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Ad vantag e s o f HVDC o ve r H VAC tr ans missio n (on p ho to : The valve hal l o f the HVDC syste m i s whe re the p o wer is mad e

read y for transmissio n; by Siemens)

Edvard

Advantages of HVDC over HVAC transmission

AC

as

preferred option

Despite alternating current being the dominant mode f or electric power t ransmission, in a number of

applications, the advantages of HVDC makes it the preferred option o ver AC transmission.

Examples include:

1. Undersea cables where high capacitance causes additional AC losses (e.g., the 250 -km Baltic Cable

between Sweden and Germany ).

2. Endpoint- to -endpoint long-haul bulk powertransmission without intermediate taps, for example, in

remote areas.

3. Increasing the capacity of an existing power grid in situations where additional wires are dif f icult o r

expensive to install.

4. Allowing power transmiss ion between unsynchronized AC distribution systems .

5. Reducing the prof ile of wiring and pylons f or a given power transmission capacity, as HVDC can carrymore power per conducto r of a given size.

6. Connecting a remote generating plant to t he dist ribution grid; for example, the Nelson River Bipole line

in Canada (IEEE 2005 ).

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7. Stabilizing a predominantly AC power grid without increasing the maximum prospect ive short- circuit

current.

8. Reducing corona losses (due to highervoltage peaks) compared to HVAC transmission lines of

similar power.

9. Reducing line cost , since HVDC transmiss ion requires f ewer conductors ; for example, two f or a

typical bipolar HVDC line compared to three for t hree-phase HVAC.

HVDC transmission is particularly advantageous in undersea power transmission. Long undersea ACcables have a high capacitance.

Example (VIDEO)

500 MW HVDC Light t ransmission interconnection

ABB has commiss ioned a 500- megawatt HVDC Light (VSC) transmission interconnection that links t he Irish

and U.K. grids, enabling cross-border power f lows and enhancing grid reliability and security of electricity

supplies.

The East West Interconnector includes a 262 km high voltage cable link of which 186 km runs subsea.

Can’t see this video? Click here to watch it on Youtube.

Consequently, the current required to charge and discharge the capacitance of the cable causes additional

power losses when the cable is carrying AC, while this has minimal effect for DC transmission. In

addition, AC poweris lost to dielectric losses.

In general applications, HVDC can carry more power per conductor than AC, because f or 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 t hickness and conductor spacing. This reduces the cost of HVDC

transmission lines as compared to AC transmission and allows t ransmiss ion line corridors to carry a

higher power density.

A HVDC transmission line would not produce the same sort of extremely low f requency (ELF)

electromagnetic f ield as would an equivalent AC line. While there has been some concern in the past

regarding poss ible harmf ul eff ects o f such f ields, including the suspicion of increasing leukemia rates, the

current scientif ic consensus does not cons ider ELF sources and their asso ciated f ields to be harmf ul.

Deployment o f HVDC equipment would not completely eliminate electric fields, as there would still be DC

electric f ield gradients between the conducto rs and ground. Such f ields are not asso ciated with health

effects.

Because HVDC allows power transmission between unsynchronized AC systems, it can help increase

system stability. It does so by preventing cascading failures f rom propagating from one part of a wider

power transmission grid to another, while st ill allowing power to be imported or exported in the event o f

smaller f ailures.

This feature has encouraged wider use of HVDC technology for its stability benef its alone. Power f low on

an HVDC transmiss ion line is set using the cont rol systems of converter stations. Power f low does not

depend on the operating mode of connected power systems.

Thus, unlike HVAC ties, HVDC intersystem ties can be of arbitrarily low transf er capacity, eliminating the

“weak tie problem,” and lines can be designed on the basis o f opt imal power f lows.

http://www.youtube.com/watch?v=ZWY4OczlH9s


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Direc t-Current (HVDC) Transmiss ion Lines

Similarly, the dif f iculties of synchronizing dif f erent operational cont rol systems at dif f erent power systems

are eliminated. Fast -acting emergency control systems on HVDC transmiss ion lines can f urther increase the

stability and reliability of the power system as a whole. Further, power f low regulation can be used f or

damping oscillations in powersystems or in parallel HVAC lines.

The advantages described above encourage the use of DC links for separating large power systems

into several nonsynchronous parts.

For example, the rapidlygrowing Indian power

system is being

constructed as several

regional power

systems

interconnected with

HVDC transmission

lines and back-to- back

converters with

centralized control of

these HVDC

elements (Koshcheev

2001).

Likewise, in China,

±800-kV HVDC will be

the main mode used to

transmit large capacity

over very long

distances f rom large

hydropower andthermal power bases.

Other applications involve long-dist ance transmission projects with few tie-ins of power supplies along the

line (Yinbiao 2005).

Advert isement

Reference: Argonne National Laboratory – The design, construction and operation of long-distance high

voltage electricity transmission technologies
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