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HVDC Light Technology Essay

Abstract—This document reveals theHVDC Light DC transmission technology.It is used in underground transmission and moreover provides point to point transmission.HVDC Light requires only two elements namely a converter station and a pair of ground cables. The new HVDC Light cable is an extruded, single-pole cable. It is ideally suited for feeding power into growing metropolitan areas from a suburban substation. HVDC Light is inherent environmentally friendly cables instead of OH transmission lines. Virtually no magnetic field. The environmental gains would be substantial, since the power supplied via the DC cables will be transmitted from efficient power plants in the main AC grid.

1. INTRODUCTION

A hundred years ago, the transformer and a new transmission and distributionbe controlled precisely and independentlycan replace overhead lines at no costcontrol capabilities that are not present oreconomically feasible to connect smallscale,Equally important, HVDC Light hasfor inefficient, polluting local generationfrequency, active and reactive power canislands, mining districts and drillingloads from a main AC-gridof each other. This technology also relieson a new type of underground cable which penalty platforms can be supplied with power frompossible even in the most sophisticated ACrenewable power generation. Renewable power generation plantssuch as diesel units.

The voltage, .
Connect small scale
Feeding remote isolated
Flexible transmission

System technology, HVDC Light, makes it the main grid,thereby eliminating the needto the main AC grid. Vice versa,using thevery same technology, remote locations asthe three phase system made it possible totransmit AC power efficiently and economically over vast distances and todistributethe power toamultitude ofusers.Since then all aspects of transmission anddistribution have developed by means oftechnical improvement and evolution. This AC transmission and distributiontechnology has made it possible to locategeneratingplants in optimum locations, andtoutilize them efficiently. This has alsoresulted in great environmental gains.Thermal plants have been located wherethey can be supplied with fuel through anefficient transportation system, therebyreducing waste and pollution.

Hydro plantshave been located where the hydroresources can be used at the greatestadvantage. And large generating plantshave meant fewer overhead lines than amultitude of smaller generating plantswould have required.However, today’s AC transmission anddistribution systems are, at least inprinciple, based on ideas that haven’tchanged much since a hundred years ago. To transmit power, step up the voltage withtransformers, transmit power, step downthe voltage and distribute power.

Despitetheir proven advantages, it is difficult andexpensive to adapt AC transmission anddistribution systems to the numerous smallscalegenerating plants that are being built,or to the increasingly complex and variableproduction and load demands.Environmental concerns and regulationsalso put heavy restrictions on building right-of-ways and on small-scale, fossilfuelledgenerating plants, such as dieselgenerating plants.These new trends require networks that areflexible. The networks must be able to copewith large variations in load and frequentchanges in productions patterns with tougher environmental regulations.Also, in such flexible networks, the powerflow and the voltages require precisecontrol in order to make the grids stable and economic.

2. TECHNOLOGY

As its name implies, HVDC Light is a DCtransmission technology. However, it isdifferent from the classic HVDCtechnology used in a large number oftransmission schemes. Classic HVDCtechnology is mostly used for large point-to-point transmissions, often over vastdistances across land or under water. It requires fast communications channelsbetween the two stations, and there mustbe large rotating units – generators orsynchronous condensers – present in theAC networks at both ends of thetransmission.

HVDC Light consists of only two elements: a converter station and a pair ofground cables. The converters are voltagesource converters, VSC’s. The output from the VSCare determined by the controlsystem, which does not require anycommunications links between the differentconverter stations. Also, they don’t need torely on the AC network’s ability to keepthe voltage and frequency stable. Thesefeatures make it possible to connect theconverters to the points bests suited for theAC system as a whole.

Power range up to 100 MW
Independent control of active and reactive power
Can feed power to AC network without ownGeneration DC

The converter station is designed for apower range of 1-100 MW and for a DCvoltage in the 10-100 kV range. One suchstation occupies an area of less than 250sq. m. (2 700 sq. ft.), and consists ofjust a few elements: two containers for theconverters and the control system, threesmall AC air-core reactors, a simpleharmonics filter and some cooling fans. 20MW:18x12m

The converters are using a set of six valves,two for each phase, equipped with highpowertransistors, IGBT (Insulated GateBipolar Transistor). The valves arecontrolled by a computerized controlsystem by pulse width modulation, PWM.Since the IGBTs can be switched on or off, the output voltages and currents onthe AC side can be controlled precisely.The control system automatically adjuststhe voltage, frequency and flow of activeand reactive power according to the needsof the AC system.The PWM technology has been tried andtested for two decades in switched powersupplies for electronic equipment ascomputers.

