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04/11/2011

Modeling and dynamic analysis of offshore wind farms in France: Impact on power system stability

KTH Master Thesis

report number

Alexandre Henry

Examiner at KTH Dr. Luigi Vanfretti

Supervisors at KTH

Dr. Luigi Vanfretti and Camille Hamon

Supervisor at EDF Dr. Bayram Tounsi

Laboratory Electric Power Systems School of Electrical Engineering

KTH, Royal Institute of Technology Stockholm, November 2011


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Alexandre Henry Page 1 / 90 KTH Master Thesis

Abstract


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Nomenclature

EWEA : European Wind Energy Association

UK : United Kingdom

EU : European union

AC : Alternating current

DC : Direct current

HVAC : High Voltage Alternating Current

HVDC : High Voltage Direct Current

PCC : Point of Common Coupling

TSO : Transmission System Operator

RTE : Réseau de transport d’électricité (French TSO)

XLPE : cross linked polythylene insulated

VSC : Voltage source converter

LCC : Line commutated converter

FACTS : Flexible AC Transmission System

SVC : Static Var Compensator

DFIG : Double Fed Induction Generator

MVAC : Medium Voltage Alternating Current

ENTSO-E : European Network of Transmission System Operators for Electricity

HFF : High Frequency Filter

FRT : Fault Ride Through


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Table of contents

ABSTRACT .............................................................................................................................................. 1

NOMENCLATURE ................................................................................................................................... 2

TABLE OF CONTENTS ........................................................................................................................... 3

1. INTRODUCTION ............................................................................................................................... 5

1.1. BACKGROUND .................................................................................................................................. 5 1.1.1. The current situation in Europe .............................................................................................. 5 1.1.2. The French situation ............................................................................................................... 7

1.2. THE GOALS OF THIS MASTER’S THESIS ............................................................................................11 1.3. THE MASTER’S THESIS CONTENT ....................................................................................................11

2. PRESENTATION OF EDF ..............................................................................................................12

2.1. EDF COMPANY ..............................................................................................................................12 2.1.1. Electricity production and consumption in France ................................................................12 2.1.2. The opening to competition in the electricity market ............................................................12 2.1.3. The EDF company today ......................................................................................................12 2.1.4. Some key data......................................................................................................................13 2.1.5. The main assets ...................................................................................................................13

2.2. EDF R&D .....................................................................................................................................13 2.3. THE RESEARCH TEAM EFESE-R12 ................................................................................................14

3. TECHNOLOGY CHOICES OF THE REFERENCE OFFSHORE WIND FARM .............................15

3.1. OFFSHORE WIND FARM TECHNOLOGIES ...........................................................................................15 3.1.1. Electrical layout of offshore wind farms : ..............................................................................16 3.1.2. Wind turbine generators: ......................................................................................................16 3.1.3. Internal cables : ....................................................................................................................18 3.1.4. Offshore substations : ..........................................................................................................18 3.1.5. Wind farm connections : Transmission cables .....................................................................19

3.2. ANALYSIS OF 6 OFFSHORE WIND FARMS ..........................................................................................19 3.3. CONCLUSION ABOUT THE STUDIED OFFSHORE WIND FARMS ..............................................................25 3.4. CHOSEN SPECIFICATIONS FOR THE REFERENCE OFFSHORE WIND FARM ............................................28

4. TECHNICAL REQUIREMENTS FOR OFFSHORE WIND FARM CONNECTIONS : SPECIFIC OFFSHORE GRID CODE ......................................................................................................................29

4.1. REQUIREMENTS ABOUT THE VOLTAGE MANAGEMENT ........................................................................31 4.1.1. Steady state conditions ........................................................................................................31 4.1.2. Reactive power/ Voltage regulation .....................................................................................32 4.1.3. Fault ride through (with grid support) ...................................................................................33

4.2. REQUIREMENTS ABOUT THE FREQUENCY MANAGEMENT ...................................................................34 4.3. SYNTHESIS ABOUT OFFSHORE GRID CODES .....................................................................................35

5. BEHAVIOR OF OFFSHORE WIND FARMS DURING A FAULT : FARM + HVDC ......................37

5.1. INTRODUCTION...............................................................................................................................37 5.1.1. HVAC transmission systems ................................................................................................37 5.1.2. HVDC transmission systems ................................................................................................37

5.2. PRESENTATION OF VSC-HVDC TRANSMISSION SYSTEMS ................................................................37 5.3. VSC-HVDC LINE AND FARM BEHAVIORS DURING A FAULT ................................................................39

5.3.1. Description of the problem ...................................................................................................39 5.3.2. Possible solutions .................................................................................................................40 5.3.3. Examples of different case studies from the literature .........................................................41

5.4. CONCLUSION .................................................................................................................................43


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6. CASE STUDY 1 : AC CONNECTION OF THE 200 MW AC REFERENCE OFFSHORE WIND FARM .....................................................................................................................................................45

6.1. MODEL OF THE WIND FARM WITH AN AC TRANSMISSION CABLE .........................................................45 6.1.1. Model of the wind turbine GE – 3 MW .................................................................................46 6.1.2. The submarine cables 33 kV and 225 kV ............................................................................47 6.1.3. The transformers ..................................................................................................................48

6.2. DYNAMIC BEHAVIOR OF THE AC/AC REFERENCE OFFSHORE WIND FARM : SIMULATIONS ....................48 6.2.1. Impact of the aggregation on the dynamic behavior of the offshore wind farm ...................49 6.2.2. Static impact of an AC 225 kV transmission line ..................................................................51 6.2.3. Dynamic behavior of the reference offshore wind farm with the AC transmission line : Compliance with the ENTSOE and RTE grid code requirements ......................................................53

6.3. CONCLUSION : CASE STUDY N°1 : AC CONNECTION OF AN OFFSHORE WIND FARM .............................68

7. CASE STUDY 2: CONNECTION OF THE AC/AC 200 MW WIND FARM ON AN ELECTRICAL NETWORK .............................................................................................................................................70

7.1. MODEL OF THE ELECTRICAL POWER SYSTEM ...................................................................................70 7.1.1. Model of synchronous generators ........................................................................................71 7.1.2. Model of lines and transformers ...........................................................................................71 7.1.3. Model of loads ......................................................................................................................71 7.1.4. Load Flow .............................................................................................................................72

7.2. DYNAMIC BEHAVIOR OF THE SYSTEM ...............................................................................................73 7.2.1. Dynamic power system stabilities ........................................................................................73 7.2.2. Definition of the simulations .................................................................................................74 7.2.3. Small signal stability .............................................................................................................75 7.2.4. Transient stability..................................................................................................................77

7.3. CONCLUSION : CASE STUDY N°2 .....................................................................................................80

8. GENERAL CONCLUSION OF THE MASTER’S THESIS .............................................................82

9. REFERENCES ................................................................................................................................83

10. APPENDICES .............................................................................................................................85

10.1. APPENDIX 1 ...............................................................................................................................85 10.2. APPENDIX 2 ...............................................................................................................................86 10.3. APPENDIX 3 ...............................................................................................................................88


Introduction

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1. Introduction

1.1. Background

Nowadays, mitigation agreements on CO2 emission have increased the proportion of

renewable energy in the total energy mix. For example, the EU aims for 2020 the 3 X 20 : 20 % of energy savings, 20 % of energy efficiency and 20 % of renewable energy. The wind power can help the EU to reach its environmental goals. Currently, the onshore wind power accounts for a total power capacity of 86 GW in Europe and the electrical power from offshore wind in Europe represented around 3 GW at the end of 2010 [1]. According to EWEA, offshore wind farms can produce 10 % of electrical demands in Europe. Therefore, despite the important construction and connection cost, the offshore wind farm market is rapidly growing in Europe and particularly in North Sea. EWEA expects 40 GW of electrical power from offshore wind by the end of 2020. Moreover, according to EWEA, the market growing of offshore wind power will be larger than 30 % per year. In fact, offshore wind has a huge potential of more than 200 GW and according to EWEA, 100 GW of offshore wind farms are already proposed. Currently, Thanet in UK is the largest offshore wind farm with an installed power of 300 MW [2] but many projects of more than 1 GW like Dogger Bank 9 GW [3] in England are currently being studied. UK, Denmark, Netherlands and Sweden are countries where the presence of offshore wind farms is currently the most significant.

The most important offshore wind turbine manufactures are currently Vestas and Siemens

whereas the most important operators of offshore wind farms are DONG Energy, Vattenfall and E.ON but with this market growing, many others will come.

1.1.1. The current situation in Europe

Nowadays, in Europe only 3 GW of offshore wind farms have been built at the end of 2010 but according to BTM Consult more than 16 GW will be installed before 2014 and 150 GW of offshore wind farms are already proposed [4]. The number of offshore wind farms connected to the electrical grid is still limited but many offshore wind farms are currently being built and planned. In order to connect all these wind farms to the onshore electrical grid, an offshore North Sea supergrid (Offshore electrical grid in North Sea) is being studied by researcher. The North Sea offshore supergrid should interconnect offshore wind farms and create more interconnections between countries [1]. According to a recent EWEA report, an offshore North sea supergrid will be profit-making for countries.

All over this Master’s Thesis, offshore wind farms larger than 100MW will be studied. The number of offshore wind farms with a rated power larger than 100 MW and with a distance farther than 10 km from the coast currently installed can be observed in Figure 1. It can be observed than the number of installed offshore wind farms with significant power is still limited, only 10 farms.


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Figure 1 : Installed offshore wind farms larger than 100 MW and farther than 10 km from the shore in May 2011 [5]

In order to estimate technologies of future offshore wind farms, it is also indispensable to look at offshore wind farms in construction and planned. For this aim, Figure 2, Figure 3 and Figure 4 represent respectively offshore wind farms currently installed, in construction and planned. On these figures, a comparison has been done between the rated power of current wind farms and future wind farms and between the distance to the shore of current wind farms and future wind farms. Today, Thanet is the largest offshore wind farm with a rated power of 300 MW and the most distant offshore wind farms have a connection distance of 40 km. However, in the near future, offshore wind farms with a rated power of 500 MW and even 1 GW will be built and they will have a connection distance of 100 km. With the continued development of offshore technologies, particularly cable technologies and wind generator technologies, it has been shown that future projects will increasingly move towards more distant and massive farms, and thus boosting the accelerated development of the sector.

Figure 2 : Power/distance cartographies of installed offshore wind farms in Europe in 2010 [6]


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0

100

200

300

400

500

600

700

800

0 20 40 60 80 100 120

Distance to the shore (km)

Po

we

r (M

W)

Figure 3 : Power/distance cartographies of the offshore wind farm in construction in Europe

[6]

0

200

400

600

800

1000

1200

1400

0 20 40 60 80 100 120 140

Distance to the shore (km)

Po

we

r (M

W)

Figure 4 : Power/distance cartographies of the offshore wind farm approved in Europe [6]

1.1.2. The French situation

In France, the offshore wind energy is not yet developed and no offshore wind farm is

currently commissioning whereas France has a huge offshore wind potential which takes advantage of many offshore regions which have interesting energy density [7] (see Figure 5). The French government targets to have 3 GW before 2015 and 6 GW before 2020. [4,5]


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Figure 5 : Distribution of wind energy density (GWh/km2) in Europe (80 m hub height

onshore, 120 m hub height offshore) [7]

Hence, the 25 January 2011, the French government has initiated a call for tender for the

construction and the exploitation of 3 GW of offshore wind farms in 5 different areas : Dieppe-Le-Tréport (750 MW), Fécamp (500 MW), Courseulles sur Mer (500 MW), Saint-Brieuc (500 MW) et Saint-Nazaire (750 MW) (cf. Figure 6) [4,5]. The goals of the French government is to choose producers in the beginning of 2012 and that the offshore wind farms are built before 2015. The call for tender about offshore wind farms in France has been officially launched on July 11, 2011 with the publication of call for tender specifications [8]. These specifications explain general characteristics about the 5 future French offshore wind farms. The most important characteristics of these farms are summarized in Table 1.


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Table 1 : The list of the selected offshore wind farm areas

Le Tréport Fécamp Courseulles Saint Brieuc Saint-Nazaire

Maximal power (MW)

750 500 500 500 750

Minimal power (MW)

600 480 420 480 420

Location Le Tréport/

Haute Normandie

Fécamp/ Haute

Normandie

Courseulles-sur-Mer / Basse

Normandie

Saint-Brieuc / Bretagne

Saint-Nazaire/ Pays de Loire

Line type HVAC HVAC HVAC HVAC C

1 ≤ 480MW HVAC

C > 480MW HVDC

Cable voltage 225 kV 225 kV 225 kV 225 kV HVAC : 225kV

HVDC : 320 kV

Number of cables

3 2 2 2 HVAC : 2

HVDC : 1 bi-cable

Number of offshore

substation 1 to 3 1 to 2 1 to 2 1 to 2 1 to 3

Distance to the coast

15-20 km 10-15 km 10-15 km 10-15 km 15-20 km

Offshore reactive power

compensation

Yes if C > 480 MW

120 MVar

Yes if C < 480 MW

80 MVar

A wind energy unit is a decentralized electrical production unit which does not fulfill the same requirements as the other classical production units. When this decentralized unit can be considered as negligible (power plant lower than 100 MW), the connection requirements are not binding and their impacts on the electrical grid are low. It was not necessary to have a perfect understanding of these units. However with the development of the offshore market, wind farms are larger and farther, which increase the impact of these units on the electrical grid. It becomes indispensable to have a better understanding of these electrical production devices. Moreover, the constraints for connecting offshore wind farms to the grid should be identified and understood.

1 C means the total capacity of the offshore wind farm


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Figure 6 : The map of the selected offshore wind farm areas

One of the particularities of this offshore wind farm call for tender is to have the PCC of offshore wind farms at the offshore substation. The grid requirements from the TSO will be applied at this level and not onshore like in England or Denmark. It means also that the transmission line will be built, maintained and kept by the French TSO, RTE (“Réseau de Transport d’Electricité”), and that the line cost will be financed thanks to a higher electricity price. The electricity price will consist of a wind farm price component and a transmission line price component. It can be noticed that transformers of offshore substations which are going to increase the voltage from 33 kV to 225 kV will be owned by offshore wind farm operators.

Currently, in France, we have a grid code for power plant connections which is applied at the PCC. However, we can imagine that RTE will create some specific grid requirements for offshore wind farms. Indeed, an offshore wind farm is a decentralized unit and cannot fulfill the same requirements. These specific offshore grid requirements are not yet defined by RTE because their fulfillments are not judged as an important criteria for choosing the offshore projects. However, it seems that the offshore wind farm will have to respect the classical RTE grid code at the PCC which will be offshore.

For an economical information, the price of the wind farm component will be situated accordingly to the Table 2. Regarding the price component of transmission line, it will be evaluated by RTE. For information, the electrical price for French consumers is around 120 €/MWh and around 50 €/MWh on the stock exchange market. []

Table 2 : The price for « Offshore wind farm » components

Pmin (€/MWh) Pmax (€/MWh)

Le Tréport 115 175

Fécamp 115 175

Courseulles-sur-Mer 115 175

Saint-Brieuc 140 200

Saint-Nazaire 140 200


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1.2. The goals of this Master’s Thesis

The aim of this study will be to study the impact of offshore wind farms on an electrical grid

and to study the dynamic behavior of an offshore wind farm and of an AC transmission line, while taking into account grid technical requirements. It will be therefore necessary to define and then simulate different grid events. The goals defined by France regarding offshore wind power seems to predict an important development of this energy source before 2020 near the French coasts, in particular the Picardy, Normandy, Brittany and Vendee coasts. It will raise problems concerning two aspects technical and regulatory which must be analyzed. The technical aspect : is dealing in the same time with the electrical layout of offshore wind farm which are different as the onshore wind farm and with the suitable technology choices in order to find the best way to transfer electrical power from the Sea to the onshore electrical grid. The connection will be done in AC (Alternative Current) or in DC (Direct current). One case study of an offshore wind farm connecting to the French power system in AC represented by an infinite bus and a power system reactance Xcc will be achieved in order to determinate the dynamic behavior of a “wind farm + transmission line”. This case study will be useful to raise the future thoughts that an offshore wind producer (such as EDF EN) are going to have in order to find the best technical solution. Another case study of the same offshore wind farm connecting to a power system will be done in order to determinate the dynamic impact of an offshore wind farm on a power system. This case study will be useful for TSOs which have the responsibility of the electrical power system stability.

