Session 7. Connecting offshore wind farms to the onshore grid

Transcription

1 Session 7 Connecting offshore wind farms to the onshore grid Dr Mike Barnes

2 Present situation: Most windfarms onshore or AC connected

3 Next generation of wind-farms will be far offshore (source TimesOnline)

4 Problem Long transmission lines (especially underwater) are a problem. Consider usual approach synchronous aggregation All parallel wind turbines are modelled as one big wind turbine Tends to neglect smoothing effect that many unsynchronised turbines have on disturbances Gives a worst-case modelling result

5 Direct (Power) Control I System vector diagram: S δ I X φ I I

6 In equation form: In equation form: * jq P I jq P I = + = *, I jq P I = + = S I X δ I I φ ( ) ( ) + + = + + = * S jq P jx I jx + + = S Q PX j QX P + = S j

7 esolving into real and imaginary parts s cosδ = Generally X>> P + QX PX Q and s sinδ = QX s cosδ = and s sinδ = Q = S X 2 PX cosδ - and P S sinδ X X And δ usually <30deg for linear control, i.e. sinδ δ and cosδ 1 Q = ( ) S X and P S X δ

8 Consider what happens as line (electrical) length increases P, S and =fixed (1pu) X varies from 0.1pu to 1pu Q = and S X P cosδ - S X 2 X sinδ Delta for 1pu P Q for given power delt ta (deg) length (X/pu) (pu) Q Length (X/pu)

9 Long lines un out of angular difference to send real power Need vast amounts of reactive power For long distances DC transmission preferred over AC. But DC has problems as well as advantages Bigger terminal costs (but then lower cost per mile) Less Q needed Some distances just not possible otherwise Use DC if more than 500 to 700 km overland, or 50 to 70km underwater

10 Source: Hitachi AC vs DC

11 Next generation windfarms

12 oltage Source Offshore footprint: Siemens

13 CS vs S HDC CS HDC 6300MW CS HDC Loss 0.8% per converter station S HDC Loss 1.1% per converter station Much smaller footprint t Now similar cost to CS 400MW S HDC ZDF

14 New Power Generation / New Solution Main advantages of S over CS: Smaller footprint (offshore platform cheaper) Power direction in cable changed by current (keep same polarity, use cheaper XPLE cable) No need for reactive power generation to supply converter ABB.co m

15 E.G. BorWin1 Keep offshore size down: -DC choppers onshore -No tap-changing transformer offshore

16 BorWin1 Protection DC over voltage protection choppers

17 BorWin1 -First wind farm installation: BorWin1=80x5MW turbines -ABB 2-level converter ZDF

18 S-HDC Platform ZDF Heavier than AC platforms lifting an issue (especially offshore)

19 Major Marine Engineering Challenge Specialist lifting ship (expensive 100,000+/day) ZDF

20 Offshore Power Electronics Issues: Maintenance / reliability Insulation (clearance), peak voltage values ZDF

21 Cable laying ZDF Cable A single point of failure - A major cost item - Hard to fix a cable fault (but they are rare)

22 Control Cascaded controller: Inner voltage generation block Current loop (or over-current protection) Power loop Multi-terminal terminal coordination (telecommunications)

23 Point-to t Point: interactions ti Previously (AC) just turbines and shore Wind Turbines and link DC Link converters Converter to shore

24 General principle Offshore: Wind turbines provide real power (P) and support local voltage Offshore converter sets AC voltage magnitude and frequency for wind farm (slack AC bus) DC Link Offshore converter injects real power (sets current) Onshore converter sets dc voltage (DC slack bus) Onshore Onshore converter injects real power Onshore converter controls local AC voltage/q Telecommunications used to adjust set-points throughout

25 Converter Operation (Siemens, 2009)

26 PWM Frequencies of selected SC-HDC projects Installation Year in Converter Switching Switching Service Pattern frequency (Hz) Gotland level Sinusoidal Tjaereborg Directlink PWM Cross Sound level ANPC 3PWM 1260 Murraylink 2002 Sinusoidal PWM 1350 Estlink level Optimum PWM 1150 (Jacobsen, 2006)

27 In service Power dc/k ac/k Transmission Project /MW length/km Hällsjön, Sweden ± Gotland, Sweden ± Directlink, Australia x60 ±80 110/ Tjaereborg, Denmark ± Eagle Pass, USA ± Back-to-back Cross Sound, USA ± / Murraylink, Australia ± / Troll A, Norway x41 ±60 56/ Estlink, Finland ± / NordE.ON ± / Caprivi Link, Namibia / Trans Bay Cable, USA ± / alhall, Norway / East-West Link, Ireland-UK ±

28 S-HDC Development Initially low-power proof-of f concept 1997 Hällsjön, 3MW, ca. 3% losses per converter Progressively more volts, more power Move from 2-level to 3-level, then back to 2-level Multi-level thought too expensive initially All manufacturers (ABB, Siemens, Alstom) selling or developing modular multi-level converters (MMCs) Key selling points: Fewer losses (less switching) No need for AC filters (smaller footprint) t)

29 MMC Starting to be Trialled San Francisco Bay BorWin2 Siemen s

30 Other Equipment: E.G. Cross- Sound Advanced Neutral Point Clamped Converter AC Filters Measurement points Pre-insertion resistance Tap-changing transformer

31 Long-term Plans National Grid Offshore Development Plan (ODIS) 2010 Integrated Scenario (still point-to-point) NGET

32 Long-term Plans 2 ODIS 2010 adial Scenario (also still point-to- to point) NGET

33 Proposed design development NGET

34 eferences Siemens The Smart Way: HDC Plus One step ahead, Company Brochure, Siemens AG, (online) Jacobson B, Jiang-Häfner, Y, ey, P, Asplund, G, Jeroense, M, Gustafsson, A and Bergkvist, M HDC with oltage Source Converters and Extruded Cables up to +/-300k and 1000MW, CIGE (online, ABB website) ABB HDC and HDC Light, accessed September National Grid, Offshore Information Development Statement (ODIS), Sept 2010, online