morelia workshop presentation william ross

Provision of Ancillary Services from Offshore

Wind Farms

William RossUniversity of Strathclyde

SupervisorsDr Olimpo Anaya-LaraProf. Stephen Finney

Contents

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• Background

• Grid Codes

• Modelling of Wind Farm and HVDC link to shore

• Results

• Future Work

• Research Objective

Driving Factors

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• Wind power installations are increasing significantly worldwide - More recently offshore

• However, investment costs for offshore wind power are much higher than those for onshore installation

• Wind speed offshore is higher and more reliable than onshore

• Average distance of off-shore wind farms continues to increase

• 2011 - 23.4km, 2012 - 29km from shore

• Projects announced for installations up to 200km from shore

(EWEA 2011, 2012, Deep Waters Reports)

Issues with Wind

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• Grid codes requiring WPPs to behave more like conventional power plants in maintaining power system stability and reliability i.e. providing ancillary services

• As installation of WPPs rises, so does their impact on the characteristics of the utility gird

• If large amount of wind gen. disconnected during survivable fault on utility side

• Includes:• Power quality • Flicker• Reliability of connection

Further imbalance resulting in a drop in system frequency over a wider region

Larger voltage depression due to increased power imbalance Eventually can lead to collapse of voltage in affected region

Research Objectives

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The research aims to:• Appraise the state-of-the-art of offshore wind power technologies and identify

Grid Code specifications for connection of large offshore wind power plant

• Enhance the available InstEE modelling platform of multi-technology offshore wind power networks to study wind power plant capability and requirements to be able to provide ancillary services.

• Design improved control approaches to deliver ancillary services from HVDC-interfaced wind power plant.

• Conduct, in cooperation with DNV, a cost-benefit analysis to weigh the impact of shorter lifetime versus increased ROI from delivery of power ancillary services to the grid.

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Ancillary Services from Offshore Wind farms

Frequency Control Ancillary Services (FCAS)

Network Control Ancillary Services (NCAS)

System Restart Ancillary Services (SRAS)

• Used to maintain the frequency on the electrical system, at any point in time, within certain range of nominal frequency, as defined in grid codes

• Control voltage at different points of the electrical network to within grid code standards

• Control the power flow on network elements to within the physical limitations of those elements.

• Reserved for contingency situations in which there has been a whole or partial system blackout and the electrical system must be restarted

primarily used to:

Grid Codes

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In most countries, grid codes apply to the wind farm at point where connects to grid or Point of Common Coupling (PCC)

Normal operation:• Frequency & voltage within ranges• Active power (P) control• Reactive power (Q) control

• Continuous operation in a limited range below and above the nominal point

• Time limited operation with possible reduced output in extended ranges

• Immediate disconnection

Grid disturbances in the form of voltage sags or swells can lead to WPP disconnections and possible cascaded loss of generation – Critical to avoid!

3 main operating zonesCommon Requirements

Behaviour under grid disturbances• Voltage ride through (VRT)• Reactive current injection (RCI)

Grid Codes

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Typical shape of continuous & reduced output regions

(An Overview of the Present Grid Codes for Integration of Distributed Generation - Traian-Nicolae - NTNU – Norway)

(Review of grid connection requirements forwind farms in the UK -O. S. Adagha)

Typical FRT requirements

Grid Codes – Unifying a mixed bunch

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Most extreme requirements for Wind Farm operation at deviations from nominl voltage and freq.

Fault ride through profile to satisfy all grid codes.

“Grid Code Requirements for Wind Power Integration in Europe” C. Sourkounis, P. Tourou

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• For large offshore wind farms at distances greater than 80 km the preferable way of transferring power to onshore is HVDC.

Grid Codes – Unifying a mixed bunch

“Grid Code Requirements for Wind Power Integration in Europe” C. Sourkounis, P. Tourou

Why HVDC?

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[“Economic limitations of the HVAC transmission system when applied to offshore wind farms”J. Machado; M. V. Neves; P. J. Santos]

Why HVDC?

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• Large amounts of reactive power required in HVAC to feed the capacitive charging current of the cables.

• For wind farms capacity with capacity <500MW, point at which HVDC becomes more economic can be as low as 50km

Using Voltage Source Converter based HVDC• VSCs use self commutating devices such as IGBTs and GTOs

that have voltage ratings close to 6.5kV • Unlike LCCs , they provide rapid, independent control of active

and reactive power.• By using Pulse Width Modulation any phase angle or

magnitude can be constructed• Lower filtering requirements, thus improving the overall converter

footprint.• Black-start capability and no restriction on multiple infeeds

Variable Speed – PMSG FRC based

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• PMSG connected to grid through back-to-back frequency converter

• PMGs also have no slip rings or brushes, and so reduced maintenance and greater reliability

• PMGs almost or about as efficient at full-load generation as standard DFIGs, but more efficient at part-loads – the most common conditions that wind turbines operate in

• Large PMGs require expensive rare earth magnets

• Fully rated PEC is expensive

Converters

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As VSWT installation is ever increasing, PEC becoming ever more critical

