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Steady State Analysis of the Interconnection of Offshore
Energy Parks
Miguel Jorge da Rocha Barros Marques
Dissertação para obtenção do Grau de Mestre em
Engenharia Electrotécnica e de Computadores
Júri
Presidente: Prof. Doutor Paulo José da Costa Branco
Orientador: Prof. Doutor Rui Manuel Gameiro de Castro
Co-orientador: Profª. Doutora Maria Eduarda de Sampaio Pinto de Almeida Pedro
Vogal: Profª. Doutora Sónia Maria Nunes dos Santos Paulo Ferreira Pinto
Setembro de 2010
i
Agradecimentos
Em primeiro lugar quero agradecer ao Professor Rui Castro pela confiança depositada em mim
desde o princípio e pelo apoio prestado em todos os momentos da elaboração desta tese. O meu
obrigado também à Professora Maria Eduarda Pedro, como co-orientadora, pela disponibilidade ao
esclarecimento de dúvidas que surgiram. Uma palavra de agradecimento ainda ao Professor Ferreira
de Jesus pela paciência e disponibilidade quase totais e que em muito contribuíram para a boa
conclusão deste trabalho.
Uma palavra ainda de forte agradecimento aos meus colegas de curso – alguns dos quais se
tornaram verdadeiros amigos – por toda a camaradagem e espírito de entreajuda, essenciais ao
longo de todo o curso.
Uma palavra muito importante de gratidão à minha família. Aos meus pais, não só por terem
viabilizado economicamente os meus estudos mas, sobretudo, pela força e carinho que me deram.
Os seus percursos pessoais e académicos foram sempre um exemplo e uma fonte de inspiração ao
longo do meu trajecto académico. À minha irmã, cujo apoio entusiasmado e companheirismo, ainda
que por vezes à distância, foi também essencial.
Um agradecimento muito especial à minha namorada, por ter estado ao meu lado em todos os
momentos, mesmo nos mais difíceis, e me ter sempre apoiado com ânimo, compreensão e amor,
tornando definitivamente mais fácil o meu percurso.
ii
Resumo
Entre as fontes renováveis de energia, a energia eólica tem ganho relevância, tornando-se uma
alternativa viável às fontes convencionais. As elevadas velocidades do vento e as vastas áreas
disponíveis no mar aumentaram recentemente o interesse global em parques eólicos offshore.
À medida que os parques eólicos offshore se tornam maiores e são instalados mais longe da
costa, a transmissão de energia para a rede em terra torna-se numa característica essencial. As
opções disponíveis são HVAC, HVDC-LCC e HVDC-VSC.
O objectivo deste trabalho é comparar a performance em regime estacionário e transitório de
dois sistemas de transmissão alternativos: HVAC e HVDC-LCC. Alguns esquemas de compensação
do excesso de energia reactiva para o HVAC são também avaliados, tal como as vantagens do
STATCOM em ambas as tecnologias. O software PSS/E é usado para simulação de todos os casos.
Os resultados obtidos mostram algumas diferenças importantes entre as tecnologias. Em regime
permanente, o comportamento do HVDC-LCC e do HVAC com compensação onshore e offshore têm
algumas semelhanças, nomeadamente no factor de potência e nas perdas. Em regime transitório, a
resposta da ligação com HVAC a um defeito na rede é muito diferente da resposta da ligação com
HVDC-LCC. A ligação com HVAC permite o “fault ride through” do parque eólico offshore enquanto a
ligação com HVDC-LCC é bloqueada, interrompendo a transmissão de energia durante o defeito.
Palavras-Chave: Parques Eólicos Offshore, HVAC, HVDC-LCC, STATCOM, PSS/E.
iii
Abstract
Among the renewable energy sources, wind energy has gained relevance, becoming a viable
alternative to conventional sources. High wind speeds and wide available area at sea have recently
increased the global interest on offshore wind farms.
As offshore wind farms become larger and are placed further from the shore, the power
transmission to the onshore grid becomes a key feature. The available options are HVAC, HVDC-LCC
and HVDC-VSC.
The objective of this work is to compare the performance in steady-state and the transient
behaviour of two alternative transmission systems: HVAC and HVDC-LCC. Compensation schemes
for excessive reactive power for HVAC are also evaluated, as well as the benefits of the STATCOM
for both technologies. The PSS/E software is used for simulation of all cases.
The results obtained show some important differences for the technologies. In steady-state, the
behaviour of HVDC-LCC and HVAC with compensation both onshore and offshore hold some
similarities, namely in the power factor and the power losses. In transient regime, the response of the
HVAC link to a fault in the grid is very different to the response of HVDC-LCC link. The HVAC link
allows the “fault ride through” of the offshore wind farm while the HVDC link is blocked, interrupting the
power transmission during the fault.
Keywords: Offshore Wind Farms, HVAC, HVDC-LCC, STATCOM, PSS/E.
iv
List of Figures
Figure 1 – EU power capacity mix: a) 2000 data; b) 2009 data [1]. ........................................................ 1
Figure 2 – Global Cumulative Installed Wind Capacity (1996-2009) [2]. ................................................ 2
Figure 3 – Annual and cumulative installed capacity (in MW) of offshore wind power in Europe (1991 –
2009) [1]................................................................................................................................................... 3
Figure 4 – Europe wind resources over open sea (Offshore Wind Atlas) [6]. ......................................... 8
Figure 5 – Cumulative share of installed capacity by country (end 2009) [1]. ...................................... 10
Figure 6 – Map of operational offshore wind farms in Europe (January 2010) (Adapted from [8]). ...... 11
Figure 7 – Offshore foundations and general characteristics [9]........................................................... 12
Figure 8 – Offshore wind turbine size evolution [9]. .............................................................................. 12
Figure 9 – General layout of an offshore wind farm [13]. ...................................................................... 16
Figure 10 – Layout of an HVAC wind farm[16]. ..................................................................................... 18
Figure 11 – Three-core XLPE submarine cable [17]. ............................................................................ 19
Figure 12 – Equivalent π-model for a submarine cable. ....................................................................... 20
Figure 13 – Maximum transmission lengths with inductive shunt compensation in both ends [17]. ..... 21
Figure 14 – (a) Schematic Diagram of an SVC; (b) an SVC in Radsted, Denmark [18]. ...................... 22
Figure 15 – (a) Schematic Diagram of a STATCOM; (b) a STATCOM installation [18]. ...................... 23
Figure 16 – Loading of an open-end cable due to capacitive charging current for different schemes of
reactive power compensation [19]. ........................................................................................................ 24
Figure 17 – Basic Configuration of a wind farm using an HVDC-LCC [7] Note: F=filter; HFF=High-
frequency filter. ...................................................................................................................................... 26
Figure 18 – Available HVDC Cables: a) OF cable; b) MI cable; c) XLPE cable [22]. ........................... 28
Figure 19 – Mass Impregnated HVDC Cable [22]. ................................................................................ 28
Figure 20 – Six-pulse Converter Bridge [23]. ........................................................................................ 29
Figure 21 – Diagram of a 12-pulse converter [23]. ................................................................................ 31
Figure 22 – HVDC configurations: a) Monopole; b) Bipolar. ................................................................. 32
Figure 23 – Basic Configuration of a wind farm using an HVDC-VSC [11]. .......................................... 34
Figure 24 – Single line diagram of the test grid used (with HVAC transmission, compensations at both
ends). ..................................................................................................................................................... 38
Figure 25 – Layout of the wind turbines in the Lillgrund wind farm [25]. ............................................... 40
Figure 26 – GE 1,5 MW voltage protection characteristics. .................................................................. 43
Figure 27 – PSS/E model of the offshore wind farm. ............................................................................ 44
Figure 28 – HVAC transmission layout in PSS/E. ................................................................................. 45
Figure 29 – HVDC-LCC transmission model in PSS/E. ........................................................................ 47
Figure 30 – STATCOM device in PSS/E. .............................................................................................. 49
Figure 31 – Legend of the values presented in the single-line diagrams. ............................................. 50
Figure 32 – Power Flow result for HVAC with offshore compensation. ................................................ 52
Figure 33 – Power Flow result for HVAC with onshore compensation. ................................................ 53
Figure 34 – Power Flow result for HVAC with offshore and onshore compensation. ........................... 54
v
Figure 35 – Power Flow result for HVAC with STATCOM compensation. ............................................ 55
Figure 36 – Power Flow result for HVDC-LCC transmission. ............................................................... 58
Figure 37 – Power Flow result for HVDC+STATCOM transmission. .................................................... 59
Figure 38 – Single line diagram of the grid used for the dynamic simulations. Note: Bus 3005, where
the fault occurs is marked in the orange rectangle. .............................................................................. 61
Figure 39 – Frequency variation of offshore buses for the Case 1 fault. .............................................. 63
Figure 40 – Voltage variation of offshore buses for the Case 1 fault. ................................................... 63
Figure 41 – Offshore wind turbine speed variation for the Case 1 fault. ............................................... 63
Figure 42 – Frequency variation of onshore buses for the Case 1 fault. .............................................. 63
Figure 43 – Voltage variation of onshore buses for the Case 1 fault. ................................................... 63
Figure 44 – Speed variation of machines on the onshore grid for the Case 1 fault. ............................. 63
Figure 45 – Frequency variation of offshore buses for the Case 2 fault. .............................................. 64
Figure 46 – Voltage variation of offshore buses for the Case 2 fault. ................................................... 64
Figure 47 – Offshore wind turbine speed variation for the Case 2 fault. ............................................... 64
Figure 48 – Frequency variation of onshore buses for the Case 2 fault. .............................................. 64
Figure 49 – Voltage variation of onshore buses for the Case 2 fault. ................................................... 64
Figure 50 – Speed variation of machines on the onshore grid for the Case 2 fault. ............................. 64
Figure 51– Frequency variation of offshore buses for the Case 3 fault. ............................................... 65
Figure 52– Voltage variation of offshore buses for the Case 3 fault. .................................................... 65
Figure 53– Offshore wind turbine speed variation for the Case 3 fault. ................................................ 65
Figure 54– Frequency variation of onshore buses for the Case 3 fault. ............................................... 65
Figure 55– Voltage variation of onshore buses for the Case 3 fault. .................................................... 65
Figure 56– Speed variation of machines on the onshore grid for the Case 3 fault. .............................. 65
Figure 57– Frequency variation of offshore buses for the Case 4 fault. ............................................... 66
Figure 58– Voltage variation of offshore buses for the Case 4 fault. .................................................... 66
Figure 59– Offshore wind turbine speed variation for the Case 4 fault. ................................................ 66
Figure 60– Frequency variation of onshore buses for the Case 4 fault. ............................................... 66
Figure 61– Voltage variation of onshore buses for the Case 4 fault. .................................................... 66
Figure 62– Speed variation of machines on the onshore grid for the Case 4 fault. .............................. 66
Figure 63 – Reactive Power injected/absorbed by the STATCOM on bus 20. ..................................... 66
Figure 64– Frequency variation of offshore buses with HVDC-LCC transmission. .............................. 69
Figure 65– Voltage variation of offshore buses with HVDC-LCC transmission. ................................... 69
Figure 66– Offshore wind turbine speed variation with HVDC-LCC transmission. ............................... 69
Figure 67– Frequency variation of onshore buses with HVDC-LCC transmission. .............................. 69
Figure 68– Voltage variation of onshore buses with HVDC-LCC transmission. ................................... 69
Figure 69– Speed variation of machines on the onshore grid with HVDC-LCC transmission. ............. 69
Figure 70 – Active Power in the converters, for the HVDC-LCC transmission. .................................... 70
Figure 71 – Reactive Power in the converters, for the HVDC-LCC transmission. ................................ 70
Figure 72– Frequency variation of offshore buses with HVDC-LCC+STATCOM transmission............ 71
Figure 73– Voltage variation of offshore buses with HVDC-LCC+STATCOM transmission. ............... 71
vi
Figure 74– Offshore wind turbine speed variation with HVDC-LCC+STATCOM transmission. ........... 71
Figure 75– Frequency variation of onshore buses with HVDC-LCC+STATCOM transmission............ 71
Figure 76– Voltage variation of onshore buses with HVDC-LCC+STATCOM transmission. ............... 71
Figure 77– Speed variation of machines on the onshore grid with HVDC-LCC+STATCOM
transmission. ......................................................................................................................................... 71
Figure 78 –Active Power in the converters, for HVDC-LCC+STATCOM transmission. ....................... 72
Figure 79 – Reactive Power in the converters, for HVDC-LCC+STATCOM transmission. .................. 72
Figure 80 – Reactive Power injected/absorbed by the STATCOM on bus 151. ................................... 72
vii
List of Tables
Table 1: Colour correspondence for each voltage level in Figure 24. ................................................... 39
Table 2: Components of the GE 1,5 MW Wind Turbine Model in PSS/E. ............................................ 42
Table 3: 150 kV cable parameters. ....................................................................................................... 45
Table 4: Onshore and Offshore transformer data [10]. ......................................................................... 46
Table 5: Equations for Rectifier and Inverter Parameters. .................................................................... 47
Table 6: Reactive power compensation alternatives. ............................................................................ 51
Table 7: HVAC selected results for different compensation alternatives. ............................................. 56
Table 8: HVDC-LCC and HVDC-LCC+STATCOM selected results. .................................................... 60
Table 9: HVAC Dynamic Cases Analysed. ........................................................................................... 62
Table 10: Reactance and Susceptance of the lines. ............................................................................. 80
Table 11: 400 kV line calculated parameters. ....................................................................................... 80
Table 12: Two-Winding Transformers Parameters. .............................................................................. 81
Table 13: Load powers. ......................................................................................................................... 81
Table 14: Shunt compensators. ............................................................................................................ 81
Table 15: Wind Turbine aggregate Power Flow Parameters. ............................................................... 82
Table 16: Unit transformer of one single wind turbine. .......................................................................... 82
Table 17: 33 kV Horns Rev cable parameters [10]. .............................................................................. 83
Table 18: 33 kV AC Cable parameters used in the modelled wind farm. ............................................. 83
Table 19: DC Line Power Flow Parameters. ......................................................................................... 83
Table 20: DC Link Converters Power Flow Parameters. ...................................................................... 84
Table 21: “CDC4T” model parameters for the DC Link. ........................................................................ 84
Table 22: STATCOM Power Flow parameters. ..................................................................................... 85
Table 23: “CSTCNT” model parameters for the STATCOM. ................................................................ 86
viii
Abbreviations
A Ampère
A Rotor Swept Area
ABB Swiss-Swedish High Tech Engineering Multinational
AC Alternating Current
B Susceptance
DC Direct Current
EEA European Environment Agency
EU European Union
EWEA European Wind Energy Association
F Farad
FACTS Flexible Alternating Current Transmission System
FP Power Factor
GE General Electric
GW Gigawatt
H Henry
HPFF High Pressure Fluid Filled
HPGF High Pressure Gas Filled
HVAC High Voltage Alternating Current
HVDC High Voltage Direct Current
Hz Hertz
Ic Charging current
Id Output DC current
IEEE Institute of Electrical and Electronics Engineers
IGBT Insulated Gate Bipolar Transistor
kA Kiloampère
km Kilometre
kV Kilovolt
kW Kilowatt
L Inductance
LCC Line Commutated Converters
LPFF Low Pressure Fluid Filled
LPOF Low Pressure Oil Filled
MI Mass Impregnated
MVA Mega Volt Ampère
Mvar Mega Volt Ampère Reactive
MW Megawatt
OF Oil Filled
PCC Point of Common Coupling
PSS/E Power System Simulator for Engineering
p.u. Per unit
Q Reactive Power
R Resistance
Rc Commutating Resistance
ix
rpm Revolutions per minute
Sb Base Apparent Power
SCFF Self Contained Fluid Filled
SCLF Self Contained Liquid Filled
SMES Superconducting Magnetic Energy Storage
STATCOM Static Synchronous Compensator
SVC Static VAr Compensator
TWh Terawatt hours
V Volt
Vb Base Voltage
VLN Line to Neutral rms Voltage
VSC Voltage Source Converters
u Wind Speed
Ud DC output voltage
X Reactance
XLPE Cross Linked Polyethylene
XT Commutating Reactance
Z0 Surface Roughness
Zb Base Impedance
α Firing Angle
β Advance Angle
γ Extinction Angle
ρ Air density
µ Delay Angle
Ω Ohm
ω Angular Speed
x
Table of Contents
1. Introduction ........................................................................................................ 1
1.1. Motivations ............................................................................................................................. 1
1.2. Thesis Objectives and Outlook ............................................................................................ 3
1.3. Thesis Structure .................................................................................................................... 4
2. Wind Energy Offshore ....................................................................................... 6
2.1. Introduction ............................................................................................................................ 6
2.2. The Wind Resource ............................................................................................................... 6
2.2.1. Wind Offshore .................................................................................................................. 7
2.3. Offshore Wind Farms ............................................................................................................ 9
2.3.1. Current Status ................................................................................................................ 10
2.3.2. Technologies and Future Trends ................................................................................... 11
2.4. Grid Integration .................................................................................................................... 12
2.4.1. Transmission Technologies ........................................................................................... 13
2.4.2. Grid Connection Requirements ..................................................................................... 14
3. Transmission Technologies for Offshore Wind Farms ................................. 16
3.1. Introduction .......................................................................................................................... 16
3.2. HVAC Transmission ............................................................................................................ 17
3.2.1. General Aspects ............................................................................................................ 17
3.2.2. Topology and Main Components................................................................................... 17
3.2.3. AC Submarine Cables ................................................................................................... 18
3.2.4. Reactive Power Compensation ..................................................................................... 21
3.2.4.1. FACTS Devices ..................................................................................................... 22
3.2.4.2. Distribution of the Compensation .......................................................................... 23
3.3. HVDC-LCC Transmission ................................................................................................... 25
3.3.1. General Aspects ............................................................................................................ 25
3.3.2. Topology and Main Components................................................................................... 25
3.3.3. DC Submarine Cables ................................................................................................... 27
3.3.4. Converter Technology ................................................................................................... 29
3.3.4.1. 6-pulse Bridge Converter ....................................................................................... 29
3.3.4.2. 12-Pulse Bridge Converter .................................................................................... 31
3.3.4.3. HVDC Schemes ..................................................................................................... 31
3.3.5. HVDC-LCC + STATCOM .............................................................................................. 32
3.4. HVDC-VSC Transmission ................................................................................................... 34
xi
4. PSS/E Modelling ............................................................................................... 35
4.1. Introduction .......................................................................................................................... 35
4.2. Grid Structure ...................................................................................................................... 35
4.2.1. Lines .............................................................................................................................. 36
4.2.1.1. 400 kV Lines .......................................................................................................... 36
4.2.2. Transformers ................................................................................................................. 37
4.2.3. Loads and Shunt Compensators ................................................................................... 37
4.2.4. Diagram of the Grid ....................................................................................................... 38
4.3. Offshore Wind Farm Model ................................................................................................. 40
4.3.1. Wind Turbines ................................................................................................................ 41
4.3.1.1. Steady-State Model ............................................................................................... 41
4.3.1.2. Transient Model ..................................................................................................... 41
4.3.1.3. Fault Ride Through Capability ............................................................................... 42
4.3.2. Step-Up Transformers ................................................................................................... 43
4.3.3. 33 kV Cables ................................................................................................................. 43
4.3.4. Layout of the offshore wind farm ................................................................................... 43
4.4. HVAC Transmission ............................................................................................................ 45
4.4.1. AC Submarine Power Cable .......................................................................................... 45
4.4.2. Onshore and Offshore Transformers............................................................................. 46
4.5. HVDC-LCC Transmission ................................................................................................... 47
4.5.1. Steady-State Model ....................................................................................................... 47
4.5.2. Transient Model ............................................................................................................. 48
4.6. STATCOM ............................................................................................................................. 48
4.6.1. Steady-State Model ....................................................................................................... 49
4.6.2. Transient Model ............................................................................................................. 49
5. Power Flow Results ......................................................................................... 50
5.1. Introduction .......................................................................................................................... 50
5.2. HVAC Power Flow ............................................................................................................... 50
5.2.1. Offshore Compensation ................................................................................................. 52
5.2.2. Onshore Compensation ................................................................................................. 53
5.2.3. Offshore and Onshore Compensation ........................................................................... 54
5.2.4. STATCOM Compensation ............................................................................................. 55
5.2.5. Discussion ..................................................................................................................... 56
5.3. HVDC Power Flow ............................................................................................................... 58
5.3.1. HVDC Configuration ...................................................................................................... 58
5.3.2. HVDC+STATCOM Configuration .................................................................................. 59
5.3.3. Discussion ..................................................................................................................... 60
xii
6. Dynamic Results .............................................................................................. 61
6.1. Introduction .......................................................................................................................... 61
6.2. HVAC Dynamic Behaviour .................................................................................................. 62
6.2.1. Case 1 results ................................................................................................................ 63
6.2.2. Case 2 results ................................................................................................................ 64
6.2.3. Case 3 results ................................................................................................................ 65
6.2.4. Case 4 results ................................................................................................................ 66
6.2.5. Discussion ..................................................................................................................... 67
6.3. HVDC Dynamic Behaviour .................................................................................................. 69
6.3.1. HVDC Configuration ...................................................................................................... 69
6.3.2. HVDC + STATCOM Configuration ................................................................................ 71
6.3.3. Discussion ..................................................................................................................... 73
7. Conclusions ..................................................................................................... 75
7.1. Final Remarks ...................................................................................................................... 75
7.2. Future Work.......................................................................................................................... 76
References .............................................................................................................. 78
Appendix A – Grid Parameters .............................................................................. 80
Appendix B – GE 1,5 MW Wind turbine aggregate “dyr” record file .................. 87
1
1. Introduction
This chapter presents an overview of some general aspects regarding wind energy and, more
specifically, offshore wind energy. The motivation that led to the execution of this thesis and the main
objectives of the work are also presented.