The regulatory aspect : The development of the offshore wind power market must be supervised by regulatory laws in order to facilitate the access to the transport power system. The requirements for the connection of offshore wind farms have to be clearly specified and maybe harmonized between the different European countries, especially if in North Sea a Supergrid is built. Currently France, RTE, does not have specific offshore grid code. Therefore, the regulatory aspect will be an essential subject of the case study n°1.

1.3. The Master’s Thesis content

This study will have two parts. The first part will deal with a characterization of a reference offshore wind farm based on existing offshore wind farms in Europe and with the definition of offshore grid requirements based on analysis of existing offshore grid codes. Between the two parts, the general behavior of an offshore wind farm with its transmission line has to be explained through a literature review. Whereas the second part will deal with the modeling of the defined reference offshore wind farm and also with the dynamic study of this offshore wind farm through two study cases.

Study case 1 : Offshore wind farm connecting to an infinite bus,

Study case 2 : Offshore wind farm connecting to an IEEE 14 bus power system. .


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2. Presentation of EDF

This chapter aims to present the EDF company . First, a brief historical description of the company will be done. Then we are going to describe EDF R&D, the research department and the research team where this Master’s Thesis has been achieved.

2.1. EDF Company

2.1.1. Electricity production and consumption in France

From the nationalization law of 1450 French companies of electricity and gas generation,

transmission and distribution from April 8, 1946, EDF (“Electricité de France”) is created. The

production park of the company, mainly hydraulic power, integrated then the fossil fired generation

(coal in the late 1950s, and oil in the 1960s) before the nuclear program in the 1970s. In parallel, the

national domestic consumption has increased significantly with the development of the household

appliances and of the electric heating.

In 2007, the French generation park has produced a total of 544.4 TWh, whose 76.9 % is nuclear

power, 10.7 % conventional thermal power, 11.6 % from hydro power and 0.7 % wind power and solar

power. Finally, the needed energy amounted to 479.9 TWh with an increase of 0.4 % from 2006. The

final electricity consumption was approximately 434 TWh, whose 284 TWh for the residential-tertiary

(65 %) and 123 TWh for the industry (28 %).

2.1.2. The opening to competition in the electricity market

Since the end of the 1990s, some European regulations have laid down the development

principles of the European energy market, in particular the introduction of competition in production

and supply and the recognition of an access to transmission and electricity distribution, whose the

regulation is provided by and independent authority. Since 1999, the French market has begun a

gradual opening to competition when the eligibility threshold to 100 GWh per year has been set up

(20% of the market). The threshold was then changed to 16 GWh per year in May 2000 (30%) and 7

GWh per year in February 2003 (37%), until the opening to all non-household customers by July 2004

(69%) and to all the consumers on July 2007.

At the same time, the need of an independent TSO associated with the imperative of non-

discriminatory access has lead in the creation of RTE (1st September 2005) for the electricity

transmission activities and in the creation of ERDF (May 1, 2008) for the distribution activities.

In late 2005, EDF has opened its capital and made its beginning on the Paris Bourse.

2.1.3. The EDF company today

The EDF Group is a leading European energy company, which is present in all aspects of the

electricity jobs from production to trading, and who wants to become more and more involved in the

gas chain in Europe. Major player in the French electricity market, EDF enjoys a location in Britain,

Germany and Italy. It has the largest production park and the first customer portfolio in Europe.


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2.1.4. Some key data

In 2007, EDF has made a turnover of 59.6 billion euros. 54 % of the turnover is related to activities

in France, 44 % activities in Europe (excluding France) and an additional 2 % for the rest of the world.

The net profit reached 4.7 billion euros and financial debt of 16.3 billion euros. In 2006, the Group had

156,524 employees worldwide, including 106,565 in France.

2.1.5. The main assets

In a newly competitive industry and in a particularly moving context, the company has unique

assets:

o The EDF company is the largest producer and marketer of electricity in Europe with an

installed capacity of 123.7 GW (128.2 GW in the world) in 2007. With the share of nuclear and

hydro power in its mix production, EDF is the largest European energy company with the less

CO2 emission and EDF has a limited exposure to oil market fluctuations.

o EDF is the first operator in the nuclear world, with its fleet of 63 GW (the first in Europe), which

represents 17% of world nuclear generating capacity.

o A key presence on European energy markets, particularly in the United Kingdom (EDF

Energy) and Italy (Edison).

o A balance between regulated activities (transmission and distribution), competitive activities

(production and consumption) and activities in Europe, providing in parallel regular income

and stable and good development perspectives.

2.2. EDF R&D

As suggested above, the area in which EDF is knows profound changes. For a perspective of competitiveness and responsible action, it will be necessary to integrate new technologies in production, storage and transmission as well as information technologies and communications. EDF R&D has 2000 people on three sites: Chatou, Clamart, Renardières in France. Its annual budget is around 400 million euros. The organization of the R&D is a matrix, and is based on:

o The directions of Programs, responsible for defining the range of activities in line with the

challenges the Group facing and in order to optimize the value created by the R&D. Four

areas are considered: production, business development, network and environment, energy

management.

o Management of Laboratories guarantees technical and scientific expertise for medium and

long terms, which are divided in 15 departments. Each department is divided into study

groups.

o Four branches acting transversely : Management-Finance, Human Resources,

Communication, Information Systems.

o Finally, the Coordination and Partnerships Directorate ensures coordination among programs,

laboratories and courses, as well as management skills and partnerships EDF R&D. These

partnerships are realized in several forms: framework agreements, joint laboratories, clusters

and targeted participation in projects of the National Agency for Research (ANR).

The main concerns of the R & D consists of 12 challenges which can be classified into the following

five themes: our planet, our optimization, power systems, customers and production. For example, the

Challenge "Preparing for the 2015 Distribution" is working on the development of power systems able

to integrate energy sources and distributed storage capacity, interaction with clients (controlling the


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charge, access to a pricing policy in real time etc.) and a possible development of electric vehicles.

The Challenges suggest changes in planning and operation of distribution power systems, which are

validated by local experiments in order to facilitate transition to the operational stage.

2.3. The research team EFESE-R12

This Master’s thesis took place within the Economics, Operations and Energy Systems Studies Department (EFESE), located at Clamart and with four groups: Operation of electrical power systems and connection (R12), Energy market and environmental regulation (R13), Economics and energy Strategy (R16) and Economics of power systems (R19). I was welcomed in the research team R12, which includes fifteen researchers and two doctoral students around topics related on power system services for centralized production, distribution power system, the connection of wind power and photovoltaic power on power systems and storage of energy in distribution networks. As part of its activities, the R12 group is involved in a number of projects covering all the issues above and is involved in many partnerships in France and also abroad.


Technology choices of the reference offshore wind farm

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3. Technology choices of the reference offshore wind farm

In this chapter, technologies which constitutes offshore wind farms and transmission lines will be detailed and explained. Then a literature review on existing offshore wind farms in Europe has been realized. These two parts will lead us to define the reference offshore wind farm which will be modelled and used later in the Master’s Thesis.

3.1. Offshore wind farm technologies

On a technical point of view, offshore wind facilities have been firstly developed thanks to onshore models, despite the differences and despite the fact that offshore wind turbines have to resist to difficult climatic conditions. However huge technological improvements have been achieved recently and wind turbine companies develop today new wind turbines which are specifically for offshore conditions. The working principles are the same as onshore wind turbine working principles, however the sea conditions induce stronger turbines : The wind turbine has to be studied to resist to the strength of the waves and of the sea currents altogether with the protection from corrosion which must be improved.

An electrical network of an offshore wind farm consists of (see

Figure 7):

Wind turbines (with their internal transformer) ;

Internal submarine cables (Network of the electricity collector);

Offshore substation (if present);

Transmission submarine cables,

Onshore substation (and onshore cables),

Connecting point to an existing grid (PCC).

Figure 7 : Classical single line offshore wind farm diagram [8]



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In this following subsection, different characteristics of an offshore wind farm are going to be presented, detailed and explained.

3.1.1. Electrical layout of offshore wind farms :

Different offshore wind farm electrical layout are presented in Figure 8 . We can find 3 different kinds of layout of the internal electrical network : AC/AC, AC/DC and DC/DC [6].

AC/AC electrical layout : In this layout, wind turbines are connected to each other and to the offshore substation thanks to AC cables whereas the transmission cable to the shore is an AC cable.

AC/DC electrical layout: Wind turbines are connected thanks to AC cables but the transmission is achieved thanks to an HVDC electrical power transmission system.

DC/DC electrical layout: Wind turbines are connected in DC and the transmission is achieved in DC.

Figure 8 : Offshore wind farm electrical layouts [6]

Currently, the type of offshore wind farms is AC/AC. However some AC/DC offshore wind farm which will be connected to the shore with a HVDC electrical power transmission system, is being built in Germany. The first offshore wind farm with a HVDC cable will be operational in Germany in the beginning of 2012.

3.1.2. Wind turbine generators:

We can divide wind turbine generators into 4 groups which are presented in Figure 9.



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Figure 9 : 4 types of wind turbine generators [9]

3.1.2.1. Type A wind generators

Type A is a fixed speed asynchronous generator, called also Danish concept, it is the oldest wind turbine generator on the market and it is disappearing. Indeed, this generator is less efficient because the rotation speed is fixed and cannot get the maximum power from the wind whereas other generators which are must more efficient but expensive (see the following wind trubine generators) become cheaper.

3.1.2.2. Type B wind generators

Type B is a variable speed asynchronous generator. The type B generator rotor is directly connected to a variable resistance, which can change the rotor current and therefore the turbine rotational speed can be variable (+/- 10%).

3.1.2.3. Type C wind generators

Type C which is called DFIG is a variable speed asynchronous generator. The rotor of this generator is connected to the grid thanks to power converters which adapt the rotor rotational speed.

3.1.2.4. Type D wind generators

Type D is a variable speed synchronous or asynchronous generator. The generator stator is electrically decoupled from the electrical grid thanks to power converters. Gearbox is one of the least efficient elements of a wind turbine because of the friction contact between two gears. The advantage of this type of wind turbine generators is that this generator with the use of a multi pole synchronous generator prevents to use a gearbox which is a critical element.

3.1.2.5. Comparison between “DFIG or full converter induction machine” and “full converter permanent magnet synchronous generators”

Currently, as we will see in the following state of the art, the huge proportion of built offshore wind turbine generators are DFIG or full converter induction machine (type D with an asynchronous generator) but a new trend is growing with the development of many full converter permanent magnet synchronous generators (type D with a synchronous generator).

Today, Vestas id developing a new 7 MW offshore wind turbine (V164) based on a medium-speed drive-train solution and on a full converter permanent magnet synchronous generators. Siemens



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develops also a 6 MW full converter permanent magnet synchronous generators without gearbox thus based on direct drive-train solution (SWT-6.0-120) . Whereas Areva sells already its model the M5000 with is a 5MW full converter permanent magnet synchronous generators with gearbox.

We wonder why the mark part of full converter permanent magnet generators are growing rapidly. Therefore we are going now to list the advantages of the permanent magnet generators compared to the DFIG or to the full converter induction generators :

The gearbox can be used with a small rotational speed ratio and we can even avoid to use the gearbox. Indeed the gearbox is a critical element for the overall efficiency of the wind turbine. Thus, with permanent magnet generators, it is possible to obtain higher overall efficiency.

Permanent magnet generators do not need rotor windings, which decreases the losses and increases the overall efficiency of the turbine

The operation cost is reduced because permanent magnet generators do not need separate excitation and slip rings, which will decrease the maintenance operation

Mechanical loads are reduced due to a slow speed of the shaft.

However, full converter permanent magnet generators are heavy (depends on the uses of a gearbox or not) and expensive.

Power converters of DFIGs are cheaper than for full converter generators. Indeed, the power which transits through this converters are 3 times less for DFIG power converters than for full converters. However, full converters are more reliable and begin to become cheaper.

3.1.3. Internal cables :

Wind turbines are connected to each other thanks to three-core submarine cables before sending electrical power to the onshore grid and the three insulated conductors are placed inside an underwater cable. In order to connect each turbine, each wind turbine has an internal transformer which rises the terminal wind turbine voltage to the internal wind farm voltage (around 33 kV). The wind turbines are then linked together with 33 kV submarine cables.

In order to connect the wind turbines to each other, it can be either a meshed connection or a radial connection. However, in order to avoid submarine cable extra cost, the radial connection is always used for the design of an offshore wind farm.

There are different submarine cable insulation technologies which are respectively mass-impregnated and cross-linked polyethylene (XLPE). However due to power capacity transfer which is more important for XLPE cables in which the electrical conductor is in copper, this technology is currently the most used. For instance, the maximum power transfer of mass-impregnated cables is 1000 MW and for the XLPE cable it is 1450 for a 2500mm² cable at 500 kV. [10] For a conductor cross section of 240mm², each radial line can connect a maximum power of 30 MW with a 33 kV XLPE insulated cable.

These cables (mass-impregnated and cross-linked polyethylene) has higher shunt capacitance, 100-200nF/km, than overhead lines, 9-13 nF/km, because of the short geometrical distance within cables. [10]

3.1.4. Offshore substations :

Offshore substations are used in order to reduce power losses during the transmission of electrical power to an onshore network bus. The transformer which is the major component of the offshore substation, will raise the voltage before the transmission line. In general, it raises the voltage from 30-36 kV to 100-220 kV.

However the offshore substation is not compulsory in the following cases :

The power of the offshore wind farm is less than 100 MW

The distance to the shore is less than 15 km



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The voltage of the electrical grid connecting point is less than 36 kV

3.1.5. Wind farm connections : Transmission cables

The connections of the offshore wind farm used either direct current (HVDC) technologies or alternative current (HVAC) technologies. The choice between these 2 technologies depends on the distance to the shore and the total power of offshore wind farms. [10]

For HVAC, the connection of wind farms to the shore is made thanks to a XLPE AC – 3 phases submarine cable with a rated voltage around 130 kV – 150 kV. Currently 245 kV submarine cables are available whereas 400kV submarine cables are under investigation. One of the issues of these cables is the limitation of the transmitted power over large distance. Indeed, higher is the transmitted power and the distance to the shore, higher power losses will be [37]. Thus, if the losses are significant it will not become economical to use HVAC transmission systems.

Concerning HVDC, the HVDC technology which is the most often used for the transmission, is the Voltage Source Converter (VSC) with XLPE cables. The VSC-HVDC technology is preferred to the LCC-HVDC technology. The rated voltage can go up until ±150 kV however some projects with a rated voltage of ± 200 kV have been approved and some projects until ±320 kV have been already ordered. The advantages of the HVDC technology is that this transmission systems can transfer more power through larger distance with less power losses than the HVAC technology.

To conclude, HVAC transmission systems are going to be used for small distance and HVDC transmission systems for larger distance.

The connection point (PCC) to the electrical grid is currently at a rated voltage around 130 - 150 kV. Indeed, the power transmission is achieved today thanks to 130-150 kV HVAC transmission systems. As connection costs to a higher rated voltage bus, for example 225 kV, is higher, it is more economically profitable to connect the wind farm to a 130 – 150 kV bus than to a 225 kV bus. However if the rated power of a wind farm is too high, it is sometimes necessary to connect directly the offshore wind farm to a 225 kV bus.

3.2. Analysis of 6 offshore wind farms

In this section, 6 offshore wind farms with different electrical layout, “Thanet”, “Lillgrund”, “Horns Rev 1”, “Lynn and Inner Dowsing”, “Barn Offshore 1” and “Nordsee Ost and Meerwind”, have been investigated in order to define the reference offshore wind farm which will be used in the case studies.

the 6 offshore wind farms can be divided as following :

1 AC/AC farm with a substation with 2 transformers and 2 HVAC cables are used to link the offshore wind farm to the onshore electrical grid ;

2 AC/AC farms with a substation with one transformer ;

1 AC/AC farm without substation ;

2 AC/DC farms using the VSC-HVDC technology.



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THANET - 300MW, AC/AC [2,11,12,13] : is an English offshore wind farm built in 2010 and situated in North Sea. It has been accepted by “The Crown Estate”

2 during the “Round Two”

3 in UK.

