© K.U.Leuven - ESAT/Electa [email protected] / May-2010 Meshed DC networks for offshore wind development Ronnie Belmans KULeuvenESAT-ELECTA.

[email protected] / May-2010© K.U.Leuven - ESAT/Electa

Meshed DC networks for offshore wind development

Ronnie Belmans

KULeuven

ESAT-ELECTA

© K.U.Leuven - ESAT/Electa [email protected] / May-2010

Overview

• Historical development of HVDC→ can we stretch to ‘supergrids’?

• VSC HVDC Offshore Wind applications Multi-terminal

• Challenges for offshore Multi-terminal Direct Current (MTDC) systems

• How to connect to AC grid


• Supergrid: Why? Harness RES, crucial role of offshore wind, but

also wave, tidal and osmotic energy. Balancing: wind - hydro - natural gas Connect remote energy sources Trading: single market


PlanningHow will the future grid look like?

Can we manage by stretching the current 380 kV grid to its limits?

Or do we need a new overlay grid?

• We must accept the limits of today’s situation

• Be aware of the “sailing ship syndrome”…

• “Stretching” was successful for

trains


Planning: How will the future grid look like?

A renewed grid vision?

2020

2050

… ?

1948

1956

1974 2008


Supergrid VisionsHow will the future DC grid look like?

source: www.airtricity.com


Supergrid VisionsHow will the future DC grid look

like?

© ABB Group Slide 7

10MP0458

Hydro power

Solar power

Wind power

DC transmission

99LFC0825

Wind300 GW25 000 km sq5000 x 10 km

Hydro200 GW

Solar700 GW8000 km sq90 x 90 km

Cables (Solar)140 pairs of5 GW and 3000 km each


Supergrid Visions How will the future DC grid look like?

http://www.mainstreamrp.com/pages/Supergrid.html

http://www.desertec.org

G. Czisch


Supergrid Visions How will the future DC grid look

like?

© ABB Group Slide 9

10MP0458

Statnett

wind-energy-the-facts.orgmainstreamrp.com pepei.pennnet.com

Statnett

wikipedia/desertec Desertec-australia.org


Electricity pioneers: AC or DC?DC is not new

• Direct Current DC Generator built by W. von Siemens and Z.Gramme

o Low line voltage, and consequently limitation to size of the system

Edison

• Alternating current AC Introduced by Nikola Tesla and Westinghouse

o Transformer invented by Tesla allows increasing the line voltage

o Allows transmitting large amounts of electricity over long distances

Work of Steinmetz


Thury SystemHVDC is not new either

• Thury system: series connected DC generators and loads

• 1889: first system (1kV)• 1906: Lyon-Moutiers

(125 kV, 230 km)• 1913: 15 Thury systems

in use• Problem: reliability

(series connection)


AC/DC Conversion

• AC became prevalent• Full DC electricity grid out of the

question. HVDC needed AC/DC conversion

• Research and development effort on Mercury Arc Valves in 20’s 40’s

• First HVDC project completed in 1955: Gotland

• Steady increase in ratings


Semiconductors

• Advances in semiconductors led to thyristor valves with many advantages Simplified converter stations Overhauls less frequently needed No risk of mercury poisoning Easy upscaling by stacking thyristors (increased voltage

levels) and parallel-connecting thyristor stacks (increasing current rating)

• Gradual replacement of mercury arc valves to thyristor valves. First replacement 1967: Gotland

• Today only 1 or 2 HVDC systems with mercury arc valves remain


Highlights

• Pinnacle: Itaipu 1984 - 1987: ±600 kV, 2 x 3150 MW

• First multi-terminal: 1987• 800 kV Shanghai-Xiangjiaba (2011), LCC

HVDC world records: Voltage (800 kV) Transmitted power (6400 MW) Distance (2071 km)


Scheme

Large inductor

Filters

Convertertransformers

valves

© K.U.Leuven - ESAT/[email protected] / May-2010

Operation: LCC or CSC HVDC

6-pulse bridge

0

Ud

LCC: Line-Commutated Converter → needs grid to commutate

CSC: Current Source Converter


LCC HVDCReactive Power Requirements

Harmonic Filters

Shunt Banks filter

converterunbalance

1,0

0,5

Id

Q

Classic

0,13

• LCC converters absorb reactive power (50% to 60% of active power).

