RK15a lines6 capacity hvdc - University of Iceland · HVDC terminal station • The design of the...

2 November 2011

1Lecture 15a Power Engineering - Egill Benedikt Hreinsson

HVDC Transmission

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HVDC TransmissionHigh Voltage Direct Current Transmission

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AC to DC Comparison

•Originally the power systems were DC•An historical struggle between Edison and

Westinghouse was called: “War of Currents”– http://en.wikipedia.org/wiki/War_of_Currents

•The AC won over DC, at least for the time being?

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HVDC Transmission• Point to point (from “A” to “B”) rather than meshed network.

No commercially available DC circuit breakers• Used exclusively for long underground/submarine cable

transmission• Flexible Computer or Electronic control of power flow• Transmission over long distances by HVDC overhead line• Recently “HVDC light”. Lower cost of AC/DC converters

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•HV is needed to transmit DC a long distance.– Semiconductor thyristors able to handle high currents

(4,000 A) and block high voltages (up to 10 kV) were needed for the widespread adoption of HVDC.

– Newer semiconductor VSC (voltage source converters), with transistors that can rapidly switch between two voltages, has allowed lower power DC.

– VSC converter stations also are smaller and can be constructed as self-contained modules, reducing construction times and costs.

Why has HVDC taken off?

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• Long distance transmission increases competition in new wholesale electricity markets.

• Long distance electricity trade, including across nations, allows arbitrage of price differences.

• Contractual provision of transmission services demands more stable networks.

• Bi-directional power transfers, often needed in new electricity markets, can be accommodated at lower cost using HVDC

Increased Benefits of Long-Distance Transmission

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• For equivalent transmission capacity, a DC line has lower construction costs than an AC line:– A double HVAC three-phase circuit with 6 conductors is needed to

get the reliability of a two-pole DC link.– DC requires less insulation ceteris paribus.– For the same conductor, DC losses are less, so other costs, and

generally final losses too, can be reduced.– An optimized DC link has smaller towers than an optimized AC link

of equal capacity.

Relative Cost of AC versus DC

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2-Nov-11

• Right-of-way for an AC Line designed to carry 2,000 MW is more than 70% wider than the right-of-way for a DC line of equivalent capacity.– This is particularly important where land is expensive or

permitting is a problem.

• HVDC “light” is now also transmitted via underground cable – the recently commissioned Murray-Link in Australia is 200 MW over 177 km.– Can reduce land and environmental costs, but is more

expensive per km than overhead line.

AC versus DC (continued)

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•Above costs are on a per km basis. The remaining costs also differ:– The need to convert to and from AC implies the terminal

stations for a DC line cost more.– There are extra losses in DC/AC conversion relative to AC

voltage transformation.– Operation and maintenance costs are lower for an

optimized HVDC than for an equal capacity optimized AC system.


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• The cost advantage of HVDC increases with the length, but decreases with the capacity, of a link.

• For both AC and DC, design characteristics trade-off fixed and variable costs, but losses are lower on the optimized DC link.

• The time profile of use of the link affects the cost of losses, since the MC of electricity fluctuates.

• Interest rates also affect the trade-off between capital and operating costs.


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• HVDC is particularly suited to undersea transmission, where the losses from AC are large.– First commercial HVDC link (Gotland 1 Sweden, in 1954) was an

undersea one.

• Back-to-back converters are used to connect two AC systems with different frequencies – as in Japan – or two regions where AC is not synchronized – as in the US.

Special Applications of HVDC

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HVDC projects around the world

Source: http://www.spectrum.ieee.org/

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HVDC submarine projects in Scandinavia

Gemmell, B.; Loughran, J. “HVDC offers the key to untapped hydro potential”, IEEE Power Engineering Review , Volume: 22 Issue: 5 , May 2002 Page(s): 8 -11

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International HVDC projects

An interestning link on HVDC : HVDC Transmission:Part of the Energy Solution? http://cohesion.rice.edu/CentersAndInst/CNST/emplibrary/Hartley%2004May03%20NanoTechConf.ppt

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15Different types of HVDC linksa) Monopolar with

earth/sea returnb) Bipolar link e.g. +/-

400 kV. Earth return in the case of a single pole failure

c) Unipolar link e.g. with 2 * -400 kV and earth/sea return

DCAC AC

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HVDC applications

• HVDC links can be used to connect 2 AC power systems with different frequencies and/or phase

• HVDC is and asynchronous connecting link

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17Lecture 15a Power Engineering - Egill Benedikt HreinssonThe functioning of the 12 pulse Graetzbridge

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HVDC terminal station

• The design of the terminal station with:– a YY transformer– a YD transformer

• A thyristor stack as a 12 pulse Gaetz bridge

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The firing angle for thyristors

The DC voltage converter for different angle.

