HVDC grids and offshore wind farms: ongoing contribution from … · 2017-04-07 · •Objectives:...

HVDC grids and offshore wind farms: ongoing contribution from BEST PATHS Demo 1

30th March 2017, Venice, Italy

Iñigo AZPIRI, Iberdrola

Carlos UGALDE, Cardiff University

Outline of the Presentation

• Introduction

• BEST PATHS Demo 1

• The ‘Open Access’ Toolbox

• Topologies under Examination

• Key Performance Indicators

• Simulation Results

• Toolbox Impact

• Demonstrator

• Conclusions

• Next steps

2

Introduction

3

Motivation

• Wind energy will be the most widely adopted renewable energy source (RES) by 2050 to contribute towards the abatement of green house gas emissions.

• Offshore wind will have an important role in the achievement of the objectives set by the European Commission:

o A more reliable wind resource with higher average wind speeds and equivalent hours.

o New projects are far from shore (>120km in many cases).

• The limitations of HVAC technology for this kind of distances means that VSC HVDC has to be considered (low losses, black-start capability and control flexibility).

• In the future, MTDC grids will facilitate a cross-border energy exchange between different countries and will enable reliable power transfer from offshore wind farms (OWFs).

• The interactions between OWFs and HVDC converters in MTDC grids have to be studied in detail.

4

BEST PATHS Demo 1 (1)

DEMO 1: MTDC grids in OWFs

• Objectives:

1. To investigate the electrical interactions between the HVDC link convertersand the wind turbine converters in offshore wind farms.

2. To de-risk the multivendor and multiterminal schemes from the point ofview of resonances, power flow and control.

3. To demonstrate the results in a laboratory environment using scaledmodels (4-terminal DC grid with 60 kVA MMC VSC prototypes and a Real TimeDigital Simulator system to emulate the AC grid).

4. To use the validated models to simulate a real grid with offshore windfarms connected in HVDC.

5


DEMO 1: MTDC grids in OWFs

HVDC equipment manufacturers provide ‘black boxes’

We intend to use ‘open models’R&D Centres TSOs

Utilities & RES developers

Independent Manufacturers(WTGs & Power

Electronics)

?

Detailed models

Simulation & Validation

6

DEMO 1 Description


The ‘Open Access’ Toolbox (1)

7

Models and Control Algorithms

• A set of models and control algorithms has been developed, simulated and assessed.

• Their portability as basic building blocks will enable researchers and designers to study and simulate any system configuration of choice.

• These have been published in the BEST PATHS website as a MATLAB/ Simulink ‘Open Access’ Toolbox: http://www.bestpaths-project.eu/.

• Full details can be found in project Deliverable D3.1. Additionally, a user manual accompanies the published models and accompanying examples.

• Specific blocks include:

o High level controllers;

o Converter stations;

o AC grid;

o DC cables;

o Wind farm.

http://www.bestpaths-project.eu/


8

At the BEST PATHS website:

• Under “Publications”, download files under “Simulation toolbox Demo 1”.

• A compressed file with the Toolbox Manual and the MATLAB files will be downloaded.

• The Toolbox was presented in the 13th IET Conf. on AC and DC Power Transmission (ACDC2017). Full paper available in the conference proceedings:

o “Open Access Simulation Toolbox for the Grid Connection of Offshore Wind Farms using Multi-Terminal HVDC Networks”

9

Converter Stations

High Level Controller

• Averaged and switched models for a modular multilevel converter (MMC)

• The combined averaged-switched model consists of two blocks:

o Power electronics block,

o Low level controller block: circulating current reference generation, circulating current controller, Nearest Level Control modulation strategy & sub-module voltage regulator.

• It allows converter operation in three control modes to cover the main control needs for different system configurations.

o Mode 0: The converter sets the voltage and frequency.

o Mode 1: DC voltage and reactive power are regulated. DC voltage vs active power droop is available.

o Mode 2: Active and reactive power are regulated. Active power vs DC voltage droop is available.

AC Grid

• AC network adapted from the classical nine-bus power system.


10

DC Cable

• The DC cable section has been modelled as a one-phase, frequency-dependent, travelling wave model.

• It is based on the universal line model (ULM), which takes into account the frequency dependence of parameters.

Wind Farm

• The aim of this model is to accurately represent the behaviour of an aggregated offshore wind farm (OWF).

• To avoid large simulation times and undesirable computer burden, simplifications have been carried out in the electrical system:

o The converter of the a wind turbine generator (WTG) is modelled with averaged-model based voltage sources.

o A current source represents the remaining WTGs of the OWF. The current injectionof the first WTG is properly scaled to complete the rated power of the whole OWF.