Indeed, the goals of the UK is to have 15 % of electricity produced by renewable energy in 2015 and 20 % in 2020. This can be compare to 3 % in 2009 [39].

This wind farm has been developed by Vattenfall whereas Vestas provided the wind turbines, Siemens the power electronic, and Prysmian the electrical cables.

Thanet can produce up to 300 MW of electrical power and it is situated 12 km from the shore but 27 km from the grid PCC. In Figure 10, we can notice a FACTS component, SVC (Static VAR Compensator), which has been installed at the PCC bus in order to control the voltage by injecting or absorbing reactive power. Indeed, it is necessary because the offshore wind farm has an important active power output and the voltage level of the connecting point is not high enough. The layout of the Thanet wind farm can be observed in Figure 10. The HVAC transmission system of this offshore wind farm is realized with 2 submarine cables and each cable can transmit 150 MW.

Figure 10 : Electrical connection of the offshore wind farm THANET

2 The Crown Estate is an English monarchy company which have two main goals: to benefit the taxpayer by

paying the revenue from their assets directly to the Treasury; and to enhance the value of the estate and the income it generates. [38]

3 Round Two is the second call for tender for offshore wind farms in UK.



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LILLGRUND – 110 MW, AC/AC [14] : is a Swedish offshore wind farm built in 2007 between Malmö and Copenhagen ( South of Sweden). It can produce up to 110 MW of electrical power and it is situated 7 km from the shore but 9 km from the grid connection point. It is currently the largest and only offshore wind farm in Sweden. The electrical layout of Lillgrund wind farm can be observed in Figure 11.

This wind farm has been developed by Vattenfall whereas Siemens provided the wind turbines and the power electronic, and ABB the electrical cables.

Figure 11 : Electrical connection of the offshore wind farm LILLGRUND



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HORNS REV 1 – 160 MW, AC/AC [15,16] is a Danish offshore wind farm built in 2002 in North Sea ( West of Denmark). It is the first large offshore wind farm to be built in North Sea and the Danish government has decided the construction of a total of 4 GW offshore wind farm before 2020. Currently there are around 800 MW of offshore wind farms [40].

It can produce up to 160 MW of electrical power and it is situated 21 km from the shore but 55 km from the grid connection point. The electrical layout of Horns Rev 1 wind farm can be observed in Figure 12. Due to the high distance until the connection point, some compensations (reactance) have been built, one in the offshore platform and another in the middle of the line at the onshore substation.

This wind farm had been developed by DONG Energy but is now operated by Vattenfall whereas Vestas provided the wind turbines, Siemens and Alstom the power electronic, and Nexan the electrical cables.

Figure 12: Electrical connection of the offshore wind farm HORNS REV 1



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LYNN AND INNER DOWSING – 194 MW, AC/AC [17,18] : is an English offshore wind farm built in 2009 which consists of 2 offshore farms Lynn and Inner Dowsing (the same power for each farm). it can produce up to 194 MW of electrical power and it is situated 6.5 km from the shore and from the grid connection point. In opposition to the offshore farm Thanet, Lynn and Inner Dowsing is part of the “Round one”

4. The particularity of this wind farm is the absence of an offshore substation

and so a MV/HV transformer. There are thus many submarine cables (6 cables) of 33kV which deliver the electrical power to the grid connection point.

The layout of Lynn and Inner Dowsing farm can be observed in Figure 13. The limit between the TSO and the producer is situated at the secondary side of the transformer. It means that the transformer are possessed by Centrica which is a leading integrated energy company and which is Lynn and Inner Dowsing wind farm owner.

This wind farm had been developed by Centrica whereas Siemens provided the wind turbines, and Nexan the electrical cables.

Figure 13: Electrical connection of the wind farm LYNN AND INNER DOWSING

4 “Round one is the first call for tender for the construction of offshore wind farm in UK.



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BARD OFFSHORE 1 – 400MW, AC/DC [19,20] : is an German offshore wind farm in construction in North Sea. 13 wind turbines are already built and connected to the electrical grid. The date of construction end is planned for 2012. It will produce up to 400 MW of electrical power and it is situated 100 km from the shore and 200 km from the grid connection point (380 kV). The offshore wind farm Bard Offshore 1 will be the first wind farm to be linked to the electrical grid thanks to a HVDC line. The 400 MW VSC-HVDC transmission system is built and will be used by one of the German TSOs, Tennet.

The electrical layout of Bard Offshore 1 wind farm can be observed in Figure 14. In this case the TSO will be the owner of the offshore substation BorWin alpha and the limit between the producer and the TSO is situated at the entry of this substation.

This wind farm has been developed by BARD Holding GmbH which provides also the wind turbines whereas ABB provides the power electronics (HVDC line) and the electrical cables.

Figure 14 : Electrical connection of the offshore wind farm BARD OFFSHORE 1



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NORDSEE OST AND MEERWIND – 576 MW, AC/DC [21, 22, 23] : Nordsee Ost and Meerwind are two German offshore wind farms in project in North Sea. They are part of a huge development of German offshore wind farm with HVDC transmission systems in North Sea. The particularity of this project is that the HVDC transmission line will be used by different offshore wind farms. These two offshore wind farms will produce up to 576 MW of electrical power and they are situated 85 km from the shore and 130 km from the grid connection point of a 380 kV bus. The commissioning of these 2 offshore wind farms is planned for 2013.

The electrical layout of the Nordsee Ost and Meerwind wind farms can be observed in Figure 15. In this case as in other cases, the TSO will be the owner of HelWin offshore substation. It means that the TSO will have the two transformers 155-25 kV and the offshore power converter of HVDC transmission line. The limit is thus situated at the entry of the substation HelWin.

Figure 15 : Electrical connection of the wind farm NORDSEE AND MEERWIND OST

It can be noticed that the German offshore wind farm are really far from the coast (100 km), which involves the use of VSC-HVDC technologies for power transmission. Indeed, Germany has strict rules about environmental protections around the coasts and it is impossible to build offshore wind farm near coasts.

The study of these 6 offshore wind farms shows us some important electrical components which will be detailed in the next section, of an offshore wind farm electrical layout.

3.3. Conclusion about the studied offshore wind farms

The Table 3 summarizes all the characteristics of studied offshore wind farms which are representative of offshore wind farms in North Sea.

In addition, 4 other offshore wind farms, “Rødsand II”, “Horns Rev 2”, “Walney 1” and “Belwind” have been investigated. It has been done in order to confirm technologies used by existing offshore wind



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farms.

Rødsand II is a Danish offshore wind farm which has been built in 2010 and have a total capacity of 207 MW. [42]

Horns Rev 2 is also a Danish offshore wind farm which has been built in 2009 and have a total capacity of 209 MW. It was the largest offshore wind farm until Thanet was opened in September 2010. [43]

Walney 1 is an English offshore wind farm which has been built in 2011 and have a total capacity of 183.6 MW. [44]

Belwind is a Belgian offshore wind farm which has been built in 2010 and have a total capacity of 165 MW. [45]

These 4 wind farms are presented in Table 4.

Table 3 : Summarizing list of the studied offshore wind farms

Studied farms Thanet Lillgrund Horns Rev

1

Lynn and Inner

Dowsing

Bard Offshore 1 (BorWin alpha)

Nordsee Ost + Meerwind :

HelWin1

Power (MW) 300 110 160 194 400 576 (288+288)

Distance to the grid

27 km 9 km 55 km 6.5 km 200 km 130 km

Country UK Sweden Denmark UK Germany Germany

Date 2010 2007 2002 2009 2012 2013

Type of electrical layout

AC/AC AC/AC AC/AC AC/AC AC/DC AC/DC

Nbr/Type of wind turbines

100/DFIG

48/Full converter induction machine

80/DFIG

54/Full converter induction machine

80/ DFIG 48/DFIG;

80/-

Radial connection of

turbines X X X X - -

Transformer (Offshore

substation) 2 1 1 0 1

4 (2 33kV/155kV ; 2 155kV/250kV)

Internal Voltage 33 kV 33kV 33 kV 33kV 33kV 33kV

Transmission line

2 AC XLPE 132 kV

AC XLPE 130 kV

AC XLPE 150kV

6 AC XLPE 33 kV

VSC-HVDC light 400

MW +/-150 kV

VSC-HVDC Plus 576 MW

250 kV

Layout available

X X X X - -



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Table 4 : 4 different existing offshore wind farms in Europe

Rødsand II Horns Rev 2 Walney 1 Belwind

Power (MW) 207 209 183.6 165

Distance to the grid ~ 10 km 42 km 15 km 52 km

Country Denmark Denmark UK Belgium

Date 2010 2009 2011 2010 Type of electrical layout - AC/AC AC/AC AC/AC

Nbr/Type of turbines

90/ Full converter induction machine



55/DFIG

Radial connection of turbines

X X X X

Transformer (Offshore substation)

- 1 1 1

Internal Voltage 33kV 33kV 33kV 33 kV

Transmission line - HVAC 150 kV HVAC 132 kV HVAC 150 kV

Layout available - X X -

Thanks to this two tables, some conclusions can be drawn about offshore wind farm electrical layouts. Offshore wind farms with an active power larger than 150 MW are really recent (commissioning since 2009) except for Horns Rev 1 which was the first large offshore wind farm in Europe. Nowadays offshore wind farms become farther and larger. One illustration of this phenomenon is the comparison between the different English “Round” launched by “The Crown Estate”. [24]

The main characteristics of built and in project offshore wind farms which have been identified, are :

The used wind turbine generators are either DFIG or full converter induction machine. However currently many full converter permanent magnet synchronous machines with or without gearbox are developed by wind turbine manufactures for a specific offshore use. Until now this technology was still too expensive but the efficiency of this turbine is superior to other kinds of turbines and this technology seems to become the standard for offshore wind turbines.

The internal voltage of offshore wind farms seems to be around 33 kV. This means that each wind turbine has its own transformer which raise the voltage from 0.69 kV (in general but it can be 1 kV for example for the turbine V90- 3.0 MW) to 33kV.

The layout of offshore wind farms is AC/AC or in the near future (2012) AC/DC in Germany. Moreover offshore wind farm turbines are connected in radial which is geometrically completely different for each farm (see appendix X)

The presence of a transformer on an offshore substation or onshore depends of the transmission distance :

- Between 0 and 10 km, the offshore wind farm do not use substation. Therefore many MVAC lines are used ( ~33 kV) for transporting the electrical power to the grid.

- Between 10 and 60 km, a preferential solution is now the use of one transformer (or many) in an offshore substation and offshore wind farms will be connected to the onshore electrical grid by one or many HVAC lines (~130 kV – 150 kV). Currently 245 kV HVAC lines are available and 400 kV HVAC lines will come. One of the problems


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is the limitation of the allowed transmitted power by these lines. If the active power of offshore wind farms is large, many lines are needed in order to transfer the electrical power to the grid, which increases connection costs.

- Superior to 60 km, the most economical and performing solution is the use of HVDC transmission systems. The used technology for connecting of offshore wind farms to an electrical grid is the VSC-HVDC transmission system. This technology seems to become the standard for larger and farther offshore wind farms.

3.4. Chosen specifications for the reference offshore wind farm

In order to be the most general as possible in the study of offshore wind farm dynamic behavior, a standard offshore wind farm model has to be define. Therefore a state of the art of existing

technologies and already installed offshore wind farms in Europe together with wind farms in project

was made in order to find a reference offshore wind farm model which has the most common

characteristics. The selected specifications of the reference offshore wind farm which will be used in the future 2 case studies are:

Power of the reference offshore wind farm : 200 MW ;

Wind turbine : Full converter synchronous generator 3 MW ;

Radial connection ; Internal voltage 33kV ; Offshore transformer 33/ 225 kV

AC/AC Wind farm: HVAC 225 kV (30 km)

AC/DC Wind farm: HVDC (50 km)5

Compensation if necessary

The characteristics of the reference offshore wind farm have been defined. In order to perform a good dynamic study of this standard offshore wind farm, technical requirements that offshore wind farms has to fulfil, have to be investigate.

5 The AC/DC wind farm, in particular offshore VSC-HVDC transmission systems, has not been modeled in this

Master’s Thesis but it can be the aim of future researches.


Technical requirements for offshore wind farm connections : Specific offshore Grid Code

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4. Technical requirements for offshore wind farm connections : Specific offshore Grid Code

The connection of an electrical energy production unity must respect some technical conditions. The grid code defines these requirements. An offshore wind farm is a production unity but it does not have the same characteristics as a classical production unity. Indeed wind intermittence is an importation limitation for wind power production because it is impossible to generate continuously the same amount of active power and wind turbines cannot entirely participate to system services as primary and secondary frequency control or voltage control. Moreover localizations of offshore wind farms are distant from the electrical grid, which can imply that technical requirements will be stricter and limited.

It is thus necessary to specify technical grid connection requirements of these production unities. There is currently in France no offshore grid code. Besides in Europe the offshore wind power market are growing and others countries need offshore wind farm requirements (or in general large wind farm requirements).

Therefore, the massive development of offshore wind in North Sea has encouraged TSOs to define the technical requirements for this type of production. The Great Britain is the first country, thanks to National Grid

6, to publish a specific grid code for offshore wind farms with all requirements and

simulations that producers have to provide in order to be authorized to connect an offshore wind farm to the English transmission power system. For instance, ones of the requirements are reactive capabilities of offshore wind farms which must consume and provide reactive power. This consuming requirement is tested by a static simulation when the PCC voltage is above the upper voltage limit, U > 1.05 Un, which confirms that offshore wind farms consume the maximum of reactive power.

After many discussions between the European TSOs which are members of the ENTSO-E (cf. Figure 16), a project about a common offshore technical referential is under development.

Figure 16 : Members of the ENTSO-E

6 National Grid is the English TSO



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ENTSO-E wants to harmonize the different electrical power systems in order to secure the European network. It will be necessary in the case of an offshore North Sea supergrid construction. The question of the location of the connection point is still decided by each TSO. Indeed, some countries, like Sweden, Denmark, United-Kingdom or Belgium, have chosen to apply the grid requirements at an onshore connection point whereas some countries, like Germany (France also , RTE will be the owner of transmission systems) prefer to manage the transmission and to apply the requirements at an offshore PCC (see Figure 17). The impact of choosing the PCC onshore or offshore will be studied in the case study n°1. However, the advantage of choosing the PCC onshore is that TSOs do not have to take care about the transmission systems and it will be cost savings. The drawback of choosing the PCC onshore is that the TSO will not be the owner of offshore transmission systems in case of a meshed connection between the offshore wind farm, which can be problematic for the stability of the overall power system.

Figure 17 : PCC of offshore wind farms defined by the TSO

The technical requirements for the offshore wind farms has been investigated for the following countries or association

Germany : Tennet which is one of the German TSOs [26]

United Kingdom : National Grid which is the English TSO (add reference)

Nordic countries : Nordel which is an association of the Nordic countries (Nordic grid code for wind turbines above 100 MW). [27]

Europe : ENTSO-E ( European network of transmission system operators for electricity) which is a European TSOs association[28]

RTE technical requirements are sometimes presented however these technical requirements are applied for classical production units.

Accordingly to ENTSO-E, offshore wind farms are grouped into 6 categories [28]: (draw figure to explain)

Configuration 1 : (Radial AC Connection) An Offshore AC System is connected to an Onshore System with one or more AC radial connection(s) to the same Onshore Grid Interconnection Point.

Configuration 2 : (Meshed AC Connection) An Offshore AC System is connected to an Onshore System at two or more Onshore Grid Points.

Configuration 3: (Radial DC Connection and AC Connection) Offshore Power Park Modules are interconnected offshore to form an Offshore AC System. The Offshore AC System is connected to an Onshore System with a radial DC connection or parallel DC connections at one Onshore Grid Interconnection Point location.

Configuration 4: (Hybrid AC/DE Solution) An Offshore AC System is connected to an Onshore System with radial AC and DC connections at two or more Onshore Grid Interconnection Point locations.



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Configuration 5: (Meshed Multiterminal DC, AC Collection) An Offshore AC System is connected to an Onshore System with multiple DC connections at two or more Onshore Grid Interconnection Point locations. The DC connections may be combined in a multi-terminal system and may also have a connection to an offshore system of another country.

