ELEC0445 - High Voltage Direct Current grids
Part 1. Line Commutated Converters
Chapter 1. Overview of HVDC applications
Patricia Rousseaux Thierry Van [email protected] 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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