A review of offshore wind power grid connection options in the Bothnian Bay

Offshore grids for wind power integrationSisu NiskanenVTT Technical Research Centre of Finland

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Contents

Finnish power system Overview of offshore wind power in Finland Foreseen power system development in Bothnian Bay

(Bothnian Bay = Gulf of Bothnia North) A case study of grid connection in Finland for 4 OWFs.

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Finnish power system

Peak load 2010/01 14320 MWh/h Annual consumption 82 TWh 2009 (88 TWh

2008) Nordic synchronous power system; Finland,

Sweden, Norway, East-Denmark Internal North-South capacity -1400…2000

MW AC connections in North of Finland to

Sweden 400 kV: -1500…1100 MW Norway 110 kV: ~200 MW

DC connections in South of Finland to Sweden: -550…550 MW, (year 2011: -

1300…1300 MW, Fenno-Skan 2) Russia (IPS/UPS-system): -1300…0

MW Estonia (IPS/UPS-system): -365…365

MW, (year 2013: -1015…1015 MW, Estlink 2)

1100 MW 1500

200

MW

200

200

0 M

W 1

400

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Wind power in FinlandOffshore wind farm project plans

Bothnian Bay sum of plans 2740…3860 MW in Finland

Not all will necessarily materialize TSO guarantees total 2000 MW sufficiently

distributed WP be able to be connected in Finnish power system over following 10 yr.

In Sweden Klocktärnan planned for 2013, 660 MW. Onshore wind farm project plan of > 2 GW near the Klocktärnan.

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Foreseen power system development in the northern end of Bothnian Bay

Eight large projects close to each other Total capacity 2400 – 3300 MW Planned schedules for construction ~2013-16 All projects will not be constructed, some will be delayed

Candidate site for a new nuclear power plant in the same area

1500 – 2500 MW Possibly by 2020

According to Finnish TSO there is a need to reinforce northern interconnection between Finland and Sweden in next 10 years

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Case study:OWFs connection options

Four offshore sites included Maakrunni 400 MW Pitkämatala 600 MW Suurhiekka 400 MW Oulu-Haukipudas 600 MW

Radial line connection alternatives:

the nearest 400 kV overhead line and a new substation

existing 400 kV substation.

One of the candicate sites for new nuclear power plant

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Case study:Grid topology

High voltage transmission lines Switching substation: PCC (Point of

common coupling) Radial transmission line Substation 400/150 kV: reactive

compensators

Underground and submarine cables

Offshore substation 150/33 kV Collecting grid: submarine cables Wind turbine: transformer,

converter and generator

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Case study: Investment costs of grid connection

400 MW OWF Transmission distance

from OWF to shore is 30 km and distance from shore to PCC is 25 km.

A substation is located to 5 km from shore to inland.

The substation is connected to the PCC by 20 km 400 kV overhead line.

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Case study: Simulation (PSS/E)

In Bothnian Bay max. connection power at 400 kV PCC is ~1000 MW. Larger complex should be divided into different connection points. 2 OWF in Bothnia Bay can be connected to the same point.

Basic model for one OWF

Total capacity 400 MW, 80 turbines, 5 MW/turbine

Collection grid 33 kV

Transmission cable 150 kV

PCC 400 kV

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Case study:Summary

In Bothnian Bay HVAC is the most economic option in OWF grid connection.

HVDC grid connection is still too expensive if overhead lines can be used in onshore.

In Finland OWF has to operate at power factor 0,95ind…0,95kap

Reactive power compensators at PCC and/or substations: submarine cables produce capacitive current OWF capacity factor is about 0.40 and transmission lines

are usually operated at capacitive power factor. The transmission distance from offshore to onshore

substation and PCC.

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Technology

Conventional high voltage AC system Grid voltage in Finland: 110, 220, 400 kV <250 MW OWF can be connected to 110 kV

transmission system with some limitations. >250 MW OWF have to connect to 400 kV transmission system.

HVDC system ABB HVDC light and Siemens HVDC plus

±150 kV, max ~570 MW ±320 kV, max ~1200 MW

LCC (Line commutated converter) Higher voltage and transmission capacity Need reactive power compensators and stronger AC

network than Light or Plus technology

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HVDC grid options

Options of small scale HVDC super grid Connection between

OWFs Parallel connection with

internal North-South transmission grid

Cross-border connection between Finland and Sweden

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Future work

Analysis on the effect of different offshore grid options in Bothnian bay on grid

Cost-benefit analysis of connection using unit commitment and dispatch models

Assess the need for additional connections and their capacity across Bothnian bay

cross border transmission capacities bottle neck operation general power transfers

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Acknowledgements

This study has been co-steered by TEKES, Fortum Oyj, wpd Finland Oy, Pohjolan Voima Oy, Fingrid Oyj

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