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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
www.windguard.de
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
www.windguard.de
• 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)
5
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
6
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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
1
www.windguard.de
•
8
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
11
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
15
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
17
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
18
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Grid connection and offshore wind farm capacities
19Source: Stiftung Offshore Windenergie
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Towards an offshore meshed grid
20
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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
21
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Example of a meshed offshore grid
22Source: PROMOTION
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Example of meshing offshore grid connections
23
Kriegers Flak combined grid solution
Source:Energienet, Tennet
Thank you
Gerhard Gerdes
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