1.1. Motivations
For a long time, conventional energy sources such as coal, oil, natural gas, hydro power and
nuclear power were used as main fuels for the production of electricity. The energy crisis that affected
the world in the 1970s and 1980s led to major changes in the hitherto energy paradigm. Reducing the
world’s dependency of fossil fuels became a priority, as the finiteness of these resources was found to
be truly concerning. The need to guarantee diversity and reliability of energy sources, together with
the fast growing concern for environmental issues also contributed to a greater interest in alternative
energy sources.
The need for a long-lasting solution to fulfil the growing worldwide energy demand, a solution that
is both environmentally friendly and economically safe was found in renewable energies. In fact,
renewable energy sources such as wind, solar photovoltaic and hydro power, to name a few, have
suffered significant increases in recent years. The installed power over the last ten years has
increased in such way that renewables account for more than 50% of new installations in the EU,
cementing a rising trend initiated over a decade ago [1]. Such data can be verified in Figure 1, where
the share of total installed power in the EU in 2000 and in 2009 is presented.
Figure 1 – EU power capacity mix: a) 2000 data; b) 2009 data [1].
In Figure 1, wind power clearly stands out as the renewable source that experienced the largest
increase in installed power: wind power’s share of total installed capacity has increased from 2% in
2000 to 9% in 2009. These facts reflect a global trend on new power installations, as 2009 was the
2
second year running where more wind power was installed than any other generating technology in
the EU [1]. Wind energy is considered one of the most promising renewable energy sources,
benefiting from a mature technology, developed especially in Europe and in the USA. It is now a totally
established technology that has increased remarkably in recent years. Wind power is, in fact, the
fastest growing renewable energy: since 1996 the installed wind capacity increased from 6100 MW to
158505 MW as it is shown in Figure 2.
Figure 2 – Global Cumulative Installed Wind Capacity (1996-2009) [2].
The fast development of wind energy throughout the world has opened up new frontiers in wind
energy generation in the form of offshore wind farms. Placing wind turbines at sea became a
possibility, as many of the windiest locations onshore are already occupied. Higher wind speeds at
marine locations and wider installation areas, together with the possibility of moving wind turbines
away from population – to which wind turbines may cause some discomfort – were the main reasons
driving the investment on offshore wind energy.
The Vindeby wind farm in the Baltic Sea, off the coast of Denmark, was the first offshore wind
farm in the world. It was built in 1992 and it consists of eleven 450 kW wind turbines. Since then, and
in particular in the last 7/8 years, offshore wind power has experienced an enormous growth, with the
installation of several new offshore wind farms. A graph representing the development of the offshore
wind power installed capacity from 1991 to 2009 can be seen in Figure 3, in which the referred growth
can be observed.
3
Figure 3 – Annual and cumulative installed capacity (in MW) of offshore wind power in Europe (1991 – 2009) [1].
The current trends for offshore wind farms indicate new offshore wind farms will be higher rated
(as a result of larger wind turbines and a higher number of wind turbines) and placed further from the
sea. The power transmission of large amounts of power over long distances, from the wind farm to the
onshore grid, makes the transmission system a key feature of the offshore wind farm installation.
Currently available technologies for the transmission system to shore are high voltage alternating
current (HVAC) and high voltage direct current (HVDC). For HVDC connections, there are two
technical options: line commutated converter based HVDC (known as HVDC-LCC) and voltage source
converter based HVDC (HVDC-VSC). The HVDC alternatives differ in the technology used for the
power converters.
The technology used for the interconnection of offshore wind farms influences in a very significant
manner the whole operation of the system. As so, the main characteristics of operation for each
transmission system have to be assessed and thoroughly studied, including steady-state analysis and
dynamic behaviour analysis of the link (when disturbances occur), making this an important field of
investigation in the near future.
1.2. Thesis Objectives and Outlook
The main objective of this thesis is to compare the performance in steady-state and the transient
behaviour of two alternative transmission systems for offshore wind farms: HVAC technology and
HVDC-LCC technology.
In steady-state, the power flow analysis of a grid with an integrated offshore wind farm is
performed, for both transmission systems. Special attention is paid to both the power factor and the
reactive power flow in the point of connection of the wind farm to the onshore grid. For the HVAC
technology, compensation schemes for the reactive power generated by the submarine cable are also
4
evaluated. Compensation with shunt reactors placed onshore, offshore and at both ends of the cable
and also the use of a STATCOM are subject to analysis based on simulations. For HVDC-LCC
technology, the configuration with a STATCOM as a way of compensating reactive power
consumption of the converters is also analysed.
The transient behaviour of the wind farm, and particularly of the connection (AC or DC Link), is
investigated. The dynamic behaviour of the system, as a response to onshore grid faults, is tested
through simulation as different faults are applied to the grid. The influence of the STATCOM is also
assessed.
All the analysis are done considering simulation results obtained after careful design,
dimensioning and testing of the offshore grid performed in PSS/E software.
1.3. Thesis Structure
The content of this thesis is divided in seven chapters.
The present chapter, chapter one, is a general introduction to the dissertation, providing
information on the current status of offshore wind energy and its integration among the global energy
panorama. Motivations, objectives and structure of the work developed in the course of the thesis are
also outlined.
Chapter two presents an overview of the most important issues regarding offshore wind farms.
The main characteristics of offshore wind farms, main advantages and constraints associated with
these wind farms, current status and grid integration issues are also approached.
Chapter three provides a closer look on the transmission system used for the connection of
offshore wind farms with the onshore grid. The chapter presents a detailed description of the three
available options: High Voltage Alternating Current Transmission (HVAC); High Voltage Direct Current
based on Line Commutated Converter Transmission (HVDC-LCC) and High Voltage Direct Current
based on Voltage-Source Converter Transmission (HVDC-VSC). Main features, operating issues and
limitations of each technology are discussed, with special focus on the HVAC and the HVDC-LCC
alternatives.
Chapter four focuses on the modelling carried out in the PSS/E software of the offshore wind farm
studied. The models used and the parameters selected for each model, for both the AC and the DC
transmission, are described with sufficient detail. Differences in modelling for steady-state analysis
and for transient behaviour analysis of the grid are also summarized.
Chapter five presents the results of the power flow simulation of the offshore wind farm. For
HVAC transmission, some alternatives for reactive power compensation are reviewed. For HVDC
transmission, the possible benefits of a STATCOM are also studied. A comparative analysis of the
transmission alternatives in steady-state is also performed.
5
Chapter six presents the results obtained for the dynamic simulation of the wind farm. Dynamic
behaviour of the HVAC or HVDC-LCC transmissions to different grid faults on the onshore grid is
analysed. The influence of the STATCOM on the dynamic behaviour of the link is studied.
In chapter seven, the main conclusions of the work are presented, together with a reference to
future studies that may offer important future contributions to the theme of this thesis.
6
2. Wind Energy Offshore
This chapter will present an overview of some general aspects of wind farms offshore: reasons
and motivations for building offshore, their advantages and disadvantages, grid integration and
transmissions options to shore.
2.1. Introduction
The increasing requirement to produce electrical power from renewable and clean energy
sources and, particularly, the major investment in wind energy made over the last 15 years, led to a
growing interest in harnessing the wind power that lies off the coast of many countries. Offshore wind
power became, therefore, the next evolution in wind power technology. A number of reasons gave rise
to this bulk investment. The offshore wind characteristics lead to a larger produced energy but the
difficult conditions at sea, the integration of the wind farm and other technical requirements have to be
seriously considered. It is however relevant to notice that after years at the starting block, the offshore
wind energy is taking off and becoming a viable alternative for energy production.
2.2. The Wind Resource
Winds are caused by pressure differences across the earth’s surface due to differential solar
heating of the earth’s atmosphere. Therefore, wind energy is strongly influenced by solar energy: the
amount of solar radiation absorbed at the earth’s surface is greater at the equator then at the poles,
resulting in faster heating of the air in the equator [3]. This warm air, heated in the equatorian regions,
rises to high altitudes and then flows toward the poles. At about 30ºN and 30ºS, the air begins to cool
and sink and so a return flow of air takes place in the lower layers of the atmosphere. Zones of high
pressure are created by descending air, as zones of low pressure are formed where air is ascending.
This horizontal gradient drives the flow of air from high to low pressure. The greater the pressure
gradient, the greater is the force on the air and the higher is the wind speed [4].
Stronger, constant and more persistent winds occur at about 10 km above the earth’s surface.
However, as placing turbines at such high altitudes is unfeasible, the area of interest is limited to only
a few tens of meters high [3].
In addition to this global phenomenon resulting in wind, there are also local effects to be
considered. The nature of the terrain has an important effect. Wind turbines utilise wind energy from a
lower area known as boundary layer. In this region, wind speed is retarded by frictional forces on the
earth’s surface, which means that wind speed increases with height (this gradient is known as wind
shear) [4].
7
2.2.1. Wind Offshore
As stated previously, the nature of the surface influences the wind profile at a given region.
Offshore, the smoothness of the surface of the ocean results in a low surface roughness (a parameter
referred to as Z0). The surface roughness length is dependent on the sea state, increasing with the
local wave conditions which are, in turn, influenced by wind itself. As a result of a low surface
roughness there is also low turbulence and wind shear on a marine environment. Offshore locations
are in fact windier and the winds are more persistent than those at an onshore location. These effects
increase with distance from shore in the downwind direction, making it more appealing to locate
offshore wind farms further from land [5].
Data collected from offshore meteorological stations has been compiled to create the wind
offshore atlas of Europe, which is presented in Figure 4. The mean wind speed at each location is
given for five different heights: 10, 25, 50, 100 and 200 m from sea level. The highest values of wind
speed are for the 200 m height, making it the most economically interesting solution for placing
offshore wind turbines.
The geographical distribution of wind speed offshore, given by Figure 4, shows that, apart from a
small region in southern France, the North Sea is the windiest location in Europe. In fact, for this
reason (and also for the fact that water depth in the North Sea increases slowly with distance from
shore) most of the wind farms in Europe are located in the North Sea, as part of solid investments by
English and Danish governments in offshore farms.
8
Figure 4 – Europe wind resources over open sea (Offshore Wind Atlas) [6].
As the power that can be extracted by a wind turbine is proportional to the cube of the wind speed
(as given by Equation (1)), the extracted power is highly dependent on the wind speed [3].
=1
2 (1)
Therefore, and given the fact that wind speed offshore is potentially higher than onshore, a 10%
increase in wind speed results in an approximate 30% increase in power production, making wind
power offshore a very interesting option.
According to the European Environment Agency (EEA) the technical potential of offshore wind by
2020 is around 25000TWh, six to seven times greater than the projected electricity demand [1]. These
figures suggest that a significant wind resource lies off the coast of many countries. The data
presented refers to European estimations but some studies done elsewhere point out similar offshore
potential in other regions of the world.
9
2.3. Offshore Wind Farms
Following the rising interest in the wind resource offshore, a way of harnessing the wind was
considered and steps were taken towards creating platforms located offshore, in order to gain
experience and test problems and potential benefits of this technology. The first offshore wind turbine
was then installed in Sweden in 1991. The following year, the Vindeby wind farm, the first of its kind,
was installed in shallow water (2-5 m deep) off the coast of Denmark. It consists of eleven 450 kW
machines, located about 3 km from the shore [5].
Small wind farms, consisting of few wind turbines (less than 10) with low ratings (around 500 kW)
were erected in other European countries but it wasn’t until the 2000s that serious investment was put
into the design and construction of offshore wind farms. Denmark has led the way, building the Horns
Rev wind farm in 2002, the largest at the time of construction and a reference for following projects.
Many new offshore wind farms have followed, and the remarkable growth in new installations
over the last 7/8 years (as can be seen in Figure 3), denotes that, in fact, offshore wind energy has
gain relevance as an alternative to consider for energy production.
The fact that offshore wind farms are located at sea offers several advantages but also poses
major challenges for potential investors. Some relevant constraints associated with the offshore wind
farms that need to be considered are the following:
• Installation and maintenance of a large structure located far from shore is a complex
procedure, given the distance and the need for special ships;
• Accessibility of turbines may prove difficult, as harsh weather conditions and extreme
waves are likely to exist at sea;
• As a consequence, materials used for wind turbines and additional equipment need to be
resistant to physical impact as well as to salt corrosion (when the wind farm is placed in
salt water);
• Overall, at present time, the construction and maintenance of offshore wind energy is
more expensive than on shore, given the higher costs of foundation, installation and
electrical connection of wind farms [4].
However, as the number and size of offshore wind farms increase, technology is likely to advance
to deal with these problems and reduce transportation and installation costs. The advantages will
outweigh the disadvantages making offshore wind farms a reliable investment [4].
Advantages of offshore wind power are as follows:
• Wind speeds are higher offshore than onshore, which means that a large potential for
power production lies offshore. In fact, the annual average full-load hours offshore is
between 3500-4000 hours, while for onshore this figure is between 1500 and 3000 (the
average being around 2000 hours) [7];
• Winds at marine locations are more persistent and less turbulent than those on land;
10
• Due to the proliferation of wind parks onshore, some countries are running out of suitable
onshore locations, as the windier sites are already occupied;
• A wider area for installation of wind turbines is also likely to be available at sea,
compared to land;
• Wind turbines that might be intrusive and cause visual and noise impact on land might be
acceptable if sited away from shore, far from population.
2.3.1. Current Status
As of January 2010 all offshore wind farms were located in Europe. According to EWEA data [1],
there are 38 operational offshore wind farms in Europe, representing a total capacity of 2055,9 MW of
installed capacity. Denmark has been a pioneer and a worldwide reference as far as offshore wind
farms are concerned. For the last few years, however, the UK became the number 1 country in
offshore installed capacity, with a total of 882,8 MW, representing almost 43% of the European total.
Complete division of installed capacity by country is presented in Figure 5.
Figure 5 – Cumulative share of installed capacity by country (end 2009) [1].
As stated previously, the North Sea is the windiest region in Europe. The relatively low water
depth (less than 20 m) of the sea has also contributed to the widespread installation of offshore wind
farms in the region. Figure 6 shows precisely that, as can be seen that most of the operational
offshore wind farms are at the North Sea.
11
Figure 6 – Map of operational offshore wind farms in Europe (January 2010) (Adapted from [8]).
Offshore wind energy is a growing market, and a significant number of offshore wind farms are
either under construction or planned for the next 10 years. Around 40 GW of installed capacity are
expected to exist by 2020 [1].