• The detailed WTG contains

o a permanent magnet synchronous generator model;

o Averaged models of machine-side and grid-side converters, including filters and the DC link;

o An LV/MV transformer and internal control algorithms.


11

System configurations have been implemented in Simulink

• A number of topologies has been modelled, simulated and analysed.

• The topologies considered constitute likely scenarios to be adopted for the transmission of offshore wind energy in future years.

• The computer simulation of the system configurations will help meeting the following objectives:

o Improve the knowledge on the integration of OWFs via HVDC links or future MTDC grids;

o Identify possible interactions between wind turbines, converters, HVCD links and/or grids, and the onshore grid;

o Reduce uncertainties from OWFs connected to MTDC and multi-vendor HVDC schemes, and, consequently, de-risk the use of these technologies.

Topologies under Examination (1)

12

GSC

Pw1

Pg1,Qg1Onshore

AC Grid #1

DC CABLE

Vdc and Q

Controller

AC Voltage

Control

Vdc_g1

Vdc_g1*fw1*|Vac_w1*|

Vac_w1

Offshore

Grid #1WFC Offshore Onshore

Qg1*

θw1*

• Easiest system configuration representing HVDC links under construction nowadays.

• Power generated by an OWF is transferred to an onshore AC grid.


Point-to-Point HVDC Link (Topology A)

13

• Three-converter terminals are connected to form an MTDC grid.

• Power is transferred from the two HVDC-connected OWFs to an onshore AC grid.


Three-Terminal HVDC System

Pw1

DC

NETWORK

AC Voltage

Control

fw1*Vac_w1*

Vac_w1

Offshore

Grid #1

WFC #1

Offshore Onshore

θw1*

GSC #1Pg1,Qg1

Onshore

AC Grid #1

(Vdc vs. P) and Q

Controller

Vdc_g1* Qg1*

Vdc_g1

AC Voltage

Control

fw12*Vac_w2*

Vac_w2

WFC #2

θw2*

Pw2

Offshore

Grid #1

14

GSC #1

Pw1

Pg1,Qg1Onshore

AC Grid #1

(Vdc vs. P) and Q

Controller

AC Voltage

Control

Vdc_g1

Vdc_g1*fw1*Vac_w1*

Vac_w1

Offshore

Grid #1

WFC #1

Qg1*θw1*

GSC #2

Pw2

Pg2,Qg2Onshore

AC Grid #2

(Vdc vs. P) and Q

Controller

AC Voltage

Control

Vdc_g2

Vdc_g2*fw2*Vac_w2*

Vac_w2

Offshore

Grid #2WFC #2

Qg2*θw2*

GSC #3

Pw3

Pg3,Qg3Onshore

AC Grid #3

(Vdc vs. P) and Q

Controller

AC Voltage

Control

Vdc_g3

Vdc_g3*fw3*Vac_w3*

Vac_w3

Offshore

Grid #3WFC #3

Qg3*θw3*

DC

NETWORK

Offshore Onshore

AC interlink

• Three offshore converter stations are connected to form an offshore AC grid.

• OWFs are connected to this grid, with offshore converter stations being connected to onshore AC grids using point-to-point links.

• The three onshore AC grids are not connected together.


Six-Terminal HVDC System with Offshore AC Links (Topology B)

15

GSC #1

Pw1

Pg1,Qg1Onshore

AC Grid #1

(Vdc vs. P) and Q

Controller

AC Voltage

Control

Vdc_g1

Vdc_g1*fw1*Vac_w1*

Vac_w1

Offshore

Grid #1

WFC #1

Qg1*θw1*

GSC #2

Pw2

Pg2,Qg2Onshore

AC Grid #2

(Vdc vs. P) and Q

Controller

AC Voltage

Control

Vdc_g2

Vdc_g2*fw2*Vac_w2*

Vac_w2

Offshore

Grid #2WFC #2

Qg2*θw2*

GSC #3

Pw3

Pg3,Qg3Onshore

AC Grid #3

(Vdc vs. P) and Q

Controller

AC Voltage

Control

Vdc_g3

Vdc_g3*fw3*Vac_w3*

Vac_w3

Offshore

Grid #3WFC #3

Qg3*θw3*

DC

NETWORK

Offshore Onshore

DC interlink

• Includes a six-terminal MTDC grid with two offshore DC links.

• Power generated by three OWFs is transferred to different onshore AC grids.

• The OWF converter stations are coupled at the DC side.


