Study of auxiliary power systems for offshore wind turbines632303/FULLTEXT01.pdf · Study of auxiliary power systems for offshore wind turbines an extended analysis of a diesel gen-set

UPTEC-ES13023

Examensarbete 30 hpJuni 2013

Study of auxiliary power systems for offshore wind turbines an extended analysis of a diesel gen-set

solution

Joakim Berggren

Teknisk- naturvetenskaplig fakultet UTH-enheten Besöksadress: Ångströmlaboratoriet Lägerhyddsvägen 1 Hus 4, Plan 0 Postadress: Box 536 751 21 Uppsala Telefon: 018 – 471 30 03 Telefax: 018 – 471 30 00 Hemsida: http://www.teknat.uu.se/student

Abstract

Study of auxiliary power systems for offshore windpower

Joakim Berggren

Until today the offshore wind power has grown in a steady pace and many new windfarms are being constructed around the globe. An important factor that is investigatedtoday in the industry are the security of power supply to the equipment needed forcontrolling the offshore system during emergency situations. When a offshore windfarm is disconnected from the external grid and an emergency case occur the windturbine generators lose their ability to transfer power and they are forced to be takenout of operation. As there are a number of loads in the wind turbines (navigationlights, sensor- and communication-apparatus, ventilation- and heating equipment etc.)they have a load demand which must be supplied in emergency mode. The GermanTransmission System operator (TSO) TenneT GmbH has set a requirement that thewind turbines is to be supplied by an auxiliary power supply (APS) in 12 hours andtherefore there is need for a long-term auxiliary power supply system. This masterthesis was assigned to investigate the most feasible APS-system. From the study of anumber of different APS's one concept was chosen. This was the diesel gen-setsolution placed on an offshore substation at sea. The system was modeled in thesoftware DIgSILENT PowerFactory where a load flow analysis validated the calculateddata and a study of the impact of transients in the system was performed.

ISSN: 1650-8300, UPTEC ES13023Examinator: Kjell PernestålÄmnesgranskare: Sandra ErikssonHandledare: Magnus Tarle, Ann Palesjö

J. Berggren Master thesis, Spring 2013

Acknowledgements

In this master thesis study, several people have been involved and I like to acknowledgethem all here.

A special thanks to my supervisors at ABB, Magnus Tarle and Ann Palesjö who steadilyhave pushed me forward and help me through the work. With interesting discussions andwith their strong energy they have supported me with all their spirit and commitment.

My evaluator at Uppsala University Sandra Eriksson have helped me with good discussionsas the master thesis work has preceded especially questions regarding generator design issues.She has helped me to organize the thesis report in a good way.

I want to thank the other co-workers at ABB / Oshore Wind Connections who haveprovided me with detailed information and with discussion that have given me many newexperiences and much new knowledge. Especially Thorulf Brattström, Mikael Svedung, JorgeBrischetto, Drasko Skenderija, Steven Coppens, Fernando Sada, Nassim Raoo and OskarBjörkbacka.

Other persons in the ABB concern I'd like to thank are:

Günter Stark and Stefan Hopp from ABB Germany which has provided me with infor-mation about diesel gen set and the dynamical transients present in the oshore windfarm.

Jan Westerlund at ABB Helsinki with the help of rating the synchronous generator inthe diesel gen set.

Rudolf Moeckli at ABB Schweiz that have help with information about the controlprocedure when starting a diesel gen set.

Marja-Liisa Marttila and Sakari Laine at ABB Vaasa with the knowledge about shuntreactors in the oshore wind farm.

Apart from the people at ABB I want to acknowledge the extensive assisting aid and infor-mation that Luis Leal from the wind turbine manufacturer REpower has given me. Withouthis help with essential information about the Wind turbine equipment this master thesis studycouldn't be completed.

With the help of contacts at dierent diesel gen set manufacturers a specic diesel gen set solution wouldn't have been possible and hereby I'd like to acknowledge this people. Theyare Jörg Haabermaas at MTU, Göran Österdahl at Wärtsilä, Peter Bojtas at MAN and KarlStapelfeldt at the company CATUM.

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Populärvetenskaplig sammanfattning

Fram till idag har havsbaserad vindkraft ökat i en jämn takt och många nya vindkraft-parker byggs eller tas i drift. Säkerhetskraven på vindkraftverken ökar idag och kraftsys-temsoperatörer (TSOs) kräver att nödkraft kopplas in till vindkraftverk inom en kort period(TenneT har ett krav på 12 timmar). När ett långvarigt avbrott (t.ex. kabelbrott) uppstårkan vindkraftsparker till havs vara utan el/energi i era månader och om inte vindkraftverkunderhålls (med värme, ventilation etc.) under denna tid kan de bli skadade. Samtidigt måstenavigationsbelysning vara tillgänglig som en säkerhet för sjöfart och ygplan.

När en vindkraftpark blir bortkopplad från det externa nätet vid ett nödfall förlorar vin-dkraftverken sin förmåga att överföra eekt och de tas ur drift. Eftersom det ändå nns ettantal laster i vindkraftverken (lanternor, sensor- och kommunikations- apparater, ventilations-och värmeanläggningar) måste de ha ett nödkraftsystem som tillgodoser eektbehovet.

Inom detta examensarbete undersöktes först olika nödkraftssystem på en grundläggandenivå för att urskilja ett eller era lämpliga system att använda som reservkraft. De un-dersökta systemen var dieselgen-set(s) på en havsbaserad plattform eller placerad vid vind-kraftverk, gas/dieselgen-set, solceller (ev. kombinerat med batterier), vågkraftverkskoncept ochfullomriktar-vindturbiner i kombination med dieselgen-sets.

Utifrån en grundläggande studie av nödkraftsystemen påvisades ett koncept som ansågsvara fördelaktigt att genomföra. Detta var ett dieselgen-set system som placerats på en havs-baserad transformatorstation kopplad till vindkraftsparken.

En modell av en vindkraftspark med ca 500 MW nominell eekt installerad i totalt 82 vin-dkraftverk konstruerades i programvaran DIgSILENT PowerFactory där en lastödesanalysvaliderade beräknade data från Microsoft Oce Excel.

Utifrån lastödesanalyser i en ytterligare DIgSILENT PowerFactory modell över den stud-erade vindkraftparken i nödläge (när vindkraftverken har laster som behöver förses med eekt)kunde olika nominella eekter på era generator typer och på en motor bestämmas. Dieselmotorn krävde ca 4 MW och den lägsta generator storleken var 4.2 MVA med shunt reaktorerinkopplade på en havsbaserad plattform.

En undersökning av transienter som är närvarande i det svaga kraftsystem (inrusningsström-mar, spänningsvariationer) som den havsbaserade vindkraftparken utgör när det är bortkop-plat från det externa nätet genomfördes. Olika koncept för att minska påverkan av de tran-sienta eekterna föreslogs efter att ha studerat olika tekniker. Två koncept som diskuteras ärspänningsrampning av vindparksnätet utan att ha vindturbinlaster inkopplade och mjukstartav vindkraftspark i nödläge där sträng efter sträng av vindturbiner energiseras tills att allalaster i vindparken tagits i drift.

Slutsatsen av de olika resultaten detta examensarbete har genererat sammanfogades tillen beskrivning av de mest fördelaktiga dieselgen-set:en (med/utan shunt reaktorer) och endiskussion angående alternativa framtida nödkrafts-koncept som eventuellt kan vara fördelak-tiga presenterades.

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Abbrevations and clarications

A AmpereAC Alternating CurrentAPS Auxiliary Power SystemCS Collector SystemDC Direct CurrentDFIG Doubly Fed Induction GeneratorGen-set Generator setHV High VoltageHVDC High Voltage Direct CurrentMV Medium VoltageO&M Operation and MaintenanceOS Oshore SubstationOnS Onshore SubstationOS Oshore SystemOWF Oshore Wind FarmOWP Oshore Wind PowerPF Power factorpu Per UnitPV Photo VoltaicRMS Root Mean SquareSG Synchronous GeneratorTC Transmission CableTrafo TransformerTS Transmission SystemTSO Transmission System OperatorUPS Uninterrupted Power SupplyV VoltVA VoltAmpereWECS Wind Energy Conversion SystemWT Wind TurbineWTG Wind Turbine GeneratorW WattWPP Wave Power PlantWPF Wave Power Farm

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Description of parameters

C Capacitancecos φ Power Factor denitionδ Load angle of GeneratorE Electromagnetic forcef Electrical Frequency of gridηmodule PV module Eciencyηtot PV system EciencyI Currentk linear constantL Inductancem Massω Angular frequencyΩ OhmP Active PowerPf Packing factorφ Numerical operator phi which determines the angle between P & QQ Reactive PowerR Resistanceρ DensityS Complex Power|S| Apparent PowerU VoltageV VolumeZ ImpedanceX Reactanceuk Relative voltage drop over transformer (no-load)

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Contents

1 Introduction 9

1.1 Emergency requirements for an oshore wind farm . . . . . . . . . . . . . . . . 9

2 Purpose and delimitations 10

3 Background 10

3.1 Oshore system description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103.2 Load demand in Wind turbines . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

4 Theory of the investigated oshore wind farm system 13

4.1 Oshore Wind turbines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134.1.1 Wind turbine generator . . . . . . . . . . . . . . . . . . . . . . . . . . . 144.1.2 UPS-system in wind turbines . . . . . . . . . . . . . . . . . . . . . . . . 15

4.2 Inter array cables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154.3 Wind turbine transformers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154.4 Load demand in Wind turbines . . . . . . . . . . . . . . . . . . . . . . . . . . . 164.5 Grid stability of electrical oshore systems . . . . . . . . . . . . . . . . . . . . . 17

5 Investigated oshore wind farm system 17

6 DiGSILENT PowerFactory 18

7 Modeling in DiGSILENT PowerFactory 20

7.1 Modeling of OWF in Normal operation . . . . . . . . . . . . . . . . . . . . . . . 207.2 Calculations of load ow in Emergency mode . . . . . . . . . . . . . . . . . . . 217.3 Modeling of OWF in Emergency mode . . . . . . . . . . . . . . . . . . . . . . . 22

8 Auxiliary power supply concepts 24

8.1 Diesel gen-set(s) used as APS-system . . . . . . . . . . . . . . . . . . . . . . . . 248.1.1 Stationary diesel gen-set concepts . . . . . . . . . . . . . . . . . . . . . . 258.1.2 Mobile diesel gen-set concept . . . . . . . . . . . . . . . . . . . . . . . . 26

8.2 Gas/diesel gen-set(s) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 268.3 Photovoltaic's (PV's) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

8.3.1 PV's on Platform . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 278.3.2 PV's on WT nacelles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

8.4 Batteries combined with PV's . . . . . . . . . . . . . . . . . . . . . . . . . . . . 298.5 Wave Power . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 318.6 Full converter wind turbines & diesel gen-set . . . . . . . . . . . . . . . . . . . . 33

9 Summary of APS-system studies 34

9.1 Diesel gen-set . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 349.1.1 On oshore substation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 349.1.2 at WT towers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 349.1.3 Mobile diesel gen-set on marine vessel . . . . . . . . . . . . . . . . . . . 35

9.2 Gas/diesel gen-set . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 359.3 Photovoltaic's (PV's) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

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9.3.1 On OS roof . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 369.3.2 On WT nacelle roof . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

9.4 Batteries combined with PV's . . . . . . . . . . . . . . . . . . . . . . . . . . . . 369.5 Full converter wind turbines & diesel gen-set . . . . . . . . . . . . . . . . . . . . 379.6 Chosen APS-system for further investigation . . . . . . . . . . . . . . . . . . . . 37

10 Sizing of diesel gen-set: Calculations 38

10.1 Sizing of diesel gen-set without wind turbine UPS-system utilised . . . . . . . . 3810.1.1 Rating of standard generator . . . . . . . . . . . . . . . . . . . . . . . . 3810.1.2 Rating of special generator . . . . . . . . . . . . . . . . . . . . . . . . . 41

10.2 Sizing of diesel gen-set with wind turbine UPS-system utilised . . . . . . . . . . 4210.2.1 Rating of standard generator . . . . . . . . . . . . . . . . . . . . . . . . 4410.2.2 Rating of special generator . . . . . . . . . . . . . . . . . . . . . . . . . 45

10.3 Reactive power compensation equipment . . . . . . . . . . . . . . . . . . . . . . 45

11 Slow start of OWF with diesel gen-set 47

12 Transient studies 48

12.1 Connection of WT loads in sequence . . . . . . . . . . . . . . . . . . . . . . . . 4912.2 Ramping up voltage over diesel gen-set . . . . . . . . . . . . . . . . . . . . . . . 50

12.2.1 Basic voltage ramping study . . . . . . . . . . . . . . . . . . . . . . . . . 51

13 Diesel gen-set summary 54

14 Discussion 55

15 Conclusions 58

16 Future Work 59

A Basic theory of Oshore wind farm systems 67

A.1 Fundamentals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67A.2 Generators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68

A.2.1 Magnetic end core heating limit . . . . . . . . . . . . . . . . . . . . . . . 71A.3 Transformers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71A.4 Cables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72

B Transients theory and calculations 73

B.1 Voltage ramping power ow calculations . . . . . . . . . . . . . . . . . . . . . . 74B.1.1 OWF equivalent data . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75

C Control systems of diesel gen-sets 75

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1 Introduction

Today, our society's energy need is mainly supplied by the use of fossil fuels. The age of thefossil fuels has led the humanity to an advanced development in the technological area and wehave created the modern society. However, as the Stone Age did not end because of the lackof stone the Age of the fossil fuels will not end because we run out of oil, gas and coal. It willend sooner. The fact is that the fossil fuels are nite energy resources which eventually willbe consumed. In the striving towards a more sustainable energy system, many countries inEurope are trying to increase the use of renewable electricity sources in the system. This hasstarted a large-scaled development in areas like oshore wind power area all over the world,especially in Europe, North America and China [1].