• Harmonic filters needed to filter AC harmonics and to provide reactive power.

• The more active power, the more reactive power is needed• Switching filters to reduce unbalance

source: ABB


CSC HVDC

• Filter requirements result in huge footprint

• Not viable for offshore application


Multi-terminalHydro Québec - New England (1992)

• Hydro Québec - New England (1992) Extended to 3-terminal Originally planned: 5-terminal

but cancelled (Des Cantons, Comerford)

Fixed direction of power


Multi-terminal Mainland Italy-Corsica-Sardinia

• 1965: monopolar between mainland and Sardinia

• 1987: converter added in Corsica

• 1990: mercury arc replaced by thyristors

• 1992: second pole added


Multi-terminal Mainland Italy-Corsica-Sardinia

• Corsica converter is parallel tap• Limited flexibility, e.g.: fast change in power flow

direction at Sardinia requires temporary shut-down of Corsican converter


Intermediate Conclusion 1

Footprint too large because of filtering requirements

There is no offshore voltage source, needed for commutation

General multi-terminal operation not feasible, only ‘pseudo-multi-terminal’

CSC for offshore multi-terminal HVDC is a dead end


VSC HVDC

• Not new development, but entirely new concept based on switches with turn-off capability

• Characteristics: No voltage source needed to commutate Very fast Very flexible: independent active and reactive

power control


VSC HVDC

• First installation: Gotland (yes, again) 1999 50 MW ±80 kV

• Subsequent installations have ever higher ratings, but ratings CSC remain out of reach


State-of-the-art

• CSC HVDC 6300 MW ±600 kV DC 785 km + 805 km

• CSC HVDC 6400 MW ±800 kV DC 2000 km

• VSC HVDC 1100 MW 350 kV DC

Existing

Currently possible

• VSC HVDC 350 MW ±150 kV DC 180 km


VSC Scheme

Large capacitor


VSC SwitchingTwo possibilities

PWM

Multi-level


-1,2 -1,0 -0,8 -0,6 -0,4 -0,2 0 0,2 0,4 0,6 0,8 1,0 1,2 Q (pu)

1,2

1,0

0,8

0,6

0,4

0,2

0

-0,2

-0,4

-0,6

-0,8

-1,0

-1,2

P (

pu)

VSC HVDCReactive power requirements

CapacitiveInductive

• Reactive power can be provided by converter

• Operation in four quadrants possible

• Voltage support• Smaller filters:

only for filtering, not for reactive power


Construction

• Less filters → reduced footprint

• Only cooling equipment and transformers outside

• Valves pre-assembled


VSC HVDC for offshore applications

Modified design for offshore applications

Troll (2005) First offshore HVDC

converter 40 MW, 70 km from shore Oil-platform



• Valhall (2010) 78 MW 292 km Oil-platform



• Borwin alpha (2010) First offshore HVDC

converter for wind power 400 MW 200 km Wind collector


Borwin alpha

1. AC side with transformers, breakers, and filters

2. AC phase reactors

3. Valves

4. DC side with capacitors and cable connections

5. Cooling equipment


VSC HVDC for wind applications

No cable length issues Wind farms are independent of power system

Do not need to run on main frequency Do not need to run on fixed frequency Wind farm topology must be re-evaluated (fixed

speed induction machines?) Multiple wind farms can be connected to

offshore grids This could lead to a ‘supergrid’ connecting

different areas with different wind profiles


Offering Ancillary Services to the Grid

• TSO’s Grid Code: “Wind turbines must have a controllable power factor”

• Grid code country-

specific• Demands at PCC

for 300 MW Minimum PF = 0,95 Required: 98,6 MVAr

Capacitive limit

Inductive limit


Offering Ancillary Services to the Grid

• Additional equipment needed such as SVC, STATCOM,… Compensate AC cable capacitance Be grid compliant

• Resonances between cable C and grid L


Reactive power control by VSC HVDC

• P,Q-controllability of onshore converter

• No additional components (STATCOM, SVC) needed


Multi-terminal VSC HVDC

• VSC HVDC only developed for point-to-point, but…• …looks very promising for MTDC

Converter’s DC side has constant voltage → converters can be easily connected to DC network.

