: XLPE cable

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Figure 3: Example voltage source converter (VSC) Table 1: Comparison between current source converterand voltage source converter technology

MI insulation is made by winding kraft paperaround the conductor until the required insulationthickness is achieved. The cable is then placed ina vacuum chamber and heated to remove any airand moisture. A viscous oil is introduced whichimpregnates the paper, increasing the electricalstrength of the insulation. A recent developmentuses polypropylene laminated papers, where thinlayers of plastic are laminated to the paperlayers, allowing the cable to operate at highertemperatures and therefore higher currentratings. HVDC, MI cables are presently availablefor voltages up to 600kV.

Cable installationsGenerally, at least two cables are required forHVDC underground/subsea transmission, unlessan earth/sea electrode is to be used at theconverter stations, although it should berecognised there can be environmental issueswith such a solution.

In a monopolar scheme, these will consist of aheavily insulated HV cable and a lightly insulatedreturn cable; whereas in bipolar designs bothcables operate at high voltages and requireheavy insulation.

Because of the reduced power capacity of cableswhen compared to overhead lines, between fourand six cables are required to match the capacityof an equivalent AC or DC overhead line. Existing cable technology limits continuous powertransfer on a single cable DC bipole circuit toapproximately 2GW, whereas a double circuit ACoverhead line may transmit over 6GW.

OnshoreThe same principles and installation techniquesfor onshore underground AC cables apply to DC(see National Grid’s ‘Undergrounding high voltageelectricity transmission’ fact sheet), with theexception that reactive compensation is notrequired for DC transmission and fewer cablesare usually needed (minimum of two cables forDC rather than three for AC).

As cables need to be spaced far apart to allow theheat generated to dissipate in the ground, a cableeasement corridor of around 12m wide will berequired for an HVDC route consisting of fourcables, with increased width to account forsection joints and access during construction andmaintenance. This causes a significantenvironmental impact in construction andreinstatement and results in ‘land sterilisation’, asthe cable corridor cannot be used for constructionor certain types of agriculture.

OffshoreIn many cases, offshore transmission is bettersuited to HVDC applications than traditional AC.Using AC, offshore cable voltages are limited to220kV, limiting power transfer capabilities unlessmany cables are used. Also, lengths of AC cableare very difficult (and consequently expensive) tojoint offshore and reactive compensation cannotbe installed mid-route, so transmission distancesare restricted.

Offshore cable installation, for both AC andHVDC, requires the use of specially designedcable-laying ships or barges. To protect the

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3000 30003000 / 4000 10001000

Figure 6: Example cable easement corridor (2 cables/pole)

“Offshore cable installation,for both AC and HVDC,requires the use of speciallydesigned cable-laying shipsor barges.”

cables from anchor strikes, they are preferablyburied in the sea bed. Specialist, remotelyoperated submersible units are needed to cutthe trench and install the cables.

Where there is a requirement for the cables tocross existing subsea equipment such aspipelines, the cables can be protected, forexample, by using concrete ‘mattresses’. Forlonger lengths of submarine cable, lengths ofcable need to be jointed together on the shipwhile at sea. This is a very complex and time-consuming operation, needing highly skilledpersonnel and a spell of good weather ofroughly three days to allow the laying ship toremain stationary while the cable is jointed.

Overhead linesOverhead lines rely on the air gap between theconductors and the ground as electricalinsulation and to provide cooling for theconductors. Conductors are suspended fromtowers by insulators made of either ceramics orpolymers, although polymer insulators aregenerally used for HVDC applications becauseof their improved pollution performance (HVDCsystems have a tendency to attractpollution/particulates).

HVDC overhead lines can be used up to 800kVand are capable of transporting more than 6GWon a single bipole route. However, transfers of thislevel have only been undertaken using CSCtechnology. As yet, VSC technology has not beenproposed to power capabilities beyond 1-2GW.

HVDC converter station installations Converter station current source convertersFigures 7 and 8 show some examples of CSCconverter stations. Converter stations require asignificant amount of land for their construction.A typical CSC converter station will take up anarea of land 200m x 120m, including indoor andoutdoor equipment.