In the future, the Oshore wind farm industry is expected to grow in a steady pace and theinstalled power is expected to increase. For Europe, there are scenarios for the developmentboth in medium term (2020) and in long term (2030) [2]. In these scenarios the largest nationsthat will produced most of the oshore wind farms (OWFs) are UK and Germany, followed byFrance, Netherlands, Belgium, Denmark and Sweden. In total the amount of installed powercould grow from present values of approximately 5 GW to 42 GW in 2020 and 126 GW in2030. However, these numbers should only be treated as indications of the increased installedpower and not as exact values.

Figure 1: The development of the oshore wind farm industry in the future, in a perspective of theyear 2020 and 2030. The increase in OWFs are shown per country in GW [2].

1.1 Emergency requirements for an oshore wind farm

The large-scale development in oshore wind power have led to increased security restrictionsof the components in an oshore wind farm during emergencies. Under these, the park iscut-o from the external grid (i.e. a cable failure) and no power can be transferred to landthrough the export cables.

In order to certicate an OWF, governmental institutions as transmission system operators(TSOs) require a complete auxiliary power supply, an APS-system that support acting wind

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turbine loads with power during extreme emergencies where the wind farm is disconnectedfrom the power grid in up to three months [3].

Due to the fact that the UPS-system consists of limited energy storage capability the WTsonly have a power supply of maximum 12 hours today [4], the need of an APS-system isrequired.

As WT manufacturer REpower has raised a question to the ABB division ABB / PowerSystems / Oshore Wind Connections (OWC) regarding the design of an APS-system, sup-plying the wind turbine loads during emergencies, this study was created with the aim ofpointing out suitable system designs.

2 Purpose and delimitations

This study will produce a technical overview of dierent auxiliary power supply systems(APS's) that must be constructed when an emergency occur. First, a distinguishing of inter-esting designs is to be performed in order to nd and clarify which systems that are theoretical& economical possible to design. The dierent cases examined are listed below in the text:

Diesel Gen-set(s)

Placed on OS

Placed at WTs

Placed on mobile watercraft

Gas/diesel gen-set

Photovoltaic cells combined with batteries

Batteries

Wave power

Full converter WT's & diesel gen-set's

The dierent system concepts are all investigated by the assumption that they all havethe potential to be a future constructed APS-system. The system(s) that could be of specialinterest is determined and can be investigated more deeply in coming sections later on. Fromthe results of this deeper study, conclusions regarding the APS-system concept(s) will betaken.

3 Background

3.1 Oshore system description

In order to understand the headline of this study, the electrical topology of an OWF is ex-plained in this chapter. The design can be classied into a number of dierent categorieswith all the necessary infrastructure and components required for the dierent designs. Theyare the transmission system (TS), the collector system (CS) and the wind energy conversion

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Figure 2: Electrical layout of an OWF with two oshore substations (example).

system (WECS) of the OWF [5]. A simple and overseeing picture which shows the overallelectric design of an oshore system is visible in Figure 2.

The Collector System (CS)

Switchgear in Wind Turbines (WTs)

Switchgear in Oshore System (OS)

Submarine inter array cables in OWF connecting the MVAC Oshore Substation(OS) with the wind turbines

The Transmission System (TS)

Submarine transmission cables from OS to land

Transmission lines or underground cables on land to the Onshore Substation (OnS)

The Wind Energy Conversion System (WECS)

Gearboxes

Transformers

Cables in the wind turbines

Electrical converters

Additional power electronics

Main structures and systems in the Wind turbines

WECS

Shaft

Tower

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Turbine

Generator

Foundation to place WT on

Additional equipment in the OS

Platform Transformer

Switchgear

Today wind farms are placed long away from shore (> 10 km) and sometimes the needof both AC and DC OS(s) are needed to minimize the transmission losses to onshore [1].Due to the technology in HVDC cables the losses are lower relative to HVAC cables and withincreased distance there is more and more economical to use HVDC instead of HVAC [1, 6].Closest to the OWF, an AC OS is always placed and this is the last link before the OWF. Thestudy is focused to investigate a APS-system placed on the AC OS or at the wind turbinesin the OWF, see the scope presented in Figure 2. In Figure 3 the geographical layout of aOWF system is seen.

Figure 3: A schematic picture of an oshore wind farm [7].

3.2 Load demand in Wind turbines

Oshore wind turbines have both constant and alternating loads that always are in need ofpower even during emergency situations when the OWF is without a power supply. The totalWT load demand consists of a number of dierent smaller loads [3]:

Heating loads

Ventilation loads

Transformer magnetisation loads1

Navigation-lights

Additional loads for operating the necessary load equipment

1See Appendix A.3 for information regarding the transformer losses.

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4 Theory of the investigated oshore wind farm system

In order to nd a suitable APS-system, a basic scenario of an oshore wind farm scenarioconsisting of 82 WTs with a rated output of 500 MW was chosen. This is a common size ofa OWF constructed today and is therefore used as a reference design, as this study doesn'tinvestigate any real OWF structure. The OWF conguration in emergency mode is shownin Figure 4. To fully understand the system design, the included dierent substructure parts(TCs, WTs, WT transformers) are described with their specic characteristics in this section.

WT loads Heating, Ventilation, NavLights,

Transformer magnetisation losses

Array with 6 WT loads

A total of 82 WTs in (82/6) strings Wind Farm rated output production

of 500 MW

Offshore substationHVAC platform

Array with 6 WT loadsTo onshore

Figure 4: The investigated scenario of a OWF with POWF ≈ 500 MW .

4.1 Oshore Wind turbines

There are a number of dierent manufacturers which constructs WTs for OWFs as Siemens,Vestas, Nordex, REpower etc. In this study, data from the German manufacturer REpower isused and the basic information of the wind turbine is shown in Table 1.

Table 1: Basic data of REpower 6M WT [8].

Wind turbine data

cosφ 0.925Rated Complex power S 6.65 MVARated Active power P 6.15 MW

The inside of the WTs structure is shown in Figure 5 and the following list shows the mostimportant equipments [9].

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Figure 5: The REpower 6M WT [9].

DFIG generator

converter

Three winding transformer

UPS-system (Backup-system consisting of lead-acid batteries)

Heating and ventilation equipment for power electronics, gearbox, etc.

Navigation lights for visualising the WT for aircrafts and ships

4.1.1 Wind turbine generator

The DFIG generator is a common wind turbine generator (WTG) type that is used in REpower6M WTs. It is built up by a variable speed constant-frequency induction generator [10]. Thegenerator has 0.66 kV as maximal rotor voltage with a stator voltage of 6.6 kV. Duringemergencies the circuit breakers from the WT transformer to the excitation of the generatorare open and only the loads are energised from the transformer, see Figure 6.

Figure 6: The DFIG REpower 6M WT when wind turbine is in APS-system mode [4].

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4.1.2 UPS-system in wind turbines

The WTs in OWFs can sometimes have an existing emergency power supply placed in connec-tion to the WT structure. The manufacturer REpower has in every WT placed a UPS-systemwhich main component is a leadacid battery with a capacity of supplying the average WTload demand of 42 kW up to 12 hours [4].

The batteries have specic charge-rates (C-rates) which describes the batteries ability tobe recharged (can also represent discharge) [11]. A high charge-rate means that a battery willbe recharge in a short time and a low charge-rate represent the opposite where the batterieshave a long recharge-time. The C-rate depends on the battery type, i.e. a lead-acid batteryis not able to recharge in a short time and has therefore a low C-rate.

The usage of the existing UPS-system together with a new APS system could be a usefulcombination in order to utilize the available emergency systems in the OWF. In that way, itcould maybe be possible to make a sustainable adaption to a APS-system which is a designof both old and new equipment in the OWF. The use of the UPS-system in the WTs togetherwith diesel gen-sets are investigated in Section 10.2.2.

4.2 Inter array cables

The TCs in the OWF are chosen to be typical MV 30 kV AC XLPE three-core cables withsimple reinforcement because of the simplicity of this cable type [12]. They have dierentcurrent ratings, cable parameters (capacitance C & inductance L) and dierent impedances(Z0, Z1 and Z2) that depends on seabed conditions and load proles/duty cycles, see Table 2.

Table 2: The dierent MV 30 kV cables used in a OWF [13].

30 kV threecore cables, XLPE 3-conductor cable parameters

Cross-section of conductor 120 mm2 240 mm2 400 mm2 630 mm2

Current rating [A] 340 480 590 715Capacitance C [µF/km] 0.19 0.24 0.29 0.35Inductance L [mH/km] 0.42 0.38 0.35 0.32Z1 & Z2 [Ohm] 0.221 + j0.13 0.123 + j0.116 0.088 + j0.107 0.067 + j0.1Z0 [Ohm] 0.416 + j0.118 0.306 + j0.104 0.262 + j0.095 0.228 + j0.088

4.3 Wind turbine transformers

There are two kinds of transformer types in the OWF, the AC OS transformer and the WTtransformer(s). The OS transformer is a step-up transformer where the voltage is transformedfrom 33 kV to 132 kV, 150 kV or 220 kV (MVAC TCs) and is the connection point in the OWFwhere transmission cables to land are connected. As an OWF can have a rated output of ahundreds of MWs the OS transformer can be large in size with > 100 MVA [14] in output.

The WT transformer used in this study is the rated three-winding transformer given byREpower [15]. The transformer is adapted for the normal mode where the WTGs are pro-ducing power to the onshore power grid. Therefore the rating of the transformer S=6.7 MVAcorresponds to the rated power of the WTG, S=6.65 MVA with PF=0.925.

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Figure 7: The WT transformer in reality [9].

Table 3: The basic data of REpower 6M WTG transformer [15].

Transformer design data

Rated power S1 6.7 MVARated power S2 6 MVARated power S3 1.1 MVA

Rated voltage U1 33 kV2

Rated voltage U2 6.6 kV3

Rated voltage U3 0.66 kV4

4.4 Load demand in Wind turbines

The load demand in a WT is made up by a number of smaller loads that contribute to the totalload demand which are previously described in Section 3.2. They consists mainly of dierentmotors and pumps which supports the heat, ventilation, navigation lights, magnetisationlosses5 etc. in the WTs. The total load demand of a REpower 6M WT is shown in Table 4.

Table 4: The load demand in a REpower 6M WT [3].

Basic WT load data

Average load demand 42 kWPeak load demand 193 kVA

2HV side of trafo3MV side of trafo4LV side of trafo5See Appendix A.3 for information regarding the transformer losses.

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4.5 Grid stability of electrical oshore systems

The Oshore wind farm in auxiliary power mode is a very weak grid operating in island modewhere the only generation is the APS-system supply. This impacts the system stability whenthe OWF is going into emergency mode and the APS-system is started from a blackout.

The relatively large amount of capacitance in the TCs in the OWF impacts the systemas the capacitance together with the inductance in the cables and in transformers giverise to resonance and voltage oscillations.

When the wind farm is energised, so called "transformer inrush currents" occurs whichcan damage the transformers or other OWF equipment. The currents can be as high as9 ·Inom of the WT transformer nominal current [3]. This phenomena is further describedin section 12.

5 Investigated oshore wind farm system

Due to the fact that the strings in OWFs consists of WTs connected in parallel by TCs, thepower ow in the connecting TCs are dierent depending on the position in the string. Forexample a cable outermost doesn't have as high current rating Irated as a TC innermost in astring. Thus the rating of the TCs can be chosen according to the required Irated. Figure 8shows a string in the studied OWF.

Figure 8: Power ow and length of TCs in a string of the studied OWF.

In each string the power ow in TC nr 1-6 is determined with the help of Equation 1,Equation 2 and Equation 3. Equation 1 describes the active power ow in a string PstringNormal

,

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Equation 2 describes the apparent power in a string SStringNormaland Equation 3 the reactive

power ow in a string QStringNormal.

PstringNormal=POWFNormal

nStrings(1)

|S|StringNormal=PstringNormal

cosφ(2)

QStringNormal=

√|S|2stringNormal

− P 2StringNormal

(3)

ITCString=|S|StringNormal√

3ULN

(4)

The currents that ow through the TCs in a string ITCStringare calculated from Equation 4.

Table 5: Powers P, Q, |S| and the currents owing in one string ITCStringof the selected OWF during

normal operational mode, where WTs produce power to the onshore power system.

From the current ratings of the TCs in a string it's possible to correlate consistent TCcable dimensions that match the currents in the string, seen in Table 6. The dierent TCsused in a OWF are compared to the specied cable dimensions described in Section 4.2.

Table 6: The currents owing in a OWF string with corresponding TC ratings.

6 DiGSILENT PowerFactory

The software DIgSILENT PowerFactory is used in power system modelling, analysis andsimulation of electric systems and includes a large variation of functions and applications thatare used to create a model of a power system which can be simulated with respect to loadow calculations (steady-state), dynamical electromagnetic transients (EMTs), continency

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analyses, short circuit analyses etc. [16]. The model parameters that are important to knowof in the software are visualized in Table 7.

Table 7: Dierent model parts in the software DiGSILENT PowerFactory [16].

Review of Model parts used in DIgSILENT PowerFactory

Model part Characteristic values PowerFactory picture

Busbar 33 kV, 16 kV and 0.66 kV

Node (connection point)

Transmission lines33 kV, rated currents for dierentconductor mm2, Z0, Z1 and Z2

WT transformer(3winding), rated power, ratedvoltages, Z0, Z1 and Z2, vectorgroup (∆ YY)

WT transformer(2windings), rated power, ratedvoltages, Z0, Z1 and Z2

External gridReference for the power systemoutside of the OWF

WT loadsAverage power demand of42 kW/WT

Circuit breakers

WTG Rated power, cos φ

Shunt reactor Rated power

Diesel genset Rated Apparent power |S|

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7 Modeling in DiGSILENT PowerFactory

7.1 Modeling of OWF in Normal operation

A model of a string in the oshore wind farm during normal operation mode was created inthe software DiGSILENT PowerFactory, in order to simulate the "loading" of the TCs forverication of the oshore wind farm layout. As the strings in the OWF have the same layoutit's possible to only investigate one string. Data was taken from Section 4.1, Section 4.2,Section 4.3 and Section 5 to implement into the model parameters which consists of:

A central busbar (symbolizing the AC OS)

TCs

WT transformers

WTGs

The model layout in non energised and in energised mode is shown in Figure 9a and Figure 9b.The loading on the TCs in a string were under 100 % not exceeding the current ratings ofeach cable selection, validating the design.