2.3.2. Technologies and Future Trends
The trends and perspective for future offshore wind farms are likely to demand new technology,
creating new challenges for the industry. As the technology develops and experience is gained, future
projects will be higher rated and placed in deeper waters, further from shore. Looking at the wind
farms proposed by developers, the industry will go beyond the so-called 20:20 envelope (20m water
depth, 20 km from shore) in which current wind farms are included.
Some experience gained from wind farm projects in land is useful but offshore wind farms differ
significantly from the onshore ones.
Submarine support structures are a very important part of the offshore project, as they represent
a significant proportion of offshore development costs. The choice of structure is determined
essentially by water depth and consistency of seabed. The currently available options for foundation
are represented in Figure 7, the more commonly used being the monopole and gravity foundations.
For deepwater application some prototypes of floating foundations are under development, with the
first experience being the Floating Hywind in Norway, at a depth of 220 m. These floating devices will
be important as the wind farms move further from shore.
12
Figure 7 – Offshore foundations and general characteristics [9].
Another issue of concern regarding the technology used in offshore wind farms is the wind
turbines used. The wind profile offshore (less turbulent and with higher average and extreme wind
speeds than onshore) along with challenging access conditions demand special requirements for the
design and manufacture of wind turbines. These include robustness, corrosion protection, high
reliability and low maintenance requirements.
Recent trends for wind turbines indicate that future offshore wind turbines will be larger machines,
with higher rotor diameter and increasingly higher capacity, as indicated in Figure 8. Planned wind
farms are expected to include turbines with rated power as high as 5 MW.
Figure 8 – Offshore wind turbine size evolution [9].
2.4. Grid Integration
Integrating an offshore wind farm in an electrical network poses a significant challenge to the grid.
The impact varies with the strength of the grid and the size of the wind farm. As the offshore wind
capacity grows, grid integration issues may arise, as increasingly large amounts of electricity are fed
into networks, either in distribution or transmission systems, at points not specifically designed for
13
such infeeds [4]. As such, the transmission technology – AC or DC – used to link the wind farm to the
onshore grid is of key relevance to meet the grid requirements of the network.
An overview of the available technologies is presented as well as a description of the main
considerations of connecting an offshore wind to the grid.
2.4.1. Transmission Technologies
The installation of offshore wind farms has given rise to new challenges regarding electrical
connections, both for the internal electric system of the wind farm and for its connection to the main
grid. Existing wind farms onshore are connected via AC cables or overhead lines to the national grids.
An AC network within the farm collects the power production of each wind turbine. The voltage level of
generation is typically around 700V, which is then stepped up to a medium voltage level (typically 33
kV) by a transformer installed in the nacelle or the tower base [4].
The same scheme was applied to the first wind farms installed offshore. However, as wind farms
tend to be larger, with longer distances between turbines and longer distance to shore, alternative
arrangements were studied. Special focus is, then, on the transmission from the wind farm to shore.
Currently available technologies for transmission system to shore are high voltage alternating current
(HVAC) and high voltage direct current (HVDC). For HVDC connections, there are two technical
options: line commutated converter based HVDC (known as HVDC-LCC) and voltage source
converter based HVDC (HVDC-VSC).
The AC connection has been used in all offshore wind farms. With the increase in the size of wind
farms, the transmission to shore is performed at higher voltage levels than the 33 kV used in
collection, usual values being 132 kV (used in the Nysted wind farm, in Denmark) or 150 kV (as is the
case of the Horn Rev wind farm). The HVAC solution is indeed the most straightforward technical
approach as it features some interesting advantages such as ease of interconnection, installation and
maintenance; operational reliability and cost effectiveness. Some disadvantages include the limits for
transmitted power, associated to the charging current on AC cables, which will limit the maximum
distance of transmission. Some scheme of reactive power compensation must then be used in AC
cable systems.
HVDC solutions might prove a reliable option for large wind farms further from shore as this
technology has some very promising characteristics. One of the most important is the ability to
transmit large amounts of power over long distances with lower losses than HVAC. Since the
transmission is done using DC cables, the transmission distance is not affected by cable charging
current. Main disadvantages include the complexity and associated cost of installation and
maintenance of the HVDC system, which may well be overcome in the future, due to economies of
scale.
In the line commutated based HVDC the conversion is performed with thyristor converters. The
HVDC-LCC is a totally established technology, as the first HVDC link dates back to 1954 in Gotland,
14
Sweden. Such proven track record has become one of the most important advantages of this option,
in addition to the features referred in the previous paragraph.
The HVDC-VSC transmission is a relatively recent technology that uses insulated gate bipolar
transistors (IGBT) converters for the inversion/rectifier operations. This technology was made possible
by advances in high-power electronics and is used as a viable alternative to thyristor based LCCs
used in “Classic HVDC”. Unlike line-commutated converters, VSCs do not need an AC source
commutation voltage; the independent control of both active and reactive power and the reduced
injection of harmonics also represent important advantages of HVDC-VSC over HVDC-LCC [4].
More detailed description of the working principles of each of the referred technologies – with
special emphasis on HVAC and HVDC-LCC – is presented in Chapter 3.
2.4.2. Grid Connection Requirements
Until some years ago wind farms were allowed, or even required, to disconnect from the grid
during a disturbance in the grid [10]. This has changed significantly, specially due to the proliferation of
large amounts of installed wind power capacity and, predictably in a near future, installed capacity
offshore. The disconnection of a large offshore wind farm would result in a significant loss of
generation that could cause some stability problems to the network. Transmission system operators
require nowadays for offshore wind farms to stay connected under certain disturbances in the grid.
These requirements are known as the fault ride through capability of the wind farm and are generally
regulated in grid codes. As established in most grid codes, only under certain circumstances shall
wind farms be disconnected from the grid following a grid fault, remaining otherwise connected in
order to assist in the stabilization of the grid frequency or the voltage during fault, providing voltage
back-up.
Apart from the fault ride through capability, other technical requirements must be fulfilled by the
wind farm, since the increasing size of offshore wind farms means that the rating of such installations
will be comparable to that of traditional generating plants on the grid. These requirements include [11]
[12]:
• Control of active and reactive power (operation under a specified range for power factor);
• Frequency range (with time durations for extreme conditions, permissible reduction at
frequency extremes, if any);
• Contribution to network stability;
• AC voltage control capability.
As the proliferation of offshore wind power increases, wind farms will be bound to meet these
demands, which may prove difficult depending, to great extent, on the transmission system used
between the wind farm and shore. The charging currents affecting AC cables represent a limitation for
15
the HVAC option and so some form of compensating the surplus reactive power generated by the
cable may prove necessary to met grid requirements. As far as HVDC is concerned, capacitive
currents are nonexistent but other issues are of concern, such as the behaviour of the DC link
following an onshore grid fault and the power factor at the connection point in HVDC-LCC, as thyristor
based converters absorb reactive power. FACTS devices can be used to reduce or eliminate these
problems, therefore enhancing the performance of the HVDC system.
Further aspects of the issues concerning connection, both in HVAC and in HVDC-LCC will be
discussed in Chapter 3.
16
3. Transmission Technologies for Offshore Wind Farms
This chapter provides an overview of the available transmission systems for offshore wind farms.
Several aspects of the connection in either AC or DC are discussed.
3.1. Introduction
One of the most important aspects of the design of offshore wind farms is the connection to the
electrical grid located in land. The electrical energy generated in the offshore wind farm requires one
or more submarine cables to transmit the power to the onshore utility grid that services the end-users.
Depending on the wind farm size and distance from shore, the connection poses a challenge, not
only to the wind farm developer, but also to the onshore network itself, as some grid code
requirements must be met.
There are currently three different transmission technologies available:
• HVAC – High Voltage Alternate Current;
• HVDC-LCC – High Voltage Direct Current with Line Commutated Converters;
• HVDC-VSC – High Voltage Direct Current with Voltage Source Converters;
These technologies present significant differences, on the operation mode and the components
used. A detailed explanation of the options is presented throughout this text, in 3.2, 3.3 and 3.4
Special focus is given to the HVAC and HVDC-LCC technologies.
The general layout of an offshore wind farm is represented in Figure 9. This scheme can refer to
both the HVAC and the HVDC transmission. The components present at the “collecting point” and at
the “wind farm grid interface” vary according to the technology used to connect the generation to the
PCC (Point of Common Coupling) with the grid.
Figure 9 – General layout of an offshore wind farm [13].
17
As of 2010 all operating offshore wind farms are connected via AC cables. However, as wind
farms become larger and are placed further from the coast, the use of HVDC solutions will become
technically and economically interesting alternatives.
3.2. HVAC Transmission
3.2.1. General Aspects
Connecting the wind farm to the grid by an AC cable is the most straightforward technical
solution, as both the power generated by the wind farm and the onshore transmission grid are AC. In
fact, all the operational offshore wind farms use HVAC for the connection link to shore. The HVAC
transmission offers some advantages over the DC solutions such as [11] [14]:
• Proven and low-cost technology;
• Easy to integrate in existing power systems;
• Low losses over small distances.
On the other hand there are some constraints of the HVAC system that limit significantly the use
of this technology, namely [11] [14]:
• There is an excessive amount of reactive power produced in the AC submarines cables;
• Increase in the cable length means increase in its capacitance which results in a reactive
power increase, resulting in a transmission distance limit for AC systems;
• Necessary use of compensation systems (shunt reactors, STATCOMS, SVC, etc) at the
ends of the cable;
• Load losses are significantly higher for longer distances;
• For large wind farms several cables may be necessary, increasing line losses.
3.2.2. Topology and Main Components
A transmission system based on HVAC technology includes the following main components [13]
[15]:
• AC based collector system within the wind farm;
• Three core XLPE HVAC submarine transmission cable(s);
• Offshore transformer(s);
• Reactive power compensation (onshore and/or offshore);
18
• Onshore transformer;
Figure 10 illustrates the layout of an HVAC based offshore wind farm, with the components
mentioned above.
Figure 10 – Layout of an HVAC wind farm[16].
The voltage used in the collecting system in the wind farm is usually medium voltage, in the range
of 33-36 kV. For relatively small wind farms, with short distances to shore, the offshore transformer
may not be necessary, so the link is done at the same voltage used in the wind farm.
3.2.3. AC Submarine Cables
The submarine cable used in the transmission system is one of the most important components
of the system, as the cost of the cable represents a large fraction of the total cost of the offshore wind
farm investment. The laying of the cable is a complex process performed with special laying vessels,
so as to guarantee the full operation of the system. The potentially severe conditions at sea and other
hazards, such as fishing or dropped objects, also demand special attention on the mechanical
resistance of the cable. Technically, different cable types have different characteristics that influence
the transmission and can even limit the maximum distance of transmission. A submarine cable has
typically the following structure [14]:
• Conductor core, typically copper or aluminium;
• Electrical Insulation, either solid dielectric (XLPE cables) or oil impregnated paper (OIP);
• Shielding, a conductor layer of paper or polymer;
• Sheathing, metallic layer used to ground the cable and also protect it from water;
• Armour, an outer metallic armature, to increase mechanical resistance;
• Optic Fibre, for communications;
The link can be achieved with either three single-core cables or one three-core cable (more than
one may be used, depending on the transmitted power). The latter is the most common type, as it
19
represents lower cable and installation costs and lower electromagnetic fields and induced current
loss, compared with separate cables.
The main types of AC cables differ in the electrical insulation used in the cable. Historically,
cables used lapped paper insulation impregnated with insulating oil. These cables are:
• High-pressure pipe-type, either fluid-filled (HPFF) or gas filled (HPGF);
• Low-pressure oil-filled (LPOF), also referred as self-contained liquid-filled (SCLF) and
Low-pressure fluid-filled (LPFF);
The need of a pumping system in both ends of the lines to circulate the fluid and the
environmental risk of pipe rupture, resulting in oil spill are serious disadvantages of these cables. As
so, these cables are only considered for very short distances.
Cross-linked polyethylene cable (XLPE) cables are currently the most cost-effective and modern
alternative. The insulation is made of solid dielectric (also called extruded dielectric or polymeric
insulated) and it presents several advantages over the fluid-filled options: higher mechanical
resistance, lower weight and, consequently, easier installation process [14]. Furthermore, XLPE
cables have lower capacitance and lower dielectric losses than the fluid-filled cables.
One three-core XLPE cable is shown in Figure 11, where the structure described above can be
seen.
Figure 11 – Three-core XLPE submarine cable [17].
The major problem concerning the connection of wind farms with AC submarine cables is the fact
that cables generate significant amounts of reactive power. This reactive power is produced by the
high shunt capacitance of cables (significantly higher than overhead lines). In the AC system, the
cable must carry the load current and the reactive current generated by the cable capacitance, which
reduces the power rating of the cable.
Considering the cable is modelled by an equivalent π-model, the submarine cable can be
represented by the scheme shown in Figure 12. The charging current at each end are represented by
and .
20
Figure 12 – Equivalent π-model for a submarine cable.
Neglecting the voltage drop across the resistance and the reactance of the cable, one can
consider that = = . As so, the total charging current affecting the cable is the sum of and
(as represented in Figure 12) and is represented by , which is given by Equation (2):
= ∙ ∙ (2)
In the equation,
• is the angular frequency (rad/s),
• C is the capacitance (F)
• V is the terminal voltage (V).
From Equation (2) it is clear that, as the cable system voltage is increased to minimize losses, the
charging currents also increase, worsening the situation.
The reactive power generated by the shunt capacitance is then given by Equation (3):
= − ∙ ∙ (3)
Note that the negative signal means the capacitance supplies reactive power to the grid. From
Equation (3) it can be seen that the reactive power generated by the cable is a function of the
capacitance and the voltage, being particularly dependent on the voltage (since it is squared).
Since the cable capacitance is distributed along the entire length of the cable, the longer the
cable the higher the capacitance and the resulting generated reactive power. As a consequence, the
load carrying capability of the cable is reduced, which means that a cable can only transmit a certain
amount of power for a given distance. This results in an obvious limitation for the length of AC links.
The maximum rating for a three-core submarine cable with a voltage rating of 150-170 kV is limited to
about 200MW, with compensation at both ends, and a maximum cable length of 200 km [11].
A comparison of the transmission capacity of different cables operated at certain voltage levels
(132, 150, 230 and 400 kV) is presented in Figure 13. The maximum distances for an AC submarine
cable system varies according to the rated voltage for the link and the maximum transmitted power
decreases with the length of the cable.
21
Figure 13 – Maximum transmission lengths with inductive shunt compensation in both ends [17].
The transmitting distance represents an obvious limitation for the HVAC system using a
submarine cable. Compensating the reactive power produced by the cable can be a solution since it
increases significantly the maximum distance of transmission. Reactive power compensation options
are discussed in 3.3.4.
The capacitance of AC cables may also lead to other undesirable effects, like overvoltages, high
harmonic currents and resonances between the cable’s capacitance and the reactance of the
generators. Real power losses within the cable also limit the distance for HVAC cable transmission.
These losses in the submarine cables derive from dielectric losses (relatively small) and ohmic losses
in the conductors (the higher component of losses), the metallic shield and the steel wire armour [14].
3.2.4. Reactive Power Compensation
The solution for the large amounts of reactive power at the cable is to compensate the reactive
power produced by absorbing reactive power, thus reducing the additional losses and increasing the
maximum transmitting distance. The compensation is usually realized by fixed or electronically
controlled shunt reactors. The fixed shunt reactor is the simplest device but the progress in FACTS
(Flexible AC transmission system) devices, such as SVC (Static VAr compensator) or STATCOM
(Static Synchronous Compensator), considerably extends the reactive power and voltage control
possibilities offered by the switched shunt reactors.
Shunt reactors are the most commonly used devices as they represent simple and robust
solutions with low installation costs. They also have the advantage of requiring no transformer for the
connection, thus having no additional power losses. One of the disadvantages is that the reactors are
22
designed for a single operational mode, usually to compensate the cable at full load. Another
disadvantage is the fact that the reactive power absorbed by the shunt reactor is proportional to the
square of the terminal voltage, as demonstrated by Equation (4), where L stands for the inductance of
the reactor.
=
(4)
3.2.4.1. FACTS Devices
The SVC and the STATCOM are part of the FACTS device family, used for voltage regulation
and power system stabilization, based on power electronics. These devices are capable of both
generating and absorbing reactive power. The flexibility of use is, therefore, the main advantage of
these equipments, since they allow the continuous variable reactive power absorption (or supply). The
reactive power is not proportional to the voltage at the bus, also another advantage of these
equipments. The FACTS devices can also contribute in the improvement of the voltage stability and
the recovery from network faults.
The similarity of the SVC and STATCOM devices led to them being sometimes referred generally
as “Static VAr Compensators”. These are, however, different equipments. The SVC (“SVC Classic” for
some manufacturers) is based on conventional capacitor banks together with parallel thyristor
controlled inductive branches. These inductive branches can either be TCR (Thyristor Controlled
Reactor), used for linear injection of reactive power or TSC (Thyristor Switched Capacitor), used for
stepwise injection of reactive power. A SVC device is represented in Figure 14, where a linear
diagram and an SVC installation (in an offshore wind farm) are represented.
Figure 14 – (a) Schematic Diagram of an SVC; (b) an SVC in Radsted, Denmark [18].
The STATCOM device uses a power electronic voltage source (VSC). The converter uses
semiconductors with turn-off capability, such as Insulated Gate Bipolar Transistors (IGBTs). The
benefits of the STATCOM (commercially known as “SVC Light” by ABB or “SVC Plus” by Siemens),
compared with the SVC, are the fact that the capacitor banks used are smaller and also there is no
23
need for big air-cored inductors. Further advantages of the STATCOM are also found in the dynamic
behaviour (such as faster transient response). A simplified schematic diagram of a STATCOM and an
example of an installation is shown in Figure 15.
Figure 15 – (a) Schematic Diagram of a STATCOM; (b) a STATCOM installation [18].
