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

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Presentation on theme: "© K.U.Leuven - ESAT/Electa / May-2010 Meshed DC networks for offshore wind development Ronnie Belmans KULeuvenESAT-ELECTA."— Presentation transcript:

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

2 © K.U.Leuven - ESAT/Electa 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 © K.U.Leuven - ESAT/Electa 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 © K.U.Leuven - ESAT/Electa 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? We must accept the limits of todays situation Be aware of the sailing ship syndrome… Stretching was successful for trains / May-2010

5 © K.U.Leuven - ESAT/Electa Planning:How will the future grid look like? A renewed grid vision? … ? / May-2010

6 © K.U.Leuven - ESAT/Electa / May-2010 Supergrid Visions How will the future DC grid look like? source:

7 © K.U.Leuven - ESAT/Electa Supergrid Visions How will the future DC grid look like? / May-2010 © ABB Group Slide 7 10MP0458 Solar 700 GW 8000 km sq 90 x 90 km Cables (Solar) 140 pairs of 5 GW and 3000 km each

8 © K.U.Leuven - ESAT/Electa Supergrid Visions How will the future DC grid look like? / May G. Czisch

9 © K.U.Leuven - ESAT/Electa Supergrid Visions How will the future DC grid look like? / May-2010 © ABB Group Slide 9 10MP0458 Statnett wind-energy-the-facts.org mainstreamrp.com pepei.pennnet.com Statnett wikipedia/desertec Desertec-australia.org

10 © K.U.Leuven - ESAT/Electa 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 / May-2010

11 © K.U.Leuven - ESAT/Electa Thury System HVDC is not new either / May-2010 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)

12 © K.U.Leuven - ESAT/Electa 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 20s 40s First HVDC project completed in 1955: Gotland Steady increase in ratings / May-2010

13 © K.U.Leuven - ESAT/Electa 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 © K.U.Leuven - ESAT/Electa Highlights Pinnacle: Itaipu : ± 600 kV, 2 x 3150 MW First multi-terminal: kV Shanghai-Xiangjiaba (2011), LCC HVDC world records: Voltage (800 kV) Transmitted power (6400 MW) Distance (2071 km) / May-2010

15 © K.U.Leuven - ESAT/Electa / May-2010 Scheme Large inductor Filters Converter transformers valves

16 © K.U.Leuven - ESAT/Electa / May-2010 Operation: LCC or CSC HVDC 6-pulse bridge 0 Ud LCC: Line-Commutated Converter needs grid to commutate CSC: Current Source Converter

17 © K.U.Leuven - ESAT/Electa LCC HVDC Reactive Power Requirements Harmonic Filters Shunt Banks filter converter unbalance 1,0 0,5 I d 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 / May-2010

18 © K.U.Leuven - ESAT/Electa CSC HVDC Filter requirements result in huge footprint Not viable for offshore application / May-2010

19 © K.U.Leuven - ESAT/Electa Multi-terminal Hydro 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 / May-2010

20 © K.U.Leuven - ESAT/Electa 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 © K.U.Leuven - ESAT/Electa 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 / May-2010

22 © K.U.Leuven - ESAT/Electa 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 / May-2010 CSC for offshore multi-terminal HVDC is a dead end

23 © K.U.Leuven - ESAT/Electa 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 © K.U.Leuven - ESAT/Electa VSC HVDC First installation: Gotland (yes, again) MW ± 80 kV Subsequent installations have ever higher ratings, but ratings CSC remain out of reach / May-2010

25 © K.U.Leuven - ESAT/Electa / May-2010 State-of-the-art CSC HVDC 6300 MW ±600 kV DC 785 km 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

26 © K.U.Leuven - ESAT/Electa / May-2010 VSC Scheme Large capacitor

27 © K.U.Leuven - ESAT/Electa / May-2010 VSC Switching Two possibilities PWM Multi-level

28 © K.U.Leuven - ESAT/Electa / May ,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 HVDC Reactive 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

29 © K.U.Leuven - ESAT/Electa Construction / May-2010 Less filters reduced footprint Only cooling equipment and transformers outside Valves pre- assembled