Configuration 6: (Meshed DC, DC Collection) An Offshore Power Park Module connected in DC to an Offshore DC System. The Offshore DC system is connected to an Onshore system with one or more DC link(s).

4.1. Requirements about the voltage management

4.1.1. Steady state conditions

(U,f) diagrams defines voltage limits and frequency limits for normal operations of offshore systems. The Figure 18 shows that for each TSO steady state requirements can be different. The squares show the area in which each offshore wind farm must operate continuously. UK (National Grid) imposes the most important frequency range in which offshore wind farms have to operate continuously.

ENTSO-E imposes a voltage range which can change depending of the countries : Central Europe, Nordic countries, United Kingdom, Irish and the Baltic countries. The green square contains (U,f) requirements of the German TSO Tennet and of Nordel. Considering the voltage, the ENTSOE contains all the TSO. It means that if all TSOs want to respect the future ENTSO-E steady state requirements, they will have to increase their steady state conditions (higher voltage range and higher frequency range).

Beyond these continuous operating ranges, the TSOs points out in the offshore grid code the minimal time in which the offshore wind farm has to stay connected to the electrical grid.

Figure 18 (U,f) offshore continuous requirements of the TSOs



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4.1.2. Reactive power/ Voltage regulation

For the TSO Tennet and the Nordic TSO association Nordel, reactive power exchanges between offshore wind farms and electrical grids must be null during voltage steady state conditions. The capacity of providing or absorbing reactive can be defined in a bilateral contract between producers and TSOs.

However, all TSOs requires that offshore wind farms have reactive capacities. The Figure 19 shows the difference between the reactive capacities in the German and British cases. In this figure, (U,Q) diagrams defined by the squares represent all points that offshore wind farms must reach. Tennet imposes more difficult requirements about providing reactive power than National Grid. An offshore wind farm which are connected in Germany must provide 41 %

7 of Pmax (maximum active power of

offshore wind farms) of reactive power whereas an British offshore wind farm must provide 31 % 8of

Pmax.

The ENTSO-E requirements take into account the (U,Q) diagram of Germany and of The united kingdom. According to ENTSO-E, an offshore wind farm must at nominal voltage provide up to 40 % of Pmax and absorb up to 35 % of Pmax (which can be seen in Figure 20). The red square is a basic requirement than all offshore wind farms must fulfil. It means that all point inside this red square must be reached by offshore wind farms.

Figure 19 : (U,Q) diagram requires by National Grid and Tennet

7 41 % of Pmax corresponds to a power factor of 0.925 at Pmax

8 31 % of Pmax corresponds to a power factor of 0.95 at Pmax



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Figure 20 : (P,Q) diagram requires by ENTSO-E

Concerning the French grid code for classical power plant, RTE requires (U,Q) diagrams for active power equal to Pmax and to Pmin. This diagram can be observed in Figure 21. (U,Q) requirements of the French grid code is less demanding as ENTSO-E requirements or Tennet grid code requirements.

Figure 21 : (U,Q) RTE requirements

4.1.3. Fault ride through (with grid support)

An offshore wind farm must stay connected during a voltage dip that is defined by the TSOs. The Figure 22 shows fault ride through curves which are imposed by ENTSO-E (Best case and worst case), UK, Germany and the Nordic countries (Sweden and Denmark) and the French (connection points that are up to 50 kV) voltage dip curve. All the curves have a maximum voltage dip level which can be 0 p.u during a limited time except for the voltage dip curve of the British case (the maximum voltage dip level is 0.15 p.u).

ENTSO-E has one voltage dip curve with 2 different slopes. All TSOs has the choice to choose between the two limits. Indeed, the worst case is situated after all other voltage dip curves.



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0

0,2

0,4

0,6

0,8

1

1,2

-0,5 0 0,5 1 1,5 2 2,5 3 3,5t (s)

Un

(p

.u)

ENTSOE (Worst Case)

ENTSOE (Best case)

France (RTE)

Nordel

United Kingdom (National Grid)

Germany (Tennet)

Figure 22 : Voltage fault ride through curve for high voltage electrical grid

In yellow, the RTE requirement about fault ride through capabilities is less demanding than ENTSO-E requirements.

4.2. Requirements about the frequency management

Offshore wind farms must have a controllable active power. The power order has to be modifiable manually or thanks to a remote controlled. TSOs can also ask offshore wind farms to provide a inertial contribution

9 thanks to an increase of their active power which should be proportional to the frequency

variation. Indeed, the participation in frequency control of power plants is important for system frequency stability. Especially if the part of offshore wind farm in the energy electrical mix is significant, offshore wind farms will have to participate in frequency control. In order to secure the system, offshore wind farms should participate in frequency control and hence they should have a controllable active power.

According to ENTSO-E offshore grid code, offshore wind farms must have a communication interface with TSOs. The following information must be provided if asking :

State of the frequency response

Active power order

Produced active power

Value of the order for the frequency response

Gap of activation of the frequency response

9 Inertial contribution is to participate in primary control.



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Available power

The TSO can ask for supplementary signals in order to verify the performance of the offshore wind farm.

The range of frequency ENTSO-E requirements defined in Table 5 is applied to offshore wind farms with the configuration 1,2 or 4. Offshore wind farms have to stay connected to the grid respecting the minimal operating times. For example, if the power system frequency is 48 Hz, offshore wind farms must stay connected during 90 min but can disconnect themselves after 90 Hz.

For the configurations 3 and 5, because of their perturbations sensitivities like some transient effect or fault on the HVDC converters, the frequency range is higher : from 46.5 Hz to 53 Hz. (explain it)

Table 5 : Requirements about the frequency variations of ENTSO-E offshore grid code

Frequency range Minimal operating time

47.5 Hz- 48.5 Hz 90 minutes

48.5 Hz – 49.0 Hz The TSO have to define the time but it will be more than 90 minutes

49.0 Hz – 51.0 Hz continuously

51.0 Hz – 51.5 Hz 90 minutes

For example, in the German case, the offshore wind farm must be capable of changing the active power in order to stay connected in case of frequency variations. (see Figure 23)

Figure 23 : Tennet requirements for frequency variations

Offshore wind farms in Germany must be capable to provide the maximum active power until 50.2 Hz. Above this limit, farms must decrease their provided active power following the slop in the Figure 23.

4.3. Synthesis about offshore grid codes

In the previous sections, the voltage and the frequency requirements has been explained. Moreover, all grid codes requires stability tests as small-signal stability or transient stability which must be defined by TSOs.

The offshore grid code prepared by ENTSO-E attempts to take into account the majority of National Grid offshore grid code requirements and of Tennet offshore grid code requirements. However, there are small differences. For instance, United Kingdom is more demanding about the frequency (huge range of frequencies) but UK is less about voltages. Indeed, the UK is an island and have more frequency variation than in the continent. Therefore, in order to avoid power system fails (blackout) the offshore wind farm must staying connected for a higher range of frequency. Concerning fault grid through requirements, ENTSO-E has the most demanding voltage dip curve.



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With the absence of a French offshore grid code, this Master’s Thesis report will focus in the case study n°1 on ENTSO-E requirements about the capacity of providing/absorbing reactive power and on voltage dip. However, the classical French grid code will be considered in case study n°1.

ENTSO-E requirements for offshore wind farm are applied at an onshore PCC. However future PCCs of French offshore wind farms will be offshore, as explained in section 1.1.2. Therefore in order to cover all the cases, the case study n°1 will consist of 2 different connection studies :

The group “wind farm + offshore substation + transmission system” will have to fulfil the ENTSO-E requirements at an onshore PCC

The group “wind farm + offshore substation“ will have to fulfil RTE classical requirements at an offshore PCC

These two connection studies are going to be more explained in the chapter of the case study n°1.

The standard offshore wind farm has been defined and now grid requirements are known. However before beginning simulations on case study n°1, the general behaviour of the group “wind farm + offshore substation + transmission system” should be identified, which will be done thanks to the following literature review.


Behavior of offshore wind farms during a fault : farm + HVDC

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5. Behavior of offshore wind farms during a fault : farm + HVDC

5.1. Introduction

It is important to understand dynamic behaviours of offshore wind farms during a fault. Indeed, the grid requirements impose that offshore wind farms must stay connected during some faults. Hence the impact of a fault at the PCC on an offshore wind farm should be identified and understood. We must first distinguished the 2 types of possible transmission systems.

5.1.1. HVAC transmission systems

For an AC transmission cable, faults which are happening at a bottom of the cable, will be seen accentuate or not at the other bottom. Therefore if a fault happens at the PCC, it will spread until wind turbines of offshore wind farm which will use their fault ride through mode in order to protect their generator and their power converters. This mode uses wind turbine crowbars

10 which will dissipate the

surplus of power and therefore protect wind turbines. These comments about the behaviour of “an offshore wind farm + a HVAC transmission line” will be useful in order to understand the case study n°1. However, the wind turbine fault ride through mode will be explained in detail at the end of the case study n°1.

5.1.2. HVDC transmission systems

HVDC transmission cases are different from HVAC cases. HVDC transmission systems will act as a barrier between offshore wind farms and onshore electrical grid. However during a fault, the transmission will not spread the voltage fault to offshore wind farms, which can involves some constraints on the HVDC line. In order to protect itself, the HVDC transmission system will have to disconnect itself. However, it is not allowed by grid code requirements. For avoiding this, faults must be spread artificially. Then offshore wind farms will be able to react consequently and the disconnection of the HVDC transmission line will be prevent. An artificial spread of faults can be done thanks to different ways but the aim is to decrease the voltage at the other end of the HVDC transmission system. [29,30,31,32] This issue are going to be discuss in the following sections.

5.2. Presentation of VSC-HVDC transmission systems

A VSC-HVDC transmission systems consists of a power converter (rectifier) built on an offshore substation and of a power converter (inverter) built onshore. The voltage source converters are situated at each extremity and they are controlled in order to transfer to an electrical onshore gird all power produced by the offshore wind farm. In Figure 24, an offshore wind farm with its VSC-HVDC transmission line can be observed.

10 Wind turbine crowbar is a resistance at wind turbine converters level of a wind turbine which will dissipate

power differences between active power produced by the mechanical power and active power that electrical grids are able to transmit.



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Figure 24 : An offshore wind farm and its HVDC line (three level topology) [31]

VSC-HVDC systems use generally a « two-level topology » technology, which is represented in Figure 25 in which each converter consists of 6 semi-conductors (iGBT), commutation inductor, harmonic filter and DC-filter.

Figure 25 : VSC two level topology [30]

The use of a « VSC two level topology» technology is due to its simplify control methods but with this technology the commutation losses are important. However a “VSC multi-level topology” technology allows to decrease the losses. This topology technology can be observed in Figure 26 .

Figure 26 : A VSC multi-level converter [46] (SM : SubModule)



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Converter control systems are important elements of VSC-HVDC transmission systems and therefore basic control strategies of VSC-HVDC converters in order to connect offshore wind farms must be explained. During a normal use of VSC-HVDC transmission systems, the goal is to transmit wind farm power to a PCC. The general operation of a HVDC line is described in the document [10]. To summarize :

The wind farm side VSC converter is responsible for transmitting the wind farm electrical power to onshore power systems. In order to achieve that the transmitted power by the HVDC line must be identical to the power produced by wind farms. In other words wind farms must see the HVDC line has an infinite bus. Therefore, the wind farm side VSC converter must keep frequency and AC internal voltage constant (f=50Hz et Vfarm=33kV).

The grid side VSC converter should control the DC voltage and can also control the reactive power. The control of the DC voltage allows an operation of the HVDC line without troubles whereas the control of the reactive power give a support of keeping the voltage at the PCC.

During a normal use, HVDC transmitted active power is adjusted to electrical power generated by wind turbines. However during a fault on grid side, offshore wind farms must react consequently in order to avoid the logout of the HVDC line. In the next sections, Issues about why it is impossible to avoid the logout of the HVDC line with the control modes defined above and about different solutions will be approached..

5.3. VSC-HVDC line and farm behaviors during a fault

In this section, a power unbalance problem of HVDC transmission systems is explained and then in order to solve this power unbalance problem, some solutions are proposed.

5.3.1. Description of the problem

In Figure 27, a representation of a HVDC transmission system can be seen with voltages, currents and powers that are involved in operations of the HVDC line.

Figure 27 : Simplify representation of a HVDC line

In normal operation time, the power balance can be written as (Eq. 1). However, during a fault which happens on grid side, the voltage at the PCC decreases, Ugrid ↓, whereas the produced active power from offshore wind farms stays unchanged. The current transmitted on the HVDC line cannot increase, which comes from a cable limitation.

Hence the current Igrid cannot increase and the voltage at the PCC decreases therefore the power transmitted to the grid Pgrid decreases. The absorbed power by the electrical grid is lower than the transmitted power from the wind farm, which creates a power unbalance . The surplus of power in the HVDC line will involve an increase of the power consumed by the capacitor , Pc ↑, and the voltage UHVDC of the line will increase. If the voltage UHVDC increases too much, the line will logout in order to protect itself. Indeed, the voltage can reach a critical value which can be dangerous for the HVDC line.



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According to a power balance and neglecting losses, the following equations has been found :

gridCfarm PPP (Eq. 1)

However : because of a grid side fault gridU

With : CtePfarm (Eq. 2)

Thus : CP HVDCU

Because : farmi ; Ci and gridi are constants and thus gridgridgrid UiP

In the following sections, some solutions will be presented in order to avoid this power unbalance problem.

5.3.2. Possible solutions

In order to avoid a brutal logout of the HVDC line, active power produced by farms must be adapted to active power that the grid can absorb. In order to solve this problem, there are 4 different solutions:

one without fault detection,

another with fault detection at the PCC (grid side converter),

two others with the detection of the fault at the wind farm side converter.

These solutions are going to be explained in the following paragraphs.

5.3.2.1. First solution : DC Chopper : without fault detection

It has been previously identified that a power surplus at the DC capacity level can involve a DC voltage increase. One of the solution is thus to add a DC chopper

11 on the HVDC line in order to

relieve the excess of active power. In Figure 28, an example of a HVDC line with a DC chopper can be observed.

Figure 28 : HVDC-VSC Line with a DC-Chopper [47]

5.3.2.2. Second solution : with fault detection at the PCC

11 A DC chopper is simply resistances in which the power surplus is dissipated



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Thanks to a measure of grid side converter voltage, it is easy to detect the fault. After the fault detection, a fast communication between grid side converters and offshore wind farms can order to wind turbines to adapt active power production. The farm power will be adjusted to the maximal power that the grid can absorb.

5.3.2.3. Third solution : With fault detection at the wind farm side converter

Network faults can be detected at wind farm converter side thanks to a measure of the DC voltage. As the DC voltage increases when a fault is happening, it is possible to detect faults. In general the detection threshold is 1.05 p.u or 1.1 p.u. Then an active power order will be sent to wind turbines much more faster than if faults were detected at the grid side converter and then an active power order is sent through fiber optical cable.

5.3.2.4. Fourth solution : With fault detection at the wind farm side converter and with modification of converter control commands.

The last solution is to change control modes of HVDC converters, in particular the control modes of the wind farm side converter. Fault detections are achieved by measuring the DC voltage of the HVDC line at the wind farm side converter and if the DC voltage is above a limit, it means that a network fault happens.

The solution here is to decrease the AC voltage of the internal wind farm network, then wind turbines are going to use their FRT (fault ride through) mode and the generated active power will decrease. The power surplus will be in this case dissipate at the crowbar of each wind turbine. The crowbar is a resistance which is at wind turbine converter level. The crowbar will dissipate the power surplus in case of a power unbalance. In wind turbines, the crowbar is the most important fault-ride through device.

The wind farm side converter must adapt the AC internal wind farm voltage. Therefore during this fault, the wind farm side converter is going to control the DC voltage of HVDC transmission systems. While the grid side converter must control the current in order to limit it and it must provide the maximal reactive current to the grid in order to support the grid voltage.

5.3.3. Examples of different case studies from the literature

5.3.3.1. Literature case study n°1 : Detection of an onshore grid fault at wind farm side converter [31]

The literature case study n°1 is an offshore wind farm with a VSC-HVDC transmission line, which undergoes a grid fault at 3.8 s and during 0.15 s. The DC voltages of the VSC-HVDC transmission system at the wind farm side converter and at the grid side converter can be observed in Figure 29.