Six-Terminal HVDC System with Offshore DC Links (Topology C)

16

GSC #4

Pg3,Qg3

Onshore

AC Grid #4

(Vdc vs. P) and Q

Controller

Vdc_g4

Vdc_g4* Qg4*

GSC #5

Pg2,Qg2

Onshore

AC Grid #5

(Vdc vs. P) and Q

Controller

Vdc_g2

Vdc_g2* Qg2*

GSC #6

Pg1,Qg1

Onshore

AC Grid #6

(Vdc vs. P) and Q

Controller

Vdc_g6

Vdc_g6* Qg6*

DC

NETWORK

Offshore Onshore

Pw1

AC Voltage

Control

fw1*Vac_w1*

Vac_w1

Offshore

Grid #1

WFC #1

θw1*

Pw2

AC Voltage

Control

fw2*Vac_w2*

Vac_w2

Offshore

Grid #2WFC #2

θw2*

Pw3

AC Voltage

Control

fw3*Vac_w3*

Vac_w3

Offshore

Grid #3WFC #3

θw3*

Pw4

AC Voltage

Control

fw4* Vac_w4*

Vac_w4

Offshore

Grid #4

θw4*

Pw5

AC Voltage

Control

fw5* Vac_w5*

Vac_w5

Offshore

Grid #5WFC #5

θw5*

Pw6

AC Voltage

Control

fw6* Vac_w6*

Vac_w6

Offshore

Grid #6WFC #6

θw6*

GSC #1

Pg6,Qg6

Onshore

AC Grid #1

(Vdc vs. P) and Q

Controller

Vdc_g1

Vdc_g1* Qg1*

GSC #2

Pg5,Qg5

Onshore

AC Grid #2

(Vdc vs. P) and Q

Controller

Vdc_g5

Vdc_g5* Qg5*

GSC #3

Pg4,Qg4

Onshore

AC Grid #3

(Vdc vs. P) and Q

Controller

Vdc_g3

Vdc_g3* Qg3*

DC

NETWORK

Offshore Onshore

WFC #4

• The DC sides of the converter stations are connected forming a meshed MTDC grid.

• This topology concentrates most of the technical challenges that will be found in the development of MTDC meshed networks.


Twelve-Terminal HVDC System with Offshore

DC Links (Topology D)

17

KPI.D1.1 – AC/DC interactions: power and harmonics

Steady state Power quality WT ramp rates

KPI.D1.2 – AC/DC Interactions – Transients & Voltage Margins

Normal operation Extreme operation

KPI.D1.3 – DC Protection Performance / Protection & Faults

Protection selectivity Peak current and clearance time

• To assess the suitability of the models and proposed HVDC network topologies, converter configurations and control algorithms, their performance has been evaluated against a set of KPIs (full details available in Deliverable D2.1 of the BEST PATHS project).

KPI.D1.4 – DC Inter-array Design

Inter-array topology

Power unbalance

Fault tolerance

Motorising capability

KPI.D1.5 – Resonances

AC systems oscillation Internal DC resonance

KPI.D1.6 – Grid Code Compliance

Active and reactive power

Fault ride-through

Key Performance Indicators

18

• Steady-state error evaluation upon:

1. Change in reactive power from 1 p.u. to 0.5 p.u. (< 1%).

2. Change in active changed from 1 p.u. to 0.5 p.u. (< 1%).

Simulation Results (1)

Assessment of KPI 1.1.1 (Topology C)

19



• Normal operation: Wind farm power output variation at once.

• KPI requirements:

1. DC voltages remain within the range 537 kV < Vdc < 704 kV.

2. Apparent power does not exceed 1.1 p.u. (1,100 MVA).

3. Terminal voltage remains within ±10% of the nominal value.

20

GSC #1

Pw1

Pg1,Qg1Onshore

AC Grid #1

(Vdc vs. P) and Q

Controller

AC Voltage

Control

Vdc_g1

Vdc_g1*fw1*Vac_w1*

Vac_w1

Offshore

Grid #1

WFC #1

Qg1*θw1*

GSC #2

Pw2

Pg2,Qg2Onshore

AC Grid #2

(Vdc vs. P) and Q

Controller

AC Voltage

Control

Vdc_g2

Vdc_g2*fw2*Vac_w2*

Vac_w2

Offshore

Grid #2WFC #2

Qg2*θw2*

GSC #3

Pw3

Pg3,Qg3Onshore

AC Grid #3

(Vdc vs. P) and Q

Controller

AC Voltage

Control

Vdc_g3

Vdc_g3*fw3*Vac_w3*

Vac_w3

Offshore

Grid #3WFC #3

Qg3*θw3*

DC

NETWORK

Offshore Onshore

DC interlink

1 2

5

3 4

6

Location 1 2 3 4 5 6

Selectivity Yes Yes Yes Yes Yes Yes

Peak current (A) 5292 5082 5272 5073 4046 4379

Clearance time (ms) 5.6 5.6 5.6 5.6 5.1 5.6


Assessment of KPI 1.3 (Topology C)

• Recorded highest DC fault current of 5.29 kA at fault location 1 (< 5.625 kA).