3.2.4.2. Distribution of the Compensation
The distribution of the reactive power compensation devices is also an important matter. Figure
16 presents several options for compensation: on one end only (offshore, in the case), on both ends or
distributed along the cable. The figure also compares the loading of an open-end cable. It is obvious
that the worst case is with compensation installed at one end only. The best solution is to install
distributed compensation along the cable, since the charging current generated would flow towards
the reactor, not increasing the rating of the cable so significantly. It is however important to notice that
the placement of reactive power compensators in the middle of the cable is likely to not be possible,
as the installation of these interstitial reactive power compensators would present an added
technological and economic challenge.
24
Figure 16 – Loading of an open-end cable due to capacitive charging current for different schemes of reactive power compensation [19].
25
3.3. HVDC-LCC Transmission
3.3.1. General Aspects
Current trends in offshore wind farms indicate future installations will be rated at hundreds of MW
and further from shore (as far as 200/300 km). These transmission requirements will not be feasible
using AC submarine cable transmission, given the limitations mentioned in 3.2. Connecting offshore
wind farms through a DC link has then been considered since there is a significant amount of
experience in transmitting large amounts of power over long distances through HVDC links.
There are two different HVDC transmission technologies: the line commutated converter HVDC
(HVDC-LCC, also referred as “Conventional” or “Classic” HVDC) using thyristors in the converters and
the voltage source converter HVDC (HVDC-VSC) which uses IGBTs.
The main advantage of HVDC-LCC over the HVDC-VSC is its proven track record, since there is
an accumulated experience of decades for this technology. The first commercial HVDC-LCC
connection was installed in 1954 and since then many other conventional HVDC links were installed
all over the world.
Some of the most important advantages that HVDC-LCC transmission offers over AC are [20]
[21]:
• Asynchronous connection, since sending and receiving end frequencies can differ;
• Transmission distance using DC is not limited by cable charging current;
• Low cable power losses;
• Higher power transmission capability per cable;
• Power flow is fully defined and controlled;
• HVDC does not transfer short circuit current.
Some of the constraints of HVDC-LCC transmission are the following [20] [21]:
• No experience in connecting offshore wind farms;
• Production of harmonics in the converter, making the use of filters necessary;
• The converters at each end consume reactive power.
3.3.2. Topology and Main Components
The HVDC-LCC transmission system for an offshore wind farm can be represented by Figure 17.
26
Figure 17 – Basic Configuration of a wind farm using an HVDC-LCC [7] Note: F=filter; HFF=High-frequency filter.
An HVDC-LCC transmission system consists of the following main components (represented in
Figure 17) [11]:
• AC based collector system within the wind farm;
• Offshore three-phase two-winding converter transformers;
• Auxiliary power set (STATCOM or Diesel generator);
• AC and DC Filters;
• Smoothing Reactor;
• DC submarine cables;
• Onshore converter station (with transformer and LCC converter) and filters.
The AC collector system is typically rated at 33 kV, as in the HVAC wind farms. This medium
voltage is stepped-up to the necessary transmission line voltage in the offshore transformers. Usually
the transformers are star-star and delta-star connected to power the 12-pulse converter, as this
configuration cancels several harmonics. The design of the transformers is critical as their insulation
must withstand the AC component of the voltage and the DC component from the thyristor valves.
Tappings are also included for proper system control.
The LCC power converters (onshore and offshore) are the most important elements in the
system, as they perform the AC/DC conversion offshore and the DC/AC conversion onshore. The LCC
converters are based in thyristor valves, capable of standing 8 kV and DC currents up to 4 kA. With
these characteristics it is possible to convert up to 1000 MW for land connections and 500 MW for
submarine transmissions [14]. In the LCC converter the current is always lagging the voltage due to
the control angle of the thyristors; hence these converters consume reactive power. For this reason,
reactive power compensation is necessary at both ends to provide reactive power to the system.
27
Capacitor banks or STATCOM devices are considered for this effect. Detailed explanation of the
functioning of the thyristor converter is provided in 3.3.4.
A line-commutated converter requires an AC voltage source for its commutation, so an auxiliary
power set is required at the offshore station to provide the necessary commutation voltage for the
HVDC-LCC connection, when there is little or no wind. The auxiliary power set is also used to supply
power to other devices in the wind farm, when the wind farm is disconnected from the grid.
The AC filters mentioned are used to absorb harmonic currents generated by the HVDC
converters, thus reducing the impact of the harmonics on the AC system. These filters also supply
reactive power to the converter station. The DC filters are used to avoid the generation of circulating
AC currents in the cable.
The smoothing reactors are large inductances connected in series with each pole of the DC link.
They are used to prevent current interruption at minimum load, limit the DC fault current, reduce
voltage and current harmonics and also prevent resonance in the DC circuit.
The DC submarine cable is approached in detail in section 3.3.3.
3.3.3. DC Submarine Cables
The submarine cable is one of the most important components of the HVDC-LCC system, since it
is the link for power transmission from the offshore wind farm.
The elements comprising a DC cable are the same as the ones in an AC cables. As so, like the
HVAC cables, the HVDC cable main technologies available differ in the electrical insulation used:
• Oil Filled (OF) cables;
• Mass Impregnated (MI) cables;
• Cross-linked Polyethylene (XLPE) cables.
These cables are represented in Figure 18.
28
Figure 18 – Available HVDC Cables: a) OF cable; b) MI cable; c) XLPE cable [22].
In Oil Filled cables (also known as SCFF – Self Contained Fluid Filled) the insulation consists of
paper impregnated with low-viscosity oil. The core of the cable is covered by a hollow shaft where oil
is circulated by pumps at both ends of the line. Oil Filled cables can be used to DC voltages up to 600
kV and great sea depths. The need for pumping at each end of the system may prove difficult, and
may limit significantly the maximum length of the cable to less than 50 km [14]. The danger of oil spill
and the need for cable protection are also obvious disadvantages of the Oil Filled cable technology.
Mass Impregnated cables have similar construction but the paper insulation is impregnated in
resin and high viscosity oil and no oil circulation system is required. It is the most used cable
technology in existing HVDC systems and so the track record and high reliability are some of the
advantages of the MI cable.
As can be seen in Figure 19, the MI cable consists of different layers that include, in addition to
the conductor and the paper insulation, layers used for mechanical protection of the copper conductor:
the polyethylene and lead sheaths, jackets and steel armour.
Figure 19 – Mass Impregnated HVDC Cable [22].
29
The MI cable technology is available for voltages up to 500 kV and transmission power of up to
800 MW. The capacity of the cable is only limited by the conductor temperature of about 50ºC.
In XLPE cables the insulating material is made of solid dielectric, also known as extruded
dielectric. It is a relatively new technology, developed to overcome some of the limitations of the
previously referred technologies. XLPE cables have all the advantages of the MI cables and
additionally can carry nominal current with a cable temperature of 90ºC. Increased bending capability,
higher mechanical resistance and lower weight are also advantages of the XLPE cables, since the
installation process is easier than for other cables.
3.3.4. Converter Technology
HVDC-LCC employs line commutated, current source converters, with thyristor valves. The use
of the thyristor grants this HVDC technology a limited operating flexibility, since the lack of turn-off
controllability of the conventional thyristor results in poor power factors and considerable waveform
distortion. Thus, some reactive power compensation and filtering must be provided.
3.3.4.1. 6-pulse Bridge Converter
The basic building block used for HVDC conversion is the three-phase, full-wave bridge referred
to as a 6-pulse or Graetz bridge, as represented in Figure 20. The term 6-pulse is due to six
commutations or switching operations per period resulting in a characteristic harmonic ripple of 6
times the fundamental frequency in the dc output voltage. Each 6-pulse bridge is comprised of 6
controlled switching elements or thyristor valves. Two thyristors are simultaneously conducting at each
instant.
Figure 20 – Six-pulse Converter Bridge [23].
The following analysis is done for the rectifier mode of operation, but similar considerations can
be made about the inverter mode. In order to conduct, the thyristor valve must be positively biased
30
(with a positive voltage between the anode and the cathode) and also a pulse has to be applied to the
gate. Therefore, the current will only flow through the valve once the firing pulse has been applied.
This enables the possibility of controlling the instant when current begins to flow through a valve, or to
commutate from one valve to another, by postponing the firing pulse. The angle between the instant at
which the thyristor valve becomes forward biased and the start of commutation (given by the firing
pulse) is referred to as the firing delay angle, α. The method of delaying the firing instant allows the
change in the average value of the DC voltage (in Figure 20) and, consequently, of the transmitted
power. As so, the average output voltage of the converter, working as a rectifier, is given by equation
(5) [24].
=3√6
∙ ∙ cos − ! ∙ (5)
Where:
• is the DC output voltage;
• is the line-to-neutral rms commutating voltage referred to the secondary of the
converter transformer;
• is the firing angle;
• ! is the commutating resistance;
• is the output DC current.
Note that the equation includes the commutation resistance that accounts for the internal voltage
drop in the converter due to commutation. The commutation delay accounts for the fact that the
commutating operation is not instantaneous, and so there is a small interval where three valves are
conducting at a time. The angle associated with this delay in commutation is called commutation
angle, µ. The value of µ is generally in the range of 15º to 25º [24].
The power factor of the converter can be proved (in [24]) to be given by equation (6):
" =3
∙ cos (6)
Therefore, it is of great interest to use a reduced value of firing delay angle, in order to have a
high power factor. The value of α is usually between 10º and 20º [24]. As the firing delay angle
increases, the converter absorbs more reactive power from the AC grid.
The inverter performs the DC/AC operation and is located at the onshore converter station.
The analysis carried out for the rectification process is equally applicable to the inverter operation,
although the inverter equations are expressed in terms of the advance angle β (β =180º-α) and the
extinction angle γ (γ= β - α). The advance angle defines the moment at which the valves of the inverter
31
start conducting and the extinction angle refers to the interval between the end of commutation and
the next zero crossing of the commutating voltage.
3.3.4.2. 12-Pulse Bridge Converter
The series-connected double bridge converter has become the standard configuration in HVDC
transmission. This converter is represented in Figure 21.
Figure 21 – Diagram of a 12-pulse converter [23].
The 12-pulse converter is a series-connection of two 6-pulse converter bridges and it requires two
3-phase transformers, one with a star-star and the other with star-delta (i.e. with 30º phase shift
between them).
This arrangement has some advantages when compared to the 6-pulse bridge converter, namely
the fact that the converter injects harmonic currents of orders n=12k±1 into the AC system, and so no
filtering of lower order harmonics, like the 5th and 7th is necessary (like in the 6-pulse converter) [24].
The alternate voltage supplied by the AC network is also more sinusoidal, meaning it has lower
harmonic content, requiring less filtering. The voltage on the DC side repeats itself 12 times during the
cycle, making it closer to the ideal continuous voltage. In the 6-pulse converter the voltage repeats
itself six times during a period at AC grid frequency.
3.3.4.3. HVDC Schemes
There are two basic configurations used in HVDC systems: the monopole and the bipolar
configurations, shown in Figure 22.
32
Figure 22 – HVDC configurations: a) Monopole; b) Bipolar.
In the monopole configuration one single DC cable is used. The return path is assured by sea
electrodes. Alternatively, due to high earth resistivity, metallic structures in the proximity or
environmental constraints, a metallic conductor may be used, either in the form of a low voltage AC or
DC cable or integrated in the cable itself.
In the bipolar scheme, two DC cables with opposite polarities are used and so the return path is
guaranteed. This configuration is the most common in HVDC transmission, since it offers some
advantages over the monopole option, namely higher transmission capacity and higher availability,
since during maintenance or outages of one pole, it is still possible to transmit part of the power. More
than 50% of the transmission capacity can be used, limited only by the overload capacity of the pole
[23].
3.3.5. HVDC-LCC + STATCOM
As referred previously, in a LCC transmission systems both converters absorb reactive power, as
in the converters the current always lags the line voltage due to the requirements for a positive
commutation voltage at the firing of the thyristors. Thus, this type of converter needs reactive power
for operation, which has to be provided by reactive power devices connected to the AC network. As
these converters depend on the line voltage for commutation, a minor disturbance in the AC voltage
might also result in commutation failures in the converters.
Capacitor banks or synchronous compensators may be used for reactive compensation at both
ends of the DC link. A more advanced solution consists of a STATCOM. The STATCOM provides the
33
necessary commutation voltage to the HVDC converter, continuous AC voltage control, fast reactive
power compensation to the network under transient conditions and removal of possible non-
characteristic harmonic interactions [18]. The STATCOM can also provide limited active power support
to the network during transient conditions, such as active power changes of the wind farm output.
However, the ability to provide active power support depends on the energy storage on the DC side,
which in the case of a conventional DC capacitor will be very limited. Larger energy storage could be
provided by means of batteries or SMES (Superconductive Magnetic Energy Storage).
The HVDC-LCC transmission system may benefit significantly from the STATCOM technology
since it can, in steady state, provide the reactive power consumed by the converter. Offshore, the
STATCOM can also work as an auxiliary power set. In AC grid fault conditions, the STATCOM can
provide reactive power to the network, thus contributing to a less severe voltage dip off the onshore
bus [12]. The working principles of a STATCOM device are mentioned in section 3.2.4.1.
34
3.4. HVDC-VSC Transmission
HVDC with voltage source converters is a fairly recent technology (first installed in 1999) and it is
known commercially as “HVDC Light”, for ABB and “HVDC Plus”, for Siemens. The basis of the
HVDC-VSC technology is the self commutated converters, with IGBT (Insulated Gate Bipolar
Transistor) valves. Figure 23 depicts a general layout of an HVDC-VSC system applied to an offshore
wind farm.
Figure 23 – Basic Configuration of a wind farm using an HVDC-VSC [11].
VSC converters are self-commutating, not requiring an external voltage source for its operation.
Also, the reactive power flow can be independently controlled at each AC network and the reactive
power control is independent of active power control. These features make VSC transmission an
interesting option for connection of offshore wind farms. In addition to the referred HVDC advantages,
the VSC technology also offers the following main benefits [11] [14]:
• Total control of active and reactive power;
• Minimum risk of commutation failures;
• Smaller size of converters and filters than HVDC-LCC;
• Possibility of the converters starting with a dead grid, not needing any start-up
mechanism (“Black-start” capability).
However, there are some constraints to the use of VSC technology. The main constraint is the
considerably lower experience in HVDC transmission, compared with the LCC option. HVDC-LCC has
more than 30 years of service experience, with high availability rates.
Also, VSC transmission has higher system power losses, when compared with an LCC system.
Typically, the power loss at full load for the converters is about 4-6% for VSC and 2-3% for LCC
transmission [11]. The IGBT valves are also much more expensive than the power thyristors used in
LCC technology.
Apart from these disadvantages, some of which easily surpassed once VSC links become
widespread and further experience is gained, the voltage source converter technology is a promising
option for HVDC in years to come.
35
4. PSS/E Modelling
This chapter describes the models used in the PSS/E software for the simulation of the
alternatives for grid connection of an offshore wind farm. The simulations performed in this software
are also described.
4.1. Introduction
In order to simulate the behaviour of an offshore wind farm and of the required transmission
system, appropriate models of the wind farm, the transmission system (either HVAC or HVDC) and
the electrical grid have to be constructed. Only then is it possible to analyse the steady-state and the
transient behaviour of the power system.
The software used to model the system and perform the simulations in the work of this thesis is
the PSS/E software. PSS/E stands for Power System Simulator/Engineering and it is a software tool
provided by Siemens Power Technologies International (PTI). It is used by most utilities in the world to
perform power system simulations, as it allows the performance of power flow analysis, dynamic
simulations and stability studies, among other features. PSS/E is composed of a comprehensive set of
programs for studies of power system transmission network performance in both steady-state and
dynamic conditions, obvious reasons for its widespread use by transmission and distribution systems
operators.
In the particular case of this thesis, the PSS/E version 30 was used for the features it offers for
network simulation: the extensive library of power system components, including wind turbine, DC
links and FACTS are extremely useful for the simulations performed in the scope of the thesis. The
wide range of possibilities available in PSS/E modelling increases significantly the complexity of the
simulation but also add to the interest of this software for network modelling.
4.2. Grid Structure
The electrical grid that will be used for steady-state and transient analysis – and in which the
offshore wind farm will be integrated – is the “savnw” network, provided by PSS/E as an example of a
relatively large grid, as it has 23 buses, 6 generators and 7 loads. In order to properly use this grid, a
few changes were made, namely to the operating frequency and voltage levels. These changes were
made taking into account values used in the European electrical grid.
Additional changes to the original “savnw” network were performed since the offshore wind farm
added (rated at 110MW) will be placed in a bus where the existing generation is of 750 MW.
Therefore, adjustments in active and reactive power of the network are required, which can be made
by adapting the power of the loads and the shunt compensators.
As so, the changes made to the original network can be summarised:
36
• The frequency was changed to 50 Hz (the frequency in use in most parts of the world,
including Portugal) since the frequency of original example grid is 60 Hz (typical of
American grids);
• The voltage levels were changed from 500 kV, 230 kV and 22 kV to, respectively, 400
kV, 220 kV and 33 kV. These voltage levels are common in most European Grids,
including Portugal;
• The 110 MW offshore wind farm was added, replacing an existing power plant of 750
MW;
• The inductive shunt compensators were removed;
• The parameters of the 400 kV lines were adapted;
In order to accommodate for the changes reported above, some network parameters have to be
adapted, which are described in the next sections. Most of the parameters of the grid are presented in
detail in Appendix A.
4.2.1. Lines
As a result of changing the grid frequency to 50 Hz, the reactance and the susceptance of the
branches have to be converted, since these parameters depend on the grid frequency. This
conversion is done using Equations (7) and (8).
#$%&' = #(%&' ×$%&'
(%&' (7)
*$%&' = *(%&' ×$%&'
(%&'
(8)
The line parameters (except those of the 400 kV lines) used in the test grid are presented in
Table 10, in Appendix A.
4.2.1.1. 400 kV Lines
The parameters of the 400 kV lines were changed to values of 400 kV lines existing in the
Portuguese grid. This decision was made upon studying the values of the existing 400 kV line
parameters and verifying these resemble the ones of a power cable, since significant amounts of
reactive power were being produced by the line. This issue does not occur in the 220 kV and 33 kV
lines.