(a) Model of an OWF string in normal operation(non energised) [16].

(b) Model of an OWF string in normal operation(energised) [16].

Figure 9: The model of an OWF string (non-energised/energised).

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7.2 Calculations of load ow in Emergency mode

Calculations of the active and reactive power transmitted in the system during emergenciesare performed in the software Microsoft oce Excel.

The active power transmitted in the system answers to the load demand of the WTs duringemergencies, PWTGEmergency

stands for 42 kW per WT including with the WT transformermagnetisation losses, see section 4.4. The copper losses in the transmission cables is neglected(Pcopper ∼ I2) as the the current is very low through a string during emergency operationcompared to normal operation. The total load demand in the WTs is POWFEmergency

and thetotal load demand answers to POWFEmergency

= 42 kW · 82WTs = 3.44 MW .

To be able to calculate the reactive power transmitted in the OWF in emergency mode andcompare it with a load ow analysis in DiGSILENT PowerFactory the following assumptionsare used:

The TCs are perfect capacitors (basic assumption as there in reality are both resistiveand inductive parameters present in a TC).

The reactive power demand is mainly coming from the capacitive part of the TC.

The load demand of 193 kVA present in eight seconds is neglected.

The reactive power in the OWF is therefore calculated with the assumption that thecapacitive part of the TCs in the OWF is the part which contributes to the load ow asXC ∼ U2 with a voltage of 33 kV (see Appendix A.4). QTC produced in the transmissionlines is calculated with Equation 5 and Equation 6.

Z =U2

|S|V A (5)

For a perfect capacitor or inductor there is no net power transfer, thus all the power inthe TCs is reactive, see Equation 6. The results of the calculations is shown in Table 8.

QTC = QCap =U2

XCV ar (6)

Table 8: The reactive power in a string of the OWF (emergency mode).

Cable number [µF/km] [µF] QTC [MVAr]

1 0.35 0.875 -0.2992 0.29 0.203 -0.0693 0.24 0.168 -0.0574 −”− −”− −”−5 0.19 0.133 -0.0466 −”− −”− −”−

QString -0.575QTot -7.86

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7.3 Modeling of OWF in Emergency mode

The design of a model for the oshore wind farm during emergencies is constructed in DiGSI-LENT PowerFactory to analyse the load ow in the system during the special operationalmode. The simulated load ow is needed to be found in order to investigate any APS-systemconcepts further.

The model is presented in Figure 10 consisting of a central busbar with connected strings.In each string the specic layout determined in Section 7.1 is used and a more detailed layoutof strings nr 1-3 in the OWF is seen in Figure 11a. Figure 11b show a detailed layout of apart of string 1-3 in the OWF.

Figure 10: The model of the OWF in emergency mode [16].

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(a) A part of the model layout ofthe OWF during an emergencysituation [16].

(b) More detailed view of the system design. A part ofstring 1 is visualised [16].

Figure 11: Detailed layout of the OWF in emergency mode.

A load ow analysis was simulated by the help of a Newton-Raphson iteration [16] in orderto nd the power ow in the system. From the results of the load ow analysis, the values ofactive power P and reactive power Q in the OWF in emergency operation mode are presentedin Figure 12.

Figure 12: Load ow in OWF during emergencies from the PowerFactory model.

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The reactive power in the PowerFactory model is diering ≈ 0.235 MVAr or 3 % fromthe calculated values, (see Table 9). This might be an eect of some inductive reactive powerreduction from TCs and a voltage level which isn't exactly 1.0 pu. The results, howevervalidates the reactive load ow in the OWF during emergencies in a good way.

Table 9: Load ow analysis in PowerFactory model and calculations.

Load ow comparison of OWF in (emergency mode)

Scenario P [MW] Q [MVAr]

DiGSILENT PowerFactory 3.44 -8.09Calculations 3.44 -7.86

8 Auxiliary power supply concepts

In this section the possible APS-systems that theoretically can be of interest as an emergencypower supply is discussed and reviewed. This include a fundamental technical evaluation inorder to synthesise the most interesting concepts that will be studied deeper later on in thereport.

8.1 Diesel gen-set(s) used as APS-system

A diesel gen-set is visualised in Figure 13 and consists of an engine, generator and a controlsystem with including an Auto Voltage Regulator (AVR).

Figure 13: A diesel gen-set consisting of a engine, synchronous generator and necessary control equip-ment as the governor of the engine and the AVR of the generator [17].

.

It is a common APS-system that is used as a backup in emergency cases. There are severalpossible diesel-gen-set conguration that are investigated, they are:

Diesel gen-set on Oshore substation

Diesel gen-set on WTs

Diesel gen-set on mobile vessel

These concepts uses dierent congurations which alter the rating of the equipment, dependingon which conguration that is studied. The following section describe the concepts in detail.

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8.1.1 Stationary diesel gen-set concepts

Basic descriptions are introduced in this section and the most important advantages anddisadvantages of the system designs are pointed out. There are two possible solutions forplacing a stationary diesel gen-set in the OWF, the diesel gen-set is either placed on theoshore substation or at the bottom of the WT tower in at every turbine. The system designsare showed in Figure 14.

(a) Diesel gen-set placed on oshore substation (Ma-

nipulated picture) [7].(b) Diesel gen-set placed at bottom of all WT towersin the OWF (Manipulated picture) [7].

Figure 14: THe stationary diesel gen-set concepts in the OWF.

The dierence between the two stationary diesel gen-set concepts are:

A diesel gen-set placed on a OS require reactive power compensation or a specialdesigned generator which can operate under-excited and consume reactive power fromthe MV 33 kV TCs in the OWF.

No magnetisation losses in the WT transformer with diesel gen-sets placed at WT towers,due to the fact that the gen-set is directly connected via a 660 V busbar to the WTloads.

Limited place at the bottom of the WT tower to place a diesel gen-set. The area alsorequire placing of a fuel tank which additonally reduces the available space.

Simple design of diesel gen-set at WTs as they are small in size, standard equipmentused in many applications today.

The total power demand in the studied OWF during to be supplied of the dierent dieselgen-set concepts are shown in Table 10.

Table 10: Power demand: Stationary diesel gen-set concepts used on AC OS or placed at WT towers.

Power demand: Stationary diesel gen-set concepts

Diesel gen-set scenario P [MW] Q [MVAr] |S| [MVA]

Placed on AC OS 3.44 -8.09 ≈ 9Placed at every WT tower 0.042 ≈ 06 ≈ 0.05

6Peaks of 193 kVA present in 8 seconds interval which demands an overrating of the power supply.

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8.1.2 Mobile diesel gen-set concept

A mobile diesel gen-set consists of a standby marine vessel which in emergencies travels outto the OWF. The system design is similar to the stationary concept of having a diesel gen-setplaced on a OS or at the WT towers, the only dierence is that the diesel gen-set are placedin the OWF when an emergency has happened. A stationary concept have the diesel gen-setplaced at the OWF at all time. The procedure to do this are explained below [4, 18]:

1. The marine vessel travels to the OS when an large emergency occur.

2. When the marine vessel has reach the OWF it places a diesel gen-set on the OS or atthe WT towers.

3. The loads in the WTs are supplied, either from the diesel gen-set on the OS or at theWT towers.

To fulll the TSO TenneT criteria and supply the WT loads in twelve hours, there are anuncertainty whether the marine vessel will have time to place diesel gen-set(s) at the OWFwhen an emergency occur. The vessel availability, diesel gen-set availability, travel time etc.impacts the time. In that case its possible that an additional short-term power supply areneeded to stationary be placed at a OWF, to handle the WT load demand until the mobilevessel has installed the long-term diesel gen-set(s).

The required diesel gen-set ratings of this APS-system are the same as for the stationarydiesel gen-set concepts, see Table 10 in Section 8.1.1.

8.2 Gas/diesel gen-set(s)

A similar concept to the diesel gen-set placed on the AC OS is the usage of an gas/diesel gen-set with a piston engine. It works in the same way as the diesel gen-set (see Section 8.1.1), butcan also run on gas and not only diesel. The advantage is that there could be less vibrationsand less pollution of the environment [19]. The exhaust gases have reduced concentrations ofnitrogen-oxides (NOx) and sulfur-oxides gases. A major disadvantage is that an additionalfuel tank for gas is required in addition to the diesel tank.

The load ow is the same as in the diesel gen-set placed on the AC OS with the genera-tor/reactive power compensation equipment handling the active and reactive power ow.

8.3 Photovoltaic's (PV's)

A sustainable energy resource that possibly could be used as a APS-system would be to utilizethe energy from the sun by using PV's. The solar cells produces electric energy for free andsupport the WT loads with power. PV eciency depends on the PV module eciency, ηmodule,the packing factor of the modules (the fraction of absorber plate area covered by the solarcells) Pf and on the eciency of the converter system (power conditioner) that deliver a goodvoltage quality to load equipment ηpc. The resulting eciency of a PV module ηtot is foundfrom the relationship between these factors and is shown in Equation 7 [20, 21].

ηtot = ηmodule ηpc Pf (7)

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From [20] a specic site consisting of 329 m2 had a total ηtot = 8 % which are used as areference for a low total eciency of the the PV system in this study. To examine the powerproduction, a study of the power production in W/m2 with dierent ηtot and weather condi-tions (sunny, partly cloudy and cloudy conditions) were examined. For simplifying reasons,the PV's are placed on a at area pointing directly vertical to the sky. Then the need of takingcare of the tilt angle and the compass directions relative to the sun that otherwise will aectthe power production were omited. The results of the examination are shown in Table 11.

Table 11: Power production in W/m2 of a PV [20, 22].

Power production in W/m2 of a PV with respect to ηtot

Weather condition Sunny Partly cloudy Cloudy

ηtot = 8 % 80 W/m2 40 W/m2 8 W/m2

ηtot = 11 % 111 W/m2 56 W/m2 11 W/m2

ηtot = 13 % 133 W/m2 67 W/m2 13 W/m2

8.3.1 PV's on Platform

The possibility to utilize some of the area on a OS roof to place PV's is investigated.

Figure 15: PVs placed on the OS roof. The dimensions of the roof is assumed to be 40 m in lengthand 20 m in width. The total area AreaOffS = 800 m2 (Manipulated picture) [7].

There is assumed that half of a typical AC OS roof area is available for installing PV'sAreaOffS = 400 m2 , shown in Figure 15. Equipment like the Helipad, diesel gen-set, crane,fuel tanks, etc. require place and therefore much of the total roof area can't be utilized.

The total power produced from the PVs placed on the OS is determined by Equation 8and the results are shown in Table 12 and Figure 16.

PPVOffS= PPV ·AOffS (8)

From the results it's visible that the PV's are not producing enough of power for supplyingthe WT load demand with only a small portion of the load demand supported. In additionthe PV's won't produce any power at night-time which must come from another power source.This can for example be the (UPS)-system in the WTs which is assumed to have an energysupply for the WT loads in 12 hours.

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Table 12: The total power POWFAPSwith dierent ηtot of the PVs placed on OS with half of platform

covered with PVs.

Power production of PV's

ηtot Sunny Partly cloudy Cloudy

8 % 32 kW 16 kW 3 kW11 % 44 kW 22 kW 4 kW13 % 53 kW 27 kW 5 kW

Figure 16: The power production from PVs versus the WTG load demand in the OWF.

8.3.2 PV's on WT nacelles

This scenario is similar to the previous section with PVs on the OS, but here the roof of theWT nacelles is utilized for power production by PV's. The PVs have been assumed to occupythe roof of the WT nacelle with a width of 6 m and a length of 15 m. The total area of theroof of one WT nacelle is AWTnacelle

= 90 m2 [23]. The total area of all WT nacelles in theOWF that consists of 82 WTs is APVWTs

= 90 m2 · 82 WTs = 7380 m2.

Similar to the previous Section 8.3.1, the total power produced by the PV's PPVWTsis

related to the total eciency of the PV system ηtot and the total area available for installingPVs, see Equation 9, the total power production is shown in Table 13 and Figure 17.

PPVWTs= PPVAWTnacelle

nWTs (9)

The power production of PV's placed on WT nacelles consists of a larger total area relativeto placing the PV's on the OS (7380 m2 ⇔ 400 m2). This increases the maximum powerproduction from 53 kW to 492 kW, please see Figur 16 and Figure 17. The results showsthat the power demand from the WTs isn't supported by only using PVs placed at the WTnacelles.

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Table 13: The total power PPVWTGswith dierent ηtot of the PVs placed on the WT nacelles.

Power production of PV's

ηtot Sunny Partly cloudy Cloudy

8 % 590 kW 295 kW 59 kW11 % 820 kW 410 kW 82 kW13 % 984 kW 492 kW 98 kW

Figure 17: The power production from PVs on WT nacelles versus the WT load demand in the OWF.The load demand is lowered as the magnetisation losses in the WT transformers are avoided by theconcept design.

8.4 Batteries combined with PV's

Another APS-system can be to use batteries in combination with PVs placed on the OS orat the WTs, then the WT loads might be supplied with enough power. The reference batteryused in the study are assumed to be Lead Acid batteries which are used in the DolWin1OS project for supporting the internal load demand of the substation [24].

A battery is built up by a number of cells that each have their own characteristics andthey are combined one battery. Such batteries have the following characteristics, shown inTable 14 [24].

The power required to be handled by the batteries are calculated with Equation 10 andthe total required power are shown in Table 15.

Ptot = PWTs − PPV (10)

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Table 14: Characteristics of OS battery for auxiliary power supply needs [24].

(a) Cell characteristics

Battery cell

Height 0.824 mLength 0.225 mWidth 0.580 mArea 0.130 m2

Volume 0.108 m3

(b) Battery characteristics

Battery

Cell voltage 2.00 VBattery voltage 110 VNumber of cells 53 cellsArea of battery 6.917 m2

Volume of battery 5.700 m3

where Ptot is the resulting required power supply from the PV/battery system, PWTs is theload demand of the WTGs and PPV is the produced power by the PVs.

