HVDC grids & its components - Torbjörn Thiringer, CTH - EERA Deep wind 2015 1(30)

Research challenges for offshore HVDC gridsand its components

Torbjörn ThiringerDep. of Energy and EnvironmentChalmers University of technologyGöteborg, Sweden

Chalmers, Göteborg

NTNU, SINTEF, Trondheim

HVDC grids & its components - Torbjörn Thiringer, CTH - EERA Deep wind 2015 2(30)

Why offshore HVDC grid ( in the North Sea ) ?

Economical benefit to make power transfer and to balance power variations in different parts of Europe with the Baltic and North Sea in-between

To long distances for AC ( could perhaps work with 15 Hz AC )

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Why offshore HVDC grid with wind turbines ?

• Interest for installing wind energy in Sea areas – space and wind speed issues

• 20-20-20

• Kyoto protocol

• Peak Oil

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SW corner of North Sea

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The supergrid

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HVDC system with offshore wind farm

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Research challengesSystem: overall operation – stability - connection to external ac-grids - cable and grid component dimensioning - grid code – insulation level –protection – current interuption – meshed vs non-meshed – sequential building strategy – dc switchgear station

DC-transformer – design – operation – extra functionality

WEC system – high efficiency – robust – direct drive? –converter-voltage levels- HVDC out

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Some thoughts/ideas/aspects

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Step-wise building

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Converter topologies2-level

3-level

n-level

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Series connected dc wind farm

dc

ac

PCC

Istack,1

Idc

Vdc

+

-

WT1,1 VWT,1,1

+

-

WT2,1 VWT,2,1

+

-

WTn,1 VWT,n,1

+

-

WT1,2 VWT,1,2

+

-

WT2,2 VWT,2,2

+

-

WTn,2 VWT,n,2

+

-

Istack,2

WT1,m VWT,1,m

+

-

WT2,m VWT,2,m

+

-

WTn,m VWT,n,m

+

-

Istack,m

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No 50 Hz transformers

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Grid Code

Required additional reactive current

-100%

-50%

Voltage drop

N

B

II

NUU

m=2

reactive currentrated current

Time in msFault occured

Line-to-line

voltageNU

U

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DC breaking

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DC breaker current

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Some research examples

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Example of system study

• Geographical locations

For hourly average wind speed data in the North Sea region

Vector ARIMA(6,0,0)

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Real exchange power

0 20 40 60 80 100-1000

-500

0

500

1000

Duration [%]Ex

chan

ge P

ower

[MW

]

NL=>NO

System dimensions

Existing HVDC transmission

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Branch power• WF 1 capacity=1700 MW

0 10 20 30 40 50-3

-2

-1

0

1

2

3

Time [hour]

Unc

ontro

lled

Bra

nch

Pow

er [p

u]

NO->WF 1NL->WF 1

0 20 40 60 80 100-3

-2

-1

0

1

2

3

Duration [%]

Unc

ontro

lled

Bra

nch

Pow

er [p

u]

NO->WF 1NL->WF 1

(84%,1)

Over loaded for about

15% of the year

Adding a wind farm of 1700 MW

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Exchange power• [WF1 WF2] capacity=[1700 2700] MW

0 20 40 60 80 100-4

-3

-2

-1

0

1

2

3

Duration [%]U

ncon

trolle

d B

ranc

h Po

wer

[pu]

NO->WF 1NL->WF 1DE->WF 2DK->WF 2WF 1->WF 2

1117MW

>30%

Peak >3pu

Multiterminal with added wind farms

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0 20 40 60 80 100-3

-2

-1

0

1

2

3

Duration [%]

Con

trolle

d B

ranc

h Po

wer

[pu]

NO->WF 1NL->WF 1DE->WF 2DK->WF 2WF 1->WF 2

Multiterminal with added wind farms

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0 0.34 0.68 1.02 1.36 1.7-600

-400

-200

0

200

400

600

WPP capacity [GW]N

PV o

f sys

tem

add

ed c

ost [

M€]

Ttariff=10 €/MWhTtariff=13 €/MWhTtariff=18 €/MWh

Reinforcing the system

0 0.34 0.68 1.02 1.36 1.7-100

0

100

200

300

400

WPP capacity [GW]

Add

ed c

ost [

M€]

Base caseReinforce-NO->WF1Reinforce-NL->WF1

0 0.34 0.68 1.02 1.36 1.7-100

0

100

200

300

WPP capacity [GW]

Add

ed c

ost [

M€]

Base caseReinforce with 216MWReinforce with 317MWReinforce with 490MW

Added cost and reinforcement

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Collection grid of offshore wind farmTypical offshore HVAC collection grid Existing VSC-HVDC collection grid

Future HVDC collection grid

Bulky 50 Hz transformers

AC Cables

Bulky 50 Hz transformers

High power density converters using HF High Power transformers

DC medium voltage cables

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The dc-transformer

Modular structure (PISO) :

Future HVDC collection grid with HF / MF transformers :

One module with HFT :

HF high power transformer + inductor

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Design and Optimization (Case Study)Optimization results

600’000 combinations of free parameters

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HF Resistance measurement

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DC-link voltage selection

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DC-link voltage and losses

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Other generators - SynRG

• 74 % of power given the same volyme

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Questions ?