Considering the base power +, = 100 / and the base voltage , = 400 1 , the base
impedance is 2, =34
5
64= 1600 Ω.
37
The line parameters: resistance, reactance and susceptance in p.u. can, respectively, be
calculated with Equations (9), (10) and (11).
!8.:. =!;< × =
2, (9)
#8.:. =#;< × =
2, (10)
*8.:. = (*;< × =) × 2, (11)
Where:
• @AB is the line resistance in Ω/1D;
• EAB is the line reactance in Ω/1D;
• FAB is the line susceptance in S/1D;
• H is the line length in 1D;
• IJ is the base impedance in Ω;
The values for the 400 kV line parameters calculated through the equations above are shown in
Table 11, in Appendix A.
4.2.2. Transformers
For the two-winding transformers in the “savnw” grid, the transformer reactance is frequency
dependent, so their values also have to be adapted to the 50 Hz frequency, according to Equation (7).
Tap changing was used in the network transformers, changing the voltage magnitude of the
transformers, and, thus regulating properly the reactive power flow.
The values of the results of the calculations for the transformers in the grid are shown in Table
12, in Appendix A.
4.2.3. Loads and Shunt Compensators
As mentioned previously, adding the offshore wind farm rated at 110 MW to bus 102 of the grid in
the place of the existing 750 MW conventional power plant leads to an imbalance of active and
reactive powers. This imbalance can be compensated by reducing the consumed power in the loads.
As so, the values of the load powers of the grid are presented in Table 13, in Appendix A.
The shunt compensators are also regulated in order to maintain the voltage in the buses at
values within reasonable values (between 0,95 p.u. and 1,05 p.u.). Table 14 (in Appendix A) outlines
the values of the shunt compensators of the grid.
38
4.2.4. Diagram of the Grid
The resulting grid, after the changes performed, is presented in the single-line diagram shown in
Figure 24, in which the offshore wind farm is already included. Bus 102, referred above, is marked in
the figure. The legend of the colours used in the figure for each voltage value is presented in Table 1.
Figure 24 – Single line diagram of the test grid used (with HVAC transmission, compensations at both ends).
39
Table 1: Colour correspondence for each voltage level in Figure 24.
Colour Rated Voltage (kV)
Red 400
Black 220
Purple 150
Dark Green 33
Blue 20
Orange 13,8
Bordeaux Red 0,7
40
4.3. Offshore Wind Farm Model
The model for the offshore wind farm is based in an existing installation: the Lillgrund Wind Farm.
This wind farm is located off the coast of Sweden, at a distance of 9 km from the Point of Connection
in the onshore grid and the transmission is achieved by a combination of an AC sea cable (7 km long)
and an AC land cable (2 km long). With 48 wind turbines, rated at 2,3 MW each, the total capacity of
the wind farm is 110 MW. Figure 25 depicts the layout of the wind turbines in the Lillgrund wind farm,
used as a reference.
Figure 25 – Layout of the wind turbines in the Lillgrund wind farm [25].
As can be seen in Figure 25, the internal grid of the Lillgrund wind farm consists of 33 kV sea
cables divided in five feeders and each of these feeders connects 9 or 10 wind turbines to the offshore
substation. The total is 48 wind turbines rated at 2,3 MW.
The modelled offshore wind farm is based on this layout. The turbines are joined in aggregates
and then connected to the offshore substation bus via 33 kV AC cables.
41
4.3.1. Wind Turbines
The wind turbines used are the GE 1,5 MW model, available in the PSS/E Wind package. This
model is of a DFIG (Doubly Fed Induction Generator) wind turbine developed by General Electric and
released for PSS/E simulation and testing.
In order to match the 110 MW of the reference wind farm, a total of 74 1,5 MW wind turbines
were used in the model. These are joined in five aggregates: two aggregates of 24 MW (16 wind
turbines for each aggregate) and three of 21 MW (14 wind turbines per aggregate). This actually adds
up to 111 MW, which is the value assumed hereby for the offshore wind farm power.
For PSS/E simulation of the wind farm, two distinct models are designed for the wind turbines: the
steady-state model (which allows the power flow simulations) and the transient model (used for
dynamic simulations).
4.3.1.1. Steady-State Model
The load flow provides initial conditions for dynamic simulations. In the Load Flow parameters of
the PSS/E, the wind turbines were modelled as five conventional generators, rated at 24 MW and 21
MW. The values specified on the existing generator record are outlined in Table 15, in Appendix A.
Note that the values in Table 15 were calculated considering the values in the Individual WTG Power
Flow Data of the PSS/E Wind User Guide [26] and multiplying by the number of lumped elements, as
recommended by the PSS/E guide.
Since the model used is of a DFIG wind turbine, both the active and reactive power can be
controlled. As so, for the wind turbine aggregates, a fixed power factor of 0,9 (injecting reactive power
in the grid) was chosen, resulting in a value for Qgen, which is 11,62 Mvar for the 24 MW aggregates
and 10,16 Mvar for the 21 MW aggregates.
The values calculated by this manner were used in the HVDC connection alternative. However,
for the HVAC transmission system, the wind turbines were regulated for a unit power factor, which
means no reactive power is generated by the wind turbines. The reason is that the AC cable already
produces a significant amount of reactive power, so an additional quantity of reactive power generated
by wind farm would deteriorate the system behaviour.
4.3.1.2. Transient Model
The transient model of the GE 1,5 MW wind turbine comprises nine models for each of the five
aggregates. These models are an accurate and complete representation of the behaviour of the GE
Wind turbine, including, for example, the simulation of wind gusts and ramps. Table 2 indicates the
models used and their description.
42
Table 2: Components of the GE 1,5 MW Wind Turbine Model in PSS/E.
Model Code Description
GEWTG1 GE Wind Turbine Generator/Converter
GEWTE1 GE Wind Turbine Electrical Control
GEWTT Two Mass Shaft
WGUSTC Wind Gust and Ramp
GEWTA GE Wind Turbine Aerodynamics
GEWTP GE Pitch Control
GEWTPT Plotting Output Variables as VARs
VTGTPA Under Voltage/Over Voltage Generator Bus Disconnection Relay
FRQTPA Under Frequency/Over Frequency Generator Bus Disconnection Relay
Values for the parameters of these models were based on typical values given in the PSS/E Wind
manual example [26]. The number of aggregated wind turbines was changed in the models to 14 for
the 21 MW aggregates and 16 for the 24 MW aggregates.
An example of the “.dyr” files used for one of the aggregates, where the parameters of each
models are written, is presented in Appendix B.
4.3.1.3. Fault Ride Through Capability
The generator model includes the Under/Over voltage protective functions (modelled by
VTGTPA). These functions are defined such that wind plants must not trip for events that are less
severe than the defined thresholds and time durations. So, the wind turbines can “ride through” a fault
that causes a voltage dip on the terminal voltage of the generator, as long as the voltage dip and
respective duration respect the curves in Figure 26.
43
Figure 26 – GE 1,5 MW voltage protection characteristics.
4.3.2. Step-Up Transformers
The 0,69/33 kV transformers that adapt the voltage at the generation buses (690 V) to the voltage
of the internal grid of the wind farm (33 kV) have the parameters presented in Table 16, in Appendix A.
As each aggregate contains 14 or 16 wind turbines, the rated power of the transformers is,
respectively, 24,5 MVA and 28 MVA.
4.3.3. 33 kV Cables
The cables used to link the five buses where the wind turbines are connected to the offshore bus
are 33 kV AC cables, XLPE insulated. The cable parameters are presented in Table 17, in Appendix
A.
Considering the base power +, = 100 / and the base voltage , = 33 1 , the base impedance
yields: 2, =34
5
64= 10,89 Ω. Therefore, using Equations (9), (10) and (11), the values in Table 17 – and
bearing in mind that * = × – the parameters for the medium voltage cables are presented in Table
18, in Appendix A. The lengths of these cables were attributed considering typical distances in
existing offshore wind farms, such as the Lillgrund wind farm.
4.3.4. Layout of the offshore wind farm
Figure 27 illustrates the offshore wind farm designed in PSS/E, where the wind turbines, the step-
up transformers and the 33 kV sea cables mentioned above are represented.
44
Figure 27 – PSS/E model of the offshore wind farm.
45
4.4. HVAC Transmission
The HVAC transmission is the simplest alternative for the transmission of the electrical power of
the offshore wind farm to the onshore grid. The main components are the submarine cable and the
two transformers: onshore and offshore. Figure 28 illustrates the HVAC transmission scheme, with the
AC cable and both transformers. Note that the 102 bus in Figure 28 corresponds to the 102 bus in
Figure 27. For proper compensation of the reactive power generated in the cable, shunt reactors (or a
STATCOM device) were applied to one or both ends of the cable. They are not, however, represented
and described in detail in this chapter, as they vary according to the case simulated.
Figure 28 – HVAC transmission layout in PSS/E.
4.4.1. AC Submarine Power Cable
The submarine power cable chosen is a 100 km 150 kV XLPE cable. 150 kV is a typical option for
offshore wind farms, used in, for example, the Horns Rev wind farm, in Denmark. Hence, using the
parameters of the submarine cable in Horns Rev, we have the following set of values: ! =
0,039 Ω 1D⁄ ; # = 0,12 Ω 1D⁄ ; = 0,19 P"/1D [10].
Considering the base power +, = 100 / and the base voltage , = 150 1 , the base
impedance is 2, =34
5
64= 225 Ω. So, using Equations (9), (10) and (11) the 150 kV cable parameters are
depicted in Table 3.
Table 3: 150 kV cable parameters.
Cable @AB
[S/AB] EAB
[S/AB] UAB
[VW/AB] Length
[AB] @X.Y. EX.Y. FX.Y. From
Bus To
Bus
20 21 0,039 0,12 0,19 100 0,0173 0,053 1,34235
46
4.4.2. Onshore and Offshore Transformers
The offshore transformer adapts the voltage from the 33 kV of the internal grid to the 150 kV of
the submarine cable. As for the onshore transformer, it is used to increase the voltage from the 150 kV
of the cable to the 400 kV of the onshore grid. Table 4 shows the offshore and onshore transformers
data.
Table 4: Onshore and Offshore transformer data [10].
Buses
Location
Rated Voltage [kV/kV]
Rated Voltage
Impedance [%]
Rating [MVA]
E [X. Y. ] Z[[ \]^ F_`aH
From Bus
To Bus
20 102 Offshore 33/150 13,8 160 0,08625
21 151 Onshore 150/400 15 200 0,075
47
4.5. HVDC-LCC Transmission
The Line Commutated Converter based HVDC technology requires a DC cable and two
converters, one onshore and another offshore. In PSS/E there are appropriate models for the
simulation of a transmission with HVDC-LCC for both steady-state and transient situations.
4.5.1. Steady-State Model
The steady-state model enables the power flow analysis of the DC Link and establishes the initial
values for dynamic simulations.
In PSS/E, the data requirements for the “Two-terminal DC Line” (as mentioned in [27]) fall into
three groups: control parameters/set points, converter transformers and DC line characteristics. In the
Load Flow Module of PSS/E 30, the two tabs refer to “DC Line” and the respective “Converters”:
Rectifier and Inverter.
Graphically, the DC line is represented by Figure 29, where the converter is represented by a
diode-like symbol. Note that the rectifier and inverter models include the converter transformer,
despite these transformers not being visible.
Figure 29 – HVDC-LCC transmission model in PSS/E.
The following values were assumed:
bcdefg= 110 /h
bcdefg= 150 1
= 18°
j = 18°
P = 20°
The current in the DC line is given by: bcdefg=
klmnofg
3lmnofg= 733 A.
The model parameters were calculated considering [27] and [24]. Some of the equations used for
the rectifier and the inverter are expressed in Table 5.
Table 5: Equations for Rectifier and Inverter Parameters.
Parameter Rectifier Inverter
Converter Power Factor cos(r) =cos( ) + cos ( + P)
2 cos(r) =
cos(j) + cos (j + P)
2
48
Transformer Nominal Power +tuvd =
bcdefg
cos(r)
AC Converter Voltage wxdyzg=
×
3√2
Converter Transformer Ratio
Du = xdyzg
|uczg
Dc = xdyzg
|uczg
Commutating Reactance #t[Ω] = #t[p. u. ] × 2,, with 2, =3gnzg
5
6n
Commutating Resistance ! =3
× #t
Using values calculated with the equations in Table 5, assuming some values recommended in
[27], the parameters used in the load flow model are reported in Table 19 and Table 20, in Appendix
A.
4.5.2. Transient Model
In order to simulate the dynamic behaviour of the HVDC link, the transient model used is the
“CDC4T”. The parameters of this model were calculated using the equations and the recommended
values in the PSS/E Manuals [28] and [29].
The parameters used in the “CDC4T” model are reported in Table 21, in Appendix A.
4.6. STATCOM
PSS/E provides appropriate models to simulate FACTS devices. A variety of such devices is
available for power flow and dynamic simulations, divided in either shunt devices (such is the case of
STATCOM), series devices and combined devices (with both series and shunt elements).
The STATCOM, as described in detail in Chapter 3 is a FACTS device, used in the network to
absorb or provide reactive power, thus providing voltage support to the network, in either steady-state
or transient situation.
In the work developed in this thesis, the STATCOM is used in the HVDC network, to inject
reactive power in the onshore bus, reducing the voltage dip originated by the fault in the AC grid. In
HVAC transmission, the STATCOM also provides this voltage support in case of a fault and,
additionally in steady-state, it is used for reactive power compensation of the AC cable.
ABB’s “PCS 6000 STATCOM double-outdoor” is used as a reference for the modelling of the
STATCOM device. It has a capacity of 2x32 Mvar.
49
4.6.1. Steady-State Model
For power flow analysis, the STATCOM is modelled as a FACTS device with the parameters
adjusted as to simulate the behaviour of this device. In both cases of the use of the STATCOM (in
HVAC and HVDC transmission) it was modelled with the same parameters.
Figure 30 is the graphic representation of a STATCOM in PSS/E.
Figure 30 – STATCOM device in PSS/E.
The parameters presented in Table 22 (Appendix A) are the most important values used in power
flow analysis of the STATCOM and they are obtained using PSS/E Manual [27] for FACTS devices.
4.6.2. Transient Model
The dynamic behaviour of the STATCOM is simulated using the “CSTCNT” model. The
parameters of this model were calculated using the equations and the recommended values in the
PSS/E Manuals [28] and [29].
The values of the “CSTCNT” model are presented in Table 23, in Appendix A.
50
5. Power Flow Results
Chapter 5 presents the results of the steady-state simulations performed on the grid with the
offshore wind farm, as described in Chapter 4. HVAC and HVDC-LCC transmission are analysed.
Discussion of the results is also performed.
5.1. Introduction
The power flow analysis carried out comprehends numerical calculations of active and reactive
power flows and node voltages. PSS/E software is used, in the scope of this thesis, for the power flow
analysis of transmission power alternatives from offshore wind farms. Special attention is given to the
Point of Common Coupling (hereby designated as PCC), i.e., the point of connection of the wind farm
with the remaining grid. The voltage at the PCC, the active power injected and the reactive power
injected/absorbed in the PCC is analysed as a part of the power flow study. Power losses for each
transmission system are also assessed.
The power flow results are presented by the single-line diagrams of the network (from the PSS/E
load flow software). The active and reactive flows at each end of the branches and the voltage
magnitude and angle at each bus are depicted in each figure of this chapter. Figure 31 depicts how
each of the values from the power flow calculations are represented in the power flow result figures.
Figure 31 – Legend of the values presented in the single-line diagrams.
5.2. HVAC Power Flow
For the HVAC transmission system, as presented in Figure 28, the power flow analysis focuses
on the reactive power flow in the submarine 100 km cable. Therefore, with the objective of
compensating the reactive power generated in the cable, some reactive power compensation options
are studied. The alternatives considered are reviewed in Table 6.
51
Table 6: Reactive power compensation alternatives.
Designation Onshore (Mvar) Offshore(Mvar)
Offshore shunt reactor 0 60
Onshore shunt reactor 80 0
Onshore and offshore shunt reactors 40 40
Offshore STATCOM 0 64
The main criteria for the choice of the value of the shunt reactors (defined in terms of reactive
power injected, in Mvar) is the power factor at the PCC, which is chosen to be approximately of 0,9, a
typical value in grid integration of wind farms [11]. As so, the shunt reactor chosen for offshore
compensation only absorbs 60 Mvar, as the 0,9 power factor at the PCC is guaranteed with this value.
An estimate of the reactive power produced by the 150 kV AC cable (data on Table 3) can be
made taking into account Equation (3). The approximate amount of reactive power produced by the
100 km cable is given by (12):
= × × × = 2 × 50 × 0,19 × 10( × 100 × (150 × 10) = 134,3 / (12)
So, for the HVAC transmission, approximately 60 % of this value of reactive power is
compensated by the schemes presented in Table 6.
Note that, for HVAC power flow, the wind turbine generators supply no reactive power, since
there is already an excess of reactive power, as a consequence of the shunt capacitance of the AC
cable. Therefore, the capability of the DFIG machines of providing voltage support to the grid, by
supplying reactive power, is not considered for the present study.
For the first case analysed (offshore compensation), the presented grid includes buses 3004, 201
and 211, closest to the wind farm. For the following power flow results, however, the results relative to
these buses are omitted as they are very similar to the results for the offshore compensation only
transmission.
52
5.2.1. Offshore Compensation
In Figure 32 the results of the grid connected by an AC cable with compensation offshore are
shown. The 60 Mvar shunt reactor (marked in the figure with the orange rectangle) absorbs 64,6 Mvar
of reactive power, given the dependency of the reactive power of the voltage at the bus (as expressed
by equation (4)).
All the bus voltages near the AC link (buses 102, 20, 21 and 151) are within acceptable values
(i.e. below 1,05 p.u.) and the power factor at PCC is approximately 0,88.
Figure 32 – Power Flow result for HVAC with offshore compensation.