Table 15: Maximal Power demand with minimal PV production (ηtot = 8%, Cloudy)

Battery power requirement

Concept Ptot [kW]

Placed in OS 3441Placed in WT nacelles 2237

The batteries must both support the WT loads on the day when the PVs lack the abilityto produced enough power and recharge the UPS-system with batteries placed in the WT thatis supplying the OWF during night-time. The total energy demand of one day with batteriesis calculated with Equation 11.

WDemandOWF= WBatteryDAY +WBatteryNIGHT = Ptottday + PWTstnight (11)

The length of the day is changing on yearly basis depending on the geographical locationwhere the site is placed the total energy demand will dier. A OWF placed in the North Seawill set the energy requirement by batteries from the darkest time of the year, the winter,with the assumption that tday = 4 hours and tnight = 20 hours.

The energy demand in [Ah] required for the batteries of one day of supply is calculatedwith Equation 12 and presented in Table 16.

WBattery =WDemandOWF

UBusbar(12)

where WBattery is the required battery capacity in [Ah] and UBusbar is the voltage on thebusbar the batteries are connected to.

It is clear that there are no possibility to use Lead-Acid batteries as a APS-system togetherwith PV's as the energy storage capability of the batteries are far to low. By using otherbattery types that are more eective and can contain more energy than Lead Acid thescenario may change, see Table 17.

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Table 16: Required capacity in [kWh], [kAh] and numbers of batteries needed to fulll the requiredenergy demand for the specied APS-system with PV's and batteries on OS or at WT nacelles.

Concept [kWh] [kAh] Batteries [Nr] Volume [m3] Weight [ton]

Placed in OS 82644 751 216 1231 48Placed in WT nacelles 54868 499 144 821 32

Table 17: Dierent battery types (lead acid, NiCd, NiMH and Liion) characteristics. The voltage/ cell, the specic energy content and required maintenance can be viewed [11].

Battery characteristics

Parameters LeadAcid NiCd NiMH Liion

Cell voltage [V] 2 1.2 1.2 3.2 3.6Specic energy [Wh/kg] 30 50 48 80 60 120 90 190Maintenance requirement 3 6 months 30 60 days 60 90 days N/A

8.5 Wave Power

There is a possibility to use a wave power farm as a APS-system and therefore it is examinedhere in a technical review of a point absorber wave power pland (WPP) concept created bythe Swedish manufacturer Seabased with roots from research at the University of Uppsala [25].The power of the ocean waves is seem in Equation 13 [26, 27] and the potential of wave powerin the world are pointed out in Figure 18.

Pwave = 0.5H2sTp (13)

where Pwave is the power in the wave, Hs is the signicant wave height and Tp is the periodtime of the wave.

Figure 18: The annual mean direction of the waves are shown with (→) and the area where wavepower plant can be placed are visualized by the thick lines around the continents [28].

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The concept is seen in Figure 19 and includes a buoy connected by a rope to a lineargenerator placed at the bottom of the sea. When the buoy moves up and down the translatorconnected to the rope that binds the buoy together with the generator are moving. The endstop consists of springs that are attached to the translator. The springs store energy duringhalf a wave cycle and simultaneously act as a restoring force in the wave troughs [25]. Themovement of the translator inside the stator give rise to a generation of electricity and apower output from the WPP. The company is today constructing units with nominal powerof 10-50 kW [29] which are designed to handle depths ranging from 20-100 m [26].

(a) A sketch of the point absorber WPPconcept Seabased [30].

(b) [A schematic picture of a wave power farm (WPF) wherethe generator structure are visible standing on the seabed, theTCs that transfer the power to an central hub/converter sta-tion and the wires with connected buoys at the surface of thesea [31].

Figure 19: The Seabased concept with substructure.

The wave climate outside of the Danish coast in the North Sea, were OWFs often areconstructed is presented in (kW/m) of signicant wave height Hs in Table 18 [27, 32]. Thewave climate at the Swedish west coast is also included in order to understand the potentialfor dierent locations.

Table 18: Average wave climate at the Baltic and Nordic Sea in kW/m with respect to the signicantwave height [27, 32].

Location Wave climate [kW/m]

North Sea 9.8Skagerrak 5.2Kattegatt 2.4

Apart from the wave climate the utility factor of the WPPs is an important economicfactor to take account for. Dierent utility factors will aect the amount of WPPs neededfor supporting the WT loads. Therefore the calculations in this report has taken this intoconsideration and the results are seen in Table 19 with initial data is taken from [27].

A large amount of WPPs of approximately 300-800 WPPs are needed to handle the WTloads. This is a large quantity that require an extensive installation and maintenance proce-

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WPP rated power [kW] 10 20 30 40 50

Utility factor [%] 0.44 0.32 0.25 0.2 0.18

(a) Power rating of WPP with corresponding utility factors.

WPP rated power [kW] 10 20 30 40 50

Required number of WPPs 783 539 460 431 383

(b) Number of WPPs required for WT load demand.

Table 19: Wave power calculations.

dure. In addition, the technology is not commercial yet and will therefore not be sutible asan APS-system today for the studied OWF. There is a possibility that this may change in thefuture as larger WPPs is created which can utilize the power in the waves more ecient.

8.6 Full converter wind turbines & diesel gen-set

A future APS-concept that may be of interest could be to utilize the wind turbines in theOWF by restart some of the wind turbines in the OWF with a few small diesel gen-sets placedat a handful number of WTs [33]. These diesel gen-sets energise the WTGs (full converterwind turbines) after a grid blackout if needed and in that way restart the WTs to normaloperation mode where they are producing power from the wind.

The full converter WT is probably a suitable type to use in this application as it cancontrol the voltage at the stator and by the converter control, the frequency on the stator.Thereby, the speed of the rotor can be used in a wide area so that the wind turbine alwaysrotates with optimal speed in order to produce the highest possible power [34]. The converterclosest to the grid can fast change the active/reactive power transferred from the WT tothe system which can contribute to maintaining stability. When an emergency occur and asystem fault is present the WT have inertial energy preserved from normal operation whichmay help restarting the system in emergency mode (diesel gen-set's available with extra powerif needed) when its operating as an APS-system [35]. Figure 20 show a schematic picture ofa full converter WT.

Figure 20: A full converter wind turbine [34].

This APS-system concept is nevertheless without implications and problems and is not

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investigated today. The power and frequency stability of the system is to be carefully con-sidered because of the weak grid, issues with black start, fault management etc. needs to besolved. The concept is further discussed in Section 14.

9 Summary of APS-system studies

The number of APS-system supplies that have been studied earlier in the text are summarizedhere in order to focus the remaining part of the study to the most feasible concept(s). Asummary of each APS-concept studied is described in the list that follows. The Diesel gen-set(s), Gas gen-set(s), Photo voltaic's (PV's), Batteries combined with PV's and nally fullconverter WT's & diesel gen-set's is reviewed.

9.1 Diesel gen-set

The dierent concepts of placing diesel gen-set(s) on oshore substation, at WT towers andmobile diesel gen-set on marine vessel are summarised here with the dierent advantages anddisadvantages.

9.1.1 On oshore substation

The diesel gen-set solution placed on the OS includes a possible concept with a motor ratedapproximately 3.5 MW and a generator that can handle the active and reactive load ow inthe system during emergencies.

Stationary solution which are able to be placed on the OS roof.

Reactive power production in the system must be handled by either generator or reactivepower compensation equipment.

Limited fuel capabilities.

O & M in continuous intervals as the gen-set under long-term standstill needs to bemaintained.

9.1.2 at WT towers

The diesel gen-set solution that is placed at WT towers have the required rating of approxi-mately 50 kW and supports the WT load demand. There is no reactive power present as thediesel gen-set is directly connected via a 660 V busbar to the WT loads.

Stationary solution which are placed at every WT.

No reactive power in the system (no TCs).

No magnetisation losses in WT transformers as the power supply is directly connectedvia a busbar and with circuit breakers to the WT loads. The magnetisation losses inthe WT transformer are 14 kW which in this case have lowered the total load demandto 28 kW.

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The limitation of available space reduces the possibility to use the concept as a APS-system, however, the exact available space is set by the WT manufacturer. By [4] thisconcept is used today for 5-10 kW loads in times when the erection of the WTs aretaking place.

9.1.3 Mobile diesel gen-set on marine vessel

The diesel gen-set solution placed on a mobile vessel that travels out to the oshore windfarm in case of an emergency could be benecial if a standby crew with a vessel for exampleis stationed near the oshore wind farm today. The concept however has some other aspectswhich are important to mention.

The fuel tank may be of great size depending on the available space on the marine vessel

Not the same limitations in diesel mobile diesel gen-set as a large piece can be placed onthe marine vessel. Then the reactive power in the TCs is less of an issue for the systemdesign.

An mobile diesel gen-set on a marine vessel could be advantageous when the reactivepower is compensated by the gen-set instead of shunt reactors as no space on platformand less maintenance is required.

The solutions feasibility additionally depends on the distance from shore the wind farmis placed.

9.2 Gas/diesel gen-set

Ägas/diesel gen-set placed on a AC OS as a APS-system has almost the same congurationas a diesel gen-set alone, see Section 9.1.3. The gas gen-set major disadvantage comparedto a diesel gen-set is related to the gas fuel. A gas/diesel gen-set require an gas fuel tank inaddition to the existing diesel tank(s) and this demand valuable space on the OS [19]. Thereare also some logistical problems when handling gas which are not fully developed for oshoresubstations today.

A gas engine is more ecient and produces less vibrations than a diesel engine.

Lower emissions of nitrogen-oxides (NOx) and sulfur-oxides from gas have an positiveenvironmental impact.

A gas turbine can be used instead of a gas engine. The turbine will have a quickerstarting sequence and a reliable operation but with the disadvantage of lower eciency(this is however not investigated in this study).

A gas gen-set needs additional equipment compared to a diesel gen-set as extra gas tanksin addition to diesel tanks and more regulation equipment.

The storage of the explosive gas are today not fully developed and therefore have somelogistical diculties [19].

The economy of a gas/diesel gen-set may be dierent compared to a diesel gen-setas additional fuel capabilities and logistical cost of handling gas has to be taken intoconsideration.

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9.3 Photovoltaic's (PV's)

From the basic feasibility study the use of PV's on part of a AC OS roof or placed on WTnacelle roof aren't enough to support the WT loads during emergencies. The oscillations inpower productions due to weather conditions and dierent ecencies ηtot of the PV systemswill make it an unreliable power supply, it is therefore not recommended that a PV system isused stand-alone as a APS-system.

9.3.1 On OS roof

The PV's placed on half of the studied AC OS isn't producing by far enough power and isonly powering approximately one WT with its loads in cloudy weather and minimal ηtot.

Not enough power production when PV's are placed on OS (32-53 kW to the loaddemand of 3444 kW).

Reactive power compensation equipment is needed to handle the capacitance in TCs.

No power supply during night-time when navigation lights are essential for the securityof the OWF.

9.3.2 On WT nacelle roof

A larger total area compared to the available AC OS area increases the power productionrelative to the other PV concept. This, however isn't enough for supplying all WT loads.

Not enough power production when PV's are placed on OS (295-492 kW comparedtothe load demand of 2296 kW).

No need of reactive power compensation equipment as the PVs fed the WT loads directlywith power via the 660 V (or similar voltage level) busbar.

No power supply during night-time when navigation lights are essential for the securityof the OWF.

9.4 Batteries combined with PV's

The concept of placing batteries in combination with PV's on the AC OS showed to beimpossible with the use of Lead-Acid batteries. The requirement of 216 batteries with a totalvolume and weight of approximately 1230 m3 respectively ≈ 48 ton are nether possible noradvantageous. The case when batteries were placed in WT nacelles was a better solution withless requirements of 144 batteries,a volume of 821 m2 and a weight of 32 ton. This is still nota realistic scenario to investigate as the WT nacelles would require ≈ 2 batteries/WT whichnot is likely to t in the nacelle or at the WT tower structure. The use of more advancedand suitable batteries may reduce the total numbers of batteries needed, but will not enablebatteries used with PV's as APS-system.

To low energy storage capability in batteries.

Not technical possible to create on AC OS as the batteries are not tted to the limitedAC OS area, the total weight is very high and the system is ineective.

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In cases when the load demand is very small (a couple of kWs) the system may betheoretical possible, however the need of reactive power compensation equipment willbe present as well as the need of recharging the batteries after ≈ 24 hours when theyare discharged.

9.5 Full converter wind turbines & diesel gen-set

The concept isn't investigated any deeper in this work and its therefore hard to draw anyconclusions. The system is not operated today and needs research before it can be commented.The concept is still discussed in Section 14 but any deeper investigations are leaved to othersto perform.

9.6 Chosen APS-system for further investigation

To simplify the review of the dierent APS-system concepts, a short summary of each conceptis seen in Table 20 where the most feasible concept was selected with respect to the technicalpossibility of constructing the APS-system together with the weighted feasibility (expectedeconomical costs of installation and operation, O & M requirements, logistical aspects, etc.).

Table 20: An overview of the investigated APSsystems that have been studied. They are all rankedin order to visualise the most feasible concept.

Summary of APS systems

Concept Technical possible Feasible Rating7

Stationary diesel gen-set on OS Yes Yes8 1Stationary diesel gen-sets at WT towers Yes Yes9 2Stationary gas/diesel gen-set on OS Yes Yes10 3Mobile diesel gen-set on marine vessel Yes Yes11 4Wave power farm No No 5PVs combined with batteries No No 6PVs on WT nacelles No No 7PVs on OS No No 8Batteries No No 9

This showed to be the diesel gen-set placed on the OS when all the aspects were evaluated,however the result of the ranking depends on the actual site where the OWF is placed. Thesummary should therefore be treated as a guideline. A deeper analyse of the the diesel gen-setplaced on the OS concept is followed in coming sections.

7Weighted feasibility (expected economical costs & technical functionality.8Reliable energy source, relatively ecient, manageable construction, limited space requirement etc.9Requirement of a low WT load demand to be feasible (≈ 5-10 kW) [4].

10Logistical problems with fuel and reduced platform area (gas tank + diesel tank) [19].11Can be feasible, needs deeper review of economical costs.

