1

Control and Operation of Multi-terminal HVDC (MTDC)

and its application for offshore wind energy integration in the North Sea

Temesgen Haileselassie,Kjetil Uhlen

Department of Electrical Power Engineering,Norwegian Univerisity of Science and Technology ( NTNU)

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Background – Why MTDC?

• Wind power and market integration: Efforts to achieve the 2020 goals (at least 20%

renewables by the year 2020)

• Suitable sites for wind farms are running out on land (or controversial). This has led to looking for new sites offshore. 1,700 MW currently operational offshore wind farms Expected to grow to 19,000 MW by 2015

• Going for offshore sites not only offers advantagesbut also brings about new challenges

Grid connection being one of them.

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The grid integration problem in the context of the North Sea

1. Considerations for developing wind farms in the North sea as far as 100-300km away from shore HVDC stands out as the more feasible solution for such

long subsea power transmission2. Availability of offshore loads (oil & gas platforms)

in the North Sea Trends to electrify oil/gas platforms of Norway from

onshore (Troll A, Valhall) by HVDC

Multi-terminal HVDC (MTDC) is a promising solution for integration of offshore wind farms, and oil/gas platforms

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An early stage scenario of MTDC in the North Sea

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The European Offshore Supergrid(Proposed by many)

VSC-HVDC is the technology to enable such offshore dc super grids.

Sintef EWEA

Airtricity

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TECHNOLOGY GAPSIn general: IGBT technology not widely used for DC transmission Suitability for offshore application must be

demonstrated Scaling up from the current capacity level of ≈ 400MW XLPE submarine cable systems must be proven for

operation at 300kV DC (current projects operate at 150kV)

For multi-terminal systems: EHV DC Circuit Breaker Control systems

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Each converter should independently control:

Active Power Control in MTDC

• the active power flow (as given by the power reference signal)

• the dc bus voltage of the converter– to manage the power balance within the DC grid

• be robust to contingencies• contribute to the balance of the ac grids

(contribute to the primary frequency droop control)

Controls should also:

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Generation station-1

Generation station-2

AggregatePgen1 Ptot

f f f

Pgen2

fmax

Power balance control in AC grids: Traditionally by frequency droop

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VSC-HVDC station-1

VSC-HVDC station-2

Aggregate

P1 Ptot

UDC

P2

UDC,max

UDC,min

UDC UDC

Inverter mode

Rectifier mode

Power flow control in DC grid :achieved by DC voltage droop

No need for communication between terminals Many converter terminals contribute to dc voltage regulation DC analogy to distributed frequency droop control in AC

systems

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An early stage scenario of MTDC in the North Sea

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Sample view of the DC droop control

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Frequency support by MTDC

• Enables sharing of primary reserves between different AC grids connected to same MTDC even though they operate asynchronously.Hence (possibly) lower cost for primary

reserves.

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Schematics of the HVDC control

DC voltage droop

controller

Frequency droop controller

AC grid HVDC

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Frequency support - Simulation examples

0 5 10 15 20 25 30

0.985

0.99

0.995

Freq

uenc

y (p

u)

0 5 10 15 20 25 30

0

10

20

30

Powe

r (M

W)

0 5 10 15 20 25 3049.6

49.8

50

50.2

50.4

Time (s)

DC v

olta

ge (k

V)Grid-1Grid-2

Grid-1Grid-2

0 5 10 15 20 25 30

0.985

0.99

0.995

Freq

uenc

y (pu

)

0 5 10 15 20 25 30

0

10

20

30

Powe

r (M

W)

0 5 10 15 20 25 3048.6

48.8

49

49.2

49.4

Time (s)

DC vo

ltage

(kV)

Grid-1Grid-2

Grid-1Grid-2

Before After

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Frequency sensitivity analysis of MTDC wrt load changes

• The objective is toanalytically determineinteraction of differentAC grids connected tothe MTDC.

• Gives a systematicway to determine thedc voltage droop (ρDC)and frequency droop(ρfC) constants.

