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Meshed DC networks for offshore wind development

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Presentation on theme: "Meshed DC networks for offshore wind development"— Presentation transcript:

1 Meshed DC networks for offshore wind development
Ronnie Belmans KULeuven ESAT-ELECTA / May-2010

2 / 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 / May-2010

3 / May-2010
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 / May-2010

4 Planning How 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? “Stretching” was successful for trains Be aware of the “sailing ship syndrome”… We must accept the limits of today’s situation / May-2010

5 Planning: How will the future grid look like?
1956 1948 A renewed grid vision? 1974 2008 ? 2020 2050 / May-2010

6 Supergrid Visions How will the future DC grid look like?
source: / May-2010

7 Supergrid Visions How will the future DC grid look like?
Hydro power Solar power Wind power DC transmission 99LFC0825 Wind 300 GW km sq 5000 x 10 km Hydro 200 GW Cables (Solar) 140 pairs of 5 GW and 3000 km each Solar 700 GW 8000 km sq 90 x 90 km © ABB Group Slide 7 10MP0458 / May-2010

8 Supergrid Visions How will the future DC grid look like?
G. Czisch / May-2010

9 Supergrid Visions How will the future DC grid look like? Statnett wikipedia/desertec Statnett © ABB Group Slide 9 10MP0458 / May-2010

10 Electricity pioneers: AC or DC? DC is not new
Direct Current DC Generator built by W. von Siemens and Z.Gramme Low line voltage, and consequently limitation to size of the system Edison Alternating current AC Introduced by Nikola Tesla and Westinghouse Transformer invented by Tesla allows increasing the line voltage Allows transmitting large amounts of electricity over long distances Work of Steinmetz / May-2010

11 Thury System HVDC 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) / May-2010

12 / May-2010
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 / May-2010

13 / May-2010
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 / May-2010

14 / May-2010
Highlights Pinnacle: Itaipu : ±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) / May-2010

15 / May-2010
Scheme Converter transformers Large inductor Filters valves / May-2010

16 Operation: LCC or CSC HVDC
Ud 6-pulse bridge LCC: Line-Commutated Converter → needs grid to commutate CSC: Current Source Converter / May-2010

17 LCC HVDC Reactive Power Requirements
1,0 0,5 I d Q Classic Shunt Banks filter Harmonic Filters 0,13 converter source: ABB unbalance 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 / May-2010

18 / May-2010
CSC HVDC Filter requirements result in huge footprint Not viable for offshore application / May-2010

19 Multi-terminal Hydro Québec - New England (1992)
Extended to 3-terminal Originally planned: 5-terminal but cancelled (Des Cantons, Comerford) Fixed direction of power / May-2010

20 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 / May-2010

21 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 Conclusion from Hydro Québec - New England and Mainland Italy-Corsica-Sardinia: Multi-terminal HVDC? Not really! / May-2010

22 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 / May-2010

23 / May-2010
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 / May-2010

24 / May-2010
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 / May-2010

25 / May-2010
State-of-the-art Existing VSC HVDC 350 MW ±150 kV DC 180 km CSC HVDC 6300 MW ±600 kV DC 785 km km Currently possible CSC HVDC 6400 MW ±800 kV DC 2000 km VSC HVDC 1100 MW 350 kV DC / May-2010

26 / May-2010
VSC Scheme Large capacitor / May-2010

27 VSC Switching Two possibilities
PWM Multi-level / May-2010

28 VSC HVDC Reactive power requirements
Reactive power can be provided by converter Operation in four quadrants possible Voltage support Smaller filters: only for filtering, not for reactive power -1,2 -1,0 -0,8 -0,6 -0,4 -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 -0,2 -0,4 -0,6 -0,8 -1,0 -1,2 P (pu) Inductive Capacitive / May-2010

29 / May-2010
Construction Less filters → reduced footprint Only cooling equipment and transformers outside Valves pre-assembled / May-2010

30 VSC HVDC for offshore applications
Modified design for offshore applications Troll (2005) First offshore HVDC converter 40 MW, 70 km from shore Oil-platform / May-2010

31 VSC HVDC for offshore applications
Valhall (2010) 78 MW 292 km Oil-platform / May-2010

32 VSC HVDC for offshore applications
Borwin alpha (2010) First offshore HVDC converter for wind power 400 MW 200 km Wind collector / May-2010

33 / May-2010
Borwin alpha AC side with transformers, breakers, and filters AC phase reactors Valves DC side with capacitors and cable connections Cooling equipment / May-2010

34 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 / May-2010

35 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 / May-2010

36 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 / May-2010

37 Reactive power control by VSC HVDC
P,Q-controllability of onshore converter No additional components (STATCOM, SVC) needed / May-2010

38 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 / May-2010

39 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 / May-2010

40 Challenges for supergrid
Technical Offshore equipment Ratings Losses Reliability MTDC Control Economical/Financial Political/Sociopolitical / May-2010

41 / May-2010
Challenges Losses Converter losses very high (> 1.3%) Special switching techniques New materials Cooling Ratings Proven power ratings low compared to CSC HVDC Proven voltage levels low compared to CSC HVDC / May-2010

42 / May-2010
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 available yet Current rises extremely fast Very fast fault detection needed Very fast and precise fault localisation needed Very fast breaker needed / May-2010

43 / May-2010
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? Unknown. / May-2010

44 / May-2010
Example Slack converter (controls DC voltage) / May-2010

45 / May-2010
Example Loss of converter / May-2010

46 / May-2010
Example Slack converter compensates 200 If 200 is higher than converter rating, DC voltage will become unacceptably high / May-2010

47 / May-2010
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 ” / May-2010

48 / May-2010
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 ” / May-2010

49 / May-2010
Challenges Other Technical compatibility Incentive mechanisms for TSOs Common, long term vision Planning / May-2010

50 / May-2010
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 / May-2010

51 / May-2010
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 / May-2010

52 / May-2010
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 / May-2010

53 / May-2010
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 / May-2010

54 Cost ratio ac / VSC HVDC (2006)
Factors: DC sea cable more expensive than DC land cable AC cable more expensive than DC cable Conclusion: If underground solution is needed, HVDC may be cheaper / May-2010

55 / May-2010
Example: Borwin 128 km DC sea cable 75 km DC land cable (less expensive) / May-2010

56 / May-2010
Example: Borwin / May-2010

57 / May-2010
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 economical and political challenges / May-2010

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