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

2. ronnie.belmans@esat.kuleuven.be / 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. ronnie.belmans@esat.kuleuven.be / 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?
wind-energy-the-facts.org
mainstreamrp.com
pepei.pennnet.com
Statnett
wikipedia/desertec
Desertec-australia.org
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. ronnie.belmans@esat.kuleuven.be / 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. ronnie.belmans@esat.kuleuven.be / 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. ronnie.belmans@esat.kuleuven.be / 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. ronnie.belmans@esat.kuleuven.be / May-2010
Scheme
Converter
transformers
Large inductor
Filters
valves
/ May-2010

16. Operation: LCC or CSC HVDCUd
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. ronnie.belmans@esat.kuleuven.be / 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. ronnie.belmans@esat.kuleuven.be / 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. ronnie.belmans@esat.kuleuven.be / 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. ronnie.belmans@esat.kuleuven.be / 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. ronnie.belmans@esat.kuleuven.be / 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. ronnie.belmans@esat.kuleuven.be / 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. ronnie.belmans@esat.kuleuven.be / 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. ronnie.belmans@esat.kuleuven.be / 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. ronnie.belmans@esat.kuleuven.be / 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. ronnie.belmans@esat.kuleuven.be / 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. ronnie.belmans@esat.kuleuven.be / May-2010
Example
Slack converter
(controls DC voltage)
/ May-2010

45. ronnie.belmans@esat.kuleuven.be / May-2010
Example
Loss of converter
/ May-2010

46. ronnie.belmans@esat.kuleuven.be / May-2010
Example
Slack converter compensates
200
If 200 is higher than converter rating,
DC voltage will become unacceptably high
/ May-2010

47. ronnie.belmans@esat.kuleuven.be / 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. ronnie.belmans@esat.kuleuven.be / 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. ronnie.belmans@esat.kuleuven.be / May-2010
Challenges
Other
Technical compatibility
Incentive mechanisms for TSOs
Common, long term vision
Planning …
/ May-2010

50. ronnie.belmans@esat.kuleuven.be / 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. ronnie.belmans@esat.kuleuven.be / 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. ronnie.belmans@esat.kuleuven.be / 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. ronnie.belmans@esat.kuleuven.be / 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. ronnie.belmans@esat.kuleuven.be / May-2010
Example: Borwin
128 km DC sea cable
75 km DC land cable (less expensive)
/ May-2010

56. ronnie.belmans@esat.kuleuven.be / May-2010
Example: Borwin
/ May-2010

57. ronnie.belmans@esat.kuleuven.be / 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