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10 Sizing of diesel gen-set: Calculations

The diesel gen-set sizing procedure is described here and the validated power ow calculatedin Section 7.2 is used as a reference to nd the right dimensions and rating of a diesel gen-setplaced on an OS. The size of the Diesel gen-set system depends on both steady-state loaddemand and transient eects studied later on in Section 12 [36].

Steady state requirements when energising the OWF in APS-system mode.

Steady state requirements when the APS-system is in operation in the OWF.

Transients (voltage drops/peaks, frequency oscillations) when energising the OWF inAPS-system mode.

Transients as (inrush currents) in equipment when switching in WT transformers afterthe OWF is energised and in APS-system mode.

10.1 Sizing of diesel gen-set without wind turbine UPS-system utilised

The diesel gen - set placed on the AC OS needs to handle the power ow transferred throughthe system which are: P = 3.44 MW and Q = -8.09 MVAr, see Section 7.2. The sign before thepowers refer to the WT loads which require active power (+), and TCs that produce reactivepower Q (-). The calculated apparent power |S| is shown to be the 8.80 MVA.

The diesel gen-set rating must answer to the apparent power demand, however this requirea special diesel gen-set solution with the ability to work well in under-excited mode (seeAppendix A.2.1). The generator is producing active power P and consuming reactive powerQ with respect to the power ow.

10.1.1 Rating of standard generator

Standard diesel gen-sets are not created to work in this area as most power systems requirea production of reactive power, but as the emergency layout of the OWF symbolises a "weakgrid" with reactive power production in TCs and no major reactive power consumption, thegenerator instead needs to produce active power and consume reactive power. To handle thereactive power, the standard diesel gen-set needs to be overrated. Manufacturers visualisesthe generators ability to work in under/over-excited mode by capability curves.

The synchronous generators have a magnetic end core heating limit that for standard dieselgen-sets are a limit of the possibility to operate under-excited, see Appendix A.2.1 for moreinformation regarding the issue.

By creating a linear function that starts in the origin (y = kx + m = kx (m = 0)) thattakes the ratio between the reactive and apparent power (Q/S) into account, a crossing pointwhere the linear function intersects with a capability curve stability limit is found. This isthe point where the magnetic end core heating limit is intersecting the linear function in acapability curve. This point will be the optimal limit of active vs. reactive power in p.u thatthe generator must handle. The reactive power will at this point be compensated and the

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Figure 21: A capability curve of a synchronous generator with a rated power of 2.2 MVA considered tobe representable for larger generator's by Leroy-Somer [37]. Operation inside the area of the capabilitycurve is required to avoid damages. Note that the red and blue lines in the diagram is specied by themanufacturer.

Figure 22: The determination of a linear function.

generator will produce enough amount of active power that is required. Figure 22 shows theprocedure to nd the linear function.

Two dierent capability curves are used (ABB 17.9 MVA and Leroy-Somer 2.2 MVA)where the adapted linear functions are shown in Figure 23.

The rating of the diesel gen-set supporting the OWF can now be calculated with Equa-tion 14 and Equation 15.

QRated = QSystem = −8.09 MVAr (14)

SRated =QRated

Q/S(15)

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(a) The capability curve of an ABB syn-chronous generator where the linear functionis intersecting the stability limit of the capa-bility curve [38]. From the intersection point,P/S=0.18 pu and Q/S=-0.42 pu.

(b) The capability curve of an Leroy-Somersynchronous generator where the linear func-tion is intersecting the stability limit of thecapability curve [37]. P/S=0.19 pu Q/S =−0.43 pu at the intersection.

Figure 23: The determination of generator ratings by the help of capability curves.

Sizing of generator

Part PRated QRated |S|Rated (Leroy-Somer) |S|Rated (ABB)

Wind farm 3.44 MW -8.09 MVAr 19.26 MVA 18.81 MVA

Table 21: The sizing of a diesel gen-set on a OS with a standard generator (overrated).

Figure 24: The sizing of a diesel gen-set on a OS with a standard synchronous generator that needsto be overrated to handle the reactive power demand.

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10.1.2 Rating of special generator

If a special designed gen-set is used the rating of the gen-set supplying the whole OWF canbe downsized. The gen-set is constructed to work in under-excited mode and the system isn'tneeded to be overrated [39]. The system design have been validated by ABB - Helsinki and isshown in Table 22.

Table 22: Special generator : Diesel gen-set rating without overrating.

Active Power P Reactive Power Q Apparent Power |S|

3.44 MW - 8.09 MVAr 8.8 MVA

Figure 25: The special diesel gen-set capability curve which shows the generators ability to operateunder-excited. You can see the slightly coloured area where the generator can operate [39].

The magnetic end core heating eects are not an issue here, the limit is outside the ca-pability curve in Figure 25 and will not limit the the generator operation. This is an eectof increased air-gap between stator and rotor (25 mm-35 mm) [39]. It usually decreases themagnetic ux in the stator (and reduce eciency), however in this case the magnetisationinstead is coming from the stator side of the generator. It should be clear that the generatorcan't consume the required Q=-8.09 MVAr if P=3.44 MW isn't produced.

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10.2 Sizing of diesel gen-set with wind turbine UPS-system utilised

The REpower 6M WTs have (UPS)-systems consists of batteries with an energy storage of12 horus, see Section 4.1.2. These systems can possibly be utilized together with the diesel gen-set(s) placed on the AC OS to limit the operational time, decrease the size of the generatorand possibly reduced fuel use, as the gen-set is less frequently in operation. By altering someof the WT loads to be supplied from diesel gen-set or batteries it could be possible to usespecial sequential power production schemes,see Figure 26.

(a) Diesel gen set sizing: 1/2 OWF in sequence (b) Diesel gen set sizing: 1/3 OWF in sequence

(c) Diesel gen set sizing: 1/4 OWF in sequence (d) Diesel gen set sizing: 1/5 OWF in sequence

Figure 26: Diesel gen set loading scheme of energising the studied OWF in sequence.

The calculation of the active power rating for the corresponding diesel gen-set sizes for allscenarios are done from Equation 16, Equation 17, Equation 18 and Equation 19.

WLoad.Gen−Set = PWT.load · (nWTs · Seq) · tSequence (16)

Where WLoad.Gen−Set is the energy needed for the gen-set, PWT.load is the WT load demand,nWTs is the number of WTs, Seq is the sequence scenario used (i.e. 1/2) and tSequence isthe operational time for the gen-set when it is in sequence until the batteries are recharge(recharge time depend on which sequence that is used).

WSupply.UPSbattery = PWT.load · (nWTs · Seq) · tDischarge (17)

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WSupply.UPSbattery is the energy stored in the batteries in WTG UPS system and tDischarge

is the discharge time for batteries. See Equation 16 for the other parameters.

Wtot = WLoad.Gen−Set +WSupply.UPSbattery (18)

where Wtot is the total energy needed for the diesel gen set to supply.

PGen−set =Wtot

tCharge(19)

where PGen−set is the active power required rating of the diesel gen-set and tCharge is the timefor all UPS batteries in the OWF to be recharged.

Figure 27 shows the active power required to be handled by the generator in sequentialoperation schemes. The results showed that the sizing in all case have a total active powerdemand of P=3.44 MW and is not depending on the sequential operation (symbolised by the C-rate of batteries (Section 4.1.2)).The dierent C-rates depending on the Sequence operationalprocedure demands dierent recharge times of the batteries, see Table 23.

Figure 27: Sequential operation of diesel gen-set vs. required C-rate of batteries.

The reactive power demand is reduced depending on the sequence used when energisingthe WT loads. The smaller sequence, the lower the reactive power demand, fewer TCs in theOWF simply doesn't have to be operation at the same time. The disadvantage is that theUPS normally doesn't operate in on/o mode. The deep discharge of the batteries may reducethe total number of operating cycles and the lifespan of the batteries are shorten.

The best concept is the sequence where 1/2 of the OWF supported by WT UPS-systembatteries and 1/2 OWF on diesel gen-set savings on the fuel is done, the reactive power inthe system is reduced from -8.09 MVAr⇒ -4.045 MVAr and the C-rate of the batteries aren'taected. In that way the batteries will be recharge on 12 hours.

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Table 23: Diesel gen-set size with UPS-system batteries utilized in the OWF.

10.2.1 Rating of standard generator

The rating of the generator calculated by data from Table 23, with the same overratinggenerator procedure used as in Section 10.1.2, calculated from the two dierent capabilitycurves, are given in Figure 28.

Figure 28: Standard generator size with UPS-system batteries utilized in the OWF.

Apparent power |S|Sequence ABB [MVA] Leroy-Somer [MVA] Average size [MVA]

Whole wind farm 19.27 18.82 19.041/2 wind farm 11.56 9.63 10.601/3 wind farm 8.70 6.58 7.641/4 wind farm 7.49 5.06 6.281/5 wind farm 6.47 4.15 5.31

Table 24: Standard generator sizing: UPS-system batteries in combination with diesel gen-set.

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10.2.2 Rating of special generator

The ratings of the generator depending on the sequence loading scheme is calculated by usingdata from Table 23. No overrating is required as the special generator [40] works well inunder-excited generator mode, shown in Figure 29.

Figure 29: Special generator sizing.

Apparent power |S|Sequence Size [MVA] ≈ [MVA]

Whole wind farm 8.79 8.81/2 wind farm 5.31 5.31/3 wind farm 4.37 4.41/4 wind farm 3.99 4.01/5 wind farm 3.80 3.8

Table 25: Special generator sizing: UPS-system batteries in combination with diesel gen-set.

The rating of the generator is down-scaled with a smaller part of the OWF in sequentialloading. This is the eect from the less reactive power in the system (i.e. it is assumed thata sequence loading of (1/2 wind farm) is operated with (1/2 of the TCs in the wind farm).

10.3 Reactive power compensation equipment

To decrease the reactive power ow produced by the TCs, a reactive power compensation unitin form of shunt reactors can be used. A shunt reactor is a piece of equipment that basically

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is an inductor that can consume reactive power built up by HV TCs [41]. The shunt reactorsizing is adapted so that the generator doesn't need to be overrated.

A shunt reactor can be rated dierently depending on which standard generator ratingthat is used. To optimise the shunt reactor size vs. the standard generator size, dierentgenerator rating and corresponding shunt reactor sizing are investigated. From a capabilitycurve by the manufacturer Leroy-Somer 2.2 MVA [37] shown in Figure 30. It is visible inthe gure that the maximum capability for the generator to work under-excited is relativelyconstant (Q/S) = const. Therefore Qmax ≈ −0.40 · Sgenerator. The shunt reactor sizing thenonly depends on the overrating of the generator.

Figure 30: Dierent generator ratings in under-excited mode (consuming Q), depending on chosen(P/S) in pu from the Leroy-Somer capability curve and Qmax/Sgenerator = −0.40. The curve is usedas a reference for other diesel gen-set standard generators [37].

The ratings of the generator is determined by Equation 20, Equation 21 and the shuntreactor by Equation 22. In Table 26, the generator and shunt reactor size is shown.

|S|Generator =Pgenerator

PF=Pgenerator

cosφ=Pgenerator

P/S(20)

where PGenerator = 3.44 MW

QGenerator =Qmax

SmaxSGenerator = −0.40SGenerator (21)

QShuntreactor = |QOS | − |QGenerator| (22)

where QOS = −8.09 MVAr

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Table 26: Generator & shunt reactor rating with respect to dierent (P/S)'s in pu.

(P/S) pu. 0.9 0.6 0.3

SGenerator 3.8 MVA 5.7 MVA 11.5 MVAQGenerator 1.5 MVAr 2.3 MVAr 4.6 MVAr

QShuntreactor 6.6 MVAr 5.8 MVAr 3.5 MVAr

Table 27: Overrated (10 %) generator and corresponding shunt reactor with respect to dierent (P/S)'s.

(P/S) pu. 0.9 0.6 0.3

SGenerator 4.2 MVA 6.3 MVA 12.6 MVAQGenerator 1.7 MVAr 2.5 MVAr 5.0 MVAr

QShuntreactor 6.4 MVAr 5.6 MVAr 3.0 MVAr

To handle the peak demand of 193 kVA of WT loads that occur during 8 seconds intervalthe generator rating is a bit overrated (10 %) which also would decrease the required shuntreactor rating, see Table 27.

The results show that with a Qmax = −0.4 ·SGenerator the shunt reactor size only dependson the generator rating. A higher ratio in this study is P/S=0.9 that will overrate the generatorbut in return the shunt reactor size is minimised and vice versa. Depending on the userrequirements either system could be benecial, the question is whether the user want toreduce shunt reactor size or generator size. it's suitable to have multiple shunt reactors with atotal reactive power which answers to the dierent generator ratings or a tap-changing shuntreactor that can vary its reactive power consumption as all TCs and WT loads may not beenergised instantaneously [41].

11 Slow start of OWF with diesel gen-set

The starting scheme of an APS-system with a diesel gen-set where the WT loads are energisedin sequence is described in this section [40, 42, 43, 44]. The concept is explained and anoverview of the system can be seen in Figure 31.

1. The engine rst power up and increase its torque on the shaft that combines the dieselengine with the generator.

2. As the rotational speed of the shaft is increased, the torque produced by the engineincreases the generator rotor speed.

3. There will be a point where the rotor windings in the generator can be excited. An AutoVoltage Regulator (AVR)12 is enabled at this point (control equipment) and a magneticux is generated and transferred through the air-gap.

12See Appendix C for more information about diesel gen-set control system

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4. The rotating magnetic ux in the stator (rotor is moving) give rise to a voltage whichincreases as the excitation of the rotor windings is increased. When the voltage hasreached Unom the generator circuit breaker is closed.

5. Each string13 of WTs in the system checks the auxiliary power themselves to connect tothe power supply with delay times between each new string that is energised until thewhole OWF is in operation.

(a) Part of the loads (WT transformers, TCs, WT loads are connected in sequencewhen the diesel engine is in cold mode.

(b) The residual loads in the system are connected and energised when the diesel engineis heated up and operates at full eciency in warm mode. When the diesel engineis at full capacity all remaining loads in the OS are energised.