• Fault is cleared within 5.6 ms by DC circuit breakers (< 6 ms).

21

Wind turbine 1

VSC-SWHFR 1

𝐶𝐷𝐶1

𝐶𝐷𝐶2

PWM generator

Modulation for square wave operation

Δ 𝑌

𝐿𝑓1

Wind turbine 2

VSC-SWHFR 2

𝐶𝐷𝐶1

𝐶𝐷𝐶2

PWM generator

Modulation for square wave operation

Δ 𝑌

𝐿𝑓2

𝐿𝑙

𝐿𝑙

𝑢 𝑎𝑏𝑐𝑆𝑊∗ 𝐷𝑒𝑙𝑎𝑦 2 =

1

6 · 2 · 𝑓𝑆𝑊

𝑢 𝑎𝑏𝑐𝑆𝑊∗ 𝐷𝑒𝑙𝑎𝑦 1 = 0

𝑉𝐷𝐶2

𝑉𝐷𝐶1

𝑉𝑃𝐶𝐶

+

-

𝑉𝑤

𝑉𝑤

𝐼𝐷𝐶 𝑖𝐷𝐶

+

-

+

-

𝑣𝑜2

𝑣𝑜1



• DC inter-array circuit with 2 (<5) wind turbines and basic control blocks.

• The system is expected to provide the target DC inter-array voltage for different power delivery cases.

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GSC #1

Pw1

Pg1,Qg1Onshore

AC Grid #1

(Vdc vs. P) and Q

Controller

AC Voltage

Control

Vdc_g1

Vdc_g1*fw1*Vac_w1*

Vac_w1

Offshore

Grid #1

WFC #1

Qg1*θw1*

GSC #2

Pw2

Pg2,Qg2Onshore

AC Grid #2

(Vdc vs. P) and Q

Controller

AC Voltage

Control

Vdc_g2

Vdc_g2*fw2*Vac_w2*

Vac_w2

Offshore

Grid #2WFC #2

Qg2*θw2*

GSC #3

Pw3

Pg3,Qg3Onshore

AC Grid #3

(Vdc vs. P) and Q

Controller

AC Voltage

Control

Vdc_g3

Vdc_g3*fw3*Vac_w3*

Vac_w3

Offshore

Grid #3WFC #3

Qg3*θw3*

DC

NETWORK

Offshore Onshore

DC interlink

input perturbation

Point

DC voltage

Grid power



• An input perturbation is included to the output of one of the wind farms.

• A 1 Hz to 100 Hz frequency sweep of DC voltage and grid power is performed.

• No resonance is found.

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GSC #1

Pw1

Pg1,Qg1Onshore

AC Grid #1

(Vdc vs. P) and Q

Controller

AC Voltage

Control

Vdc_g1

Vdc_g1*fw1*Vac_w1*

Vac_w1

Offshore

Grid #1

WFC #1

Qg1*θw1*

GSC #2

Pw2

Pg2,Qg2Onshore

AC Grid #2

(Vdc vs. P) and Q

Controller

AC Voltage

Control

Vdc_g2

Vdc_g2*fw2*Vac_w2*

Vac_w2

Offshore

Grid #2WFC #2

Qg2*θw2*

GSC #3

Pw3

Pg3,Qg3Onshore

AC Grid #3

(Vdc vs. P) and Q

Controller

AC Voltage

Control

Vdc_g3

Vdc_g3*fw3*Vac_w3*

Vac_w3

Offshore

Grid #3WFC #3

Qg3*θw3*

DC

NETWORK

Offshore Onshore

DC interlink

f changed from 50 Hz to 52 Hz



• A frequency step from 50 to 52 Hz is applied at AC Grid #1.

• No power steady state error (< +/-5%) is exhibited, with a settling time of 1.1 s (< 10%).