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Equations for power balances

(1)Li Ci GiP P P∆ + ∆ = ∆

(2)i ii

Gi Gi

f fP P

ρ ∆=∆

(3)i i DC DCCi Ci

fCi DCi

f f U UP Pρ ρ

∆ ∆ ∆ = −

10 (4)

.

n

Cjj

P

n total number of HVDC stations=

∆ =

−

∑

PG

PL

PC

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After solving (1)-(4) we get the frequency sensitivity expression

Frequency sensitivity matrix

1 1 2 2 1 1 1

2 1 1 2 2 2 2

1 1 2 2

1 1 2 2

C fC Ci fCi Cn fCn pu

C fC Ci fCi Cn fCn pu

i C fC i C fC i i Cn fCn ipu

n C fC n C fC n Ci fCi n npu

K q P q P q P fq P K q P q P f

q P q P K q P f

q P q P q P K f

ρ ρ ρρ ρ ρ

ρ ρ ρ

ρ ρ ρ

− ⋅ ⋅ ∆ − ⋅ ⋅ ∆ ⋅ ⋅ ⋅ ⋅ ⋅ ⋅ ⋅

⋅ − ⋅ ∆ ⋅ ⋅ ⋅ ⋅ ⋅ ⋅ ⋅ ⋅ ⋅ − ∆

1

2

L

L

Li

Ln

PP

P

P

∆ ∆ ⋅

= ∆

⋅ ∆

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Simulation study of for a four terminal system

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Terminal No

AC Grid size (GW)

Converter size (GW) ρfci (Converter

freq. droop)ρDCi

South Norway 12 (6 GW/Hz) 1.0 0.05000 0.025

Netherlands 6 (3 GW/Hz) 0.8 0.00833 0.025England 8 (4 GW/Hz) 1.0 0.01250 0.025Offsh. Windfarm 0.2 0.2 ∞ (insensitive) ∞ (insensitive)

Resulting sensitivity matrix:1 1

2 2

3 3

4 4

312.9 34.29 28.57 05.71 218.6 22.86 07.14 34.29 251.4 0

0 0 0 5

pu L

pu L

pu L

pu L

f Pf Pf Pf P

∆ ∆− ∆ ∆− = ∆ ∆− ∆ ∆−

UDC=400 kV, ρi =0.04

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Simulated shut down of generation (1.64 GW) in the UK ac grid (ΔPG3=1.64 GW)

150 200 250 300 350 400 4500.95

1

Freq

uenc

y (p

u)

150 200 250 300 350 400 450

-500

0

500

Cov

erte

rpo

wer

(MW

)

150 200 250 300 350 400 450385

390

395

Time (s)

DC

vol

tage

(kV)

Nor. Neder. UK

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Conclusions• MTDC has the potential to fully integrate power markets

between asynchronous areas.• Can be operated in a similar manner as ac grids (with the

dc voltage droop control)• No need of fast communication between converter

terminals, instead DC voltage droop control is used.• Primary reserves can be traded between asynchronous

areas (with frequency droop on the converter)• With primary reserves exchange by MTDC, cost of

operating spinning reserves can be reduced.

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Thank you.

WWW.NOWITECH.NO

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Remarks• With the proposed analytical method, the

observed and the predicted frequency changeswere in good agreement.

• The method, can further be used for small signalstability study of MTDC connected ac grids.

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Observed

Calculated

1 1

2 2

3 3

0.00055 0.2053

0.00064 0.1068

0.00660 0.3120

0.0048

pu L

pu L

pu L

DCpu

P GWP GWP GW

U

ω

ω

ω

∆ = − ∆ =

∆ = − ∆ =

∆ = − ∆ = −

∆ = −

1 1

2 2

3 3

0.0007 0.203

0.0007 0.110

0.0066 0.314

0.0055

pu L

pu L

pu L

DCpu

P GWP GWP GW

U

ω

ω

ω

∆ = − ∆ =

∆ = − ∆ =

∆ = − ∆ = −

∆ = −

Comparison of Results