UVoltage

over Generator

The rotor windings are

excited (80 – 90 % of

rated speed of diesel engine)

Urated = 100 %, fel = 50 Hz Generator synchronised to the grid

5 - 10 seconds

Generator breaker are closed now: Switchgear ON

Diesel engine mode: Cold Diesel engine mode: Warm

Base load of diesel gen – set system: Switchgear, Breakers,

Control equipment etc.

Srated = 0.6 pu

Srated = 1.1 pu

½ hour – 1 hour

Continency mode

Srated = 1 pu

Frequency oscillations fel and dynamical transient control when OWF loads are energised

fel

Energising of OWF loads: OffS Transformer for diesel gen-set Transmission cables in OWF Wind turbine transformers Wind turbine loads

Wh

ole O

WF en

ergised

Figure 31: Sequential starting procedure for WTs in OWF [18, 42, 43, 44, 40, 45].

12 Transient studies

There are dierent transients that occur in the OWF when it is in APS-system mode and inisland operation. The stability of the power system is aected by these transients when theAPS-system is energised and the need of studying these issues are required.

Voltage transients as a result of the diesel control system together with wind farmresonances.

13By [18] it's possible to energise string by string of WT loads in the OWF.

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Inrush currents in WT transformers and other equipment when they are energised fromsystem blackouts. This is the case when the APS-system of the OWF are starting up.

The voltage transients in the system occur due to the harmonic content in the circuit ofthe OS and is a result of a MV circuit consisting primarily of the 33 kV TCs in the OWF thatbuilds up capacitance. The capacitance can create resonance loops in the MV circuit whichare transferred to the LV circuit and the generator may be forced out of synchronisation [36].

Inrush currents in transformers and in other equipment is a well known problem in powersystems and specially when a weak grid is started up and energised. This is the case with theAPS - system energising the OWF. Therefore the phenomena is important factor that has tobe considered when designing a APS-system. The transient phenomena is not modelled andvalidated in this study but previous work regarding the topic have been done [36, 46, 47]. In[46] it was shown that the worst inrush current occurred when the last WT transformer thefarthest away from the OS was energised. The inrush current is causing the following eectson the OWF when energising in APS-system mode:

High currents (9 ·Inom of REpower 6M WT transformers can occur) [4] that can damagethe transformers.

The diesel gen-set may trip because of the fact that the currents aren't linear withrespect to time and contains many harmonics which creates oscillating torques for thediesel gen-set to handle [36]. The generator therefore might fall out of synchronisationwhen the dynamical transients rise as protection devices disconnect the equipment. Asa results the APS-system breaks down.

Sympathetical inrush currents which occur when transformers are connected in parallelamplies the problem [48]. An OWF is a good example where transformers are connectedin parallel.

Concepts to decrease impact of transients:

1. The energising sequence of the system can be done without having the WT trans-formers connected to the rest of the oshore system. The WTs are not energisedin the beginning when the system is started up as the circuit breakers to the WTstransformers are open. When the system is in operation, the WT transformers canbe taken into operation in sequence where a part of a string a string or only oneWT transformer is energised at a time [43]. To connect the WT transformer thereis a couple of ways to do this accordingly to reduce inrush currents.

2. By slowly increase the voltage over the diesel gen-set having all the WT transform-ers connected but not the WT loads the dynamical transients might be avoided.Not until the whole OS including with (Diesel gen-set, TCs, control equipment,WT transformers etc) are connected the circuit breakers are closed and WT loadsconnected.

12.1 Connection of WT loads in sequence

The diesel gen-set soft starting procedure are performed, see Section 11 and the WT loads areenergised with a sequence operational control discussed here.

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A time interval large enough to avoid large inrush currents: tunit. By using timers forexample with a specied time interval which controls the circuit breakers of the WTtransformers the dynamical transients could be limited. When a circuit breaker of aWT transformer is closed the next WT transformer that follows in sequence have atimer set to start. In the studied OWF the total amount of timers needed are 82 if everyWT transformer are to be energised in sequence.

By measuring the frequency oscillations in the oshore system foscillate when it's ener-gised (inrush currents present) together with the excitation time of the oshore systemtexcitation the circuit breakers of the WT transformers can be controlled. When the fre-quency deviation is below a certain level in the system where stability is reached thenext unit in sequence can be connected.

The control equipment sends a binary signal to the WTs and tell the system when it'stime to be energised, then the next WTs have the same scenario until the whole windfarm is energised.

Power and frequency control by the AVR in the system design in order to keep them inthe specied limits.

12.2 Ramping up voltage over diesel gen-set

To control the energising of the OWF, the use a special voltage ramping sequence can possiblybe performed. In the text that follows and in Figure 32 the basics of the concept are presented.

Figure 32: The voltage ramping sequence scheme in the OWF when the OS rst is energised and thenthe breakers connecting WT loads are closed.

Motor start⇒ The speed of the motor reach rated rpm and the diesel engine are poweredup to rated power at t < 0 s.

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All breakers to the whole oshore system consisting of diesel gen-set, platform trans-formers, array cables to WTs, array cables between WTs, WT transformers are switchedin the system when t=0 s.

The voltage at t=0 s is U=0 V.

the voltage is increased in a time period that satisfy the criterions with minimizeddynamical transients present.

When nominal voltage has reach Unom and the system is energised the system is readyto connect the WT loads to the rest of the system.

The breaker is closed and the WTs loads in the OWF are energised.

The manufacturer REpower has set a requirement on (< 2 s) ramping scheme of theWT loads when the breaker is closed. A longer ramping (> 2 s) require that new controlequipment (voltage supervision of circuit breaker, protection systems for the equipment towithstand maximized inrush currents and new feeder equipment to motor components) [49].A numerous of dierent diesel gen-set manufacturers and contacts were asked wether this isperformable, with the result that it probably not can be done today. The generator reactiontime is to long (the eld current in the rotor windings set the limit). A ramping with ≈ 10 scould be managed over the WT loads [42].

12.2.1 Basic voltage ramping study

A basic study of a equivalent APS-system model design with a diesel gen-set is conducted.The diesel gen-set is in sequence operation when half of the OWF is supporting the WT loadsin 12 hours and the UPS-system batteries in another 12 hours. The characteristics of thesystem are seen in Figure 33 and listed in Appendix B.1.1.

Figure 33: The equivalent circuit with half of the OWF energised.

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The equivalent circuit includes one AC OS transformer, one string with the total lengthof all strings in the OWF design and one WT transformer with the rated power output of allWT transformers in the OWF all together.

Two arbitrarily long ramping scenarios were used: tRamp = 15 min and tRamp = 30 min.For both these scenarios, (POWF and QOWF ) owing in the system during voltage rampingare calculated (see Appendix B.1.1) and the results was presented to dierent diesel gen-setmanufacturers for further investigation whether the concept of reducing dynamical transientsare executable. Table 28 and Figure 34 shows the power ow in the systems.

Table 28: The power ow in the oshore system when a voltage ramping of an OWF equivalent isperformed.

Figure 34: The power ow in the oshore system with the voltage ramping scenario of tRamp = 15 min.

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By all the dierent manufacturers, no manufacturer opposed the concept but as no realproject with this concept is tested (that I know of), it's dicult to say wether a voltage rampingscenario are possible to perform. There are a number of dierent aspects and challenges thatneeds to be resolved rst before a real system can be constructed. They are listed here:

Special designed gen - set ⇒ eld current limitations when the voltage is low = highcurrent ⇒ Insulation and heat damage must be prevented.

The system is simplied, a real OWF consists of multiple strings and WT transformerswhich will create cluster issues. As the cable length (the TCs capacitance = reactivepower will be dependent on the length of the line, and the type of line (area of conductor,capacitance, resistance, inductance) this will also aect the power ow in the systemwhich not has been studied in this text.

In a real system there are possibilities to change a number of variables in the systemfor the voltage ramping: The transformers (variable tap changer transformer ⇔ nonvariable transformer), WT load interaction14, transformer saturation15 etc.

The loads in the WTs have to be examined deeper. They can be divided into impedanceloads, voltage loads etc. that have specic requirements. The voltage independent loadsmay experience problems when the voltage ramping sequence is performed.

14The loads operate dierently (constant power loads, constant voltage loads, etc.)15Hysteresis losses

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13 Diesel gen-set summary

A Diesel gen-set specication includes information about the minimum technical requirementsfor the system design of the Diesel gen-set. In the specication the size and descriptions of theengine and the generator is presented in order to optimize the system design. As this studyhas focused on making a feasibility study with technical content the specication is limited tothe basic information of the engine and generator. The ratings of the engine and the dierentgenerator sizes are previously described in Section 10.1.2 and Section 10.2.2 and Section 10.3.The results are shown in Table 29.

Table 29: The specication of dierent diesel gen-set congurations.

SpecicationNumber Component P [MW] Q [MVAr] S [MVA] Design UPS-system utilized

−−−− Engine 3.44 −−−− −−−− −−−− −−−−1 Generator 3.44 -8.09 8.8 Special No16

2 Generator 3.44 -4.045 5.3 Special Yes17

3 Generator 3.44 -8.09 19.26 Standard No4 Generator 3.44 -4.045 11.56 Standard Yes

5 Generator 3.44 −1.718 4.2 Standard Yes6 Generator 3.44 −2.519 6.3 Standard Yes7 Generator 3.44 −5.020 12.6 Standard Yes

All ratings are up-scaled approximately 10 % to handle the small "peaks" of 193 kVA inWT loads that may occur [4], see Table 30 and Table 31 for more details.

Table 30: Generator sizings with an 10 % increase rated output.

Generator ratings (10 % increased)Number Component S [MVA] Design UPS-system utilized

1 Generator 9.7 Special No2 Generator 5.8 Special Yes3 Generator 21.2 Standard No4 Generator 12.7 Standard Yes

5 Generator 4.2 Standard No6 Generator 6.3 Standard No7 Generator 12.6 Standard No

16No use of the (UPS)-system batteries in the WTs17batteries in the WTs with 1/2 wind farm in sequence, C-rate corresponding to 12 hours recharge time.18Shunt reactors are used, P/S=0.919Shunt reactors are used, P/S=0.620Shunt reactors are used, P/S=0.3

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Table 31: Engine sizing with an 10 % increase rated output.

Engine ratingComponent P (≈) [MW]

Engine 3.8

The specication shows that there are a number of dierent concepts that are possible todesign with the lowest generator rating of 4.2 MVA21. This however require reactive powercompensation unit(s) with a capability to consume 6.4 MVAr22.

When no reactive power compensation equipment is used, the lowest rating of the generatoris found to be 5.8 MVA23, although a special generator is required which could be more dicult(expensive) to design. The engine is sized at 3.8 MW.

14 Discussion

A number of dierent aspects regarding the work in the APS-system study are presented here.It includes information about the dierent APS-concepts with a deeper discussion of the dieselgen-set sizing.

Aspects regarding the standard diesel gen-set:

1. The diesel gen-set ratings proposed to be used in this study are related to thespecic design of the studied OWF consisting of 82 WTs, a dierent layout wouldhave an impact on the ratings and the requirements could be dierent.

2. Shunt reactor(s) in combination with a diesel gen-set could be preferable to consumea part of the reactive power in the system, with the advantage that the generatorrating can be down-scaled.

3. From the assumptions taken in this study, the diesel gen-set is the most preferablesolution to be used today. It could possibly be of interest to install a gas/dieselgen-set instead and utilize gas as fuel with increased reliability.

4. Fuel tank size is an important factor to be investigated (need to refuel gen-sets atsome time). The limited space of placing tanks will have an impact on the availablefuel reserves.

5. O&M is required in a specied time intervals, this includes at least monthly inspec-tions [33].

6. In the master thesis work the assumption that one diesel gen-set is placed on theOS is undertaken. This however may not be the optimal conguration, as multiplediesel gen-set with the same total rating could be used. Then the redundancy of theAPS-system would be increased (larger number of diesel gen-sets, less probabilitythat all will fail at the same time).

21see nr 5 in Table 30.22See Table 2723see nr 2 in Table 30.

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Aspects regarding the special diesel gen-set

1. The special designed generator will require a design that today is not yet devel-oped. To accurately get a diesel gen-set layout, diesel gen-set manufacturers mustinvestigate the issue further.

2. The special designed generator is rated to precisely handle the active and reactivepower requirements in the oshore wind farm (at max P when Q is compensatedin OWF). In reality the rating of the generator should be overrated at least 10 %to handle the "peaks" in 193 kVA of WT load demand.

Aspects regarding the oshore power system in emergency mode:

1. A lower voltage level in the system could possibly decrease the generator ratings asthe reactive power in the TCs ∼ U2. This however will require extensive additionalcontrol equipment and additional switch gear that are costly [43].

2. Resonance frequency in the electrical system is not investigated. This may haveimpact on the diesel gen-set design and may be investigated in future studies.

Future APS-concepts:

1. A gas turbine instead of a gas/diesel engine will have impact on the APS-systemoperation. The start-up sequence would be shorter, but the logistical problemswhen handling gas needs to be under control to produce a sustainable concept.

2. In the future, OWFs are possibly interconnected with each other or an power sys-tems and not only connected to land. The need of a APS-system is probably nolonger required as emergency power can be transferred from multiple sources ifone transmission link is damaged and taken out of operation. The power supplyredundancy is high and the system will operate with a greater stability than before.

Full converter WTs & diesel gen-set:

1. With the help of an park controller, it might be possible to control the system[43]. This is a regulation system that can analyse the OWFs consumption andproduction of power and from this the diesel gen-sets can act in order to maintaina stable network.

2. The WTs in a OWF can possibly be constructed so that they are working au-tonomous and rapidly react on power ow changes in the network. In cases whenthe wind farm is going back into normal operational mode from auxiliary powermode this is automatically controlled by this function.

3. The turbocharger which boost the diesel engine with extra air needs time before itcan deliver the required amount of air. Therefore when the APS-system is energisedthere could be voltage dierences present which makes the system unstable untilthe diesel gen-set are warmed up [44].

Reducing the load demand (heating) in the WTs:

1. The high load demand of 42 kW in REpower (28 kW without WT transformermagnetisation losses) contains a larg part which is heat demand inside WTs. An

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optimisation of the environment inside the WT's may reduce the heating demand.The structure could be divided into dierent sections with the aim of minimisationof the load demand.