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Figure 29 : Switching of control modes during grid fault : (a) DC voltage on WFSC; (b) DC voltage on GSC

It can be noticed that if the DC voltage exceeds a level, the control modes of converters will change in order to maintain the DC voltage at a value just above 1 p.u (in the example 1.025 p.u). When the DC voltage become less than 1 p.u, classical converter control modes come back.

5.3.3.2. Literature case study n°2 [30]

The literature study case n°2 compares the used of the third solution and on the forth solution, it means that one will impose a new wind farm power order and the other will adapt the AC voltage of the offshore wind farm in order to decrease the produced active power. The Figure 30 represents the results if a simulation where a fault happens in the grid system. The right column corresponds to solution 3 and the left column corresponds to solution 4. The grid fault can be identified in the first row. In the second row, wind farm voltages has been draw. We see in the third solution where the converter mode are unchanged, that the voltage stays constant and in the fourth solution it drops. Whereas, wind farm currents drop in the third solution and stay unchanged in the fourth. In the two solutions, the active power provided to the grid will drop. Thanks to the fourth row, we see that active power of the fourth solution drops faster than active power of third solution.



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Figure 30 : Simulation results for a three-phase fault in the HV grid with coordination of HVDC and WF with output power reduction through: (left column) wind farm active power control;

(right column) voltage control

The two methods are equivalent and aims to protect the HVDC line. However, it seems that the method 4 allows to obtain a faster response.

5.4. Conclusion

Thanks to a literature review, the aim of this part was to present general “wind farm + transmission line” behavior during grid faults. Concerning an AC transmission line, faults will spread until offshore wind farms. Concerning an DC transmission line, it has been observed that HVDC lines cannot allow a fault ride through capacity with classical control modes of HVDC converters. In order to solve this problem, 4 solutions has been suggested :

Relieve the power excess into a DC Chopper.

Communication between onshore electrical grids and offshore wind farms with a new active power order.

Detection of faults at wind farm side converters thanks to the measure of the HVDC transmission voltage and communication of a new active power order.

Detection of faults at wind farm side converters thanks to the measure of the HVDC transmission voltage and change of HVDC converter control modes.



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The case study n°2 have shown that the most efficient, fastest, solution is the solution 4. [30] About faults inside offshore wind farm, they are extremely seldom because internal electrical power system has a small AC grid and buried cables. However, if it happens, each wind turbine uses their mode Fault Ride Through. In a HVDC transmission system case, HVDC converters should not change their control modes during this kind of faults.


Case study 1 : AC connection of the 200 MW AC reference offshore wind farm

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6. Case study 1 : AC connection of the 200 MW AC reference offshore wind farm

The first case study which will be studied in this Master Thesis, will consist of analyzing dynamic behaviors of a 200 MW AC/AC offshore wind farm linked to an electrical grid with an AC sub marine cable of 30 km. The choice of the reference offshore wind farm has been detailed in 3.4. In this case study, the electrical power system will be represented by an infinite bus with a system reactance.

The modeling of the reference offshore wind farm and of the AC transmission system has been done on EMTP-RV which is a specialized software for the simulation and analysis of transients in power systems [41].

The goal is to simulate grid faults or voltage steps at the PCC. Then it will be to attempt to answer the problem which is to know if an offshore wind farm with a classical electrical layout is capable of fulfilling some offshore specific grid code requirements like ENTSOE grid code or the classical RTE requirements.

The first part will be the offshore wind farm and its transmission cable modeling. Then some grid events will be simulated at the PCC in order to study dynamic behaviors of the “wind farm + AC transmission cable” system.

It will be developed later but when ENTSOE requirements are applied, the PCC will be situated onshore whereas when classical RTE requirements are applied, the PCC will be situated offshore at the offshore 33 / 225 kV transformer secondary side.

6.1. Model of the wind farm with an AC transmission cable

Thanks to the specifications of case study which have been detailed in 3.4, the offshore wind farm with the AC transmission cable have been modeled on EMTP-RV. The Figure 31 shows different components of offshore wind farms which will be modeled and designed. I.e. the layout of the internal electrical network, wind turbines, the 33 / 225 kV transformer and the 225 kV transmission cable.

Figure 31 : Study case of the reference offshore wind farm with an AC transmission cable



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Internal electrical network layouts of offshore wind farms depend on ground geographies, wake effects, localizations of offshore substations and of electrical cables. Under different constraints and after having analyzed existing wind farm layouts, it seems that the chosen layout in Figure 31 is the most standard among existing wind parks.

The modeling will be done thanks to an aggregation of each line. The distance between two wind turbines is around 500 m and between two line around 800 m. The aggregation impact will be evaluated later in this chapter.

6.1.1. Model of the wind turbine GE – 3 MW

The chosen wind turbine model is the full converter permanent magnet synchronous machine whose model is represented in Figure 32.

Figure 32 : Model which represents a full converter permanent magnet synchronous generator [40]

12

The Figure 32 shows the topology of a permanent magnet synchronous generator associated with a power converter. The used model is a GE machine model which clarifies different control modes for the two converters and for the generator. The regulation blocs are explained in Appendix 1 and are classified as following :

The wind model ;

The model of the machine which are represented into 3 bloc : control of the rotor, of the turbine, and of the electrical power ;

The mean model of the converters ;

The protection blocs of the generator and of the converter ;

In this GE wind turbine model, the produced reactive power of the reference wind farm can be controlled through 3 different ways :

Constant reactive power order

Constant power factor order

Voltage regulation

12 OVP represents the crow-bar of the wind turbine



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6.1.2. The submarine cables 33 kV and 225 kV

With EMTP-RV, it is possible to realize a model of 33 kV and 225 kV submarine cables with their geometrical characteristics. It is thus necessary to know the physical configuration of an XLPE 3-phase submarine cable. The XLPE technology will be used for 33 kV cables and will be also used for 225 kV cables. In Figure 33, a 3-phase cable with an optic cable can be seen. The optic cable allows to transfer information between offshore wind farms and onshore electrical networks. In appendix 2, all components of a 3-phase XLPE submarine cable are explained.

Figure 33 : 3-phase submarine 33 kV cable technology

In order to visualize the geometrical distribution and the stacking of different strata which form a submarine cable, a figure of a XLPE submarine cable seen from above (cf. Appendix 2) is drawn in order to determinate cable electrical characteristics. Thanks to the geometrical data detailed in appendix 2, these electrical characteristics have been obtained in EMTP-RV with the bloc “cable data” (see Figure 34). Finally, through the calculation bloc “PI circuit from cable data” (in Figure 34), EMTP-RV can generate impedance and admittance matrix for equivalent PI submarine cables models.



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Figure 34 : Electrical data creation of a 30 km cable in EMTP

6.1.3. The transformers

Transformers are important elements of an offshore wind farm electrical layout. Indeed, each 3 MW wind turbine must be associated with a transformer 0.69 / 33 kV. Moreover, an offshore substation can be built in order to host a transformer 33 / 225 kV. The used equivalent model of transformers in EMTP is a simplified model which is represented by an impedance (cf. Table 1).

Table 1 : Transformer data of the reference offshore wind farm

Transformer (kV) R (p.u) L (p.u)

0.69/33

Nominal power = 3 MVA 0 0.06

33/225

Nominal power = 250 MVA 0.00375 0.15

6.2. Dynamic behavior of the AC/AC reference offshore wind farm : Simulations

In this section, the aggregation impact on the dynamic behavior of the offshore wind farm and the cable impact on the offshore wind farm will be first presented in order to conclude respectively on the aggregation validation and on submarine cable influence.

Then connection studies has been performed on 3 different requirements :

+ 2 % voltage step

(P,Q) and (U,Q) diagrams of the reference wind farm.

Voltage dips

This simulation aims to check small signal stability, reactive capabilities and low voltage fault ride



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through.

In order to compare classical grid requirements (RTE grid code) and offshore grid requirements (ENTSO-E) and ,in addition, in order to compare the impact of the PCC localization, 2 parallel studies has been performed :

Offshore PCC (neglecting 225 kV transmission system) with RTE requirements

Onshore PCC with ENTSO-E requirements

It can be reminded here that RTE decides to apply grid requirements at an offshore PCC. However, in the majority of cases in Europe the PCC is onshore and ENTSO-E grid requirements are required at an onshore PCC.

6.2.1. Impact of the aggregation on the dynamic behavior of the offshore wind farm

Because of the power level of the reference wind farm,200 MW , it is difficult to model the 80 wind turbines which are part of the internal network. In order to reduce the computation time, it is possible to aggregate each line of the reference offshore wind farm which has 8 lines and 10 turbines per line. The principle of a N wind turbine aggregation is to calculate different outputs for one wind turbine and then to multiply outputs by N. Outputs are generally active, reactive and mechanical powers, terminal voltage and current. However, it is necessary to evaluate the impact of a such aggregation on study results.

Therefore, a comparison has been realized on the terminal wind turbine voltage level in the following two cases :

Totally aggregated wind farm : A wind turbine of 30 MW for each line which is equivalent to 10 wind turbines of 3 MW ; (see Figure 35)

Figure 35 : Aggregated line

Partially aggregated wind farm : One line is not aggregated (see Figure 36) and the 7 others are aggregated.

Figure 36 : Non-aggregated line

30 MW

10 * 3 MW



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A voltage dip simulation based on the ENTSO-E fault ride through curve has been performed at an

onshore PCC. A voltage profile which represents a grid fault which happens at 10s, has thus been applied at the PCC. In Figure 37, the single line equivalent diagram of the simulated system can be

seen. The wind farm is replaced in the first case by 7 aggregated lines and 1 non-aggregated line and in the second case by 8 aggregated lines. The simulation results are presented in Figure 38.

Figure 37 : single line equivalent diagram of the tested system

Figure 38 : Aggregation impact on terminal wind farm voltage during a voltage dip

The dynamic behavior of the totally aggregated farm is the same as the dynamic behavior of the farm with 1 non-aggregated line. Figure 38 shows that the voltage level of an aggregate line is identical with the voltage level of the first, fifth and last wind turbines of a non-aggregate line. Table 2 shows voltage deviations in comparison to the non-aggregation line voltage. Voltage deviations during pre-faults and post-faults are really small and during faults, voltage deviations increases but they improve voltages during faults.

The small difference on voltage levels of a same line depends particularly on two factors which are the supplied or absorbed reactive power by each wind turbine and the length of 33 kV submarine cables. The hypothesis of aggregating the wind farm does not induce important errors on dynamic behaviors of the reference wind farm. It is therefore enough to conclude that the aggregation of the reference offshore wind farm will have no significant impact on future simulations. It will even mitigate voltage



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dips.

However, it is necessary to determinate the impact of 33 kV internal cables and of the 225 kV connection cable on the wind farm behavior.

Table 2 : Voltage deviations compared to the aggregated line voltage

Pre-fault During the

fault Post-fault

1st wind turbine voltage

difference (%) 0 - 0.3 - 0.05

5th wind turbine voltage

difference (%) + 0.2 + 3.4 + 0.15

Last wind turbine voltage difference (%)

+ 0.3 + 5 + 0.25

Observation N°1

The dynamic behavior comparison of a non-aggregated line and of an aggregated line shows that the voltage profile is the same with a negligible difference. The internal layout of offshore wind farms does not have an important influence on the propagation of voltage dips between wind turbines which form the farm. This observation concerns obviously the radial layout because it is the only studied layout in this Master Thesis.

6.2.2. Static impact of an AC 225 kV transmission line

Depending on the PCC localization, onshore or offshore, the 30 km 225 kV sub-marine cable will not play the same role about the voltage profile propagation during a fault. Indeed, when the PCC is onshore, the 225 kV cable will be part of the test system however when the PCC is offshore, the 225 kV will not be part of the test system which can be seen in Figure 39.

Figure 39 : Wind farm single line equivalent diagram with PCCs

Moreover, 33 kV cables can have an impact on the wind farm internal network voltage profile. Indeed 33 kV cables can increase or decrease the voltage level. In order to characterize the impact of submarine cables on wind farm voltages, three scenarios have been simulated thanks to load flow simulations :

Scenario 1 : The wind farm produces 200 MW and does not provide reactive power (Q = 0



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MVar).

Scenario 2 : The wind farm produces 200 MW and provides 64 MVar of reactive power.

Scenario 3 : The wind farm produces 200 MW and consumes 64 MVar of reactive power.

For each scenario, a critical analyze between voltage profiles with and without the 225 kV connection cable is provided. For this analysis, the PCC voltage level is keeping at 1 p.u. These load flows have been done in order to characterize the influence of sub-marine cables on voltages depending on the reactive power produced by wind turbines.

Scenario 1

If the offshore wind farm does not produce reactive power, the 225 kV cable which have a capacitive behavior, increases the voltage by 0.7 % from the onshore grid and compensates the reactive consumption of the 33 / 225 kV transformer. The voltage at the offshore wind farm bus (33kV) stays equal to the nominal voltage. In this case, inside the offshore wind farm, 33 kV cables can be the source of voltage increases thanks to their capacitive behaviors (around 0.2 % and 0.6 % depending of the cable length). However without the 225 kV cable, the voltage of the wind farm bus (33 kV) is inferior to the nominal voltage (0.992 p.u) because of the offshore transformer.

Figure 40 : Load flows of the studied system with 0 reactive power production. With the 225 kV cable (ENTSO-E case) and without the 225 kV cable (RTE case)

Scenario 2

The provided reactive power by the offshore wind farm increases the capacitive behavior of the 225 kV cable which increases the voltage by 1.4 % (0.7 % in the scenario 1) from the onshore grid. The transformer 33 / 225 kV increases also the wind farm bus voltage. Without the 225 kV cable, the voltage profile is high (1.031 p.u) but it is inferior to the voltage measured with the connection cable (1.045 p.u).

Figure 41 : Load flows of the studied system with 64 MVar reactive power production. With the 225 kV cable (ENTSO-E case) and without the 225 kV cable (RTE case)



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Scenario 3

The scenario 3 is simulated for an wind farm operation point equal to tan(φ) = - 0.32. The reactive power provides by the 225 kV cable is not enough to increase the voltage and to compensate reactive power consumed by the transformer and the wind farm. Beside, with or without cable, wind farm voltages are identical because there is not increasing effect of the cable.

This case shows that wind farm steady states can be found where the AC transmssion system reactive power benefits is lost.

Figure 42 : Load flows of the studied system with - 64 MVar reactive power production. With the 225 kV cable (ENTSO-E case) and without the 225 kV cable (RTE case)

The 3 studied scenarios show that the 225 kV cable can have an important impact on the voltage profile of offshore wind farms because of its capacitive behavior. Indeed, this submarine cable increases wind farm voltages by 1% depending on the reactive power provided by the farm and the cable length (longer is the cable and more the cable is capacitive). A range of steady states in which the connection sub-marine cable is still increasing the voltage can be found and this range is:

Tan (φ) = [-0.28, 0.52]

Beyond this range, reactive power consumed by the offshore wind farm and transformers is the source of voltage decreases.

About wind farm dynamic behavior study on a voltage dip, the transmission sub-marine cable can play an important role because this cable decreases low voltage constraints. Indeed, during low voltage (UPCC < 0.8 p.u) at the onshore PCC bus, voltages in p.u will be higher at wind turbine buses than at the PCC bus. However, if the voltage is high (UPCC > 1.1 p.u) at the onshore PCC, wind turbines will face an voltage increase at their terminal (Uturbine > UPCC). The impact of voltage faults on the cable behavior must now be studied.

Observation N°2

When the PCC is situated offshore, wind farm operators will not beneficiate of the cable capacitive benefit. If the PCC is onshore the transmission cable will decrease low voltage constraints and increase high voltage constraints. The voltage fault type knowledge at the PCC is indispensable in order to determinate the submarine cable benefit. The submarine will simply raise offshore wind farm voltages. Low voltage => Positive benefit of the sub-marine cable High voltage => Negative benefit of the sub-marine cable

6.2.3. Dynamic behavior of the reference offshore wind farm with the AC transmission line : Compliance with the ENTSOE and RTE grid code requirements

Before building and connecting offshore wind farms to the French transmission electrical power system, it is indispensable that French wind producers choose a suitable dimensioning of wind farm



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components and of technologies in order to respect TSO’s requirements. Nowadays, RTE does not give information about future grid requirements which will be specific to French offshore wind farms apart from the PCC which will be offshore. The study case of an 200 MW reference offshore wind farm connecting with a 30 km AC transmission line will be realized based on the ENTSOE offshore requirements, detailed in chapter 4, and on RTE classical requirements which are in the French grid code (referential technique [36]). RTE requirements have been slightly explained in chapter 4 and will be explained before each simulation.

