© K.U.Leuven – ESAT/Electa Challenges to Climate Policy: Integrating Network Regulation Meshed DC networks for offshore wind development Ronnie Belmans.

© K.U.Leuven – ESAT/Electa

Challenges to Climate Policy: Integrating Network Regulation

Meshed DC networks for offshore wind development

Ronnie BelmansFlorence, 5 November 2010

[email protected] / November-2010

© K.U.Leuven – ESAT/Electa [email protected] / November-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• Technical• Economic/financial• Political/Sociopolitical• Standardization

• How to connect to AC grid

2


Supergrid: Why?

• 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

3


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?

4

• 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?

5

2020

2050

… ?

1948

1956

1974 2008


Supergrid VisionsHow will the future DC grid look

like?

6

source: www.airtricity.com


Supergrid VisionsHow will the future DC grid look

like?

7

© 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?

8

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

http://www.desertec.org

G. Czisch


Supergrid Visions How will the future DC grid

look like?

9

© ABB Group Slide 9

10MP0458

Statnett

wind-energy-the-facts.org

mainstreamrp.com pepei.pennnet.com

Statnett

wikipedia/desertec Desertec-australia.org


CSC: Classical HVDC

• 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

10


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)

11


CSC HVDC

• Filter requirements result in huge footprint

• Not viable for offshore application

12


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

13


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

14


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’

15

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 control16


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

17


State-of-the-art

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

18

• CSC HVDC 7200 MW ±800 kV DC 2000 km

• VSC HVDC 1100 MW ± 320 kV DC

Existing

Currently possible

• VSC HVDC 350 MW ± 150 kV DC 180 km


Construction

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• 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

20



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

21



• Borwin alpha (2010)• First offshore HVDC converter for wind power• 400 MW• 200 km• Wind collector

22


Borwin alpha

• AC side with transformers, breakers, and filters

• AC phase reactors• Valves• DC side with capacitors

and cable connections• Cooling equipment

23


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

24


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

25

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

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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

27


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

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VSC for offshore multi-terminal HVDC looks promising


Challenges for supergrid

• Technical• Offshore equipment• Ratings• Losses• Reliability• MTDC Control

• Economic/Financial• Political/Sociopolitical• Standardization

29


Challenges

• Losses• Converter losses were very high (> 1.3%) but

improvements are made (now 1%) Special switching techniques New materials

• Cooling

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

30


Challenges• Reliability

• DC Fault leads to complete shutdown To protect IGBTs from fault current, they are blocked Anti-parallel diodes keep conducting the fault current No DC breakers are present 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.

• Problem DC breaker not commercially available yet, but should

come out of the laboratories soon Current rises extremely fast

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

31


Challenges

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

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

What if voltage controlling converter fails? Other voltage control method needed. Which one? Grid codes will be needed

32


Technical challenges by auxiliary equipment and

maintenance• Pumps, fans, cooling in harsh and remote

environment (also valid for windturbine itself)

• Maintenance• Training and availability of personnel• Accessability in winter

33


Challenges

• Economical/Financial issues• Different generation and load scenarios• Cost/benefit of scenarios• Electricity prices• Financial demand per scenario• Financing by not directly involved TSO’s• Realization and ownership of the Supergrid• European funding• Potential investors

34

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, Norway,…• Political stability of regions• Start up of regulation now for the starting projects

for which technology is ready to go: Point to point Star connected (Kriegers Flak, Channel area, Dogger

Bank)

35

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


ChallengesStandardization

• General justification for standards• Reducing variety of technology competition• Interoperability avoid lock-in

• For HVDC• Competition? Several manufacturers/vendors in

the market Fairly OK

• Interoperability? Not at all! Need for standards

36


ChallengesStandardization

• Interoperability: in a context of meshed DC grids very important

• Today different systems are incompatible

37


Challenges Standardization

• What should be standardized?• Minimum minimorum to allow integration in a DC

grid• Voltage levels

• Necessary to avoid excessive integration costs• Cable sizes• Footprints• Cubicle sizes• Voltage control

• Optimal interoperability• Power electronics• Filters• Short circuit current• Protection• Communication• EMF-EMC

38


Challenges

• Other• Technical compatibility• Common, long term vision• Planning• …

39


Connection to AC grid

• Connection to AC grid• Close to shore

Reinforcement AC grid needed– OHL AC– Underground AC cable

• To strong, inland AC bus Overhead DC Underground DC

40


Example: Borwin

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

41


Example: Borwin

42


Conclusions• CSC HVDC

• Stretching not possible Too large Grid voltage needed

• VSC HVDC• Stretching possible

Small footprint Passive grid operation Technical characteristics suited to wind applications Offshore applications proven

• Technical challenges remain… DC breaker Fast fault detection and localisation Losses Ratings DC voltage control

• …but can be solved• Need to further look into economic and political challenges• Standardization required

43

© K.U.Leuven – ESAT/Electa Challenges to Climate Policy: Integrating Network Regulation Meshed DC networks for offshore wind development Ronnie Belmans.

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