DC equipment attracts pollution/particulatesfrom the atmosphere and so the valves andtherefore DC switching arrangements, and oftensmoothing reactors, are housed in large climate-controlled buildings up to 22m high. Theconverter transformers are outdoors and cangenerate significant levels of audible noise. The CSC converter’s requirement for harmonicfiltering and reactive compensation make asignificant contribution to the size of theconverter station (which can be greater than50% of the overall footprint).

Voltage source convertersThe footprint of a VSC converter station isconsiderably smaller than that of a CSC station,as there is no need for reactive compensationequipment. The land needed for a typicalonshore VSC converter station is approximately120m x 60m x 22m. All equipment is generallyindoors with the exception of the connection tothe AC grid.

To reduce the footprint further, the VSCconverter can be constructed on two levels.

Figure 7: Ballycronan More converter station Figure 8: Sellindge 2GW converter station

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Source: Siemens Plc

VSC is the preferred technology for theconnection of wind farms located far from theshore where the distance makes an ACconnection uneconomic and unfeasible.

Operational issuesControllabilityOne of the key benefits of an HVDC system isthe ability to precisely control power flows. In ACsystems the power, like water, flows along thepath of least resistance, which in some casescan result in unequal distribution of poweracross the transmission system. HVDC systemsallow the operator to precisely control the flowacross the circuit, which may help alleviateissues on the wider power system.

Impact of faults on the AC systemAC overhead line systems are very resilientunder fault conditions. If an AC overhead linesuffers a lightning strike then the power flowingin the network redistributes itself very quicklyand without any need for external intervention.

Adjusting power flows on an HVDC link is notinstantaneous and requires the intervention ofthe operator or control system. VSC-based

systems are faster at responding to changes insystem conditions than CSC-based systems,where polarity reversal must take place tochange the direction of power flow.

An additional weakness, specific to CSCtechnology, is that faults on the AC system canintroduce ‘commutation failure’ in the valves ofthe converter stations. As CSC converters taketheir voltage signal from the AC network,disturbances in this voltage, such as might beseen after a fault, can interrupt the powertransfer. This may exacerbate the problems onthe AC network.

HVDC system reliabilityHVDC systems suffer all of the potential issues ofAC technology but with additional failure modesintroduced at the converter stations.

Overhead lines generally experience more faults, such as lightning strikes, than buriedtransmission systems, although repair andmaintenance is considerably simpler on anoverhead line than on a cable. Cables requireextensive excavation to identify the location of thefault and repair it. This is worse for submarinecable systems, where repair means raising thecable from the sea bed, removing the damagedsection and replacing it with a new section. A faulton a submarine cable is therefore very serious andcan result in the HVDC system being out ofservice for up to six months.

Short term overloadA useful characteristic of AC systems is theirability to withstand overloads for a short period oftime. AC equipment can exceed its current ratingfor short periods before the conducting elements

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Figure 9: Example offshore wind farm connection using VSC HVDC

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become too hot. This can allow the systemoperator valuable time to reconfigure the networkand return power flows to normal levels.

However, in HVDC systems the valve elementsheat up very quickly and cannot operate abovetheir rating. This means that the capability ofHVDC systems to deal with temporary overloadconditions is limited unless the converter stationhas been designed with this in mind (at extra cost).

Multi-terminal operationThe AC transmission system is protected byhigh voltage circuit breakers which allowisolation of faulty sections of the network. Thesame is not true for HVDC systems becausethere are no commercially available HVDC circuitbreakers at present.

Consequently, HVDC systems tend to be point-to-point systems and not interconnected, as anysingle fault would shut down the entireinterconnected HVDC network.

There are only three terminal applications in theworld, each with much less operational flexibilitythan AC systems. The properties of VSC HVDCsystems in theory lend themselves better tomulti-terminal operation over CSC systems,although no such system has yet been built.

This limitation of HVDC systems is fundamentalwhen considering the future expandability of atransmission network. It is a relatively simplematter to, for example, connect a new powerstation to the AC system, whereas to introduceextra infeeds or outfeeds into an HVDC systemis not easily achieved.

EconomicsCapital costsThe capital costs of HVDC installations can bemuch higher than for equivalent overhead linetransmission routes. It is very difficult to give adirect comparison, however, as the cost of anyHVDC and AC project is dependent upon amultitude of project-specific factors such astechnology used and route length. Under somecircumstances, HVDC may be more economicthan equivalent AC transmission; generally thelonger the route length the more competitiveHVDC becomes. The break-even distance willvary depending upon the installation type butcan be over hundreds of kilometres.

