application of ac dc in the connection of wind farms...offshore wind farm connections in operation...

Application of AC – DC in the connection of wind farms

G.J.Gerdes

Deutsche WindGuard GmbH

www.windguard.de

EU Round table

16th of May 2018

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Outline

• AC/DC/AC Inverter in wind turbines

• HVDC and AC in German wind farm feeders

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DC in Wind Turbines

2

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Direct ConnectionDirect Connection

Grid coupling of wind turbines

Disadvantage:

• fluctuations in wind are directly transformed into power fluctuations

• this having an impact on mechanical loads (torque) and electrical power quality

• Low controllability

3

Gen

generatorGear box

rotor

grid

Grid frequency

inverter system

coupled

=Decoupled

≠

Advantage:

• lower mechanical stress high controllability

• Improved electrical power quality

• Higher aerodynamic efficiency

Generating frequency

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Downtime due to inverter failures in wind turbines

4Source: Elforsk

From research study “Investigation of converter failure in wind turbines”, Elforsk

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• Technical availability calculated for 5 wind farms

– Covers: standstills due to technical failures of the wind turbines and repair

– Excludes: regular maintenance, external faults (grid, meteorology, owner stops)

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Example of annual availability of selected wind farms

Year11 WTG Full

inverter5 WTG Full

inverter4 WTG

double fed IG5 WTG

double fed IG5WTG direct

coupling

2017 97.7% 99.6% 99.1% 97.7% 98.3%

2016 98.2% 96.7% 98.9% 99.1% 96.1%

2015 99.9% 99.3% 99.0% 97.2% 98.3%

2014 99.1% 96.0% 98.9% 97.7% 95.5%

Average 98.7% 97.9% 99.0% 97.9% 97.1%

Standstill 1.3% 2.1% 1.0% 2.1% 3.0%

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Development of Offshore wind farm connection in Germany

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Planning of offshore connections started in 2002: Total capacity planned from 2010 on

6,3 GW

6,8 GW

2,4 GW

9,5 GW

3

3

4

Total capacity: 25 GW

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•

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Ecological concerns

• Extension of areas:

– National park: along most of the coast line

– Shipping routes: the remaining coast line

• Compromise:

– Very few drillings underneath the national park area

– Limited number through shipping lines

Resulting in cables with high transport capacity

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Target:

• to minimize the number of connecting cables

• To maximize the transport capacity of each cable

• To minimize the ecological impact (electromagnetic fields, heating of see ground, etc.)

First plans for connecting offshore wind farms

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Available technology in 2004 for maximumtransport capacity of each cable

• 3-phase AC current:

– Three-conductor cable : 170kV, 200MW to 245kV, 250MW

– Single conductor cables: 420kV, 400MW

– Largest distance offhsore 120km

• DC current:

– Conventional HVDC (Thyristor): 800MW to 1 GW, 800kV (±400kV)

– Bipolar Transmission (Monopolar cable) ±600kV, 2500MW

– HVDC-IGBT up to 145kV, 250 - 300MW

• GIL:

– At that time realisation of 400kV at 2000MVA

– Higher capacities are theoretically possible

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Technology and transmission distance

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0 60 120 180 240

Co

st

Length of Line[km]

HVAC

HVDC

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Offshore wind farm connections in operation or construction – North Sea

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ConnectionCommissioning

date

Cabel

length

Offshore

Cabel

length

Onshore

Cabel

length

total

Technology CapacityVoltage

Level

DC

platform

Supplier

Alpha Ventus 2009 60 km 6 km 66 km HVAC 62 MW 110 kV AC

Riffgat 2014 50 km 30 km 80 km HVAC 113 MW 155 kV AC

Nordergründe 2017 28 km 4 km 32 km HVAC 111 MW 155 kV AC

BorWin1 2012 125 km 75 km 200 km HVDC 400 MW ±150 kV ABB

BorWin2 2015 125 km 75 km 200 km HVDC 800 MW ±300 kV Siemens

BorWin3 u. constr. 2019 130 km 30 km 160 km HVDC 900 MW ±320 kV Siemens

DolWin1 2015 75 km 90 km 165 km HVDC 800 MW ±320 kV ABB

DolWin2 2017 45 km 90 km 135 km HVDC 916 MW ±320 kV ABB

DolWin3 u. constr. 2018 80 km 80 km 160 km HVDC 900 MW ±320 kV Alstom

HelWin1 2015 85 km 45 km 130 km HVDC 576 MW ±250 kV Siemens

HelWin2 2015 85 km 45 km 130 km HVDC 690 MW ±320 kV Siemens

SylWin1 2015 160 km 45 km 205 km HVDC 864 MW ±320 kV Siemens

HelWin2 2015 85 km 45 km 130 km HVDC 690 MW ±320 kV Siemens

Total 1 133 km 660 km 1 793 km 7 822 MW only DC 7 536 MW

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Source: TenneT

HVDC offshore wind farm connections in the North Sea

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Offshore development in Germany until 2025

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5.4 GW

0.8 GW

1.5 GW

0.02 GW

1.5 GW

1.6 GW

OWT (feeding in)

Installed OWT (no feed-in)

Construction in Progress

Final Investment Decision

Grid Connection Capacityassigned

Acceptance of Bid

Available in Tender 2018

Expected Development

by 2020 (7.7 GW)

Expected Development

by 2025 (10.8 GW)

Until 2030: 15GW, tender starting 2021

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Development of offshore transmission and wind farm capacities

• Transmission capacity:

– 2020 7.8 GW

– 2021 to 2030 15.0 GW

• Offshore wind farm capacities:

– Today 5.4 GW

– 2020 7.8 GW

– 2025 10.8 GW

– From 2021 on 700-900 MW/year

– 2030 15.0 GW

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Grid connection and offshore wind farm capacities

19Source: Stiftung Offshore Windenergie

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Towards an offshore meshed grid

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Development of a meshed offshore grid (MOG) in the North Sea

• Several studies have been conducted by the EU, showing the overall benefit for the participating countries

• Purpose of a MOG is

– the transport of offshore wind power to land

– the connection of markets and thus

– more electricity trading and levelling of electricity market prices in the EU

• A project to accelerate the development of a MOG is currently running: PROMOTION

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Example of a meshed offshore grid

22Source: PROMOTION

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Example of meshing offshore grid connections

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Kriegers Flak combined grid solution

Source:Energienet, Tennet

Thank you

Gerhard Gerdes

g.gerdes@windguard.de

04451 9515 110

www.windguard.dewww.windguard.de

www.windguard.de 26

Personal DC Experience, long time ago

• Stand alone system, 1983

• consumption approx. 10000kwh/a

• Electricity supply: wind turbine, photovoltaics, Back-up CHP-unit

• Storage: Lead acid Batteries and later Hydrogen

• Battery Storage: 170kWh

• System tension: DC, 208 VDC

Experience:

Huge switches

Huge auxiliary drives (pumps etc.)

Even ohmic loads were difficult

Sine wave (and even trapeze) inverters were very expensive