William Ross is researching how offshore wind farms connected to the grid via HVDC transmission can provide ancillary grid services like frequency and voltage control. He is modeling an offshore wind farm and HVDC link using voltage source converters to study their capability to deliver these services according to various grid codes. His objectives are to evaluate grid integration requirements, enhance modeling of offshore wind power networks, design control approaches, and analyze the costs and benefits of ancillary service provision. He is simulating scenarios like ramping wind farm power output and fault ride-through to test the control systems.

1. Provision of Ancillary
Services from Offshore
Wind Farms
William Ross
University of Strathclyde
Supervisors
Dr Olimpo Anaya-Lara
Prof. Stephen Finney

2. Slide 2
Contents
William Ross
• Background
• Grid Codes
• Modelling of Wind Farm and HVDC link to shore
• Results
• Future Work
• Research Objective

3. Slide 3
Driving Factors
William Ross
• 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)

4. Slide 4
Issues with Wind
William Ross
• 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

5. Slide 5
Research Objectives
William Ross
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.

6. Slide 6 William Ross
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:

7. Slide 7
Grid Codes
William Ross
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 zones
Common Requirements
Behaviour under grid disturbances
• Voltage ride through (VRT)
• Reactive current injection (RCI)

8. Slide 8
Grid Codes
William Ross
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 for
wind farms in the UK -O. S. Adagha)
Typical FRT requirements

9. Slide 9
Grid Codes – Unifying a mixed bunch
William Ross
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

10. Slide 10 William Ross
• 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

11. Slide 11
Why HVDC?
William Ross
[“Economic limitations of the HVAC transmission system when applied to offshore wind farms”
J. Machado; M. V. Neves; P. J. Santos]

12. Slide 12
Why HVDC?
William Ross
• 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

13. Slide 13
Variable Speed – PMSG FRC based
William Ross
• 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

14. Slide 14
Converters
William Ross
As VSWT installation is ever increasing, PEC becoming ever more critical
Two-level PWM converter
Lower 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

15. Slide 15
Wind Turbine Modelling
William Ross
Noise signal injected into 𝑃g
∗
Reflect wind variability in the output
power to illustrate mitigation effect of
the VSC-HVDC transmission system.
𝑄g
∗
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

16. Slide 16
Developing Link Control
William Ross
WG
WG
WG
WG
SEC REC
GSCMSC
LgLf
Vdc2
DC cable
Grid
Vdc1
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

17. Slide 17
-
+
-
+
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
Developing Link Control
William Ross
Outer loops provide
reference values 𝑖 𝑑
∗
& 𝑖 𝑞
∗
for current controller on
each end
For GS-VSC
Maintaining DC link voltage
Maintain AC voltage synchronised
to onshore AC grid

18. Slide 18
Developing Link Control
William Ross
-
+
-
+
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

19. Slide 19
Simulation with ramp up in power
delivered from wind farm
William Ross

20. Slide 20
More results
William Ross

21. Slide 21
FRT on Point to Point VSC HVDC
William Ross

22. Slide 22
Low cost Diode/VSC Hybrid design
William Ross
• 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

23. Slide 23
Previously…
William Ross
• 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)

24. Slide 24
Simulation Results
William Ross
DC link voltages at SEC and REC respectively

25. Slide 25
 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
William Ross

26. Slide 26
Future Work
William Ross
• 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

27. Slide 27
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
William Ross
• ETP annual conference 2014 – Dundee, Scotland
 Poster Presentation
• ETP annual conference 2015 – Glasgow, Scotland
 Oral Presentation
“Wind Farm Technologies towards Ancillary Services”

28. Slide 28
Others activities
William Ross
• 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

29. Slide 29 William Ross
Thank you!