• Extension to ‘pseudo-multi-terminal’ systems straightforward: e.g. star-connections


Intermediate Conclusion 2

Footprint can be made small enough for offshore applications because of limited filtering requirements

No offshore voltage source needed Offshore operation is proven for point-to-point

connection General multi-terminal operation possible because

DC side has constant voltage

VSC for offshore multi-terminal HVDC looks promising


Challenges for supergrid

• Technical Offshore equipment Ratings Losses Reliability MTDC Control

• Economical/Financial• Political/Sociopolitical


Challenges

• Losses Converter losses very high (> 1.3%)

o Special switching techniqueso New materials

Cooling

• Ratings Proven power ratings low compared to CSC HVDC Proven voltage levels low compared to CSC

HVDC


Challenges

• Reliability DC Fault leads to complete shutdown

1. To protect IGBTs from fault current, they are blocked

2. Anti-parallel diodes keep conducting the fault current

3. No DC breakers are present

4. The fault needs to be cleared by opening AC breakers For MTDC, a DC fault would lead to loss of whole MTDC grid.

This is not acceptable. The fault needs to be cleared selectively at DC side.

Problemo DC breaker not available yeto Current rises extremely fast

Very fast fault detection needed Very fast and precise fault localisation needed Very fast breaker needed


Challenges

• Reliability DC voltage needs to remain within small band Problem:

o If only one converter controls DC voltage, DC voltage can become unacceptably low in MTDC grid

o What if voltage controlling converter fails?o Other voltage control method needed. Which one?

Unknown.


Example

Slack converter

(controls DC voltage)


Example

Loss of converter


Example

Slack converter compensates

200

If 200 is higher than converter rating,

DC voltage will become unacceptably high


Challenges

• Economical/Financial issues Different generation and load scenarios Cost/benefit of scenarios Electricity prices Financial demand per scenario Realization and ownership of the Supergrid European funding Potential investors

Source: ENTSO-E, “Ten-year network development plan 2010-2019”


Challenges

• Political/Sociopolitical issues Legal and regulatory framework Social acceptance of the Supergrid Permitting processes, harmonization of national rules European policy on DSM New areas to be incorporated: Russia, Nordic,… Political stability of regions Risk of terrorist attacks

Source: ENTSO-E, “Ten-year network development plan 2010-2019”


Challenges

• Other Technical compatibility Incentive mechanisms for TSOs Common, long term vision Planning …


Connection to AC grid

• Connection to AC grid Close to shore

o Reinforcement AC grid needed OHL AC Underground AC cable

To strong, inland AC buso Overhead DCo Underground DC


Sea vs Land cable

• Land cable Light: weight limits maximum

section length. Less joints needed

Small bending radius: smaller drums can be used

Easy installation No armour


Sea vs Land cable

• Sea cable Heavy Transported by ship in very

long sections Large bending radius: huge,

ship-mounted drums Armoured by galvanised steel Water tight by lead sheaths


AC vs DC cable

• AC cables Three-phase → three conductors Reactive compensation at regular intervals

• DC sea cable more expensive than DC land cable• AC cable more expensive than DC cable


Cost ratio ac / VSC HVDC(2006)

Conclusion:If underground solution is needed, HVDC may be cheaper

• Factors:• DC sea cable more expensive than DC land cable• AC cable more expensive than DC cable


Example: Borwin

• 128 km DC sea cable• 75 km DC land cable (less expensive)


Example: Borwin


Conclusions

• CSC HVDC Stretching not possible

o Too largeo Grid voltage needed

• VSC HVDC Stretching possible

o Small footprinto Passive grid operationo Technical characteristics suited to wind applicationso Offshore applications proven

Technical challenges remain…o DC breakero Fast fault detection and localisationo Losseso Ratingso DC voltage control

…but can be solved Need to further look into economical and political challenges

© K.U.Leuven - ESAT/Electa [email protected] / May-2010 Meshed DC networks for offshore wind development Ronnie Belmans KULeuvenESAT-ELECTA.

Documents

leuven esatelecta planning

czisch slide

future dc grid

future grid

kv grid

current source converter

abb group slide

ac grid