ELEC0445 - High Voltage Direct Current grids

Part 1. Line Commutated Converters

Chapter 1. Overview of HVDC applications

Patricia Rousseaux Thierry Van Cutsemt.vancutsem@ulg.ac.be www.montefiore.ulg.ac.be/~vct

February 2018

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Chapter 1. Overview of HVDC applications Principle of HVDC links

Principle of HVDC links

HVDC links embedded in AC systems

Rely on converters :I rectifier1: from AC to DCI inverter2: from DC to AC

1redresseur2onduleur

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Chapter 1. Overview of HVDC applications Historical perspective

Historical perspective

At the beginning (end of 19th century) : two struggling partiesI first generators producing Direct Current (DC) - Gramme, EdisonI first generators producing Alternating Current (AC) - Ferranti, Tesla

Have a look at : war of the currentshttps://www.youtube.com/watch?v=dIfIRj0Crc8

the AC system won :I possibility to increase and lower the voltage thanks to the transformer⇒ transmission of higher powers possible

I creation of a rotating field easy with three-phase AC windings

I impossibility to raise the DC voltage ⇒ impossibility to transmit largepowers with DC

I limitation of the power of early converters : a few kW onlyI difficulty of interrupting a DC current.

Revival of DC technology in the ‘50s

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Chapter 1. Overview of HVDC applications Historical perspective

Historical perspective (cont’d)

Advances in power electronics : converters can carry larger currentsthrough higher voltages ⇒ higher power ratings ⇒ transmissionapplications possible

1882 : Marcel Deprez (France) and Oskar Von Miller (Germany, AEG)design the first transmission link between a DC source and a DC load;15 kW 2 kV 56.3 km

mid ‘30s : mercury-arc valve rectifiers made available.They open the way to HVDC transmission link projects

1945 : first commercial project of HVDC transmission in Germany.Not commissioned and moved to USSR (Moscow-Kashira) in 1950;60 MW 200 kV 115 km, with buried cables

1954 : first commercial HVDC submarine installation : from Gotlandisland to Sweden20 MW 100 kV 98 km

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Chapter 1. Overview of HVDC applications Historical perspective

Historical perspective (cont’d)

Up to the mid ‘60s,due its higher cost, HVDC was favoured onlywhere AC met operational difficulties, e.g. sea crossing

late ‘60s : advent of high power thyristor-based valve converters

1975 : 1st long-distance HVDC transmission using thyristor valveconverters : Cahora Bassa in Mozambique

1920 MW 533 kV 1420 km, with overhead line

thyristor ratings have grown up to V = 9 kV and I = 4 kA (per thyr.)

late ‘90s : high power transistor-based components become available :IGBT, MOSFET

development of Voltage Source Converters. They allow controllingboth the active and the reactive power at each terminal of the link.

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Chapter 1. Overview of HVDC applications First application : Power transmission over long distances

First application : Power transmission over long distancesthrough overhead lines

Long AC lines require reactive power compensation / voltage support andfor distances larger than 600-800 km, HVDC is more economical

Examples :

Pacific DC inter-tie along Westcoast of USA :1360 km 3100 MW ± 500 kV

Cahora-Bassa line in Mozambique :1420 km 1920 MW ± 533 kV

Hydro-Quebec DC line :1018 km 2000 MW ± 450 kV

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Chapter 1. Overview of HVDC applications First application : Power transmission over long distances

Smaller investment costs

initial investment is higher forDC (due to converters) but

with increasing distance, reactivepower compensation is requiredfor an AC line

break-even distance 600-800 km

comparison of towers : identicaltransmission capacity of 3 GW,a) 735 kV AC b) 500 kV DC

smaller Right-of-Way for DCcorridor

reduced footprint7 / 18

Chapter 1. Overview of HVDC applications First application : Power transmission over long distances

Lower losses, higher thermal capacity

At a similar voltage level (RMS phase-to-phase vs. DC pole-to-ground) :

a DC line can transmit more than twice the power of an AC line

with about half the losses of an AC line.