53
5.2.2. Onshore Compensation
The most important remark regarding Figure 33 is the fact that using onshore compensation only
leads to overvoltage on the offshore bus. Other values for the onshore reactor where experimented
but led to negligible improvements on the magnitude of the voltage of bus 20. The power factor at the
PCC is, however, acceptable, since the shunt reactor (marked in orange rectangle on the figure)
absorbs a large portion of the reactive power produced at the cable: 83,4 of the 128,3 Mvar produced
by the cable.
Figure 33 – Power Flow result for HVAC with onshore compensation.
54
5.2.3. Offshore and Onshore Compensation
Figure 34 shows the power flow result for a scheme of compensating reactive power at both ends
of the cable by two 40 Mvar reactors. The compensators are marked in orange on the figure. No
overvoltages occur in any bus, onshore or offshore.
Figure 34 – Power Flow result for HVAC with offshore and onshore compensation.
55
5.2.4. STATCOM Compensation
Figure 35 presents the results for the transmission using an offshore STATCOM. The results are
very similar to the use of offshore compensation with a shunt compensator, since the amount of
reactive power consumed by the STATCOM and the reactor are identical. The STATCOM is shown
inside the orange rectangle on the figure.
Figure 35 – Power Flow result for HVAC with STATCOM compensation.
56
5.2.5. Discussion
The compensation alternatives studied bare some similarities but also important differences
between them.
All the schemes were designed with the purpose of obtaining a power factor of approximately 0,9
(injecting reactive power in the grid) at the PCC. Issues like reactive power circulation, bus voltages,
power rating of the cable and power losses are analysed.
Table 7 shows the results obtained for the power factor at the PCC, the voltages at each end of
the cable and the active and reactive power at each end of the cable for the four configurations
studied. The values are taken from the power results presented above.
Table 7: HVAC selected results for different compensation alternatives.
Power Factor at the PCC
V onshore bus (p.u.)
V offshore bus (p.u.)
P at the offshore end (MW)
P at the onshore
end (MW)
Q at the offshore
end(Mvar)
Q at the onshore
end(Mvar)
Offshore 0,88 1,023 1,038 110,8 108,8 76,2 60,3
Onshore 0,93 1,021 1,070 110,8 108,3 10,8 128,3
Offshore and Onshore
0,94 1,020 1,046 110,8 108,8 55,1 82,1
STATCOM offshore
0,88 1,023 1,037 110,8 108,8 77,9 58,4
Compensating onshore only is the option that leads to less favourable results. The excess of
reactive power at the onshore bus leads to an excessive voltage magnitude at bus 20, as seen in
Figure 33. The influence of bus voltage amplitude on the transmission of reactive power is thus
demonstrated by the results. Another important issue to consider is the power rating of the AC cable.
In the onshore compensation scheme the power transmitted by the cable reaches its peak near the
onshore bus. This may lead to the overloading of the cable, a direct consequence of the significantly
high amount of reactive power transmitted through the cable.
The other compensation alternatives present more satisfactory results. As far as the power rating
of the cable is concerned, compensating offshore only leads to a more evenly distributed loading of
the cable (as presented in Figure 16) as similar amounts of reactive power flow to each end of the
cable. The bus voltages at each end of the cable (bus 20 offshore and bus 21 onshore) are below 1,05
p.u. (as can be seen in Table 7) so no overvoltage exists, since the excess of reactive power at the
offshore bus no longer exists. Therefore, compensation offshore is proven to be necessary, in order to
maintain the stability of the bus voltages. It is, however, important to notice that the installation of
compensators offshore is likely to be more expensive and technically challenging than the placement
of such devices at the onshore substation.
The STATCOM compensation presents very similar results to the compensation offshore using a
shunt reactor. This option was introduced since the STATCOM device may offer benefits for the
system in its dynamic behaviour, as approached in Chapter 6.
57
The power losses of the system are the same for all compensation schemes, since the devices
used for compensation are considered as being lossless. The active power generated by the wind
farm is 111 MW and the active power at the PCC is 108,8 MW. Therefore, the losses for the HVAC
transmission can be calculated by:
= − kww
=
111 − 108,8
111= 1,98%
In Table 7, it is obvious that for any of the cases studied the power factor achieved is close to the
0,9 power factor, set as an objective of the compensation. Without compensation of any sort, the
reactive power injected in the grid would be very high, leading to a much deteriorated power factor at
the PCC. The bus voltages would also be above 1,05 p.u., due to the excess of reactive power.
The solution considered to be the most effective and cost-efficient, given the smaller size of the
compensators is the offshore and onshore compensation. This configuration is considered for the
dynamic simulations in Chapter 6.
58
5.3. HVDC Power Flow
The HVDC power flow analysis is carried out for two configurations: one without STATCOM and
another with a STATCOM at the onshore bus (bus 151). Note that for the studies with HVDC
transmission the wind turbines are working with a 0,9 power factor (injecting reactive power in the
grid). The objective is once again to guarantee an approximate 0,9 power factor at the PCC.
The active and reactive power flows and the voltages at the buses at each end of the link are
compared for these two HVDC transmission options and also taking into account the HVAC
transmission power flow results.
5.3.1. HVDC Configuration
Figure 36 depicts the single-line diagram with the power flow results for the HVDC transmission.
Figure 36 – Power Flow result for HVDC-LCC transmission.
59
5.3.2. HVDC+STATCOM Configuration
Figure 37 shows the power flow results for the HVDC+STATCOM alternative. The STATCOM can
be seen at bus 151, injecting reactive power in steady-state. The introduction of the STATCOM in the
system influences the power flows at the nearby lines.
Figure 37 – Power Flow result for HVDC+STATCOM transmission.
60
5.3.3. Discussion
Comparing with the HVAC transmission, the HVDC configuration holds no reactive power issues,
since these issues yield from the shunt capacitance of the AC cable, inexistent in HVDC.
Table 8 presents some results for the two HVDC configurations studied, including power factor at
the PCC, voltage magnitudes on both ends of the transmission scheme and active and reactive
powers circulating on both ends of the submarine cable.
Table 8: HVDC-LCC and HVDC-LCC+STATCOM selected results.
Power Factor at the PCC
V onshore
bus (p.u.)
V offshore
bus (p.u.)
P at the offshore end (MW)
P at the onshore
end (MW)
Q at the offshore
end(Mvar)
Q at the onshore
end(Mvar)
HVDC-LCC 0,89 1,012 0,978 110 109 44 54
HVDC-LCC+STATCOM 0,89 1,016 0,978 110 109 44 55
From Table 8 it can be noted that the two configurations bare similarities. Offshore, as the wind
farm is the same, the voltage magnitude and the active and reactive powers transmitted in the cable
are equal. Onshore, the influence of the STATCOM – although slight – can be noted, as the voltage is
higher, due to the injection of reactive power by the STATCOM.
As expected, the HVDC-LCC converters consume reactive power, which is a consequence of
having lagging current. Therefore, as can be seen in Table 8, the rectifier (located offshore) consumes
44 Mvar and the inverter (onshore) consumes 54 Mvar and 55 Mvar, for the HVDC and for the HVDC-
STATCOM transmission, respectively. As so, the power factor at the PCC is, for both cases, 0,89
(consuming reactive power).
The use of a STATCOM is justified in steady-state for the injection of reactive power in bus 151,
in order to compensate the reactive power absorbed by the inverter. So, in the power flow analysis,
the STATCOM is working as a capacitor bank, injecting 65 Mvar – its maximum value for the bus
voltage at bus 151. In this case, the use of the STATCOM changes the reactive power that flows to
and from the lines connected to bus 151, because of the behaviour of the LCC converter (consuming
reactive power). For the purpose of this study, in steady-state, capacitor banks could have also been
used but the STATCOM is chosen as to also provide support to the grid in transient situations.
The power losses in the HVDC transmission are very similar to the ones in HVAC (around 2%),
as the generated power is the same (111 MW) and the power at the PCC is similar (109 MW). So the
power losses are given by:
=111 − 109
111= 1,8%
In HVDC, the losses are originated in the DC line (characterized by a resistance). The line
reactance and capacitance concepts do not exist and so neither does reactive power in the link.
61
6. Dynamic Results
Chapter 6 presents the results of the dynamic simulations performed on the grid under analysis.
The response of the HVAC and HVDC-LCC transmission systems to an AC grid fault is analysed and
results are discussed.
6.1. Introduction
The analysis of the dynamic behaviour of the offshore wind farm comprehends the response of
the wind farm to voltage and frequency disturbances in the grid.
The study of this work is focused on the response of both the offshore wind farm and the onshore
grid to a three-phase fault in an onshore bus, bus 3005, as marked in Figure 38.
Figure 38 – Single line diagram of the grid used for the dynamic simulations. Note: Bus 3005, where the fault occurs is marked in the orange rectangle.
62
The behaviour of some parameters of the grid, namely the voltage and the frequency in each bus,
during and immediately following the disturbance are analysed for the two transmission systems under
analysis: HVAC and HVDC-LCC.
The parameters of the grid that are presented in the results, in 6.2 and 6.3, are the frequency and
voltage variation in the offshore and onshore buses and the speed variation of the offshore and
onshore machines. For the HVDC transmission the active and reactive power in the rectifier and
inverter are also presented.
The analysis is carried out for the two transmission alternatives and for different fault types, with
the objective of assessing the fault ride through capability of the offshore wind farm, i.e. the
requirement for the wind farm to stay connected to the grid during the disturbance, thus contributing to
the reestablishment of the normal operation. The fault ride through capability of the wind turbines in
the offshore wind farm is guaranteed by the under/over voltage disconnection relays of the wind
turbine generators. These devices allow the operation of the wind turbines even when the terminal
voltage decreases. This capacity of “riding through” a fault is limited to defined voltage dips and fault
durations. So, and according to the implemented fault ride through characteristic (as defined in Figure
26), the wind turbine will only trip if the fault that occurs across the terminals of the machine are
outside the defined limits.
6.2. HVAC Dynamic Behaviour
In order to evaluate the behaviour of the offshore wind farm connected to the onshore by an
HVAC link, different dynamic cases are analysed, as presented in Table 9.
Table 9: HVAC Dynamic Cases Analysed.
Case Fault Duration
(ms) Fault Severity
Reactive Compensation Scheme
Case 1 100 Moderate Shunt reactors at both ends
Case 2 300 Moderate Shunt reactors at both ends
Case 3 100 Severe Shunt reactors at both ends
Case 4 100 Severe STATCOM offshore
Cases 1 and 2 are two different short-circuits with similar severity but different clearance times,
simulating two different types of protections: fast and slow operating. Case 3 represents a more
severe short-circuit, with a higher short-circuit power. All these three cases are studied with the
compensation at both ends of the cable (as proposed in 5.2.3). Case 4 has the same conditions as
Case 3, except for the fact that the compensation is assured by a STATCOM placed offshore and
possible benefits of this device are studied.
The results for the outlined cases 1 to 4 are presented in 6.2.1 to 6.2.4, in the proper order. In
6.2.5 the most significant aspects regarding the results obtained are discussed.
63
6.2.1. Case 1 results
Figure 39 – Frequency variation of offshore buses for the Case
1 fault.
Figure 40 – Voltage variation of offshore buses for the Case 1
fault.
Figure 41 – Offshore wind turbine speed variation for the Case
1 fault.
Figure 42 – Frequency variation of onshore buses for the Case
1 fault.
Figure 43 – Voltage variation of onshore buses for the Case 1
fault.
Figure 44 – Speed variation of machines on the onshore grid
for the Case 1 fault.
0 0.5 1 1.5 2 2.5 3 3.5 449.98
50
50.02
50.04
50.06
50.08
50.1
50.12
50.14
Time [s]
Fre
qu
en
cy
[H
z]
Bus 90001
Bus 90002
Bus 90003
Bus 90004
Bus 90005
Bus 20
Bus 102
0 0.5 1 1.5 2 2.5 3 3.5 40.85
0.9
0.95
1
1.05
1.1
Time [s]
Vo
lta
ge
Ab
so
lute
Va
lue
[p
.u.]
Bus 90001
Bus 90002
Bus 90003
Bus 90004
Bus 90005
Bus 102
Bus 20
0 0.5 1 1.5 2 2.5 3 3.5 4
2999.7
2999.8
2999.9
3000
3000.1
3000.2
3000.3
3000.4
3000.5
Time [s]
Ma
ch
ine
Sp
ee
d [
rpm
]
Bus 90001
Bus 90002
Bus 90003
Bus 90004
Bus 90005
0 0.5 1 1.5 2 2,5 3 3.5 4 4.5 549.98
50
50.02
50.04
50.06
50.08
50.1
50.12
50.14
Time [s]
Fre
qu
en
cy [
Hz]
Bus 101
Bus 211
Bus 3011
Bus 3018
Bus 206
Bus 151
Bus 21
0 0.5 1 1.5 2 2.5 3 3.5 40.8
0.85
0.9
0.95
1
1.05
1.1
1.15
Time [s]
Vo
ltag
e A
bs
olu
te V
alu
e [
p.u
.]
Bus 101
Bus 211
Bus 3011
Bus 3018
Bus 206
Bus 151
Bus 21
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 52999
3000
3001
3002
3003
3004
3005
3006
3007
3008
3009
Time [s]
Mac
hin
e S
pe
ed
[rp
m]
Bus 101
Bus 211
Bus 3011
Bus 3018
Bus 206
64
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 52995
3000
3005
3010
3015
3020
Time [s]
Mach
ine S
peed
[rp
m]
Bus 101
Bus 211
Bus 3011
Bus 3018
Bus 206
6.2.2. Case 2 results
Figure 45 – Frequency variation of offshore buses for the Case
2 fault.
Figure 46 – Voltage variation of offshore buses for the Case 2
fault.
Figure 47 – Offshore wind turbine speed variation for the Case
2 fault.
Figure 48 – Frequency variation of onshore buses for the Case
2 fault.
Figure 49 – Voltage variation of onshore buses for the Case 2
fault.
Figure 50 – Speed variation of machines on the onshore grid for the Case 2 fault.
0 0.5 1 1.5 2 2.5 3 3.5 449.95
50
50.05
50.1
50.15
50.2
50.25
50.3
50.35
Time [s]
Fre
qu
en
cy
[H
z]
Bus 90001
Bus 90002
Bus 90003
Bus 90004
Bus 90005
Bus 20
Bus 102
0 0.5 1 1.5 2 2.5 3 3.5 40.85
0.9
0.95
1
1.05
1.1
1.15
1.2
1.25
Time [s]
Vo
ltag
e A
bso
lute
Valu
e [
p.u
.]
Bus 90001
Bus 90002
Bus 90003
Bus 90004
Bus 90005
Bus 102
Bus 20
0 0.5 1 1.5 2 2.5 3 3.5 4
2999.7
2999.8
2999.9
3000
3000.1
3000.2
3000.3
3000.4
3000.5
Time [s]
Mac
hin
e S
pee
d [
rpm
]
Bus 90001
Bus 90002
Bus 90003
Bus 90004
Bus 90005
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 549.95
50
50.05
50.1
50.15
50.2
50.25
50.3
50.35
Time [s]
Fre
qu
en
cy
[H
z]
Bus 101
Bus 211
Bus 3011
Bus 3018
Bus 206
Bus 151
Bus 21
0 0.5 1 1.5 2 2.5 3 3.5 40.8
0.85
0.9
0.95
1
1.05
1.1
1.15
1.2
Time [s]
Vo
lta
ge
Ab
so
lute
Va
lue
[p
.u.]
Bus 101
Bus 211
Bus 3011
Bus 3018
Bus 206
Bus 151
Bus 21
65
6.2.3. Case 3 results
Figure 51– Frequency variation of offshore buses for the Case 3
fault.
Figure 52– Voltage variation of offshore buses for the Case
3 fault.
Figure 53– Offshore wind turbine speed variation for the Case 3
fault.
Figure 54– Frequency variation of onshore buses for the
Case 3 fault.
Figure 55– Voltage variation of onshore buses for the Case 3
fault.
Figure 56– Speed variation of machines on the onshore grid
for the Case 3 fault.
0 0,5 1 1,5 2 2,5 3 3,5 4 4,5 549.8
49.9
50
50.1
50.2
50.3
50.4
50.5
50.6
Time [s]
Fre
qu
en
cy [
Hz]
Bus 90001
Bus 90002
Bus 90003
Bus 90004
Bus 90005
Bus 20
Bus 102
0 0,5 1 1,5 2 2,5 3 3,5 4 4,5 50.4
0.5
0.6
0.7
0.8
0.9
1
1.1
Time [s]
Vo
lta
ge A
bs
olu
te V
alu
e [
p.u
.]
Bus 90001
Bus 90002
Bus 90003
Bus 90004
Bus 90005
Bus 102
Bus 20
0 1 2 3 4 5 6 7 8 9 102970
2975
2980
2985
2990
2995
3000
3005
3010
3015
3020
Time [s]
Ma
ch
ine
Sp
ee
d [
rpm
]
Bus 90001
Bus 90002
Bus 90003
Bus 90004
Bus 90005
0 0,5 1 1,5 2 2,5 3 3,5 4 4,5 5 5,5 6
50
50.1
50.2
50.3
50.4
50.5
50.6
50.7
Time [s]
Fre
qu
en
cy
[H
z]
Bus 101
Bus 211
Bus 3011
Bus 3018
Bus 206
Bus 151
Bus 21
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5
0.4
0.5
0.6
0.7
0.8
0.9
1
1.1
1.2
1.3
Time [s]
Vo
ltag
e A
bso
lute
Valu
e [
p.u
.]
Bus 101
Bus 211
Bus 3011
Bus 3018
Bus 206
Bus 151
Bus 21
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 62995
3000
3005
3010
3015
3020
3025
3030
3035
Time [s]
Mach
ine S
peed
[rp
m]
Bus 101
Bus 211
Bus 3011
Bus 3018
Bus 206
66
6.2.4. Case 4 results
Figure 57– Frequency variation of offshore buses for the Case 4
fault.
Figure 58– Voltage variation of offshore buses for the Case 4
fault.
Figure 59– Offshore wind turbine speed variation for the Case 4
fault.
Figure 60– Frequency variation of onshore buses for the Case 4
fault.
Figure 61– Voltage variation of onshore buses for the Case 4
fault.
Figure 62– Speed variation of machines on the onshore grid for
the Case 4 fault.