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20Lecture 15a Power Engineering - Egill Benedikt HreinssonChange of power flow in a HVDC system

Source: http://www.spectrum.ieee.org/

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21Lecture 15a Power Engineering - Egill Benedikt HreinssonSymbols and the composition of semiconductor parts

• Light triggered thyristor

Gate

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The Cross Section of aHigh Voltage Thyristor

• The valve is the basic power-switching element of a converter. It consists of series-connected, fully protected thyristorlevels, each having high power thyristors of up to 125mm diameter, 8.5kV rating

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Thyristor characteristics

• Anode current as a function of voltage for a thyristor

Anode current

Anode voltage

Characteristic curve for forward cutoff current

A characteristic line for forward current

Characteristic curve for current break-through backwards

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A Thyristor unit

•A thyristor unit with 7 water cooled thyristors

LTT=light triggered thyristor

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The thyristor valve hall

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Dannebo converter station(Fennoskan)

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Baltic Cable converter station

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Baltic Cable converter station

Q comp

AC filter

ActiveDC filter

Transformer building

Valve hall

DC line

Smoothing reactor

AC switchyardAC line

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Valve cooling system diagram

HVDC : ThyristorValve Technology

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The evolution of a thyristor’s current and voltage capacity 1970-1995

01

23456

1970 1975 1980 1985 1990

Nominal currentthrough thyristor

Maximum cut offvoltage for thyristor

Current (kA)

2

4

6

8Voltage (kV)

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HVDC link between Norway and Denmark (Skagerak)

AC transmissionlines

Reactive power generation

Back-up generation

Harmonics filters

High passfilters

Thyristor valves

Thyristor valves Harmonics filters

High passfilters

Reactive power generation

Back-up generation

AC transmissionlines

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32Lecture 15a Power Engineering - Egill Benedikt HreinssonThe submarine cable between Sweden and Finland (Fennoskan)

• The copper conductor is 1200 mm2

• The cable weights 54 kg/m

• Double armouring• Transmission

capacity 500 MW• Voltage 400 kV DC• Length 200 km• Commissioning 1989

A SINTEF employee with a cable sample

Heimild: http://www.energy.sintef.no

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A plan for submarine cable project in Malaysia

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HVDC cable cross

section

Source: Vattenfall, Stockholm, Sweden

Fennoskan(Sweden-Finland)

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The Break-even Distance for HVDC

Gemmell, B.; Loughran, J. “HVDC offers the key to untapped hydro potential”, IEEE Power Engineering Review , Volume: 22 Issue: 5 , May 2002 Page(s): 8 -11

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Typical HVDC Break-Even Distances

Source: Arrillaga (1998)Source: Arrillaga (1998)

An interesting link : HVDC Transmission:Part of the Energy Solution? http://cohesion.rice.edu/CentersAndInst/CNST/emplibrary/Hartley%2004May03%20NanoTechConf.ppt

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Cost of HVDC converter stations

The cost of the DC/AC converters shows a significant economies of scale

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38Lecture 15a Power Engineering - Egill Benedikt Hreinsson 38From a HVDC submarine cable factory

The cable is wound on a horizontal roll and is then delivered to the ship

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Submarine cable and cable laying vessel

Slide 14 of 21

Left: submarine cable with all layers shown- Top right: cable laying vessel and the crew at work

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N. American Transmission Regions

Four major independent asynchronous networks, tied together only by DC interconnections:1. Eastern Interconnected Network – all regions east of the Rockies except ERCOT and Quebec portion of the NPCC reliability council.2. Quebec – part of the NPCC reliability council.3. Texas – the ERCOT reliability council.4. Western Interconnected Network – the WSCC reliability council.

Four major independent asynchronous networks, tied together only by DC interconnections:1. Eastern Interconnected Network – all regions east of the Rockies except ERCOT and Quebec portion of the NPCC reliability council.2. Quebec – part of the NPCC reliability council.3. Texas – the ERCOT reliability council.4. Western Interconnected Network – the WSCC reliability council. Source: Arrillaga (1998)Source: Arrillaga (1998)

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• Proposed Neptune Project: 1,000 km 1,200 MW submarine cable from Nova Scotia to Boston, New York city and NJ.– Take natural gas energy to NY with less visual impact, while

avoiding a NIMBY problem in NY and allowing old oil-fired plant in NY to be retired.

– Help improve network stability and reliability.– The southern end has a summer peak demand, the northern end a

winter one, so a bi-directional link allows savings from electricity trade.

Neptune Project

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HVDC Energy delivery in the North Sea

Valhall

Hod

UlaTambar

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Valhalla Norwegian oil project

Flank South

Flank NorthEkofisk

Lista

Valhall Complex

New PH

Pipeline from HOD

HVDCCable

Oil Pipeline to Ekofisk

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~

=

ACFilter

Transformer Phasereactor

Converter

Åna-SiraConverter Station

300

kV

0 kV

-150 kV

DCFilter

DCFilter

~=

Phasereactor

Transformer

ACFilter

Converter

11 k

V

ValhallConverter Station

Lista

System• 78 MW is needed to deliver 150mbd /

175mmscf/d, offshore • The main components:• Power delivered form the shore• HVDC converter stations – on land and offshore

(ABB)• HVDC cable includes a fibre optic cable (Nexans)

Norwegian oil platforms, 290 km

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Norned45

700 MW , 580 km link available 2008 between Norway and thNetherlands.Transmission capacity auctioned 1 day ahead. Price differences in the APX and Nordpool

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Britned

1000 MW, 280 km link to take into operation 2010 between the UK and Netherlands

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China

http://www.ptd.siemens.de/artikel0707_low.pdf

Several DVDC links in China

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History repeats itself!!

• Back to Edison !• Use DC power !

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References• Gemmell, B.; Loughran, J. “HVDC offers the key to

untapped hydro potential”, IEEE Power Engineering Review , Volume: 22 Issue: 5 , May 2002 Page(s): 8 –11

• The rise of high-voltage, direct-current systems by Narain G. Hingorani, Consultant (1996) http://www.spectrum.ieee.org/

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