Two-level PWM converterLower cost due to its maturity/simplicity

High frequency harmonics(additional cost in EMI-filters)

Lower total harmonic distortion

Multilevel Converters

Smaller switching losses

Back-to-back PWM converter NPC cell and ANPC cell

Complex control

May also use uncontrolled diode rectifier on wind farm end• Cheap, robust, lack of control

Wind Turbine Modelling

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Noise signal injected into

Reflect wind variability in the output power to illustrate mitigation effect of the VSC-HVDC transmission system.

is set to zero for unity power operation

Fully-rated converter decouples generator from collection network

Only grid-side converter modelled in detail

Further simplified by modelling wind farm as single very large wind turbine

Developing Link Control

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WG

WG

WG

WG

SEC REC

GSCMSC

LgLfVdc2

DC cable

GridVdc1

BGBWF

For initial investigation:Wind turbines modelled as DC source connected to Grid-Side Converter in order to simplify the model and reduce simulation time

For further simplicity, the entire offshore wind farm is represented by one equivalent unit.

VSC as two level converter

+

-+

ωL

PI

-+

-+ ∑ PI

ωL

PI ∑ PI

dq

abc

Vdc2

V*dc2

vsd

vsd

isd

isq

|V*BG=1|

|VBG|

PWM

Md

Mq

i*sd

i*sq

ud

uq

-

+

+

-+

+

Outer Controllers Inner Controllers

Id max

Id min

Iq max

Iq min


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Outer loops provide reference values & for current controller on each end

For GS-VSC

Maintaining DC link voltage

Maintain AC voltage synchronised to onshore AC grid


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-+

-+

ωL

PI

-+

-+ ∑ PI

ωL

PI ∑ PI

dq

abc

vd

v*d = 1pu

vsd

vsd

isd

isqvq

PWM

Md

Mq

i*sd

i*sq

ud

uq

-

+

+

-+

+

Outer Controllers Inner Controllers

Id max

Id min

Iq max

Iq min

v*q = 0pu

For WF-VSC

Control offshore AC voltage and frequency through d and q components

Simulation with ramp up in power delivered from wind farm

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More results

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FRT on Point to Point VSC HVDC

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Low cost Diode/VSC Hybrid design

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• HVDC link using 12P-Rec and an VSC• 12PREC utilized to deliver

part of the wind farm power• Reduced power rating of the

VSC

• the SEMMC is used to control the offshore AC grid voltage at a constant magnitude and frequency

Previously…

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• Cost of power semiconductor devices and gate drivers ~ 75.2% and 45.6% compared with those of the NPC-based HVDC and MMC based HVDC transmission systems

• Power losses of the converters for NPC-based and MMC-based HVDC links are about 1.6% and 1.0%, respectively

• Average power loss of the converters then becomes 0.733%

(A Cost-Effective Converter System for HVDC Links Integrated with Offshore Wind Farms – T. H. Nguyen, D Lee, C. Kim)

Simulation Results

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DC link voltages at SEC and REC respectively

It is important that Vdc3 remains balanced with Vdc1 and Vdc2

This could be approached either by using measurements of Vdc3 directly or using control of real power injected through VSC

Both these approached will be tested and compared against benchmark model

Conclusions from simulations

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Future Work

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• Modelling of novel hybrid topology to achieve similar performance comparable to benchmark model. Then build on this to begin expanding modelling of wind turbines .

• Consolidate results from existing model, test limitations, and expand control to provide frequency support also

April - May

April

• Investigate addition of Novel Frequency measurement technique for provision of frequency support services

May-July

Presentations to date

• Deepwind conference (Trondheim, Norway) 2014 Oral presentation with my colleague Ioannis Antoniou

“The Future of HVDC” Ioannis Antoniou, Dr Olimpo Anaya-Lara, Prof. Stephen Finney

• Deepwind conference (Trondheim, Norway) 2016 Poster presentation

“Provision of Ancillary Services from Large Offshore Wind Farms” Dr Olimpo Anaya-Lara, Prof. Stephen Finney, Prof. Aurelio Medina-Rios

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• ETP annual conference 2014 – Dundee, Scotland Poster Presentation

• ETP annual conference 2015 – Glasgow, Scotland Oral Presentation

“Wind Farm Technologies towards Ancillary Services”

Others activities

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• Part of group funded by IET to organise, create and give presentation to groups of 100-300 high school kids (~14 years old) in 4 Scottish cities on importance of Power Engineering to promote interest for higher studies

• Member of newly formed IEEE IAS student chapter for Strathclyde & part of organising 2 day workshop with guests from Distinguished Lecturers program

• Also beginning to plan organisation of full conference in 2018

• Upcoming conference

Abstract submitted for UPEC 2016 conference in September“Providing Ancillary Services from Large HVDC-connected

Offshore Wind Farm Using Low Cost Hybrid Converter”Dr Olimpo Anaya-Lara, Prof. Stephen Finney, Prof. Aurelio Medina-Rios

William Ross

Thank you!