Figure 63 – Reactive Power injected/absorbed by the STATCOM on bus 20.
0 1 2 3 4 5
49.95
50
50.05
50.1
50.15
50.2
50.25
50.3
50.35
50.4
Time [s]
Fre
qu
en
cy
[H
z]
Bus 90001
Bus 90002
Bus 90003
Bus 90004
Bus 90005
Bus 20
Bus 102
0 1 2 3 4 50.5
0.6
0.7
0.8
0.9
1
1.1
1.2
1.3
Time [s]
Vo
lta
ge
Ab
so
lute
Va
lue
[p
.u.]
Bus 90001
Bus 90002
Bus 90003
Bus 90004
Bus 90005
Bus 102
Bus 20
0 1 2 3 4 5 6 7 8 9 102980
2985
2990
2995
3000
3005
3010
3015
Time [s]
Ma
ch
ine
Sp
ee
d [
rpm
]
Bus 90001
Bus 90002
Bus 90003
Bus 90004
Bus 90005
0 1 2 3 4 5
50
50.1
50.2
50.3
50.4
50.5
50.6
50.7
49.9
Time [s]
Fre
qu
en
cy
[H
z]
Bus 101
Bus 211
Bus 3011
Bus 3018
Bus 206
Bus 151
Bus 21
0 1 2 3 4 50.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
1.1
1.2
Time [s]
Vo
ltag
e A
bso
lute
Valu
e [
p.u
.]
Bus 101
Bus 211
Bus 3011
Bus 3018
Bus 206
Bus 151
Bus 21
0 1 2 3 4 5 62995
3000
3005
3010
3015
3020
3025
3030
3035
Time [s]
Mach
ine S
peed
[rp
m]
Bus 101
Bus 211
Bus 3011
Bus 3018
Bus 206
0 1 2 3 4 5-50
-25
0
25
50
75
100
Time [s]
Reacti
ve P
ow
er
[MV
Ar]
67
6.2.5. Discussion
Observing the results, some similarities are encountered between the figures obtained for the
different analysed cases. An important conclusion that can be drawn is that the behaviour of the
offshore wind farm is largely governed by the behaviour of the wind turbines, and, more specifically,
the DFIG machines modelled.
When the fault occurs, the electromagnetic torque drops, caused by the terminal voltage drop of
the generators. This causes the machine to accelerate, a behaviour noticed in all cases under
analysis. When the fault is cleared (either 100ms or 300ms after the application), the machine speed
decreases until it reaches a stable value close to the initial. For case 3 and 4 (Figure 53 and Figure
59), the rotor speed variation does not reach the initial value within the 10 seconds of the simulation,
unlike cases 1 and 2 (Figure 41 and Figure 47). This is caused by the aggravated severity of the short
circuit applied in these cases, which also causes the machine to accelerate to a much higher speed
than the speed in cases 1 and 2. The speed variation of the machines in the onshore grid is very
similar for all four cases: the machines accelerate during the fault and then decelerate following the
disturbance.
The electrical frequency in the offshore buses also increases, followed by the stabilization to the
initial 50 Hz. Similar results are found for onshore buses, for all the cases studied.
The voltage dips in offshore and onshore buses present some differences in the cases. Cases 1
and 2 present voltage dips less profound than the ones in Cases 3 and 4, due to the difference in the
short-circuit severity. In Cases 1 and 2 the voltage at the offshore buses reach approximately 0,9 p.u.
(Figure 40 and Figure 46). In Case 3, the voltage at the offshore wind farm buses reach about 0,6 p.u.
In Case 4 the voltage decreases to approximately 0,65 p.u.
The influence of the STATCOM installed offshore can be seen in case 4. The figures resemble
the ones in case 3, since the fault applied is the same. However, the STATCOM provides the voltage
support it was supposed to. The voltage dip in the offshore buses (in Figure 58) is less severe
(approximately 0,65 p.u.) than the voltage dip in case 3 (Figure 52) (0,6 p.u.). The wind turbine speed
increases slightly less than in the case 3. The effect of the STATCOM in the onshore buses (Figure
60, Figure 61 and Figure 62) is not significant, derived from the fact that the STATCOM is placed
offshore. Figure 63 shows the reactive power injected/consumed by the STATCOM, which explains
the reason for the improved behaviour of the offshore buses as a result of the action of the
STATCOM. This device is consuming reactive power before the fault (working as a reactor). During
the fault, the STATCOM injects reactive power, thus contributing to the rise of voltage in all offshore
buses, making the dips less profound. Following the clearance of the fault, the STATCOM slowly
tends to consume reactive power, as in steady-state.
In all four cases, the recovery of the voltage magnitudes, the machine frequencies and the
machine speeds indicate that stability is reached in all four cases, regardless of the differences in the
behaviour of the cases. This means that the faults to which the grid is subjected allow the fault ride
through of the wind farm, as these faults are not severe enough to allow the disconnection of the
68
machines by the under/over voltage relays. If more severe faults and/or with a larger fault duration
(outside the characteristic of the voltage relays, defined in Figure 26) were to be applied to the grid,
the disconnection of the machines would occur, thus having the wind farm disconnected from the grid.
This case was not, however, studied in this work.
69
6.3. HVDC Dynamic Behaviour
For HVDC transmission between the offshore wind farm and the grid the study carried out is
similar to the one for HVAC transmission. In this case, the fault applied is a 100 ms moderate fault
(similar to the one applied in Case 1, for HVAC transmission). Figure 70 and Figure 71 are present
only for HVDC transmission as they show the active and reactive power in the converters.
6.3.1. HVDC Configuration
In this section the results for the HVDC+STATCOM are presented. Note the additional figure,
Figure 80, which represents the reactive power injected or absorbed by the STATCOM.
Figure 64– Frequency variation of offshore buses with HVDC-LCC
transmission.
Figure 65– Voltage variation of offshore buses with HVDC-LCC
transmission.
Figure 66– Offshore wind turbine speed variation with HVDC-LCC
transmission.
Figure 67– Frequency variation of onshore buses with HVDC-
LCC transmission.
Figure 68– Voltage variation of onshore buses with HVDC-LCC
transmission.
Figure 69– Speed variation of machines on the onshore grid with
HVDC-LCC transmission.
0 0.1 0.2 0.3 0.4 0.50
50
100
150
200
Time [s]
Fre
quen
cy [
Hz]
Bus 90001
Bus 90002
Bus 90003Bus 90004Bus 90005
Bus 102
0 0.5 1 1.5 2 2.5 30
0.2
0.4
0.6
0.8
1
1.2
1.4
Time [s]
Vo
ltag
e A
bso
lute
Valu
e [
p.u
.]
Bus 90001
Bus 90002
Bus 90003
Bus 90004
Bus 90005
Bus 102
0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.53000
3000.05
3000.1
3000.15
3000.2
3000.25
Time [s]
Mac
hine
Spe
ed [
rpm
]
Bus 90001
Bus 90002Bus 90003Bus 90004
Bus 90005
0 1 2 3 4 5 6 7 8 9 1049.8
49.85
49.9
49.95
50
50.05
50.1
50.15
50.2
Time [s]
Fre
qu
en
cy [
Hz]
Bus 101
Bus 211
Bus 3011
Bus 3018
Bus 206
Bus 151
0 0.5 1 1.5 2 2.5 30.8
0.85
0.9
0.95
1
1.05
1.1
1.15
Time [s]
Voltage A
bsolu
te V
alu
e [p.u
.]
Bus 101
Bus 211
Bus 3011
Bus 3018
Bus 206
Bus 151
0 1 2 3 4 5 6 7 8 9 102985
2990
2995
3000
3005
3010
Time [s]
Mach
ine S
peed
[rp
m]
Bus 101
Bus 211
Bus 3011
Bus 3018
Bus 206
70
Figure 70 – Active Power in the converters, for the HVDC-LCC
transmission.
Figure 71 – Reactive Power in the converters, for the HVDC-LCC
transmission.
0 0.5 1 1.5 2 2.5 3-150
-100
-50
0
50
100
150
Time [s]
Ac
tiv
e P
ow
er
[MW
]
Rectifier
Inverter
0 0.5 1 1.5 2 2.5 30
10
20
30
40
50
60
70
80
Time [s]
Re
ac
tiv
e P
ow
er
[Mv
ar]
Rectifier
Inverter
71
6.3.2. HVDC + STATCOM Configuration
Figure 72– Frequency variation of offshore buses with HVDC-
LCC+STATCOM transmission.
Figure 73– Voltage variation of offshore buses with HVDC-
LCC+STATCOM transmission.
Figure 74– Offshore wind turbine speed variation with HVDC-
LCC+STATCOM transmission.
Figure 75– Frequency variation of onshore buses with HVDC-
LCC+STATCOM transmission.
Figure 76– Voltage variation of onshore buses with HVDC-
LCC+STATCOM transmission.
Figure 77– Speed variation of machines on the onshore grid with
HVDC-LCC+STATCOM transmission.
0 0.1 0.2 0.3 0.4 0.50
50
100
150
200
Time [s]
Fre
quen
cy [
Hz]
Bus 90001
Bus 90002
Bus 90003Bus 90004
Bus 90005
Bus 102
0 0.5 1 1.5 2 2.5 30
0.2
0.4
0.6
0.8
1
1.2
1.4
Time [s]
Vo
lta
ge
Ab
so
lute
Valu
e [
p.u
.]
Bus 90001
Bus 90002
Bus 90003
Bus 90004
Bus 90005
Bus 102
0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.53000
3000.05
3000.1
3000.15
3000.2
3000.25
Time [s]
Mac
hine
Spe
ed [
rpm
]
Bus 90001
Bus 90002Bus 90003Bus 90004Bus 90005
0 1 2 3 4 5 6 7 8 9 1049.8
49.85
49.9
49.95
50
50.05
50.1
50.15
50.2
Time [s]
Fre
qu
en
cy [
Hz]
Bus 101
Bus 211
Bus 3011
Bus 3018
Bus 206
Bus 151
0 0.5 1 1.5 2 2.5 30.8
0.85
0.9
0.95
1
1.05
1.1
1.15
1.2
1.25
Time [s]
Voltage A
bsolu
te V
alu
e [p.u
.]
Bus 101
Bus 211
Bus 3011
Bus 3018
Bus 206
Bus 151
0 1 2 3 4 5 6 7 8 9 102985
2990
2995
3000
3005
3010
Time [s]
Mach
ine S
peed
[rp
m]
Bus 101
Bus 211
Bus 3011
Bus 3018
Bus 206
72
Figure 78 –Active Power in the converters, for HVDC-
LCC+STATCOM transmission.
Figure 79 – Reactive Power in the converters, for HVDC-
LCC+STATCOM transmission.
Figure 80 – Reactive Power injected/absorbed by the STATCOM on bus 151.
0 0.5 1 1.5 2 2.5 3-150
-100
-50
0
50
100
150
Time [s]
Acti
ve P
ow
er
[MW
]
Rectifier
Inverter
0 0.5 1 1.5 2 2.5 30
10
20
30
40
50
60
70
80
Time [s]
Reacti
ve P
ow
er
[Mvar]
Rectifier
Inverter
0 1 2 3 4 5 6-80
-70
-60
-50
-40
-30
-20
-10
0
Time [s]
Re
acti
ve P
ow
er
[MV
Ar]
73
6.3.3. Discussion
From the results for HVDC and HVDC+STATCOM transmission options, the most relevant
conclusion is that the offshore wind farm connected in such ways does not possess fault ride through
capability, i.e., the wind farm does not remain connected to the grid during the disturbance. Rather,
the voltage drop causes commutation failure in the inverter, which means that the current does not
transfer from one semiconductor switch to another. The consequence of the commutation failure is the
bypassing of the inverter by short-circuiting its input and opening its output. The bypassing occurs at
time 0,502 s – Figure 70 and Figure 78 depict this behaviour (similar in both transmission systems).
During the disturbance, with the inverter bypassed, the rectifier continues to circulate a lower level of
direct current through the shorted inverter, resulting in a lower active power drawn by the converters
as a consequence of the voltage dip (represented in Figure 65 and Figure 73). Active and reactive
power at the rectifier and the inverter continue to flow during the fault until the AC voltage at the
rectifier finally goes to zero, at which time the DC link is blocked, at 0,602 s. The active and reactive
power circulated through the converters become equal to zero.
As the frequency of the offshore buses increases very significantly (as noted in Figure 64 and
Figure 72) the over frequency generator bus disconnection relays try to disconnect the wind
generators. The under-voltage relays also step in, due to the voltage dip in generator buses. Finally,
the offshore machines trip, the voltage at the offshore buses drops to zero (Figure 65 and Figure 73).
After the disconnection of the machines, the frequency values are not relevant, so the time interval
shown in Figure 64 and Figure 72 is only of approximately 0,5 seconds. At this point, all the buses
located offshore are isolated from the onshore grid, as they are disconnected.
From the grid point of view, the wind turbine speed (Figure 66 and Figure 74) is not relevant after
the fault so the time interval does not include the time period following the clearance of the fault, since
the machines are disconnected.
From the point of view of the onshore grid, the disconnection of the offshore wind farm is a loss of
a 110 MW generator. As so, the frequency of the onshore buses increases but it does not reach the
value it had before the disturbance, 50 Hz. This is caused by the fact that the governors of the
onshore generators have first order control and so the reference level of power is the same before and
after the fault, even though the amount of active power has changed. This means that the frequency
establishes in a lower value, approximately 49,85 Hz (Figure 67 and Figure 75). Similar reasoning is
applied to the speed of the onshore machines (Figure 69 and Figure 77). The voltage drops at the
onshore buses are, much like all the other results here analysed, identical for both transmission
alternatives. The voltage drops, on bus 151, to about 0,8 p.u. (Figure 68 and Figure 76).
All the results are analysed together for the HVDC and the HVDC+STATCOM transmissions as
the differences between these two are negligible. The conclusion is that the use of the STATCOM
does not improve the performance of the transmission system. The objective was that this device
would inject reactive power in the onshore bus (bus 151) during and immediately after the grid
disturbance and thus contributing to a less severe voltage dip in the bus at which it is connected.
74
However, as can be verified in Figure 80, the STATCOM is already injecting the maximum reactive
power it can supply before the fault and so its action is very limited, as it does not have the ability of
injecting more than 70 Mvar. As such, the benefits of the STATCOM to the dynamic behaviour of an
HVDC-LCC are not significant enough to consider the HVDC+STATCOM a viable alternative to the
HVDC transmission. The utilization of a larger STATCOM could, in theory, solve this limitation.
The disconnection of the wind farm during the fault becomes the only viable solution due to the
operating problems of the LCC converters. Limited improvements are available: increasing the
minimum extinction angle of the inverter is possible although it represents an increased reactive power
consumption of the converter.
It is noticeable that the behaviour of the wind farm is largely influenced by the behaviour of the
HVDC-LCC transmission technology, and, it this particular case, the limitation it has when dealing with
commutation failures.
The reconnection of this offshore wind farm, following the grid fault, is not simulated in this work,
but it is likely that this reconnection leads to severe transient phenomena.
75
7. Conclusions
In this chapter, the main conclusions of the work are presented, together with a reference to
future studies that may offer important future contributions to the theme of this thesis.
7.1. Final Remarks
The development of global wind energy over the last 15 years has been remarkable. In particular,
Europe has invested strongly in new installations. In the last two years (2008 and 2009) more wind
power was installed than any other generating technology in the EU. The significantly growing wind
energy has given rise to the creation of offshore wind farms. In just 10 years the installed capacity of
offshore wind energy has reached 2 GW and the previsions indicate this energy source will experience
a breathtaking increase, with forecasted scenarios of 40 GW of installed capacity by 2020.
The higher wind speed offshore, the available area of installation and the possibility of moving
away from population possibly disturbing wind turbines have been the main driven forces behind the
recent developments on offshore wind energy. As trends indicate that future wind farms will be larger
and further from shore, the transmission technology from the offshore wind farm to the onshore grid
has gained relevance and HVAC and HVDC solutions are under analysis.
The objective of this thesis was to compare two transmission technologies, HVAC and HVDC-
LCC, by carrying out steady-state and dynamic analysis of the offshore wind farm and the onshore
grid for both transmission alternatives. The offshore wind farm was modelled in PSS/E software as a
110 MW wind power plant placed 100 km from the shore.
The power flow analysis showed that the steady-state performance of HVAC and HVDC is
satisfactory, even though different operation issues arise for each technology.
For HVAC the main issue is the large amount of reactive power produced at the submarine cable.
As such, some compensation schemes were studied, in order to absorb the surplus reactive power.
The application of a compensator device of some sort (a reactor or a STATCOM) located offshore was
found to be necessary. The scenario of onshore compensation only (no offshore device) led to an
unacceptable voltage rise on the offshore bus voltage, as a result of the excessive amount of reactive
power. The performance of the remaining compensation schemes was similar: the compensation at
both ends of the cable, the compensation offshore only (by a shunt reactor) and the STATCOM
offshore compensation all lead to a high power factor at the point of connection of the wind farm to the
remaining grid (around 0,9, injecting reactive power in the grid). The application of a STATCOM
offshore leads to almost the same results as the shunt reactor offshore. The offshore and onshore
compensation alternative led to the most favourable results: lowest reactive power injected in the
onshore grid and an even distribution of the cable loading (low power rating of the cable). The need of
smaller shunt reactors (40 Mvar) are also an advantage of this alternative.
76
The HVDC link allows the transmission of the power generated at the wind farm with low power
losses (around 2%) and no reactive power problems are originated by this technology. However, both
line-commutated converters (rectifier and inverter) always consume reactive power, which can be
compensated. The use of a STATCOM (as part of the HVDC+STATCOM alternative considered)
provides that compensation, supplying reactive power to the onshore grid.
The dynamic behaviour of the offshore wind farm and the onshore grid was studied for the
response of this power system to an AC grid fault. In this case, the results differ very much according
to the transmission system.
The offshore wind farm connected to the grid by HVAC presents favourable results, as the wind
farm is able to stay connected to the grid during the disturbance. This fact is valid for all three bus
faults applied (differing in duration and severity) which means this wind farm is able to “ride through”
the fault. The performance is improved by the application of the STATCOM, as this device injects
reactive power during the fault, reducing the voltage dips and also reducing the frequency rise and
offshore wind turbines speed increase.