2. Another concept could be to simply placing gas holders inside the WTs to produceheating mainly where it is most required required. The heating demand may notbe the same anywhere in the WT structure.

Transients in the system while energising:

1. The proposed concepts of slow staring the system and ramping of the voltage overthe system without WT loads connected require more modeling and simulation work.Questions like how the transformer energising is aected of voltage ramping, howthe control of the voltage ramping specically should be performed and whether itwill require additional protection devices are raised.

Voltage ramping of system:

1. Voltage ramping of < 2 seconds of WT loads when the OWF is energised and thebreakers connecting the loads are closed is probably not possible to perform. Thefast ramping would create frequency oscillations and the voltage of the system couldbe destabilized. The generator needs time to react on the increased load demand(normally takes ≈ 5-10 seconds) because of the limitations in the derivative of eld

current (in the rotor windings of the generator) vs. time, ∂If∂t .

UPS-system in WTs:

1. Battery use is not validated by UPS manufacturer. By REpower an operation of 12hours discharge / 12 hours recharge could have an impact on the battery lifetimeand reduce the number of cycles a battery can be operational.

Model development:

1. The model is simplied and doesn't investigate the system thoroughly. Short-circuitsimulations are not investigated, no real transients study is performed etc.

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15 Conclusions

From a number of dierent APS-system studies the diesel gen-set solution placed on the OSin the OWF was pointed out as the optimal solution in the scope of the study. The rating ofthe gen-set was specied with respect to dierent possible system congurations, with reactivepower compensation equipment, a special designed generator and a standard generator. Theresults showed that the minimal diesel gen - set rating was S=4.2 MVA with reactive powercompensation in form of shunt reactor(s) with a total capacity to consume approximately6.4 MVAr.

The minimum rating of a special designed gen-set which can work well in under-excitedmode, was found when UPS-system batteries in WTs in combination with diesel gen-set wereused. Half of the WT loads in the wind farm is loaded by batteries in 12 hours and theother half is loaded by diesel gen-set(s). The constructed generator rating would then beS=5.8 MVA.

If a standard diesel gen-set with a generator that is not especially constructed for operationin under-excited mode are used, an overrating of the generator is required. When the UPS-system batteries are in use in combination with diesel gen-set, the rating would be S=9.7 MVA.

Concepts to decrease the transients that aects the OWF system when it's energised canpossibly be designed. The voltage ramping concept is especially interesting as it may minimisethe inrush current transients and largely eects the system design. The design of such aconcept is briey described and an equivalent circuit of the studied OWF were examinedwith the result of approximately 500 kW (466 kW) and 4.4 MVAr (4.37 MVAr) in the systemwith no WT loads energised (no-load losses). The results were sent to dierent diesel gen-setmanufacturers where no manufacturer pointed out that this concept couldn't be accomplished.Although, no manufacturer has deeply studied the concept before.

Otherwise, the slow starting sequence of an OWF are pointed out as an advantageoussystem design to decrease transients, with the downside that there will take longer time toenergise the OWF and the WT loads.

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16 Future Work

The study of a possible APS-system concepts are aected by the load demand of the WTsstudied in this master thesis study. With a lower WT load demand the conclusions of thisreport need to be revalued and another APS-system could be more advantageous than theproposed.

The specic design of the APS-system are not studied in this report and is left for furtherinvestigations as the aim of this master thesis study was to perform a study of many dierentconcepts with a immerse study in the most advantage APS-system design. The details of thediesel gen-set operational guideline are left to be investigated.

The dynamical transients in the OWF at APS-system mode need to be investigated furtherin order to nd an optimal system design which minimize the transients. The voltage rampingscenario needs to be conrmed and the idea of the concept established with the diesel gen-setmanufacturers. Then the system design can be conrmed or rejected.

The energy storage in the batteries and the ability to fast recharge/discharge need to bethoroughly examined. The on and o switching of the batteries may have an impact on thelifetime of the components, dierent battery types, energy storage, maintenance requirements,etc.

The dierent loads in WTs have to be examined further. Questions like how they areoperated and whether they are constant power loads, constant voltage loads, impedance loadsetc. will have an impact on the operational requirements for the used APS-system. In overall,specic details of the system design must be investigated before any specied concept can becreated in the future.

A concept to reduce the capacitive reactive power in the TCs could be to utilise the LineSide Converter (LSC) in the DFIG REpower 6M WT to consume reactive power. The LSCinductance may be utilised to consume reactive power even at PWT ≈ 0 MW when the WTis in emergency mode, and the OWF is isolated from land with no power transmission toland [50]. According to [23] a 2 MW rated DFIG WT is able to produce QL ≈ 5 % of PLSC

independent of the wind speed. This would answer to 0.1 MW/WT and for the studied OWFused in this study the total of 82 WTs would consume 8.2 MVAr. This is the whole oshorewind farm's reactive power ow (note there there is a dierent rating of 2 MW/WT compareto 6 MW/WT in my OWF).

If this concept is possible to use it would signicantly reduce the diesel gen-set's sizing asreactive power would be consumed in the LSC's and the large reactive power build-up fromthe TCs is compensated without shunt reactors and/or diesel gen-set's. A small internal loaddemand for the LSC's (cooling of power electronics) are present and must be supplied by theAPS-system but the overall rating of the APS-system may be down-scaled and cheaper toproduce. It is not certain that this idea is viable, but a future analysis may be benecial toperfor to validate/reject the idea. The concept was not studied in this study, due to limitedinfo from manufacturers.

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List of Tables

1 Basic data of REpower 6M WT . . . . . . . . . . . . . . . . . . . . . . . . . . . 132 30 kV cables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 153 REpower 6M WTG Transformer . . . . . . . . . . . . . . . . . . . . . . . . . . 164 REpower 6M WTG Load demand . . . . . . . . . . . . . . . . . . . . . . . . . . 165 Powers in one string of the selected OWF . . . . . . . . . . . . . . . . . . . . . 186 Currents in a string of selected OWF system . . . . . . . . . . . . . . . . . . . . 187 Summary of model parts in PowerFactory . . . . . . . . . . . . . . . . . . . . . 198 The reactive power in a string of the OWF (emergency mode). . . . . . . . . . . 219 Load ow analysis of PowerFactory model and calculations . . . . . . . . . . . 2410 Power demand: Stationary diesel gen-set concepts . . . . . . . . . . . . . . . . . 2511 Power production in W/m2 of PVs with given eciencies . . . . . . . . . . . . . 2712 Total active power production of PVs placed on oshore substation roof . . . . 2813 Total active power production of PVs placed on WT nacelles . . . . . . . . . . 2914 Oshore substation battery . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

(a) OS battery cell characteristics . . . . . . . . . . . . . . . . . . . . . . . . 30(b) OS battery characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . 30

15 Power need: Batteries and PV's combined . . . . . . . . . . . . . . . . . . . . . 3016 Needed capacity for batteries combined with PV's . . . . . . . . . . . . . . . . 3117 Characteristics of dierent battery types . . . . . . . . . . . . . . . . . . . . . . 3118 Wave climate at the Baltic and Nordic Sea . . . . . . . . . . . . . . . . . . . . . 3219 Wave power calculations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

(a) Wave power concept investigation: Seabased in the North Sea . . . . . . . 33(b) Wave power concept investigation: Seabased in the North Sea . . . . . . . 33

20 Summary of APSconcepts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3721 The sizing of a diesel gen-set on a OS with a standard generator (overrated). . 4022 Special generator : Diesel gen-set rating without overrating. . . . . . . . . . . . 4123 Sequential operation of Diesel gen-set:Table . . . . . . . . . . . . . . . . . . . . 4424 Standard generator sizing: UPS-system batteries in combination with diesel

gen-set. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4425 Special generator sizing: UPS-system batteries in combination with diesel gen-set. 4526 Generator & shunt reactor rating with respect to dierent (P/S)'s in pu. . . . . 4727 Overrated (10 %) generator and corresponding shunt reactor with respect to

dierent (P/S)'s. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4728 Power ow for voltage ramping scenario . . . . . . . . . . . . . . . . . . . . . . 5229 The specication of dierent diesel gen-set congurations. . . . . . . . . . . . . 5430 Generator sizings with an 10 % increase rated output. . . . . . . . . . . . . . . 5431 Engine sizing with an 10 % increase rated output. . . . . . . . . . . . . . . . . . 55

List of Figures

1 Future oshore Wind Power development . . . . . . . . . . . . . . . . . . . . . 92 The electric system layout of a OWF . . . . . . . . . . . . . . . . . . . . . . . . 113 Drawing of an OWF with it's dierent substructures . . . . . . . . . . . . . . . 124 Electric layout of selected OWF . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

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5 REpower 6M WT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 146 DFIG WTG . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147 Real picture of REpower 6M WT Transformer . . . . . . . . . . . . . . . . . . . 168 Power ow in TCs in a string . . . . . . . . . . . . . . . . . . . . . . . . . . . . 179 The model of an OWF string (non-energised/energised). . . . . . . . . . . . . . 20

(a) Model of an OWF string . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20(b) Model of an OWF string . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

10 The model of the OWF in emergency mode [16]. . . . . . . . . . . . . . . . . . 2211 Detailed layout of the OWF in emergency mode. . . . . . . . . . . . . . . . . . 23

(a) Model part of OWF: Emergency1 . . . . . . . . . . . . . . . . . . . . . . . 23(b) Model part of OWF: Emergency2 . . . . . . . . . . . . . . . . . . . . . . . 23

12 Load ow in OWF during emergencies . . . . . . . . . . . . . . . . . . . . . . . 2313 Diesel gen-set schematics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2414 THe stationary diesel gen-set concepts in the OWF. . . . . . . . . . . . . . . . 25

(a) Diesel gen-set container placed on oshore substation . . . . . . . . . . . 25(b) Diesel gen-set containers placed at WT towers . . . . . . . . . . . . . . . 25

15 Basic layout of PVs placed on oshore substation . . . . . . . . . . . . . . . . . 2716 Power production vs WTG load demand . . . . . . . . . . . . . . . . . . . . . . 2817 Power production vs WT load demand with WT nacelles . . . . . . . . . . . . . 2918 Wavepower potential on earth . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3119 The Seabased concept with substructure. . . . . . . . . . . . . . . . . . . . . . . 32

(a) Seabased WPP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32(b) A schematic picture of a wave power farm . . . . . . . . . . . . . . . . . . 32

20 A full converter wind turbine . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3321 Leroy-Somer capability curve . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3922 Determine diesel size: Linear function adaption . . . . . . . . . . . . . . . . . . 3923 The determination of generator ratings by the help of capability curves. . . . . 40

(a) Diesel gen-set sizing: ABB capability curve . . . . . . . . . . . . . . . . . 40(b) Diesel gen-set sizing: Leroy-Somer capability curve . . . . . . . . . . . . . 40

24 Standard generator sizing with overrating . . . . . . . . . . . . . . . . . . . . . 4025 Special gen-set capability curve . . . . . . . . . . . . . . . . . . . . . . . . . . . 4126 Diesel gen set loading scheme of energising the studied OWF in sequence. . . 42

(a) Diesel gen set sizing: 1/2 OWF in sequence . . . . . . . . . . . . . . . . . 42(b) Diesel gen set sizing: 1/3 OWF in sequence . . . . . . . . . . . . . . . . . 42(c) Diesel gen set sizing: 1/4 OWF in sequence . . . . . . . . . . . . . . . . . 42(d) Diesel gen set sizing: 1/5 OWF in sequence . . . . . . . . . . . . . . . . . 42

27 Sequential operation of Diesel gen-set vs. required C-rate . . . . . . . . . . . . 4328 Diesel gen-set size with UPS-system batteries. . . . . . . . . . . . . . . . . . . . 4429 Diesel gen-set size with UPS-system batteries. . . . . . . . . . . . . . . . . . . . 4530 Dierent generator ratings depending on (P/S) in pu of the generator . . . . . 4631 Sequential starting procedure for WTs in OWF . . . . . . . . . . . . . . . . . . 4832 Voltage ramping sequence scheme . . . . . . . . . . . . . . . . . . . . . . . . . . 5033 The equivalent circuit with half of the OWF energised. . . . . . . . . . . . . . . 5134 Power ow for voltage ramping scenario . . . . . . . . . . . . . . . . . . . . . . 5235 Generator motion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69

(a) Salient pole generator (0°) . . . . . . . . . . . . . . . . . . . . . . . . . . . 69(b) Salient pole generator (90°) . . . . . . . . . . . . . . . . . . . . . . . . . . 69

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36 Equivalent circuit of synchronous generator . . . . . . . . . . . . . . . . . . . . 6937 Phasor diagram of generator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7038 Quadrant diagram of generator modes and load modes . . . . . . . . . . . . . . 7039 Electrical equivalent diagram of a transformer . . . . . . . . . . . . . . . . . . . 7140 Pi link of a transmission cable . . . . . . . . . . . . . . . . . . . . . . . . . . . 73

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A Basic theory of Oshore wind farm systems

A.1 Fundamentals

The basic electrical knowledge of an electric grid is described in this section that is needed fora good understanding of the subject investigation presented in this master thesis study.

The relationship between the voltage and current is made up by Ohms law, shown inEquation 23

U = |Z| · I =√R2 +X2 · I V (23)

where Z is the impedance, R the resistance, X the reactance and I is the average current.The linetoline voltage are denoted with a subscript LL and the linetoneutral voltage

are denoted with the subscript LN in the following equations. The relationship between thelinetoline voltage and the lineto neutral voltage are given in Equation 24 below.

|ULL| =√

3|ULN | (24)

The average active power P in a singlephase system are described in Equation 25.

P1ph = |ULN ||I| cosφ =|ULL|I| cosφ√

3W (25)

where |ULN | is the RMS value of the linetoneutral voltage, |ULL| is the linetoline voltage,|I| is the average current and cosφ is the power factor.

The average threephase active power P equals three times the singlephase active power,shown in Equation 26:

P3ph = 3|ULN ||I| cosφ =√

3|ULL|I| cosφ W (26)

The reactive power Q of a singlephase system is given in Equation 27.