In order to achieve 2 connection studies, ENTSOE type for an offshore electrical producer and RTE type for a classical electrical producer, a set of simulations will be achieved on the referential case study of the 200 MW AC offshore wind farm presented in the chapter 3.1. In order to determinate the offshore wind farm dynamic behavior whose the technology is based on offshore wind farms existing in North Sea, Table 7 presents grid events that will be simulated and studied.

In the classical producer – RTE case, according to the call for tender specifications detailed in section 1.1.2, the PCC is considered offshore and all simulations must be done from this point. Therefore, the 225 kV transmission cable will not be taking account.

Table 3 : Simulations for the connection study of the reference offshore wind farm

Grid requirements Classical producer - RTE Offshore producer - ENTSOE

PCC Offshore Onshore

A step of + 2 %

A 2% voltage order step of the wind farm voltage level. The voltage at the infinite bus is

constant at 1 p.u.

A + 2 % voltage step at the PCC

Constructive capabilities P/Q and U/Q diagram P/Q and U/Q diagram

Low voltage fault ride through RTE fault ride through curve for

a producer connecting to the 225 kV network

ENTSOE fault ride through curve for an offshore producer

6.2.3.1. A voltage step of + 2 %

6.2.3.1.1. RTE conditions

According to the RTE grid code [36], a voltage order step of + 2% must be applied to the wind farm voltage regulation. This voltage order step is made in order to test small signal stability of power plants. At the PCC level, the onshore electrical network is represented by a reactance Xcc and an infinite bus. The Xcc reactance can be situated between a and b :

For a power plant > 250 MW and ≤ 800MW : a = 0,05 p.u ; b = 0,54 p.u;

For a power plant ≤ 250 MW : a = 0,05 p.u ; b=0,30 p.u.

Hence, the single line equivalent system to study is represented in Figure 43.

Figure 43 : + 2% step – RTE requirements – PDL offshore



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RTE requirements about this simulation are that :

Power plants must stay stable ;

The PCC voltage response time, Tr, at ± 5 % of the difference between the initial voltage and the final voltage must be inferior to 10s ;

The active power damping time at the PCC at ± 1 % of the final value must be inferior to 10s.

The simulation result (voltage and power at the PCC) after a voltage order step of + 2 % in the offshore wind farm (RTE requirements) can be observed in Figure 44 and Figure 45, for Xcc = 0.05 p.u and Figure 46 and Figure 47 for Xcc = 0.3 p.u

Xcc = a = 0.05 p.u Initial conditions at the PCC : Q = 0, P = 200 MW, Uα= 235 kV

Figure 44 : Voltage at the PCC during a voltage order step of + 2 % - RTE requirement

Success criteria

OK - RTE

The offshore wind farm stays stable during a voltage order step of + 2 % with a Xcc = a reactance. The response time, Tr, at 5 % of the difference between

the final voltage and the initial voltage (cf. Figure 44) stays inferior to 10 s and is around 4 s.

A



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Xcc = a = 0.05 p.u Initial conditions at the PCC : Q = 0, P = 200 MW, Uα= 235 kV

Figure 45 : Active power at the PCC during a voltage order step of + 2 % - RTE requirement

Success criteria

OK - RTE

The ± 1% active power damping time the PCC at is inferior to 10s. The active power at the PCC reaches its final value after 9 s.

A

Xcc = b = 0.3 p.u Initial conditions at the PCC : Q = 0, P = 200 MW, Uα= 235 kV

Figure 46 : Voltage at the PCC during a voltage order step of + 2 % - RTE requirement

Success criteria

OK - RTE

The offshore wind farm stays stable during a voltage order step of + 2 % with a Xcc = b reactance. The response time, Tr, at 5 % of the difference between

the final voltage and the initial voltage (cf. Figure 46) stays inferior to 10 s and is around 5 s.

A



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Xcc = b = 0.3 p.u Initial conditions at the PCC : Q = 0, P = 200 MW, Uα= 235 kV

Figure 47 : Active power at the PCC during a voltage order step of + 2 % - RTE requirement

Success criteria

OK - RTE The active power damping time at the PCC at ±1% is inferior to 10s.

6.2.3.1.2. ENTSOE conditions

The ENTSOE offshore grid code states that offshore wind farm smust stay stable during a + 2 % voltage step which happens at the PCC. In this ENTSOE case, the PCC is assumed to be onshore (cf. Figure 48). The Xcc reactance of the RTE case is not required in this case . Hence, in this case, the AC submarine cable is part of the simulated power system.

Figure 48 : + 2 % voltage step – ENTSOE requirement – PDL onshore

The simulation results, active power at the PCC and terminal wind turbines voltage, after a + 2 % voltage step at the PCC onshore (ENTSOE requirement), are observed in Figure 48 and Figure 49. The requirement is that the offshore wind farm stay stable.



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Initial conditions at the PCC : Q = 0, P = 200 MW, Uα= 225 kV

Figure 49 : Terminal voltage of a wind turbine during a voltage step of + 2 % at the PCC - ENTSOE requirement

Success criteria

OK - ENTSOE

The wind turbine terminal voltage of the wind farm stays stable during a + 2 % voltage step at the onshore PCC. The voltage reaches its nominal value 1

p.u within 15 s.

A

Initial conditions at the PCC : Q = 0, P = 200 MW, Uα= 225 kV

Figure 50 : Active power at the PCC during a + 2 % voltage step – ENTSOE requirement

Success criteria

OK - ENTSOE

The offshore wind farm active power at the onshore PCC stays stable after a voltage step of + 2 %.



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6.2.3.1.3. Conclusion

The studied offshore wind farm fulfills RTE and ENTSOE requirements when the voltage step is applied for a wind farm voltage order (RTE case) and when the voltage step is applied at the onshore PCC (ENTSOE case).

6.2.3.2. P/Q and U/Q diagrams

In order to be authorized to connect power plants to the electric power system, every producers must provide (U,Q) capability diagrams of the installation and for different operating points. This diagram must especially be drawn for different values of the active power. The (U,Q) diagram is a trapezoid which encircles the required operating points. Each TSO requires a (U,Q) diagram which must be respected by each power plant who wants to be connected to the electrical network. Offshore wind farms must be capable of changing their reactive power inside the diagram and in the limits of allowable voltage values. In the case of RTE, all producers must be capable of reaching a number of important points [36]. However, if the constructive capabilities of power plants do nor allow to reach these operating points, producers can use some local compensation ( passive reactive compensation, FACTS, etc.) which can be built after the connection of power plants.

Different operating points have been simulated at constant voltage (1 p.u) in order to determinate the (P,Q) diagram of the offshore wind farm and at constant power (1 p.u and 0,2 p.u) in order to determinate (U,Q) diagrams.

Initial conditions at the PCC : Un = 1 p.u, Uα= 225 kV

-0,8

-0,6

-0,4

-0,2

0

0,2

0,4

0,6

0,8

1

0 0,2 0,4 0,6 0,8 1

P/Pmax

Q/P

ma

x

Onshore PCC

Offshore PCC

ENTSOE grid code

Offshore wind

turbine alone

Figure 51 : (P,Q) diagram of the studied offshore wind farm

Success criteria – ENTSOE

If Offshore PCC : OK

If Onshore PCC : NOK

The (P,Q) diagram which is required by ENTSOE is included in the (P,Q) diagram of the farm and of the “ farm + transformer 33/225 kV” (Offshore PCC). However, it is not the case with the (P,Q) diagram of the “farm +

transformer + 225 kV sub-marine cable”.



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The submarine cable capacitive behavior and its impact on the “farm + cable” operation has been detailed in chapter 3.2.2. Indeed, the obtained (P,Q) diagram when the PCC is offshore, is shifted compared with the diagram required by ENTSOE and does not allow to reach operating points when the “wind farm + submarine cable” system is absorbing reactive power. This gap is due to the transmission submarine cable which produces reactive power. Therefore when the PCC is onshore, the (P,Q) diagram will be shifted and the power plant capability will provide more reactive power than to consume reactive power.

To conclude the offshore wind farm could comply with ENTSOE constructive capability requirements for reactive power if the PCC is offshore. However, if the PCC is onshore, the presence of the 225 kV sub-marine cable must be compensated by an additional element in order to reach requirements about reactive power consumption.

Initial conditions at the PCC : P = 1 p.u, Uα= 225 kV

-0,8

-0,6

-0,4

-0,2

0

0,2

0,4

0,6

0,8

1

0,8 0,85 0,9 0,95 1 1,05 1,1 1,15

U/Un

Q/P

max Onshore PCC

ENTSOE

Offshore PCC

Figure 52 : (U,Q) diagram of the studied offshore wind farm – ENTSOE requirement


If offshore PCC : NOK

If onshore PCC : NOK

The onshore and offshore (U,Q) diagrams do not completely cover the (U,Q) diagram required by ENTSOE.

The same trend with and without the 225 kV submarine cable has been found. Indeed, the (U,Q) diagram at the onshore PCC (with the consideration of the 225 kV cable) can produce too much reactive power as necessary but does not reach the points where reactive power consumption is necessary for a voltage between 0,9 p.u and 1,075 p.u.



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Initial conditions at the PCC : P = 1 p.u, Uα= 225 kV, Pmin = 0,2 p.U

-0,8

-0,6

-0,4

-0,2

0

0,2

0,4

0,6

0,8 0,9 1 1,1 1,2

U/Un

Q/P

max

(U,Q) diagram at Pmin

RTE diagram at Pmin

(U,Q) diagram at Pmax

RTE diagram at Pmax

Figure 53 : (U,Q) diagram of the studied offshore wind farm – RTE requirement


OK

(U,Q) diagrams of the studied wind farm encircles all the points which are required by RTE at Pmax and Pmin.

RTE requires that every power plants have to encircle the minimal operating area which is delimited by the blue dash line in Figure 53. At minimal power, the 3 tops of the red triangle are required by RTE. In our case, the studied offshore wind farm allows to respect all the RTE operating point at Pmax and Pmin.

However, it is interesting to notice that the Pmin value choice is not specified in the RTE grid code. Concerning wind farms, it is difficult to choose the value of the minimal power. This point will have to be defined by RTE in future requirements for the connection of offshore wind farms in France. In this study, Pmin has been chose at the value 0.1 p.u, which corresponds to an active power higher than system losses and thus will let wind turbines working.

Observation N°3

Every power plants which does not respect capability areas required by TSOs, will need to add some additional components depending on needs. In the case of an onshore PCC, the sub-marine cable behavior might have some limitations for consuming reactive power. In this case, some capacitive elements must be added. The location of the PCC can have an impact on (U,Q) and (P,Q) diagram fulfillments for the same offshore wind farm.



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6.2.3.3. Low voltage fault ride through

6.2.3.3.1. Dynamic behavior of the reference offshore wind farm during RTE and ENTSOE voltage dips

The dynamic behavior of offshore wind farms during a voltage dip is part of simulations which are required by TSOs. Success criteria are often the no logout of power plants when voltage curves stays within the FRT voltage curve defined by grid codes. The low voltage fault ride through study of the reference offshore wind farm is achieved as explained in Figure 54. The offshore wind farm must stay connected to electrical networks following a FRT curve defined by ENTSOE (applied at the onshore PCC) and a second curve defined by RTE (applied at the offshore PCC). The goal is to understand the behavior of wind turbines and limitations which can involve that the reference offshore wind farm does not stay connected to electrical power systems.

Figure 54 : simulation schema for voltage dips

In the specific ENTSOE offshore grid code (cf. section 4.1.3), pre-fault conditions in which the offshore wind farm must be, are not described. ENTSOE gives the choice to TSOs to define pre-fault conditions and post-fault conditions. In this study, the behavior of the reference offshore wind farm will be studied according to 2 scenarios :

o Control voltage of offshore wind farms ;

o 0 reactive power order at the PCC.

Concerning RTE grid code, initial conditions are clearly defined and success criteria is that power plants have to stay connecting when voltages at the offshore PCC stay above RTE fault ride through curve.



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Voltage dip ENTSO-E, control voltage mode

Wind turbines of the farm are used with the control voltage mode. The FRT curve is applied at the PCC onshore. The voltage dip is mitigated at wind turbine levels thanks to the capacitive behavior of the 225 kV cable.

Indeed, when the voltage at the PCC is 0 V, it will be mitigated at 0,2 p.u at the wind farm level.

Voltages of the offshore wind farm stay inside the FRT voltage curve.

Figure 55 : Wind farm voltage during ENTSOE voltage dip

When the voltage dip happens, electrical power of the offshore wind farm goes from 1 p.u to 0 p.u. Mechanical power will become null one second later. Indeed, wind turbines will slow down since it is impossible to evacuate active power to the grid. This operation behavior shows the control mode of wind turbines during a voltage dip.

The recovery of the network voltage at the PCC is followed by the electrical recovery with a delay of some seconds. Finally, the mechanical power is recovered 5 s after once the blades are working again.

Figure 56 : The electrical power and the mechanical power of the offshore wind farm during an ENTSOE voltage dip

Observation :

The voltage level of wind turbines stays inside the FRT curve required at the PCC. In these simulation conditions (farm with the control voltage mode), this wind farm is capable of staying connecting to electric power system after an ENTSO-E voltage dip. However, wind turbine manufactures must certificate that wind turbines are going to stay connecting after an ENTSO-E voltage dip otherwise the connection will be refused by TSOs.

A



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Voltage dip ENTSO-E, reactive at the PCC = 0 VAR

The reactive power at the PCC is being kept at 0 VAR during pre-fault and post-fault. This condition is required by RTE. Because of the capacitive effect of the 225 kV cable, the offshore wind farm has to consume reactive power in order to satisfy test conditions.

A voltage change of the farm happens at 0.9 p.u. This is due to a control mode change of the wind farm operation which provides reactive power during the dips and is forced to consumes reactive power in steady state (U > 0.9 pu).

Figure 57 : Wind farm voltage during ENTSOE voltage dip

The behavior of electrical and mechanical powers are almost the same as for the previous simulation where the farm uses the control voltage mode in steady state.

Electrical power of the farm is negative in the time range [5.5s, 7s] since the crow bar

13

works as a load.

Figure 58 : The electrical power and the mechanical power of the offshore wind farm during an ENTSOE voltage dip

Observation :

The voltage level of wind turbines is not always above the voltage dip required at the PCC. In these simulation conditions ( null reactive at the PCC), the farm is not assured to stay connecting, thus some additional guarantees from wind turbine manufacturers should be given.

13 A resistance in parallel with the continue bus of each wind turbine which protects the turbine in case of network

fault.



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RTE voltage dip, reactive power at PDL = 0 VAR

The reactive power at the offshore PCC is being kept at 0 VAR in steady state. The wind turbine voltage follows the RTE low voltage fault ride through curve.

Figure 59 : Wind farm voltage during a RTE voltage dip

Wind turbines has the same behavior as for the ENTSOE case.

Figure 60 : The electrical power and the mechanical power of the offshore wind farm during an RTE voltage dip

Observation :

The wind turbine voltage stay above the voltage dip curve required by RTE. In these test conditions, the offshore wind farm can be connected on the RTE electrical power system.



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6.2.3.3.2. Behavior analysis on the studied offshore wind farm during voltage dips

The dynamic behavior study of an offshore wind farm during a voltage dip depends extremely on the used simulation model. Despite the difficulty of achieving this type of transitory, it is necessary to understand the different control modes which exist in the wind turbine model GE 3 MW full converter for degraded operation.

Furthermore, accordingly the performed simulations during a RTE and ENTSOE voltage dips, there is obviously a specific control mode when the terminal voltage of the wind turbine is lower than 0.9 p.u. Whatever pre-fault and post-fault operation conditions, the wind farm changes its control mode during the voltage dip and attempts to provide reactive power in order to relieve the electric power system. In the same time, the crow bar is working in order to protect the DC bus of the wind turbine full converter. The DC bus can see a suddenly increase of the DC voltage because of electrical power losses although the mechanical power is still significant.