The HVDC transmission, however, gives totally different results. The AC onshore grid fault
causes commutating failure of the inverter, which is bypassed instantly after the disturbance. The
tripping of the machines leads to the blocking of the DC link. The offshore buses are disconnected
from the grid and the power generation is lost. This means that the converter technology governs the
behaviour of the wind farm, since the limitations of the LCC converters cause the wind farm to be
disconnected. The disconnection during the fault and the subsequent reconnection after the fault is the
solution for operating one such system, although the reconnection is not analysed in the course of the
thesis. The STATCOM does not improve the performance of the transmission, since it is already, in
steady-state, supplying reactive power to the grid, not having, therefore, the capability of injecting
further amounts of reactive power (since it is already injecting initially the maximum reactive power it
can).
In conclusion, one can state that if HVAC is used to implement the grid connection, the response
of the wind farm to disturbances is determined by the wind turbines, since the connections themselves
are passive elements. With the DC link, the wind turbines are electrically decoupled from the analysed
system. As so, the response of the wind farm is governed by the technology in the LCC converter.
7.2. Future Work
The modelling of the wind farm may be the starting point to future simulations, with the possibility
of exploring other options, resolving some of the limits found on the course of this thesis work.
The limited action of the STATCOM that has been encountered may be overcome by using larger
devices, with larger capability of reactive power injection. Two STATCOM devices working together
can also be considered, if limited space is not a problem.
77
For HVDC transmission, in particular, some auxiliary power devices may be used to absorb the
surplus active power when an AC grid fault occurs, and by doing so, reducing the frequency instability
of the offshore grid. Large batteries and more technologically advanced solutions, like
Superconducting Energy Storage (SMES) systems can be considered in the future.
Applying this study to different scenarios can also be advantageous to the knowledge of these
systems. Working with a larger wind farm, installed further from the shore can be considered. The
power losses are likely to be higher and, in AC, the longer cable will need to have larger compensation
schemes. For increased distances, the use of HVDC is likely to be more favourable, since the HVAC
will reach its limit.
Finally, one of the most important advances in HVDC transmission, the HVDC-VSC (voltage
source converters) may well prove to be a valid alternative for the connection of offshore wind farms.
This technology, owing to the turn-off capability of the IGBTs of the converters, allows total control of
active and reactive power, which could represent fault ride through capability for offshore wind farms
connected with HVDC-VSC.
78
References
[1]. EWEA - European Wind Energy Association. www.ewea.org. [Online] [Cited: 26 April 2010.]
[2]. GWEC - Global Wind Energy Council. www.gwec.net. [Online] [Cited: 25 June 2010.]
[3]. Castro, Rui. Folhas da Cadeira de Energias Renováveis e Produção Descentralizada. 2008.
[4]. Freris, Leon. Renewable Energy in Power Systems. s.l. : Wiley, 2008.
[5]. Manwell, James F., McGowan, Jon G. and Rogers, Anthony L. Wind Energy Explained:Theory,
Design and Application. s.l. : Wiley, 2002.
[6]. Wind Atlases of the World. www.windatlas.dk. [Online] [Cited: 24 April 2010.]
[7]. Danish Energy Authority. www.ens.dk. [Online] [Cited: 26 April 2010.]
[8]. Delivering offshore wind power in Europe - EWEA Report. www.ewea.org. [Online] [Cited: 29 April
2010.]
[9]. E-ON Offshore Factbook. www.eon.com. [Online] [Cited: 29 April 2010.]
[10]. Enquist, Johan. Fault Ride-Through of Offshore Wind Parks - Master of Science Thesis. s.l. :
Chalmers University of Technology, 2007.
[11]. Ackermann, Thomas. Wind Power in Power Systems. s.l. : Wiley, 2005.
[12]. Xu, Lie and Andersen, Bjarne R. Grid Connection of Large Offshore Wind Farms Using HVDC.
s.l. : Wiley Interscience, 2005.
[13]. Lazaridis, Lazaros P. Economic Comparison of HVAC and HVDC Solutions for Large Offshore
Wind Farms under Special Considerations of Reliability - Master's Thesis. s.l. : Royal Institute of
Technology, 2005.
[14]. Alegría, Iñigo Martinez de, et al. Transmission alternatives for offshore electrical power. s.l. :
Elsevier, 2008.
[15]. Negra, N. Barberis, Todorovic, J. and Ackermann, T. Loss Evaluation of HVAC and HVDC
transmission solutions for large offshore wind farms. s.l. : Elsevier, 2006.
[16]. Zubiaga, M., et al. Evaluation and selection of AC transmission lay-outs for large offshore wind
farms. s.l. : IEEE, 2009.
[17]. Nexans. www.nexans.com. [Online] [Cited: 7 May 2010.]
[18]. Siemens Energy. www.energy.siemens.com. [Online] [Cited: 18 May 2010.]
79
[19]. Wiechowski, W. and Eriksen, P. Selected Studies on Offshore Wind Farm Cable Connections -
Challenges and Experience of the Danish TSO. s.l. : IEEE, 2008.
[20]. Wright, Sally D., et al. Transmission Options for Offshore Wind Farms in the United States. s.l. :
AWEA, 2002.
[21]. Kirby, N. M., et al. HVDC Transmission for Large Offshore Wind Farms. s.l. : IEEE, 2002.
[22]. Prysmian Cables. www.prysmian.com. [Online] [Cited: 25 May 2010.]
[23]. High Voltage Direct Current Transmission - Proven Technology for Power Exchange.
www.energy.siemens.com. [Online] [Cited: 15 March 2010.]
[24]. Sucena Paiva, José Pedro. Redes de Energia Eléctrica - Uma Análise Sistémica. s.l. : IST
Press, 2005.
[25]. Technical Description Lillgrund Wind Power Plant. s.l. : Vattenfall, 2008.
[26]. PSS/E Wind Modelling Package for 1.5/3.6/2.5 MW Wind Turbines - User Guide. 2009.
[27]. PSS/E 30 Users Manual. 2004.
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[29]. PSS/E 30 Program Application Guide Volume II. 2004.
80
Appendix A – Grid Parameters
This appendix presents the parameters used in the test grid, represented in Figure 24.
A.1. Onshore Grid Parameters
A.1.1. Line Parameters
Table 10: Reactance and Susceptance of the lines.
Line E[
[X. Y. ]
F[
[X. Y. ] From
Bus
To
Bus
153 154 0,0375 0,08333
153 154 0,045 0,125
153 3006 0,01 0,025
154 203 0,0333 0,08333
154 205 0,00278 0,075
154 3008 0,01833 0,25
203 205 0,0375 0,06667
203 205 0,0375 0,06667
3001 3003 0,00667 0
3002 3004 0,045 0,075
3003 3005 0,045 0,075
3003 3005 0,045 0,075
3005 3006 0,025 0,05833
3005 3007 0,02083 0,05
3005 3008 0,04167 0,1
3007 3008 0,02083 0,05
A.1.2. 400 kV Line Parameters
Table 11: 400 kV line calculated parameters.
Line H
[AB]
@AB
[S/AB]
EAB
[S/AB]
FAB
[S/AB] @X.Y. EX.Y. FX.Y. From
Bus
To
Bus
151 152 100 0,0292154 0,3215977 3,514E-06 0,00183 0,0201 0,5623
151 152 100 0,0292154 0,3215977 3,514E-06 0,00183 0,0201 0,5623
151 201 50 0,0292154 0,3215977 3,514E-06 0,00091 0,01005 0,28115
152 202 80 0,0292154 0,3215977 3,514E-06 0,00146 0,01608 0,44984
81
152 3004 70 0,0292154 0,3215977 3,514E-06 0,00128 0,01407 0,39361
201 202 50 0,0292154 0,3215977 3,514E-06 0,00091 0,01005 0,28115
201 204 40 0,0292154 0,3215977 3,514E-06 0,00073 0,00804 0,22492
A.1.3. Transformer Parameters
Table 12: Two-Winding Transformers Parameters.
Buses E[ [X. Y. ] From
Bus To
Bus 101 151 0,01133
152 153 0,00417
201 211 0,01771
202 203 0,01354
204 205 0,0125
205 206 0,01111
3001 3002 0,0125
3001 3011 0,00833
3004 3005 0,01354
3008 3018 0,07083
A.1.4. Loads
Table 13: Load powers.
Bus _H []
_H [\_]
153 200 100
154 500 450
154 400 350
203 200 100
205 1000 700
3005 100 50
3007 200 75
3008 100 50
A.1.5. Shunt Compensators
Table 14: Shunt compensators.
Bus F`Y []
151 0
153 150
82
154 300
201 300
203 100
205 300
A.2. Offshore Wind Farm Parameters
A.2.1. Wind Turbines
Table 15: Wind Turbine aggregate Power Flow Parameters.
24 MW Aggregates
21 MW Aggregates
Pgen [MW] 24 21
Pmax [MW] 24 21
Pmin [MW] 1,1 1
Qgen [Mvar] 11,62 10,16
Qmax [Mvar] 11,62 10,16
Qmin [Mvar] 0 0
Mbase [MVA] 26,72 23,38
Xsource [p.u.] 0,8 0,8
A.2.2. Step-up transformers
Table 16: Unit transformer of one single wind turbine.
Transformer Parameters Value
Unit Rating [MVA] 1,75
Unit Rated Voltage [kV/kV] 0,69/33
Unit Impedance [%] 5,75
Unit X/R 7,5
83
A.2.3. 33 kV Cables
Table 17: 33 kV Horns Rev cable parameters [10].
Cable Parameters Value
Resistance [Ω/km] 0,042
Reactance [Ω/km] 0,11
Capacitance [µF/km] 0,28
Table 18: 33 kV AC Cable parameters used in the modelled wind farm.
Cable Length
[AB] @X.Y. EX.Y. FX.Y. From
Bus To
Bus
1 102 1 0,003857 0,010101 0,000957
2 102 0,8 0,003085 0,008081 0,000766
3 102 0,6 0,002314 0,006061 0,000574
4 102 0,4 0,001543 0,00404 0,000383
5 102 0,2 0,000771 0,00202 0,000191
A.3. HVDC-LCC Modelling Parameters
A.3.1. Steady-State DC Line Parameters
Table 19: DC Line Power Flow Parameters.
Parameter Value
Line Number 1
Control Mode Power
Rdc [Ω] 2,0
Rcmp [Ω] 0,0
Delti [p.u.] 0,0
Setval [MW] 110
Vschedule [kV] 150
Dcvmin [kV] 0,0
Vcmode [kV] 145
Metered Rect
CCC Itmax 20
Ccc Accel 1,0
84
A.3.2. Steady-State Converter Parameters
Table 20: DC Link Converters Power Flow Parameters.
Parameter Rectifier Inverter
Bus Number 102 151
Max Firing Angle [º] 18 19
Min Firing Angle [º] 8 18
Bridges in Series 1 1
Primary Base [kV] 33 400
Commutating Resistance [Ω] 0 0
Commutating Reactance [Ω] 14,22 13,20
Measuring Bus 0 0
Trans Ratio [pu] 3,36 0,28
Tap Setting [pu] 0,91875 0,91875
Max Tap Setting [pu] 1,5 1,5
Min Tap Setting [pu] 0,51 0,51
Tap Set [pu] 0,00625 0,00625
Reactance of CCC [Ω] 0 0
AC Tx From Bus 0 0
AC Tx To Bus 0 0
AC Tx Id 1 1
A.3.3. Transient Model Parameters
Table 21: “CDC4T” model parameters for the DC Link.
Parameter Value
ALFDY [º] 8
GAMDY [º] 18
TVDC [s] 0,05
TIDC [s] 0,05
VBLOCK [pu] 0,6
VUNBL [pu] 0,65
TBLOCK [s] 0,1
VBYPAS [kV] 75
VUNBY [pu] 0,8
TBYPAS [s] 0,1
RSVOLT [kV] 145
RSCUR [A] 73,33
VRAMP [pu/s] 5
CRAMP [pu/s] 5
85
C0 [A] 0
V1 [kV] 100
C1 [A] 586,67
V2 [kV] 147
C2 [A] 733,33
V3 [kV] 150
C3 [A] 733,33
TCMODE [s] 0,1
A.4. STATCOM Parameters
A.4.1. Steady-State Parameters
Table 22: STATCOM Power Flow parameters.
Parameter Rectifier
Device Number 1
Terminal Bus 0
Control Mode Normal
P Setpoint [MW] 0,00
Q Setpoint [MVAR] 0,00
V send setpoint 1,02
Shunt Max [MVA] 64
RMPCT [%] 100
Bridge Max 0
V term max [pu] 1,1
V term min [pu] 0,9
V series max [pu] 1
I Series Max [MVA] 64
Dummy Series X [pu] 0,05
V Series Reference Sending end Voltage
86
A.4.1. Transient State Parameters
Table 23: “CSTCNT” model parameters for the STATCOM.
Parameter Value
T1 [s] 0,1
T2 [s] 0,1
T3 [s] 0,1
T4 [s] 0,1
K 25,116
Droop [pu] 0,03
Vmax 999
Vmin -999
Icmax [pu] 1,25
ILmax [pu] 1,25
Vcutout [pu] 0,2
Elimit [pu] 1,2
Xt [pu] 0,1
Acc 0,5
STBASE [MVAr] 64
87
Appendix B – GE 1,5 MW Wind turbine aggregate “dyr”
record file
The following data was used for the dynamic simulation of one aggregate of GE 1,5 MW wind
turbines. Similar records were written for the other aggregates.
90001 'USRMDL' 1 'GEWTG1' 1 1 2 11 3 5 0 14 1.5000 0.8000 0.50000 0.90000 1.1100 1.2000 2.0000 0.40000 0.80000 5.0000 0.20000E-01/ 90001 'USRMDL' 1 'GEWTE1' 4 0 10 62 18 7 90001 0 0 1 0 0 1 0 0 0 0.15000 18.000 5.0000 0.0000 0.0000 0.50000E-01 3.0000 0.60000 1.1200 0.40000E-01 0.43660 -0.43600 1.1000 0.20000E-01 0.45000 -0.45000 5.0000 0.10000 0.90000 1.1000 40.000 0.50000 1.4500 0.50000E-01 0.50000E-01 1.0000 0.15000 0.96000 0.99600 1.0040 1.0400 1.0000 0.95000 0.95000 0.40000 1.0000 0.20000 1.0000 0.25000 -1.0000 11.000 25.000 3.0000 -0.90000 8.0000 0.25000 10.000 1.0000 1.7000 1.1100 1.2500 5.0000 0.0000 0.0000 10.000 0.25000E-02 1.0000 5.5000 0.10000 -1.0000 0.10000 0.0000 / 90001 'USRMDL' 1 'GEWTT' 5 0 1 5 4 3 0 5.2900 0.0000 0.0000 1.8800 2.3000 / 0 'USRMDL' 0 'WGUSTC' 8 0 3 6 0 4 90001 '1 ' 0 9999.0 5.0000 30.000 9999.0 9999.0 30.000 / 0 'USRMDL' 0 'GEWTA' 8 0 3 9 1 4 90001 '1 ' 0 20.000 0.0000 27.000 -4.0000 0.0000 1.2250 35.250 72.000 1000.0 / 0 'USRMDL' 0 'GEWTP' 8 0 3 10 3 3 90001 '1 ' 0 0.30000 150.00 25.000 3.0000 30.000 -4.0000 27.000 -10.000 10.000 1.0000 / 0 'USRMDL' 0 'GEWTPT' 8 0 2 0 0 17 90001 '1 ' / 0 'USRMDL' 0 'VTGTPA' 0 2 6 4 0 1 90001 90001 '1' 0 0 0 0.1 5.0 0.2 0.08 / 0 'USRMDL' 0 'VTGTPA' 0 2 6 4 0 1 90001 90001 '1' 0 0 0 0.2 5.0 0.733 0.08 / 0 'USRMDL' 0 'VTGTPA' 0 2 6 4 0 1 90001 90001 '1' 0 0 0 0.3 5.0 1.267 0.08 / 0 'USRMDL' 0 'VTGTPA' 0 2 6 4 0 1 90001 90001 '1' 0 0 0 0.4 5.0 1.8 0.08 / 0 'USRMDL' 0 'VTGTPA' 0 2 6 4 0 1 90001 90001 '1' 0 0 0 0.5 5.0 2.333 0.08 / 0 'USRMDL' 0 'VTGTPA' 0 2 6 4 0 1 90001 90001 '1' 0 0 0 0.6 5.0 2.867 0.08 / 0 'USRMDL' 0 'VTGTPA' 0 2 6 4 0 1 90001 90001 '1' 0 0 0 0.7 5.0 3.4 0.08 / 0 'USRMDL' 0 'VTGTPA' 0 2 6 4 0 1 90001 90001 '1' 0 0 0 0.8 5.0 3.933 0.08 / 0 'USRMDL' 0 'VTGTPA' 0 2 6 4 0 1 90001 90001 '1' 0 0 0 0.9 5.0 4.467 0.08 / 0 'USRMDL' 0 'VTGTPA' 0 2 6 4 0 1 90001 90001 '1' 0 0 0 0.0 1.1 0.1 0.08 / 0 'USRMDL' 0 'VTGTPA' 0 2 6 4 0 1 90001 90001 '1' 0 0 0 0.0 1.15 0.0 0.08 / 0 'USRMDL' 0 'FRQTPA' 0 2 6 4 0 1 90001 90001 '1' 0 0 0 47.0 55.0 0.02 0.08 / 0 'USRMDL' 0 'FRQTPA' 0 2 6 4 0 1 90001 90001 '1' 0 0 0 47.5 55.0 10.0 0.08 / 0 'USRMDL' 0 'FRQTPA' 0 2 6 4 0 1 90001 90001 '1' 0 0 0 45.0 51.25 30.0 0.08 / 0 'USRMDL' 0 'FRQTPA' 0 2 6 4 0 1 90001 90001 '1' 0 0 0 45.0 52.0 0.02 0.08 /
