23-12-2009

Challenge the future

DelftUniversity ofTechnology

HVDC grid connection of offshore wind power

Ir. Arjen A. van der Meer, Electrical Sustainable Energy

2/21HVDC grid connection of offshore wind power

Contents

•Present offshore wind parks

•Grid connection

•VSC-HVDC

•Future trends

•Conclusions

3/21HVDC grid connection of offshore wind power

1st generation offshore wind parks

•Typical rating 100 – 200 MW

•Distance from the cost < 25 km

•Operation like a conventional power plant

4/21HVDC grid connection of offshore wind power

Offshore wind parks in NL

•OWEZ:

• 36 X 3 MW

• Wind park directly connected to the mainland (33 kV)

• ±12 km from the shore

•Prinses Amaliawindpark:

• 60 X 2MW

• MV/HV transformer located offshore

• Grid connection through 150 kV cable

• ±25 km from the shore

5/21HVDC grid connection of offshore wind power

Typical AC grid connection

MV strings

wind turbinesconnected at MV

onshoreMV/HV substation

offshore MV/HVsubstation (platform)

HV exportcable circuit

6/21HVDC grid connection of offshore wind power

2nd & 3rd generation wind parks

• Will have larger power rating (>500 MW)

• Will be located further from the shore (>>25 km)

• Will be clustered together

• Will show behavior like a large power plant

• In some countries, the TSOs are responsible for grid connection

7/21HVDC grid connection of offshore wind power

Limitations of AC transmission

0

250

500

750

1000

0 50 100 150 200 250 300

Cable length (km)

Cur

rent

(A)

reactive current max. active current current limit

cable is a ‘distributed capacitor’

at AC voltage, a charging current must be provided to keep voltage

thermal limit determines maximum current

maximum possible active current reduces with cable length

8/21HVDC grid connection of offshore wind power

DC transmission

• Solves most problems

• Only resistive losses, unlimited connection distance

• No reactive current compensation needed, more power per cable

• But, at the cost of

• Higher losses

• Higher investment costs

• More maintenance

9/21HVDC grid connection of offshore wind power

VSC-HVDC versus LCC-HVDC

• LCC-HVDC

• Operated through thyristors

• Dedicated for bulk power transfer

• Needs a strong grid to communitate against

• VSC-HVDC

• Operated through IGBTs

• Offers black-start capability

• Good controllability

• Feasible for systems ≤800 MW

• Smaller footprint

10/21HVDC grid connection of offshore wind power

Consequences of VSC-HVDC

• Modern wind parks are required to behave as a conventional

power plant

• VSC-HVDC creates an electrical barrier between 2 grids

• Control system now determines grid interaction

• Disadvantage: fault ride-trough behaviour

• Grid support during disturbances

11/21HVDC grid connection of offshore wind power

Low voltage ride-through

• Onshore converter has maximum current rating;

• Direct voltage quickly rises due to the power imbalance;

• Only few milliseconds are available for active power reduction;

12/21HVDC grid connection of offshore wind power

fast power reduction

• Increase of offshore frequency;

• Reduction of offshore network voltage;

• power reduction signal to wind turbines;

• Dissipation of excess energy in dynamic braking resistor

f Vac

Pref

13/21HVDC grid connection of offshore wind power

Low voltage ride-through: results

• Power reduction methods:

• Reliability is an issue

• Additional control systems needed in wind turbines

• Braking resistor: robust but expensive

14/21HVDC grid connection of offshore wind power

Grid support during disturbances

U

t0 t1 t2

U0

Umin

Un

15/21HVDC grid connection of offshore wind power

Grid support during disturbances

• Reactive current injection to support voltage restoration

• Minimize geographical impact of the fault

• Wind turbines/ VSCs are able to inject reactive power any time

• Different grid codes require different behaviour

• Common variants:

• No reactive power support

• Reactive power boosting

• Continuous voltage control

16/21HVDC grid connection of offshore wind power

Grid support: results

• Reactive current injection by VSC-HVDC improves dynamic performance

• Some support methods are more efficient than others

• Voltage support is ineffective for wind parks connected through long

AC-cables, VSC-HVDC can overcome this problem

17/21HVDC grid connection of offshore wind power

What does the future hold?

18/21HVDC grid connection of offshore wind power

Multi-terminal VSC-HVDC

• Increased need for cross-border interconnection capacity

• Utilization factor increases

• Reliability improves

• Synergies with other offshore applications

• Economic feasibility is an important prerequisite

19/21HVDC grid connection of offshore wind power

Multi-terminal DC: challenges

• Wind power dispatch amongst connection points

• Implementation into market environment

(how to earn money with the additional applications)

• Availability of DC circuit-breakers

20/21HVDC grid connection of offshore wind power

Kriegers flak

• 650 MW

• Planned between Germany,

Denmark, and Sweden

21/21HVDC grid connection of offshore wind power

Conclusions

• Next generation Wind Power Plants requires re-thinking of

connection technologies

• VSC-HVDC offers major advantages above other technologies

• VSC interface needs to fulfill grid-code compliance