During a voltage dip, the wind turbine goes through 4 operation steps :

Figure 61 : The difference steps of the wind turbine fault ride through mode

Step 0 : The wind farm put itself in FRT mode when the PCC voltage goes from 1 to 0 pu.

Step 1 : The network voltage fault takes place when the mechanical power is at its maximal. The important difference between the injected active power on the electrical network and the provided power by the generator will involve a brutal increase of the wind turbine DC bus voltage. In order to avoid the destruction of the DC bus, a resistor, the crow bar, is used and consumes the energy produced by the wind turbine. In the same time, the wind turbine imposes a reactive power order to support the PCC voltage.

Step 2 : As it is impossible to inject electrical power to the network during voltage dips, wind turbine blades are turned off in order to decrease the produced mechanical power. Thanks to the DC bus capacitor, the DC bus is capable of providing electrical power stored in the capacitor when the voltage at the PCC is higher than 0.6 p.u (see Figure 56).

Step 3 : The threshold voltage from which wind turbines find back their pre fault operation mode is around 0.88 p.u. Blades become functional some seconds after the end of the fault (around 3s) .

The Figure 62 and Figure 63 shows the different steps of the operating mode of the offshore wind farm during a fault.



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Figure 62 : Powers influencing the wind turbine behavior during a fault

Indeed, the wind farm is working with the FRT mode after the step 1, which explains that the crowbar is instantanously working to consume active power surplus (see Figure 62). The electrical active power decreases at 0.05 p.u. With the FRT mode, blades are turn off in order to decrease the mechanical speed and to put mechanical power at 0 p.u.

When the PCC voltage stays below 0.88 p.u, the offshore wind farm are working with the FRT mode. Above this voltage threshold, electrical power increases and finds back its nominal power of 1 p.u 5s after the end of the fault (time when the PCC voltage exceed 0.88 p.u). Without mechanical power, the injected electrical power on the electric power system is insured by DC buses. For the ENTSO-E voltage dip type, DC buses provide electrical power to the grid during 5 s. This electrical energy sizes the used capacitors.

The FRT mode of the modeled offshore wind farm has an impact on the reactive power exchanged with the grid. Indeed, wind turbines attempt to support electrical network with the injection of reactive power when the voltage of the PCC is null. The reactive power order is fixed by a reactive current Iq

which is at its maximal value 0.4 p.u when the voltage is include in [0 pu, 0.42 pu ]. Above 0.42 p.u, the wind farm decreases the reactive order for an ENTSO-E voltage dip. To conclude the provided reactive power during the fault is a function of the PCC voltage.



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Figure 63 : Reactive power supply of the wind farm

Observation n°4

Every offshore wind producer must certificate that wind farm are capable of staying connecting to the electric power system during a voltage FRT curve required by TSOs. Prove that the voltage at the wind turbine terminal stays (or not) above the voltage dip curve is not enough.

Observation n°5

The pre-fault and post-fault steady state condition choice can influence the respect of the voltage FRT curve required for a defined wind farm. Indeed, the reactive power injection during faults and the active power furniture to the network are highly correlated to the design of static converters and generators coupled to the DC bus. If for a particular wind farm configuration is not enough to stay connecting during a low voltage dip, two actions are possible for producers :

1- Choose a better control: faster response, better choice of the threshold voltage 2- Add some reactive components

6.3. Conclusion : Case study n°1 : AC connection of an offshore wind farm

Concerning the connection of a 200 MW AC/AC offshore wind farm connecting with a 30 km sub-marine cable and to a 225 kV voltage level, a set of issues has been underlined and analyzed.

Concerning the modeling part, the wind farm aggregation is possible because the voltage constraint is really closed between each wind turbine for a radial electrical layout. The 225 kV transmission cable has a capacity behavior which can impact the design of the wind farm in order to satisfy the TSOs’ requirements. For example, the choice of an onshore PCC can influence the no-respect of a (U,Q) diagram whereas for the same wind farm is will be enough if the PCC was offshore. It is due to the transmission cable which shifts the diagram to the reactive production.



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According to the performed simulations in this case study, the wind farm 200 MW fulfills the following RTE requirements :

+ 2 % step of the voltage order : the farm dynamic responses fulfill the time constants require by the RTE grid code.

(U,Q) diagram at Pmax and Pmin : the (U,Q) diagram shows that the 200 MW farm is capable of reaching all the required points at Pmax and Pmin.

Low voltage fault ride through : It has be demonstrated that the terminal voltage of wind turbines stays in the voltage FRT curve required by RTE.

The design of the studied farm is not enough in order to satisfy all the tests requires by ENTSOE. The transmission cable was a brake for the fulfillment of the (U,Q) diagram. It leads us to say that the choice of the PCC localization can be important for the design of the offshore wind farm because the “wind farm + cable” system will not have the same limits and characteristics that the wind farm alone.


Case study 2: Connection of the AC/AC 200 MW wind farm on an electrical network

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7. Case study 2: Connection of the AC/AC 200 MW wind farm on an electrical network

In the previous chapter, a connection study of the reference offshore wind farm to an infinite bus has been studied. This connection study has been realized based on ENTSOE offshore requirements and on RTE classical requirements.

In this chapter, the impact of this reference offshore wind farm on power system stability will be studied. Stability problems involve by the integration of massive offshore wind farms will be identified. In order to answer this issue, it will be necessary to model the IEEE 14-bus power system and to explain power system stability issues. Then simulations will be run in order to characterize offshore wind farm impacts.

7.1. Model of the electrical power system

In order to achieve this case study n°2, an IEEE 14 bus power system has been used [48]. The single line diagram of the power system can be observed in Figure 64 . However, the used system is a three-phase system.

Figure 64 : IEEE 14-bus test system

The modeling of different system components will be detailed in the following sections.[48, 49]



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7.1.1. Model of synchronous generators

The modeling of synchronous generators can vary from a classical model to really detailed ones. The classical generator model is modeled by a voltage source with constant magnitude behind a transient reactance (x’d). However, transient phenomena appears when the disturbance point is located near the generator and φf cannot be anymore considered as a constant. In the one axis model, the variation of φf is considered. The effect of the high speed generators rotors must can also be considered, which means that the q-axis of the rotor will be equipped with a short circuit damper winding which represents this phenomenon. The two axis model takes into account this winding. The dynamic of the two-axis generator model is given by the following equations :

In the IEEE 14-bus system, transient and subtransient phenomena are represented for generator models. Therefore, the model is a two-axis model which respects the previous dynamic equations.

Moreover, for time domain simulation, it is important to represent the excitation system. Indeed, the excitation controller are linked with the reactive power production of generators. Therefore, simple Automatic Voltage Regulators (AVR) are used. With the use of an AVR, Ef is not anymore constant and the dynamic equations of a generator using the two-axis model with AVR are given by :

These equations govern the dynamic of the IEEE 14-bus generators which will be used.

7.1.2. Model of lines and transformers

In the IEEE 14-bus standard system, the lines are represented by their single line diagram. It means that their impedance Zline and capacitance Cline are given. However in this case study, a three line diagram is needed. Therefore, the positive, negative and zero sequences has used to define the lines. The positive, negative and zero sequences are given by :

Positive sequence Negative sequence Zero sequence

Zline Zline 3*Zline

Cline Cline 0.5*Cline

Concerning transformers, an impedance and a voltage tap ration can simply represent them.

7.1.3. Model of loads

Static load which are used in the IEEE 14-bus system, can be modeled according to 3 static representations :

constant impedance load model (a = b = 2) : active and reactive powers are proportional to the square of the voltage,

constant current load model (a = b = 1) : active and reactive powers are proportional to the voltage

constant power load model (a = b = 0) active and reactive powers are constant

These load models can be described by the following reactive and active power equations :



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b

a

V

VQQ

V

VPP

)(*

)(*

0

0

0

0

Static load model may also be a composition of these tree different load model and may be frequency dependant. Dynamic load model represents for instance electrical motors which consumes a massive amount of electrical power and these components can have a significant impact on power system stability especially on voltage stability.

In the IEEE 14-bus system, loads will be only represented by static constant power load model. However, it can be a good perspective of study to use a more detailed load model for the IEEE 14-bus system.

7.1.4. Load Flow

In order to understand the static behavior of the modeled power system, a load flow has been run. The load flow has been realized without additional compensation other that the ones in Figure 64 and without the reference offshore wind farm. Results of the load flow has been listed in Table 6. For the case study, the base power Sb has been chosen at 200 MVA and the base voltage at 225 kV.

Table 6 : load flow of the IEEE 14-bus power system without offshore wind farms

Bus no Bus Type P (MW) Q (MVar) V (p.u) θ (°)

1 PV 232 - 51,1 1 2,9

2 Slack 12,9 56,8 1 0

3 PV - 94,2 21 0,987 - 4,2

4 PQ - 47,8 0 0,983 - 2,8

5 PQ -7,6 - 1,6 0,988 - 2

6 PV - 11,2 16,5 0,928 - 33,1

7 PQ 0 0 - -

8 PV 0 24 0,981 - 33,3

9 PQ - 29,5 - 16,6 0,943 - 34,1

10 PQ - 9 - 5,8 0,937 - 34,2

11 PQ - 3,5 - 2 0,928 - 33,6

12 PQ - 6,1 - 1,6 0,920 - 33,7

13 PQ - 13,5 - 5,8 0,919 - 33,8

14 PQ - 14,9 - 5 0,923 - 34,6

Power plants are situated in buses 1 and 2, whereas a huge industrial consumer is situated in bus 3 and small consumers are located in buses 6 to 14. The lowest voltages are situated at load buses and especially at bus 13. PV buses, bus 3, bus 6 and bus 8, do not keep the voltage at 1 p.u, which means that the synchronous compensators provide already their maximum active power to the system and cannot provide more reactive power in order to support the voltage.

The offshore wind farm are located around coasts and at the ends of electric power systems where there are consumption loads. Therefore, it has been chosen that the reference offshore wind farm will be connected at bus 13. Indeed, generally at the ends of electric power transmission system, there are



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some low voltage problems.

Observation n°1

The load flow shows that voltage problems can appear in the IEEE 14 bus system. Indeed, synchronous generators cannot provide supplementary reactive power. Finally, lines between the area n°1 formed by bus 1,2 and 3 and the area n°2 formed by other buses can be overloaded due to huge amount of active power which goes from area n°1 to area n°2.

7.2. Dynamic behavior of the system

The power system model has been realized and static analysis of this system without the reference offshore wind farm has been achieved. In this section, dynamic behavior of a power system with an offshore wind farm will be studied. First power system stabilities will be explained and these stabilities will be tested on the case study n°2. The used offshore wind farm is the reference offshore wind farm which has been modeled for the case study n°1.

7.2.1. Dynamic power system stabilities

Electrical power system has to deal with many stability problems. These dynamic stability problems will be slightly explained in the following sections.

7.2.1.1. Frequency stability

In a power system, the active power must be balanced. It means that the produced active power must be instantly consumed elsewhere including losses. Indeed, electrical power cannot be stored

14.

However, when there is a power unbalance in an electric power system, the frequency will decrease if the production is too low or will increase if the production is too high. Indeed, the power deficit will slow down rotor speed of power system generators whereas the power surplus sill speed up rotor speed of power system generators. Therefore in a power system, the frequency must stay stable in order to avoid system collapse. Primary and secondary controls are the two controls which prevent frequency instability. [49]

Due to the wind intermittence, offshore wind farm will create frequency instability. However, this stability problem will not be studied in this Master Thesis. Offshore wind farms may be used in the future for primary and secondary controls. Thus, it can be interesting to investigate in future researches this possibility.

7.2.1.2. Rotor angle stability

In a power system, generators have to synchronized even after being subjected to a disturbance. This loss of synchronism is traduced by an increase of angular swings of some generators compared to other generators. This stability problem can be divided into two stabilities :

Small signal stability

Transient stability

Small signal stability is the ability of the system to stay synchronized when subjects to a small disturbance. In this case, the power system can be linearized. Whereas transient stability is the ability of the power system to stay synchronized when subjects to a large disturbance.

14 It is not totally true. There is a small storage capacity of 4s thanks to the inertia of generator and there are other

small storage capacity like batteries but it is negligible.



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7.2.1.3. Voltage stability

In a power system, voltage must be keeping a nominal value otherwise electrical components connected to the electric power system will be damaged. There are two voltage instability, a fast voltage instability due to fast restoring load and a slow voltage instability due to slow dynamic devices.

This stability problem will not be studied in this Master Thesis.

7.2.2. Definition of the simulations

Different types of network events will be used in order to characterize the impact of the offshore wind farm on the power system. The 3 event cases are the following:

Case 1 : The active load of bus 3 is disconnected at during 100 ms.

Case 2 : A three phase fault occurs at on the line 2->5 next to bus 5 and the fault is cleared by opening the faulted line.

Case 3 : An outage of the compensation source n°3 happens.

These network events are going to be first simulated on the IEEE 14-bus power system without the offshore wind farm. Then there are going to be simulated on the IEEE 14-bus power system with the offshore wind farm connecting to the bus where the voltage is the lowest.

Thanks to the load flow achieved on the power system, it has been decided to connect the offshore wind farm on Bus 13. The offshore reference wind farm will be control in order to provide a null reactive power at the connecting bus 13. This requirement is a wind farms classical requirement. In this following dynamic simulations, rotor angle deviations of generators, speed deviations of generator, active power transmitted from bus 3 to bus 4 and the voltage at bus 13 have been drawn and analyzed.

Figure 65 : Single line diagram of the power system without the offshore wind farm and with



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the fault cases

7.2.3. Small signal stability

Small signal stability is the stability of the power system when a small disturbance happens. It means that we can linearize the system. Therefore, eigenvalues of the system can be found and analyzed. However, in our case due to the wind farm model, the software cannot linearize the system. Therefore, in order to characterize the dynamic behavior of the system, simulation on a small disturbance has been run. This small disturbance corresponds to the case n°1.



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Figure 66 : Rotor angle and P34 dynamic behaviors after a case n°1 disturbance on the power system without the offshore wind farm

Figure 67 : Rotor angle and P34 dynamic behaviors after a case n°1 disturbance on the power system with the offshore wind farm

It can be seen thanks to the previous simulation that the offshore wind farm has not significant impact on the small signal stability of the power system. Indeed, the magnitude of the rotor angle deviation is identical in the case without the farm and in the case with the farm and rotor angles reach is finale value after 10s in the two cases. Concerning the power transferred on the line 3-4, the only difference is the active power amount which is transferred. Indeed, it is due to the 200 MW farm add on bus 13.



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Observation n°2

The reference offshore wind farm does not have a significant impact on power system small signal stability. It will only modify power flows. However harmonics will appear on the power system due to the used of power electronics at the wind farm level.

7.2.4. Transient stability

Transient stability is the ability of the power system to stay stable when a large disturbance happens. It depends a lot of initial conditions, the localization and the type of disturbance. In order to characterize the dynamic behavior of the system, simulation on two large disturbances has been run. These large disturbances correspond to the case n°2 and the case n°3. The first one is a classical large disturbance which is a three phase fault on a line which is cleared by opening the line. The second is the loss of a synchronous compensator.

In the following simulation, a three phase fault on the line 34 happens at 10s during 100 ms and is cleared by opening the lines.




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Figure 69 : Rotor angle and P34 dynamic behaviors after a case n°2 disturbance on the power system with the offshore wind farm

the previous simulation shows that the offshore wind farm has not significant impact on the power system instability produced by this large disturbance. Indeed, the magnitude of the rotor angle deviation has decreased with the presence of the reference offshore wind farm. The second swing amplitude for the compensator 4 is 0,5 p.u when there is no offshore wind farm and 0,4 p.u when the offshore wind farm is connected to the grid. Concerning the power transferred on the line 3-4, the amplitude of swings has decreased when the offshore wind farm is connected and the active power deviation is lower.

Observation n°3

The offshore wind farm has decrease power system transient instability. The response time and the magnitude of rotor angle and power oscillations have decreased with the presence of the offshore wind farm. The stress on the system is lower. Moreover the active power flow deviation is less because the offshore wind farm allows a better active power flow repartition.