Due to the new, high powerIGBTs, the PWM technology can now beused for high power applications as electricpower transmission.HVDC Light can be used with regularoverhead transmission lines, but it reachesits full potential when used with a new kindof DC cable. The new HVDC Light cable isan extruded, single-pole cable. As anexample a pair of cables with a conductorof 95 sq mm aluminum can carry a load of30 MW at a DC voltage of +/-100KV.Handling the cable is easy. Despite its large power-carrying capacityit has a specific weight of just over 1 kg/m.Contrary to the case with AC transmission;distance is not the factor that determinesthe line voltage. The only limit is the costof the line losses, which may be lowered bychoosing a cable with a conductor with alarger cross section. Thus, the cost of apair of DC cables is linear with distance.

Insulation: 5.5 mm triple extruded
Screen: Copper wire
Sheath: HDPE
Weight: 1.05 kg/m
Voltage: > 100 kV DC
Current: > 300 A
Power: > 30 MW
Conductor: 95 mm^2Aluminum

A DC cable connection could be more costefficientthan even a medium distance ACoverhead line, or local generating unitssuch as diesel generators.The converter stations can be used indifferent grid configurations. A singlestation can connect a DC load or generatingunit, such as a photo-voltaic power plant,with an AC grid. Two converter stationsand a pair of cables make a point-to pointDC transmission with AC connections ateach end. Three or more converter stationsmake up a DC grid that can be connected toone or more points in the AC grid or todifferent AC grids.

An HVDCLight network can be configured radial or meshed,like any network.

The DC grids can be radial with multi-dropconverters, meshed or a combination ofboth. In other words, they can beconfigured, changed and expanded in muchthe same way AC grids are.

3. APPLICATIONS

3.1 OVERHEAD LINES

In general, it is getting increasingly difficultto build overhead lines. Overhead lineschange the landscape, and the constructionof new lines is often met by public resentment and political resistance. Peopleare often concerned about the possiblehealth hazards of living close to overheadlines. In addition, a right-of-way for a high voltage line occupant valuable land. Theprocess of obtaining permissions forbuilding new overhead lines is alsobecoming time-consuming and expensive.Laying an underground cable is a mucheasier process than building an overheadline.

A cable doesn’t change the landscapeand it doesn’t need a wide right-of-way.Cables are rarely met with any publicopposition, and the electromagnetic fieldfrom a DC cable pair is very low, and also astatic field. Usually, the process ofobtaining the rights for laying anunderground cable is much easier, quickerand cheaper than for an overhead line.A pair of HVDC Light cables can beplowed into the ground. Despite their largepower capacity, they can be put in placewith the same equipment as ordinary, AChigh voltage distribution cables. Thus,HVDC Light is ideally suited for feedingpower into growing metropolitan areasfrom a suburban substation.

3.2 REPLACING LOCAL GENERATION

Remote locations often need localgeneration if they are situated far awayfrom an AC grid. The distance to the gridmakes it technically or economicallyunfeasible to connect the area to the maingrid. Such remote locations may be islands,mining areas, gas and oil fields or drillingplatforms. Sometimes the local generators use gas turbines, but diesel generators aremuch more common.An HVDC Light cable connection could bea better choice than building a local powerplant based on fossil fuels.

Theenvironmental gains would be substantial,since the power supplied via the DC cableswill be transmitted from efficient powerplants in the main AC grid. Also, thepollution and noise produced when thediesel fuel is transported will be completelyeliminated by an HVDC line, as the needfor frequent maintenance of the diesels.Since the cost of building an HVDC Lightline is a linear function of the distance, abreak-even might be reached for as shortdistances as 50- 60 km.

HVDC Light lowest cost
AC + Overhead line
HVDC Light + cable
Cost inside
AC grid

Distance from the AC grid eliminate local diesel Cost/kWh

3.3 CONNECTING POWER GRIDS

Renewable power sources are often builtfrom scratch, beginning on a small scaleand gradually expanded. Wind turbine farms is the typical case, but this is alsotrue for photovoltaic power generation.These power sources are usually locatedwhere the conditions are particularlyfavorable, often far away from the mainAC network. At the beginning, such aslowly expanding energy resource cannotsupply a remote community with enoughpower.

