A Demonstrator for Experimental Testing Integration of Offshore Wind Farms With HVDC Connection

S.D'Arco, A. Endegnanew, SINTEF Energi

BEST PATHS PROJECT

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• Validate the technical feasibility, impacts and benefits of novel grid technologies,

• Five large-scale demonstrations• Deliver solutions that allow for transition from High Voltage Direct Current (HVDC) lines to HVDC grids;

• Upgrade and repower existing Alternating Current (AC) parts of the network;

• Integrate superconducting high power DC links within AC meshed network

BEyond State-of-the-art Technologies for re-Powering AC corridors and multi-Terminal HVDC Systems

LARGE SCALE DEMONSTRATIONS

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1. HVDC in offshore wind farms and offshore interconnections

2. HVDC-VSC multivendor interoperability

3. Upgrading multiterminal HVDC links

4. Innovative repowering of AC corridors

5. DC Superconducting cable

From HVDC lines to HVDC grid

Upgrading ofexisting AC grids

DEMO 1 Objectives

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• To investigate the electrical interactions between HVDC link converters and wind turbine converters in offshore wind farms.

• To de-risk the multivendor and multiterminal schemes: resonances, power flow and control.

• To demonstrate the results in a laboratory environment using scaled models (4-terminal DC grid with MMC VSC prototypes and a Real Time Digital Simulator system to emulate the AC grid).

• To use the validated use the validated models to simulate a real grid with offshore wind farms connected in HVDC.

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Substation Wind farm

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8

Substation Wind farm

HVDCHVAC

Demonstrator overview

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• Three-terminal scheme MMC with • MMC with HB cells, 18 cells and 6 cells per arm,

• MMC with FB cells, 12 cells per arm

• Wind farm emulator

• National smart grid laboratory

Wind farm emulator

National Smart Grid Laboratory

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• Laboratory formally opened in September 2016 after a major upgrade

• Jointly operated by NTNU and SINTEF

• Reconfigurable layout with multiple ac and dc bus

• Power electronics converters• 2 level VSC 60 kVA, MMC 60 kVA

• Electrical machines• Synchronous generators, Induction

machines

• Real-time simulator

Real-time simulation and PHIL capabilities

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• OPAL-RT based real time simulator platform• 5 parallel cores,• 2 FPGAs for IO and small time step simulation,• Fiber optic communication

• Egston Compiso Grid emulator• 200 kVA rated power• 6 individual outputs• > 10 kHz bandwidth• Connected to the OPAL-RT system via fiber optics with 4 µs update rate for measurements and

references

Demonstration of HVDC transmission systems connected to offshore wind farms

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• Designed and built 3 MMC prototypes

• Tested the converters in point to point and multiterminal configurations

• Planned PHIL experiements with real time model of a wind farm

GSC

Pw1

Pg1,Qg1Onshore

AC Grid #1

DC CABLE

Vdc and Q Controller

AC VoltageControl

Vdc_g1

Vdc_g1*fw1*|Vac_w1*|

Vac_w1

Offshore Grid #1

WFC Offshore Onshore

Qg1*

θw1*

Pw1

DC NETWORK

AC VoltageControl

fw1*Vac_w1*

Vac_w1

Offshore Grid #1

WFC #1

Offshore Onshore

θw1*

GSC #1 Pg1,Qg1Onshore

AC Grid #1

(Vdc vs. P) and Q Controller

Vdc_g1* Qg1*

Vdc_g1

AC VoltageControl

fw12*Vac_w2*

Vac_w2

WFC #2

θw2*

Pw2

Offshore Grid #1

MMC Converters

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• Three MMC converters were designed from scratch• MMC with HB cells, 18 cells per arm• MMC with FB cells, 12 cells per arm• MMC with HB cells, 6 cells per arm

• Built and successfully tested at full rating• 42 modules• 144 power cell boards• 1764 capacitors

Power cell boards

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Assembling stages

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Converter performance test

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Conv12 700UDC, 100% active current Id (-81.2A) ● Phase C upper arm voltage, ● Phase C Lower arm voltage, ● Phase

C output voltage, ● Phase C arm current

Conv18 700UDC, 24.3kW,7.8kVar ● Phase C upper arm voltage, ● Phase C Lower arm voltage, ● Phase

C output voltage, ● Phase C arm current

Point-to-point and multiterminal configurations

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• Tests to evaluate the accuracy of the models to represent the demonstrator

28 29 30 31 32 33

-80

-60

-40

-20

0

20

40

60

80

Time (s)

Id (A

)

DemonstratorModel

4 4.01 4.02 4.03 4.04 4.05-20

-10

0

10

20

30

40

Time (s)

Arm

Cur

rent

(A)

ModelModelDemonstratorDemonstrator

4 4.01 4.02 4.03 4.04 4.05660

670

680

690

700

710

720

Time (s)

Cel

l Vol

tage

(V)

Wind farm emulator

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• Wind farm model is adapted to run in the 200 kVA high-bandwidth grid emulator

• PHIL implementation combining the real time simulator and the grid emulator• Flexibility in the model simulated

• Possibility to reproduce faster dynamics

IA

IB

IC

IA *

IB *

IC *

UA

UB

UC

Real Time Wind Farm Model UA*

UB*

UC*

Grid EmulatorReal Time Simulator

Current References

Voltage Measurements

Interaction of an offshore wind farm with an HVDC

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Low costModerate costHigh cost

• Complex issues • Noise, randomness of event timings, and

hardware design

• Numerical simulations are widely accepted and cost effective • Test a wide variety of different cases, however,

the fidelity of the results is difficult to assess.

• Hardware power-in-the-loop (HIL) simulation offers a good balance between test coverage and fidelity.

PHIL experiment: Wind farm connected to VSC-based HVDC

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Grid emulator

~=VSC

Transf. vabc

HWGE

Measurement points

vabc

iabc*

iabc*

VSC controller (Island mode)

HVDC dc voltage

refeence

Wind farm simulation

Real-time simulator

f 1s-2π

vd *=1 puvq*=0

abcdq

• Simulated wind farm• Input: Wind speed and measured voltage

• Output: Grid emulator reference current

• Hardware• Two-level VSC generates a three-phase ac

voltage with a fixed frequency

• The close-loop behaviour of the PHIL setup was stable

Simulation Hardware

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ResultsW

ind

spee

d (m

/s)

Angu

larv

el. (

rpm

)Po

wer

(pu)

Volta

ge(V

)Cu

rren

t(A)

Conclusions

• Power hardware-in-the-loop (PHIL) approach combines hardware devices with software simulation.

• The hardware part allows a high fidelity of the results whereas, the software simulation part allows an extensive study of different cases at a reasonable cost

• Grid integration of wind farm using VSC-based HVDC system was evaluated in PHIL experiment as a proof of concept.

• In the future work ,PHIL implementation using modular multilevel concepts will be studied

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