In the following simulation, the compensator 5 is disconnected at 10s during 100 ms and the fault is cleared by reconnecting the compensator.



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The power system transient stability has been affected on this large disturbance when the offshore wind farm is connected. The compensator 4 swing amplitude has increased from 0,2 p.u to 0,5 p.u. Concerning the power it is identical and the response time at 5% is higher when the offshore wind farm is connected.

Observation n°4

This large disturbance have created more stress on the power system when the offshore wind farm is connected. Therefore, the type of disturbance with the localization of the disturbance is really important to determinate the ability of the system to stay transient stable.

7.3. Conclusion : case study n°2

In the case study n°1, a connection study has been achieved which the reference offshore wind farm. In the case study n°2, the IEEE 14 bus power system has been modeled in order to study the impact of the reference offshore wind farm on this power system.

It has been decided to study rotor angle stability, small signal stability and transient stability. Therefore, 3 different power disturbances have been studied : One load disconnection, one three phase fault on a line and one synchronous generator disconnection.

In the two first parts, the offshore wind farm improves rotor angle stability of the power system whereas in the last case, the offshore wind farm decreases the power system stability and increase the stress on the power system. It can be noticed that the last disturbance is closer to the offshore wind farm.

The impact of the offshore wind farm will involve much stress during fault next to the power system. During this kind of disturbance, the rotor angle will have higher oscillation amplitude and the voltage of the connecting bus will be impacted.

However, the offshore wind farm will improve rotor angle stability when disturbance happens far from the offshore wind farm and will allow to get a better repartition of the electrical power production but all these conclusions are true when wind speed allowed energy production. Therefore, conventional power plant has always needed in case of null wind power production. (solutions load shedding, battery better use of hydro power ).

Voltage problems at the PCC of the offshore wind farm can be solved thanks to the production/consumption of reactive power. But sometime inductive elements has to be added. Voltage drops are created generally because the production units are not close to consumption point. In the



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offshore wind farm case, the production will be closer and will avoid voltage drops.

Concerning rotor angle stability (small signal stability and transient stability), if many wind farms are connected to the grid, it will be useful to have some FACTS or HVDC devices in order to decrease these instabilities.


General conclusion of the Master’s Thesis

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8. General conclusion of the Master’s Thesis


References

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EDF R&D


9. References

[1] EWEA, The European Wind Energy Association, http://www.ewea.org/index.php

[2] Description of the offshore wind farm Thanet, Vattenfall, http://www.vattenfall.co.uk/en/thanet-offshore-wind-farm.htm

[3] New UK offshore wind farm licences are announced, BBC News, 2010-01-08, http://news.bbc.co.uk/2/hi/business/8448203.stm

[4] “Politique industrielle de la France : discours du president à Saint Nazaire”, 25 january 2011, http://www.elysee.fr/president/les-actualites/discours/2011/politique-industrielle-de-la-france-discours-du.10523.html

[5] « Eoliennes offshores : La France se lance », 31 january 2011, http://www.meretmarine.com/

[7] Europe’s onshore and offshore wind energy potential, EEA technical report, No 6/2009, http://www.energy.eu/publications/a07.pdf

[6] Livrable R&D, H-R12-2010-01257-FR, Bayram Tounsi, 5 may 2011 : « Développement de la filière éolienne offshore et impact sur le raccordement »

[7] BTM Consult, http://www.btm.dk/news/offshore+wind+power+2010/?s=9&p=&n=39

[8] Wind Energy the facts, http://www.wind-energy-the-facts.org/en/

[9] Wind Power Systems lectures, Katherine Elkington, KTH

[10] “Perspectives on Power Electronis and Grid Solutions for Offshore Wind farms”, Hans-Peter Nee and Lennart Ängquist” November 2010.

[11] http://www.warwickenergy.com/pdf/ThanetNTSlr.pdf

[12] LORC, Lindoe Offshore Renewables Center tests and demonstrates technology harvesting renewable energy offshore, http://www.lorc.dk

[13] “Siemens to connect Thanet offshore wind farm to the British power grid”, Siemens, http://www.siemens.co.uk/en/news_press/index/news_archive/siemens_to_connect_thanet_offshore_wind_farm_to_the_british_power_grid.htm

[14] Description of the offshore wind farm Lillgrund, Vattenfall, http://www.vattenfall.se/sv/lillgrund-wind-farm.htm

[15] “CaseStudy-European Offshore Wind Farms”, POWER Pushing Offshore Wind Energy Regions

[16] Official website of the offshore farm Horns Rev,http://www.hornsrev.dk/index.en.html

[17]Description of the offshore wind farm : Lynn and Inner Dowsing, Centrica, http://www.centrica.com/index.asp?pageid=923&project=project4&projectstatus=operational#project4

[18] Description of the power electronics of the offshore wind farm Lynn and Inner Dowsing, Siemens,

http://www.energy.siemens.com/us/en/power-transmission/grid-access-solutions/references.htm#content=2008%3A%20180%20MW%20offshore%20wind%20farms%20Lynn%20and%20Inner%20Dowsing%2C%20UK

[19] Offical website of the offshore wind farm Bard Offshore 1 http://www.bard-offshore.de/en/projects/offshore/bard-offshore-1

[20] Presentation « Défis posés par le raccordement des parcs éoliens offshore aux réseaux de distribution allemands », Transpower, November 2009

[21] Description of the power electronics of the offshore wind farm HelWin 1, Siemens, http://www.energy.siemens.com/us/en/power-transmission/grid-access-solutions/references.htm#content=2013%3A%20576%20MW%20offshore%20HVDC%20PLUS%20link%20HelWin1%2C%20Germany

http://www.ewea.org/index.php

http://www.vattenfall.co.uk/en/thanet-offshore-wind-farm.htm

http://www.vattenfall.co.uk/en/thanet-offshore-wind-farm.htm

http://news.bbc.co.uk/2/hi/business/8448203.stm

http://www.elysee.fr/president/les-actualites/discours/2011/politique-industrielle-de-la-france-discours-du.10523.html

http://www.elysee.fr/president/les-actualites/discours/2011/politique-industrielle-de-la-france-discours-du.10523.html

http://www.meretmarine.com/

http://www.btm.dk/news/offshore+wind+power+2010/?s=9&p=&n=39

http://www.wind-energy-the-facts.org/en/

http://www.warwickenergy.com/pdf/ThanetNTSlr.pdf

http://www.lorc.dk/

http://www.siemens.co.uk/en/news_press/index/news_archive/siemens_to_connect_thanet_offshore_wind_farm_to_the_british_power_grid.htm

http://www.siemens.co.uk/en/news_press/index/news_archive/siemens_to_connect_thanet_offshore_wind_farm_to_the_british_power_grid.htm

http://www.vattenfall.se/sv/lillgrund-wind-farm.htm

http://www.vattenfall.se/sv/lillgrund-wind-farm.htm

http://www.hornsrev.dk/index.en.html

http://www.centrica.com/index.asp?pageid=923&project=project4&projectstatus=operational#project4




http://www.bard-offshore.de/en/projects/offshore/bard-offshore-1

http://www.bard-offshore.de/en/projects/offshore/bard-offshore-1

http://www.energy.siemens.com/us/en/power-transmission/grid-access-solutions/references.htm#content=2013%3A%20576%20MW%20offshore%20HVDC%20PLUS%20link%20HelWin1%2C%20Germany




References

KTH EPS -

EDF R&D


[22] Website of the responsible of the project Meerwind, http://www.windmw.de/en_index.html

[23] Website of the offshore wind farm Nordsee Ost, http://www.rwe.com/web/cms/en/400596/rwe-innogy/renewable-energies/wind/wind-offshore/developing-sites/nordsee-ost/

[24] Website of « The Crown Estate », http://www.thecrownestate.co.uk/round3

[25] http://www.thewindpower.net/offshore_wind_farms_list.php

[26] Requirements for Offshore Grid Connections in the transpower Grid, transpower, 30 avril 2010

[27] Nordic Grid Code, Nordel, 2007

[28] ENTSOE-E Draft Requirements for Grid Connection Applicable to all Generators, Entsoe, march 2011

[29] Ranjan Sharma, Tonny W. Rasmussen, Kim H.Jensen, Vladislav Akhmatov “HVDC solution for offshore Wind Park Comprising turbines euipped with full range converters” , Siemens wind power A/S and Department of electrical Engineering, Technical University of Denmark

[30] Christian Feltes, Holger Wrede, Friedrich Koch, Istvan Erlich, “Fault ride through of DFIG based Wind farms connected to the grid through vsc based HVDC link”

[31] lie Xu, “Power electronics options for large wind farm integration : VSC-based HVDC transmission” IEEE

[32] Miguel montilla-djesus, “System of control in an offshore wind farm with HVDC link”, Universidad de Los Andes-Merida

[33] T. Weber, D. Mushamalirwa, M. Deschatres, C. Hilberg, P. Maibach, W. Janssen, T. Leske, S.Kehrer et D. Saint-Andre “Grid integration of offshore windfarm Cote d’albatre to the french transmission grid” cigre 2010.

[34] Ministère de l’écologie et du développement durable, des transports et du logement, « Cahier des charges de l’appel d’offres n° 2011/S 126-208873 portant sur des installations éoliennes de production d’électricité en mer en France métropolitaine »

[35] the “syndicat des énergies renouvelables”, http://www.enr.fr/

[36] Référentiel technique RTE – article 8.3 : http://clients.rte-france.com/htm/fr/mediatheque/telecharge/reftech/15-05-08_article_8-1_v1.pdf

[37] Kala Meah*, Student Member, IEEE, and Sadrul Ula, Senior Member, IEEE, Comparative Evaluation of HVDC and HVAC Transmission Systems

[38] The Crown Estate, http://www.thecrownestate.co.uk/home.htm

[39] Department of Energy and Climate Change, http://www.decc.gov.uk

[40] EWEA LARGE SCALE INTEGRATION OF WIND ENERGY IN THE EUROPEAN POWER SUPPLY, www.ewea.org

[41] http://www.emtp.com/

[42] http://www.siemens.com/press/en/pressrelease/?press=/en/pressrelease/2010/renewable_energy/ere201010003.htm

[43] http://www.dongenergy.com/Hornsrev2/EN/about_horns_rev_2/About_the_Project/Pages/about_the_project.aspx

[44] http://www.dongenergy.com/walney/about_walney/about_the_project/pages/about_the_project.aspx

[45] http://belwind.eu/fr/home

[46] KTH EE research projects, http://www.kth.se/en/ees/omskolan/organisation/avdelningar/e2c/research/projects

http://www.windmw.de/en_index.html

http://www.rwe.com/web/cms/en/400596/rwe-innogy/renewable-energies/wind/wind-offshore/developing-sites/nordsee-ost/

http://www.rwe.com/web/cms/en/400596/rwe-innogy/renewable-energies/wind/wind-offshore/developing-sites/nordsee-ost/

http://www.thecrownestate.co.uk/round3

http://www.thewindpower.net/offshore_wind_farms_list.php

http://www.enr.fr/

http://clients.rte-france.com/htm/fr/mediatheque/telecharge/reftech/15-05-08_article_8-1_v1.pdf

http://www.thecrownestate.co.uk/home.htm

http://www.decc.gov.uk/

http://www.ewea.org/

http://www.emtp.com/

http://www.siemens.com/press/en/pressrelease/?press=/en/pressrelease/2010/renewable_energy/ere201010003.htm

http://www.siemens.com/press/en/pressrelease/?press=/en/pressrelease/2010/renewable_energy/ere201010003.htm

http://www.dongenergy.com/Hornsrev2/EN/about_horns_rev_2/About_the_Project/Pages/about_the_project.aspx

http://www.dongenergy.com/Hornsrev2/EN/about_horns_rev_2/About_the_Project/Pages/about_the_project.aspx

http://www.dongenergy.com/walney/about_walney/about_the_project/pages/about_the_project.aspx

http://belwind.eu/fr/home

http://www.kth.se/en/ees/omskolan/organisation/avdelningar/e2c/research/projects


Appendices

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EDF R&D


[47] S. K. Chaudhary, Student Member, IEEE, R. Teodorescu, Senior Member, IEEE, P. Rodriguez, Member, IEEE, and P. C. Kjær Member, IEEE , "VSC Chopper Controlled Resistors in VSC-HVDC Transmission for WPP with Full-scale Converters"

[48] Sameh Kamel Mena Kodsi, Student Member, IEEE and Claudio A.Canizares, Senior Member, IEEE, “Modeling and simulation of IEEE 14 bus system with FACTS controllers”, technical report, 2003

[49] Dynamic and static analysis of power systems, KTH compendium

10. Appendices

10.1. Appendix 1

Protections

Under/Over Voltage

Over Speed

Under Frequency

Generator/Converter Model

GE FULL CONVERTER MODEL

Power signal

Synthetic_scope

WTG

Pord

Pitch

Wg

Pelec

pstl

TurbineControl

Turbine_Control

Converter

current

limiter

IQcmd

IPcmd

pdbrQelec

Qord

Pord

Pelec

Vterm

pstl

Converter_Current_limit

G/C

mo

de

l

Vins

phaseIQcmd

IPcmd

I_o

ut

Trip

Generator_Converter

pstlP_pu

Freq_pu

P control

Active_Power_Control

pdbr

WtP_pu

Wg

RotorRotor

Wind PowerVw

WtPitch

P_pu

Wind_Power

scope

spd

scope

ptch

PROD1

2

WgbyTrip

PROD1

2

PbyTrip

scope

wspd

Q control

Vterm Qord

Pelec

Reactive_Power_Control

Protections

Vterm

Trip

Vw

Wg

TripRD

Protections

PROD1

2

WgbyTripRD

scope

pm

f(u)1

pm_MW

Me

as

ure

I3p

ha

se

I3phaseO

Freq_pu

phase

VinsVterm

Qelec

Pelec

Measures

Vw

Page Vterm

PageVterm

PageVterm

PageVterm

Page Pelec

Page Pelec

Page Qelec

PageQelec

Page freq

Pagefreq Page TripRD

PageTripRD

Page Trip

Page Trip

Page Trip

Figure 72 : GE – 3 MW model ins EMTP

Modélisation du bloc « Wind power »

Le bloc wind power a pour but de fournir la puissance mécanique de la turbine en fonction de la vitesse du vent. Pour pouvoir calculer la puissance mécanique que produit l’éolienne, 3 entrées sont indispensables la vitesse du vent, l’angle de rotation des pales éoliennes (le pitch) et la vitesse de rotation de l’axe rotorique.

La formule bien connue de la puissance mécanique extraite du vent est la suivante :

),(***2

3

CpVwAPmécanique (Eq. 1)


Appendices

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Avec la densité de l’air en kg/m^3, A l’air couvert par les pâles de l’éolienne en m², Vw la vitesse

du vent en m/s, et Cp est le facteur de puissance fonction de qui est le facteur de puissance et

qui est l’angle des pales éoliennes.

Vw

wKb * (Eq. 1)

Finalement ),( Cp est calculé en fonction de et de multiplié par des coefficients ji ,

10.2. Appendix 2

The geometrical characteristic of the 33 kV and 225 kV des câbles 33 kV et 225 kV

33 kV 250 mm² cable 225 kV 500mm² cable

Distance du conducteur au centre du câble (mm)

25.12 55.8

Distance du centre du conducteur jusqu'à l’extérieur de

la gaine (mm) 21.75 48.25

Rayon du conducteur (mm) 9.05 13.25

Rayon de l’isolant (mm) 18.25 39.75

Rayon de l’écran (mm) 20.25 42.45

Rayon intérieur du câble (mm) 46.9 104.05

Rayon extérieur à l’armure (mm) 52.9 110.05

Rayon extérieur (mm) 59.45 117.5

Enfouissement (m) 1 1


Appendices

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1 Conducteur; 2 L’écran du conducteur ; 3 L’isolation XLPE ; 4 L’écran de l’isolation ; 5 L’écran ; 6 Le séparateur; 7 La gaine ; 8 Eléments de remplissage ; 9 Bande de liaison ; 10 « Bedding » ;11 L’armure ; 12 « Serving ».


Appendices

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10.3. Appendix 3

Documents

Modeling and dynamic analysis of offshore wind …vanfrl/documents/mscthesis/2011_AH...Modeling and dynamic analysis of offshore wind farms in France: Impact on power system stability