An HVDC Light link could be anideal solution in such cases.First, the link could supply the communitywith power from the main AC grid,eliminating the need for local generation.The HVDC Light link could also supply thewind turbine farm with reactive power for the generators, and keeping the powerfrequency stable.When the power output from the windgenerators grows as more units are added,they may supply the community with asubstantial share of its power needs. Whenthe output exceeds the needs of theCommunity, the power flow on the HVDCLight link is reversed automatically, and thesurplus power is transmitted to the mainAC grid.

Wind power
Small scale hydropower
HVDC Light
Extruded cable
Distant ac- grid

Waste gas is usually burned at offshoredrillingplatforms, since it is too expensive,or technically difficult, to use the gas for power generation and transmit it by an ACcable to the main grid on the shore. Thus,the energy content of the gas is wasted, andthe primitive burning process is source ofpollution. With an HVDC Lightunderwater cable transmission, the gas canbe used as gas turbine fuel, supplying boththe platform and the main AC grid withpower. The process of burning the gas ingas turbines would also produce much acleaner exhaust than simple burning woulddo.The DC underwater cable network could easily be extended to other offshoreplatforms.

3.4 ASYNCRONOUS LINKS

Two AC grids, adjacent to each other butrunning asynchronously with respect toeach other, cannot exchange any powerbetween each other. If there is a surplus ofgenerating capacity in one of the grids itcannot be utilized in the other grid. Each ofthe networks must have its own capacity of peak power generation, usually in the formof older, inefficient fuel fossil plants, ordiesel or gas turbine units. Thus, peakpower generation is often a source ofsubstantial pollution, and their fueleconomy is frequently bad.A DC link, connecting two such networks,can be used for combining the generationcapacities of both networks. Cheap surpluspower from one network can replace peakpower generation in the other. This willresult in both reduced pollution levels andincreased fuel economy. The powerexchange between the networks is alsovery easy to measure accurately.

4. ADVANTAGES

* Transmission by HVDC Light saves the environment by replacing local fossil-fueled generation withtransmission from main AC-grid. * Connecting small scale renewable power to main AC –grid. * HVDC Light is inherent environmentally friendlycables instead of OH transmission lines. * Virtually no magnetic field. * No ground currents because of bipolar transmission.

5. CONCLUSION

HVDC Light technology saves theenvironment by replacing remote fossilfuelledgenerators with cost-efficienttransmission of power from efficient andclean, large-scale generation productionunits. The efficiency of a modern, largescale, thermal generating plant is usually 25percent higher than that for a modernsmall or moderate scale diesel generatorplant,Vice versa, HVDC Light provides aconvenient and cost-effective way forconnecting renewable and non-pollutingenergy sources as wind power farms andphotovoltaic power plants to a main grid.The HVDC Light technology in itself hasstrong environmental benefits.

Since poweris transmitted via a pair of underground cables, the electromagnetic fields from thecables cancel each other. Any residual fieldis a static field, as opposed to the powerfrequencyfields radiated from AC cables.Since HVDC Light transmissions arebipolar, they do not inject any currents intothe ground. Ground currents can disturbcommunications systems or causecorrosion on gas or oil pipelines.A pair of light-weight DC cables can beeasily plodded into the ground at a costthat is comparable to or less than for acorresponding AC overhead line. Asopposed to an overhead line, anunderground cable pair has no visualimpact at all on the landscape. Usually it’salso much easier to obtain permissions andpublic approval for a cable transmissionthan for an overhead line, especially inresidential areas.

ACKNOWLEDGEMENT

Our sincere thanks to HOD and FACULTIES for encouraging us to prepare the above document. A special thanks to IEEE.org

REFERNCES

[1] K. Eriksson, “HVDC Light™ and development of Voltage SourceConverters”, IEEE T&D 2002 Latin America, São Paulo, Brazil, March. [2] L. Carlsson, G. Asplund, H. Björklund, M. Åberg, “Present trends inHVDC converter station design” IV SEPOPE Conference, Foz doIguacu, Brazil. [3]IEEE explorer.org


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