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Chapter 1. Overview of HVDC applications Second application : submarine power transmission

Second application : submarine power transmission

AC cables have large capacitance.Maximal acceptable length :50-70 km.For larger distances, HVDC is theonly (reasonable) solution

Examples from Europe :I NorNed link between Norway

and The Netherlands (2008)580 km 700 MW ± 450 kV(LCC type)

I Nemo project between Belgiumand UK (2019)140 km 1000 MW ± 400 kV(VSC type)

I connections of off-shore windparks in North Sea to thecontinental European grid source ENTSOe (www.entsoe.eu)

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Chapter 1. Overview of HVDC applications Third application : DC link in AC grid, for power flow control

Third application : DC link embedded in AC grid, forpower flow control

power flows in AC lines cannot be controlled directlyI determined by line impedances and Kirchhoff lawsI partially controllable by phase shifting transformers

power flows in HVDC links can be controlled directly (through thecontrol of converters)

I can be used to limit loop flows and overloading of AC linesI participate in the trading of power

Examples :I ALEGrO (Aachen Liege Electric Grid Overlay) project of HVDC link

between Belgium and Germany (2019-2020)100 km (49 in Belgium) 1000 MW buried cable

I France - Spain DC interconnection :65 km buried XLPE cable ± 320 kV DC 2000 MW 3

3transfer capacity between France and Spain : 1400 MW in AC lines10 / 18

Chapter 1. Overview of HVDC applications Third application : DC link in AC grid, for power flow control

France - Spain DC interconnection

power flow reversal in 150 millisecondsinvestment cost : 700 Me

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Chapter 1. Overview of HVDC applications Fourth application : interconnection of asynchronous AC systems

Fourth application : interconnection of asynchronous ACsystems

1 Two AC networks with different nominal frequencies.Back-to-back connection (rectifier and inverter in same substation)

I Melo HVDC link between Uruguay(50 Hz) and Brazil (60 Hz)500 MW ± 79kV

I Shin Shinano HVDC link betweenWestern (60 Hz) and Eastern (50Hz) power grids of Japan

600 MW ± 125 kV

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Chapter 1. Overview of HVDC applications Fourth application : interconnection of asynchronous AC systems

2 Two AC networks with identical nominal frequency but differentfrequencies (not interconnected for size reasons)

I Highgate back-to-back HVDC link between Quebec and Vermont200 MW ± 57 kV

I McNeil HVDC link between Alberta and Saskatchewan150 MW ± 42 kV

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Chapter 1. Overview of HVDC applications Fifth application : Multiterminal DC grids

Fifth application : Multiterminal DC grids

1 Radial DC link with more than two DC terminals

I a few systems are in operation today with proven technology

I example : the Sardinia-Corsica-Italy link (SACOI)

3 terminals. The 2-terminal Italy-Sardinia link was initiallybuilt, and the Corsica terminal installed at a later stage

I more elaborate control scheme than for a two-terminal link

I can be also used to connect off-shore windparks

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Chapter 1. Overview of HVDC applications Fifth application : Multiterminal DC grids

2 Meshed DC gridsI still under investigation

Example : North Sea DC grid project : connection of wind parks inNorth Sea through a meshed DC grid (part of the European AC-DCsupergrid)

I main technological challenges :F DC circuit breakersF protection against faults in DC gridF grid power flow control

I see report of CIGRE WG B4.52 ‘’HVDC Grid Feasibility study”, 2013

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Chapter 1. Overview of HVDC applications Two technologies

Two technologies

Line Commutated Converters (LCC)

uses thyristors

large power ratings

large harmonics filters

strong AC grid required

oil based (mass impregnated)cables

active power control

not adequate for off-shoreapplications

cheaper

lower losses

possible commutation failure

Voltage Source Converters (VSC)

uses IGBTs

medium power ratings

less harmonic filter needed,smaller footprint

operates with weak AC systems

XPLE (plastic) insulated cablesare possible

independent active and reactivepower control

off-shore applications possible

black start capability

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Chapter 1. Overview of HVDC applications Two technologies

LCC technology

also called Current Source Converter (CSC) or “classic HVDC”thyristors are fired with some intentional delayextinguished by current passing through zeroAC current always AC lags voltage ⇒ consumes reactive powerin normal operation, DC current is kept constantDC current cannot be reversedpower is reversed by reversing the voltage polarity

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Chapter 1. Overview of HVDC applications Two technologies

VSC technology

basic block : 6-pulse IGBT converter

Pulse Width Modulation (PMW) used to adjust the AC voltage

in normal operation, DC voltage is kept constant

DC voltage polarity does not change

power is reversed by reversing the current.

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