Q = |ULN ||I| sinφ =|ULL|I| sinφ√

3V ar (27)

where |ULN | is the RMS value of the linetoneutral voltage |ULL| is the linetoline voltage,|I| is the average current and φ is the angle between the active power P and the reactivepower Q.

The reactive power Q of a threephase system equals three times the singlephase activepower, given in Equation 28

Q = 3|ULN ||I| sinφ =√

3|ULL|I| sinφ V ar (28)

The complex power for a single S is given by the formula in Equation 29.

S = UI∗ = P ±Q V A (29)

The apparent power |S| is given in Equation 30.

|S| = |ULN ||I| =√P 2 +Q2 V A (30)

|S| = 3|ULN ||I| =√

3|ULN |I| V A (31)

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A.2 Generators

The basic generator theory are described in this section with a focus on the synchronousmachine as it is used in this master thesis study. First the basics of magnetic elds andelectromagnetism is explained in order to give an understanding of the generator theory. Agenerator is based on the fact of the laws of electromagnetism together with the rules formagnetic circuits [51].

When a current ows through a conductor, a magnetic eld is created that rotates clockwiseto the motion of the current. This magnetic eld will introduce a magnetomotive force (mmf)given from Ampere's law, see Equation 32.

Hl = NI = Fm (32)

where Fm is the mmf, H is the magnetic eld strengh, l the distance around the lines ofthe magnetic eld, N number of turns in a coil and I is the current carried in each turn in acoil [51].

This magnetic force will aect other conductors if ferromagnetic materials is present, i.e.iron. Therefore the magnetic eld will produce a magnetic ux density B that depends onthe permeability µ by surrounding materials of the conductor, see Equation 33.

B = µH = µ0µrH (33)

where µ0 is the permeability of free space, µr is the relative permeability in a ferromagneticmaterial, B is the magnetic ux density and H the magnetic eld strength.

The magnetic ux φ corresponds to B and is given in Equation 34

φ = BA (34)

Where A is the cross-section area and B the magnetic ux density.

By the previous displayed facts of magnetic circuits and by electromagnetism law's, thefundamental generator theory can be explained. From Faraday's law of induction it can beseen that a electromagnetic force (emf) is induced when a conductor is subjected for a varyingmagnetic eld [52, 51]. The law is written in Equation 35.

Ei = −N ∂φ

∂t(35)

Where Ei are the induced voltage (emf) which is a function of the number of turns for a coilN and the time-derivative of the magnetic ux ∂φ/∂t.

This law is realised in a synchronous generator when the rotor windings are excited witha eld current If that give rise to a magnetic ux from the rotor poles to the stator armature.When the rotor of the generator is rotating, the magnetic eld rotates and the magnetic uxdensity B at the stator coil changes with it. As a result of the change in magnetic ux in thestator coils a voltage (emf = Ei) are induced in the coils at the stator teeth. The operationalprocedure are shown in Figure 35a and Figure 35b [51].

The synchronous machine used as a generator operate at synchronous speed which iswhen the speed of the rotor is equal to the rotational speed of the the magnetic eld [53] andoperate as described in Figure 35a and Figure 35b. The characteristics operation procedures

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(a) Salient pole generator (0°) [51] (b) Salient pole generator (90°) [51]

Figure 35: Generator motion

in a synchronous generator can be explained by the formula shown in Equation 36 where theinternal resistance of the generator Ri is neglected.

Figure 36: The equivalent circuit of a synchronous generator with the resistance of the stator windingneglected.

Ei = jωLI + U = jXdI + U (36)

where ˆ denotes phasors, Ei is the machines internal emf (the induced voltage in the statorwindings), I is the stator current, Xd is the machines internal reactance and U is the systemvoltage.

In complex form with the complex Equation 36 becomes the following equation [52].

Ei(cosδ + jsinδ) = U + (jXd)I(cosφ− sinφ) (37)

where the power factor angle is φ which is the phase angle between the current I and thevoltage Us. The angle is dierent depending if the current leads or lags the voltage which isa factor of the type of load the generator are exposed of. If the generator are connected toa purely resistive load the angle is zero. The load angle δ is the angle between noload andload voltage of the generator. The angle is a result of the loading of the generator where themagnetic ux in the rotor is tilted compared to the magnetic ux in the stator. In Figure 37you can see a phasor diagram of an overexcited generator which is consuming reactive powerQ.

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Figure 37: A phasor diagram of a synchronous generator that is operating in over-exited mode with alagging current. The generator is Producing active and reactive power to the grid.

By controlling the excitation current in the rotor windings in the generator, the machinecan work in either under-excited or over-excited mode. In under-excited mode the generatorproduce reactive power to the external grid (Q > 0) and in over-excited mode a generatorconsume reactive power (Q < 0). In this way, a generator can control the power ow in aelectrical system and keep the stability of the grid. In Figure 38 this is further described.

+ Q

- P

- Q

+ P

SS

Overexcited generator E cos (φ) > U

(The generator Produces P and Q)

Underexcited generatorE cos (φ) < U

(The generator produces P and consumes Q)

Controlled by the excitation current in

the rotor

The signs of the powers P and Q are referering to the generator side (see it from a generator perspective)Generator

sideLoad side

Inductance

Resistance

Capacitance

ResistanceCapacitive load

(Produces reactive power Q)

Generator inductive

Load capacitive

Inductive load (Consumes reactive power Q)

Generator capacitive

Load inductive

Figure 38: Under-excited mode (P > 0, Q < 0) where the generator works as a inductor (I leads U)and over-excited mode (P > 0, Q > 0) where the generator works as an capacitor (I lags U).

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A.2.1 Magnetic end core heating limit

The heating of the end core is created by the magnetic ux distribution above the statorcore (mainly in the radial direction of the stator) that give rise to eddy currents in the stator[54]. In under-excited mode the generator receives power from the stator side and additionalheating created by eddy currents limits the operation scheme.

A.3 Transformers

The no-load losses and the loading losses in a transformer are shown here. The no-load lossesis the magnetisation (iron) losses which occurs in the iron / steel core. They occur when aexcitation current (approximately 0.5 % of the load current) ows through the primary windingresults in a small voltage drop across the resistance of the winding and a small inductive dropin the inductance of the winding. This is represented by R0 and the XM , see Figure 39. Thevoltage drop will introduce a impedance on the primary winding of transformer which is shownas R1 och X1 in Figure 39.

Figure 39: A schematic gure of a transformer, with all it's electrical properties both on the primary(sending ...S) and secondary (receiving ...R) side of the transformer.

The no-load losses exist whenever a transformer is energised and primarily depends onthe voltage U and the frequency f. Therefore, when the transformer has reach its normaloperational mode the no-load losses are approximately constant and will not further increasewith an increased loading current. The load losses however, is increased when the transformerhave been energised and a load current ows through the transformer windings. In short theload losses are given from the formula P = I2R in the transformer windings - R1 and R2

shown in Figure 39) and consist of the following parts:

Eddy currents losses caused by the leakage eld of the transformer

Stray losses due to the eect of a leakage magnetic eld inside the internal metallicstructures of the transformer.

Auxiliary losses are the power required for cooling the transformer (usually not includedin the "losses")

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A.4 Cables

The properties of a TC are explained in the following text [55].

For a MV AC cable design (< 60 kV) threecore cables are manufactured and used.

The cables used in this study are of "Höchstädter" type where all three single conductorsin the design have their own sheath (i.e. the cable works as there are three single-corecables).

The isolation material that has been used in the TCs that are present in this masterthesis study are the plastic material XLPE (crosslinked polyethylene) as plastic is asolid insulator.

In detail a TC design have a number of dierent parameters to spoke of. The value of theparameters dene the cable types and are essential for the understanding of how a TC isfunctioning. The series inductance L, the shunt capacitance C and the series resistance R [56].Here follows a list of equations that describes the parameters.

Rac =Ploss

|I|2Ω (38)

where the Rac are the eective resistance, Ploss is the a conductors real power loss, and I isthe RMS conductor current.

The series resistance contributes to the ohmic losses in a TC and can be described byEquation 39.

Ploss = |I|2 ·Rac =U2

RacW (39)

The three phase ohmic losses in a TC are three times as big as the power loss for oneconductor, see Equation 40.

P3Ph = 3 · Ploss = 3 · (|I|2 ·R) W (40)

The series impedance Z which includes both the series resistance R and the reactanceXL give rise to a voltage drop (series) along a TC. The series impedance for a three-phasethree-core conductor cable are described in Equation 41 [55].

Z =U

I= R+ jX Ω (41)

where Z is the impedance, R the resistance and X the reactance that can consist of both ainductive XL = jωL and capacitive part XC = 1

jωC = − jωC [55].

The inductive part of a TC will consume power, however the inductive part of the reactanceis small compered to the capacitive part XC that increase the power in the TC. Therefore theinductive part XL can be neglected, QXL

≈ 0 V ar

The reactive power QTC produced in the transmission lines are then produced by thecapacitance (simplication) and is shown in Equation 42.

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QTC = QCap =U2

XCV ar (42)

The complex power S and the apparent power |S| of a TC can be dened by Equation 29and Equation 30 and is presented below:

STC = P3Ph + jQTC (43)

|STC | =√P 23Ph +Q2

TC (44)

In this master thesis study, a PI-model of the TC system are used, the characteristics ofthis model are described in order to highlight the properties of the model [57].

Used for stationary simulations and calculations that has low frequency oscillations.

Representative when short cables and transmission lines are used in simulations.

A PI-model is simple to use. The cable parameters as capacitance C, inductance L andthe resistance R are implemented from the technical specication of a cable design. Inorder to calculate the current and voltage along a transmission line a number of PI-linkscan be modeled together.

Figure 40: A PIlink that describes a TC, the inductance L are neglected due to the fact that thecapacitive part is much greater.

B Transients theory and calculations

The theory behinds the inrush current phenomena are explained and A transformer coreusually consists of iron, a material that can transfer magnetic ux φ. When a voltage isapplied to the primary side of an unloaded transformer, a current will ow through the primarywinding of the transformer. This will induce a magnetic ux through the core material andcause magnetization of the transformer. The current is called magnetization current Im. Insituations when a transformer is taken out of service or in any way is disconnected from theelectricity grid the magnetization current is zero (Im = 0) and no current is running throughthe windings. However some magnetic ux in the iron core is still present, a residual ux can

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therefore be observed. This is called the remanent ux φr. When a disconnected transformeris reconnected to the electricity grid at an instant such that the system voltage initiates aux φmag in the transformer core that ows in the same direction as the remanent ux thetotal ow will be φtot = φr + φmag. Due to the fact that the core only can handle a certainamount of ux it will go into saturation and draw a large current from the grid that suppliesthe transformer [55]. This will result in inrush currents in the transformer.

B.1 Voltage ramping power ow calculations

The power ow when the system was energised can be characterised into a few dierent sources.The main active power ow comes from the resistive losses in TCs and in transformers (P =3I2R) together with the magnetisation losses (iron losses) in the OS- and WT- transformer.

Equation 45, Equation 46, Equation 47, Equation 48, Equation 49 where used to determinethe resistive losses.

IOS =|S|√3U≈ QTC√

3U(45)

POffSTrafo= 3 · IOS

2 ·ROffSTrafo(46)

PWTTrafo= 3 · IOS

2 ·RWTTrafo(47)

PTC = 3 · IOS2 ·RTC (48)

PResitivelosses = POffSTrafo+ PWTTrafo

+ PTC (49)

PmagOffStrafo=|S|baseWTtrafo

|S|baseOffStrafo

· PmagWTtrafo(50)

The total active power in the system POS where determined by Equation 51.

POS = PResitivelosses + PmagWTtrafo+ PmagOffStrafo

(51)

The reactive power QOS in the system consists of the inductive reactive power in the OStransformer (QXL

and the reactive capacitive power in the TCs QXC. The inductive reactive

power in the WT transformer is neglected as the impedance of the WT transformer is lowcompared to the OS transformer).The TC mainly consists of capacitive reactive power andthe inductive part is neglected as its is ∼ I2 and the current when ramping the system is low.

The impedance base of the OS transformer and the WT transformer are calculated fromEquation 52.

ZbaseOffStrafo=

Ubase2

|S|baseOffStrafo

(52)

XbaseOffStrafo= ukOffStrafo · ZbaseOffStrafo

(53)

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The inductive reactive power from the OS transformer is seen in Equation 55.

QXL= 3XbaseOffStrafo

I2 (54)

The capacitive reactive power in the TC are seen in Equation 56

QXC= U2ωC (55)

The total reactive power in the system are given by Equation 56

QOS = QXC−QXL

(56)

B.1.1 OWF equivalent data

Most advantageous diesel gen - set system design: Half OWF energised by diesel gen-set/UPS - system batteries in WTs. P = 3.44 MW and Q = -8.09 MVAr/2 = -4.045MVAr, |S| ≈ = 5.3 MVA, Unom = 16 kV

OS transformer rating in proportion to diesel gen - set rating SOfftrafo = 5.3 MVA

TC proportional to (41/6) in OWF, total length of string lstring = 2.5 + (0.7 · 5)=6 km.Total length of TC 6 · (41/6)) = 41 km

TC of 33 kV 630 mm2 copper MVAC type: Rcable = 0.067 Ohm/km · 41 km = 2.747Ohm

TC 33 kV 630 mm2 copper MVAC type: Ccable = 0.35 µF/km · 41 km = 14.35 µF

The WT transformer equivalent which represent half of the OWFs WT transformershave the rating of ≈ S = (41/6) · 6 · 6.7 MVA = 275 MVA

C Control systems of diesel gen-sets

A diesel gen-set is not only consisting of an engine and a generator, it also includes a controlsystem consisting of regulators as the auto-voltage-regulator (AVR), eld current regulator(FCR), power factor regulator (PF), Reactive power regulator etc. and limiters that keep thesynchronous generator in a safe and stable operation area [58].

The AVR is an essential part of the control system in a diesel gen-set as it protects thegenerator and regulates the voltage (reactive power regulation) [17, 43].

For controlling the grid frequency (active power regualation), the speed governor of therotor in the generator is controlled by the diesel engine governor that increases/decreases theproduced torque on the shaft when the load demand is changed. Thereby the frequency isstabilised kept at its rated number [17].

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