Toolbox Impact (27th March 2017)

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o Universities include the Aalborg University, KU Leuven, the Fukui University of Technology, the Imperial College of London, the Technical University of Denmark, the University College of Dublin, Ensam, the Technical University of Darmstadt, the Technical University of Eindhoven, the Pontifical Comillas University, Cardiff University and the University of Strathclyde.

o Research centres include the KTH Royal Institute of Technology, the SuperGrid Institute, GridLab, IREC (Institut de Recerca en Energia de Catalunya) and L2EP (Laboratoire d’Electrotechnique et Electronique de Puissance, Lille).

o Companies include Siemens, Tractebel, Sarawak Energy, Energinet.dk, DNV GL and IBM Research.

Type of organisation

University

Research centre

Company

Purposes of use

Evaluation

Research

Testing

Information

• Toolbox and user manual uploaded on BEST PATHS website on 14th February.

• Presentation at 13th IET Conf. on AC and DC Power Transmission (ACDC2017); advertisement via social media and on project website.

• The toolbox has been downloaded by 33 different users between 16th February and 27th March.

• Users who downloaded the toolbox come mostly from Universities for research purposes.

• 486 new users have been recorded on the website since the toolbox has been uploaded.

http://bestpaths-project.eu/en/publications

Demonstrator (1)

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• Existing laboratory in SINTEF (Trondheim, Norway) is being upgraded with 3 additional MMCs

• Full Bridge and Half Bridge designs

o Half bridge cells with 18 cells per arm

o MMC unit with full bridge cells with 12 cells per arm

o MMC unit with half bridge cells with 6 cells per arm

• Central control level (1 for 3 MMCs)

o OPAL-RT based

o Control of circulating currents and grid currents, reference generation for modulation, Grid synchronization etc.

• Arm control level (6 per MMC)

o Control functions as sorting and balancing, signal distribution

o Separate custom control board - FPGA based

• Block/Rack control level

o Measurement and status inputs, gate control signal generation.

o Separate FPGA based control board

• Power module board

o Includes capacitors and MOSFETS

o Measurement, switching, low level protection

Central control

Unit(OPAL-RT)

Arm controlboard

Insulation

FPGA

uP

SFP

SFP

SFP

SFP

Block

Block controlboard

Drivers

Measurements

Drivers

Measurements

Drivers

Measurements

FPGA

Drivers

Measurements

Drivers

Measurements

Drivers

Measurements

FPGA

Drivers

Measurements

FPGA

FPGA/ARM

FPGA/ARM

Power cell board

Arm

Converter

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Demonstrator (2)

• Assembly of converter hardware completed:

• 42 modules.

• 144 power cell boards.

• 1764 capacitors.

• Communication between OPAL-RT (centralised controller) and the converters units almost completed.

• Implementation of wind emulator in cooperation with Gamesa has been initiated.

• Deliverable 8.1 with demonstrator specification submitted to EC (public version available in the BEST PATHS website).

27

Current status (March 2017)

Demonstrator (3)

Conclusions

28

Main Contributions of this Work

• A MATLAB/Simulink ‘Open Access’ Toolbox for the simulation of grid-connected OWFs using HVDC grids has been presented.

• The toolbox models are portable, enabling users to employ them as basic building blocks to assess different topologies.

• A set of KPIs to assess system performance and a number of topologies representing future scenarios where OWFs are integrated to onshore grids have been presented.

• The simulated configurations exhibit a good performance and the tested KPIs are fully met.

• A real-time MTDC demonstrator is currently under development.

• The main contribution of this work is the provision to TSOs, utilities, manufacturers and academic institutions with an open access toolbox to generate the necessary knowledge for the development, construction and connection of MTDC systems –aiming to help de-risking the use of MTDC grids for the connection of OWFs.

Next steps

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Tasks for 2017

• Commissioning of the demonstrator.

• Simulation of the demonstrator architecture using the models developed in Demo 1.

• Definition of tests to validate the models.

• Run tests both in the laboratory and in the computer.

• Publication of results (Deliverable 8.2, scheduled for December 2017).

Stakeholder meeting

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Trondheim, May 11th

•Location: SINTEF premises, Trondheim (Norway)

•Guided visit to the laboratory & updates from all demos

Register at www.bestpaths-project.eu/en/news-events

Questions?

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The authors gratefully acknowledge the financial support provided by the EU FP7 Programme through the project “BEyondState of the art Technologies for re-Powering AC corridors & multi-Terminal HVDC Systems” (BEST PATHS), grant agreement number 612748.

Iñigo AZPIRI

[email protected]

Dr Carlos UGALDE

[email protected]

www.bestpaths-project.eu

HVDC grids and offshore wind farms: ongoing contribution from … · 2017-04-07 · •Objectives:...

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Transcript of HVDC grids and offshore wind farms: ongoing contribution from … · 2017-04-07 · •Objectives:...