30 © K.U.Leuven - ESAT/Electa / May-2010 VSC HVDC for offshore applications Modified design for offshore applications Troll (2005) First offshore HVDC converter 40 MW, 70 km from shore Oil-platform

31 © K.U.Leuven - ESAT/Electa VSC HVDC for offshore applications Valhall (2010) 78 MW 292 km Oil-platform / May-2010

32 © K.U.Leuven - ESAT/Electa / May-2010 VSC HVDC for offshore applications Borwin alpha (2010) First offshore HVDC converter for wind power 400 MW 200 km Wind collector

33 © K.U.Leuven - ESAT/Electa 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 / May-2010

34 © K.U.Leuven - ESAT/Electa / May-2010 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

35 © K.U.Leuven - ESAT/Electa / May-2010 Offering Ancillary Services to the Grid TSOs 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 I nductive limit

36 © K.U.Leuven - ESAT/Electa 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 © K.U.Leuven - ESAT/Electa / May-2010 Reactive power control by VSC HVDC P,Q-controllability of onshore converter No additional components (STATCOM, SVC) needed

38 © K.U.Leuven - ESAT/Electa Multi-terminal VSC HVDC VSC HVDC only developed for point-to-point, but… …looks very promising for MTDC Converters 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 © K.U.Leuven - ESAT/Electa 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 / May-2010 VSC for offshore multi-terminal HVDC looks promising

40 © K.U.Leuven - ESAT/Electa Challenges for supergrid Technical Offshore equipment Ratings Losses Reliability MTDC Control Economical/Financial Political/Sociopolitical / May-2010

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

42 © K.U.Leuven - ESAT/Electa 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. Problem o DC breaker not available yet o Current rises extremely fast Very fast fault detection needed Very fast and precise fault localisation needed Very fast breaker needed / May-2010

43 © K.U.Leuven - ESAT/Electa 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. / May-2010

44 © K.U.Leuven - ESAT/Electa Example / May-2010 Slack converter (controls DC voltage)

45 © K.U.Leuven - ESAT/Electa Example / May-2010 Loss of converter

46 © K.U.Leuven - ESAT/Electa Example / May-2010 Slack converter compensates 200 If 200 is higher than converter rating, DC voltage will become unacceptably high

47 © K.U.Leuven - ESAT/Electa 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 / May-2010 Source: ENTSO-E, Ten-year network development plan

48 © K.U.Leuven - ESAT/Electa 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 / May-2010 Source: ENTSO-E, Ten-year network development plan

49 © K.U.Leuven - ESAT/Electa Challenges Other Technical compatibility Incentive mechanisms for TSOs Common, long term vision Planning … / May-2010

50 © K.U.Leuven - ESAT/Electa Connection to AC grid Close to shore o Reinforcement AC grid needed OHL AC Underground AC cable To strong, inland AC bus o Overhead DC o Underground DC / May-2010

51 © K.U.Leuven - ESAT/Electa 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 © K.U.Leuven - ESAT/Electa Sea vs Land cable / May-2010 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

53 © K.U.Leuven - ESAT/Electa AC vs DC cable AC cables Three-phase three conductors Reactive compensation at regular intervals / May-2010 DC sea cable more expensive than DC land cable AC cable more expensive than DC cable

54 © K.U.Leuven - ESAT/Electa / May-2010 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

55 © K.U.Leuven - ESAT/Electa Example: Borwin 128 km DC sea cable 75 km DC land cable (less expensive) / May-2010

56 © K.U.Leuven - ESAT/Electa Example: Borwin / May-2010

57 © K.U.Leuven - ESAT/Electa Conclusions CSC HVDC Stretching not possible o Too large o Grid voltage needed VSC HVDC Stretching possible o Small footprint o Passive grid operation o Technical characteristics suited to wind applications o Offshore applications proven Technical challenges remain… o DC breaker o Fast fault detection and localisation o Losses o Ratings o DC voltage control …but can be solved Need to further look into economical and political challenges / May-2010


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