Converter stationsDC converter stations represent a significantportion of the cost of any HVDC installation. CSC converter stations are generally moreexpensive than VSC converter stations becauseof the reactive compensation and harmonicfiltering requirements. However, VSC convertershave presently only been installed at lowermaximum ratings than CSC converters so, interms of capital cost per unit of power, theyhave generally been more expensive.

CablesWhen used in AC systems, long HVDC cableroutes incur a high capital cost. However, DCcables normally:� require two cables per circuit rather than three;� do not require reactive compensation mid-route;

and � are more highly rated than equivalent AC cables.

Table 2 shows the costs of a number of typicalHVDC cable projects.

Table 2: Cost comparison of typical HVDC projects

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Name Capacity/Technology Cost and Distance

Eirgrid East – West interconnector between Republic of Ireland and Wales http://www.interconnector.ie/projects/east-westinterconnector/projectactivity/

500MW VSC Cable Estimated cost €600m; 245kmProject cost includes on-shorereinforcement works.

BritNed (Tennet and National Grid) – interconnector between England andThe Netherlandshttp://www.nationalgrid.com/NR/rdonlyres/88FF9856-8D4E-47F9-85DB-B8BDB3CCF24B/17288/BRITNED2.pdf

1000MW CSC Cable,Bipole

Estimated cost €600m; 260kmProject cost includes on-shorereinforcement works.

Neptune RTS – interconnector New Jersey to Long Islandhttp://www.neptunerts.com/default.asp?Ss=5&Pg=7

660MW CSC Cable,Monopole

Estimated cost $600m; 105 km

www.nationalgrid.com

Overhead linesAs in AC networks, overhead lines are the mosteconomic means of power transmission inHVDC systems.

Transmission lossesConverter stationsThe process of converting AC power to DC isnot 100% efficient. Power losses occur in allelements of the converter station: the valves,transformers, reactive compensation/filtering andauxiliary plant. CSC converter stations incur aloss of 0.5-1% per converter station (1-2% for alink including a converter station at each end).VSC converters suffer higher losses in the regionof 1-2% per converter (2-4% per link).

As there is a cost associated with lost power,these losses can significantly increase the costof a converter station over its lifetime. Whencompared to AC transmission, the converterstation losses render HVDC transmissionconsiderably less efficient than AC transmissionover short distances.

Cables and overhead linesOver short distances, the losses in both cablesand overhead lines are very small. Losses in ACtransmission are in the region of 0.7%-1% per

100km for an overhead line route and 0.3%-0.6% for a cable system. DC transmissionlosses are slightly less for a given power transfer.Consequently, very long lengths of cable oroverhead line are required before the losses inthe converter stations are offset and HVDCbecomes more efficient than AC.

MaintenanceAnother cost that must be considered whenevaluating transmission projects is the cost tooperate and maintain the equipment. Maintenancecosts for DC systems are generally higher than AC because of the complex converter stationsthat require regular, specialist maintenance.

RefurbishmentAC transmission has an expected lifetime ofapproximately 60-80 years, with mid-liferefurbishment of overhead lines, which are the largest capital component, needed after 40 years. In contrast, HVDC systems have a shorter life expectancy of 40 years, and largeparts of the converter stations (valves andcontrol systems) are likely to need replacing after 20 years.

Glossary & abbreviationsAC Alternating current

Bn Billion

CSC Current source converter

Converter Part of an HVDC system which either convertsAC electricity to DC or alternatively converts DCelectricity to AC

Commutation Part of the process of converting alternatingcurrent to/or from direct current

DC Direct current

GW Gigawatt (one thousand Megawatts)

HV High voltage

Hz Hertz

Inversion The conversion of DC electricity to ACelectricity (as performed by an inverter).

km Kilometre

kV Kilovolt (one thousand volts)

MI cable Mass impregnated insulated cable

MW Megawatt (one million watts or one thousandkilowatts)

Reactive power A form of power which is required to maintainvoltage on the transmission system

Rectification The conversion of AC electricity to DCelectricity (as performed by a rectifier).

VSC Voltage source converter

XLPE cable Cross-linked polyethylene insulated cable

June 2010

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