Study of auxiliary power systemsfor offshore wind turbines: an extended analysis of a diesel gen-setsolution

(1)

Juni 2013

Study of auxiliary power systems

for offshore wind turbines

an extended analysis of a diesel gen-set

solution

(2)

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

Study of auxiliary power systems for offshore wind

power

Joakim Berggren

Until today the offshore wind power has grown in a steady pace and many new wind farms are being constructed around the globe. An important factor that is investigated today in the industry are the security of power supply to the equipment needed for controlling the offshore system during emergency situations. When a offshore wind farm is disconnected from the external grid and an emergency case occur the wind turbine generators lose their ability to transfer power and they are forced to be taken out of operation. As there are a number of loads in the wind turbines (navigation lights, sensor- and communication-apparatus, ventilation- and heating equipment etc.) they have a load demand which must be supplied in emergency mode. The German Transmission System operator (TSO) TenneT GmbH has set a requirement that the wind turbines is to be supplied by an auxiliary power supply (APS) in 12 hours and therefore there is need for a long-term auxiliary power supply system. This master thesis was assigned to investigate the most feasible APS-system. From the study of a number of different APS's one concept was chosen. This was the diesel gen-set solution placed on an offshore substation at sea. The system was modeled in the software DIgSILENT PowerFactory where a load flow analysis validated the calculated data and a study of the impact of transients in the system was performed.

Examinator: Kjell Pernestål Ämnesgranskare: Sandra Eriksson Handledare: Magnus Tarle, Ann Palesjö

(3)

Acknowledgements

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

A special thanks to my supervisors at ABB, Magnus Tarle and Ann Palesjö who steadily have pushed me forward and help me through the work. With interesting discussions and with 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 discussions as 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 have provided me with detailed information and with discussion that have given me many new experiences and much new knowledge. Especially Thorulf Brattström, Mikael Svedung, Jorge Brischetto, Drasko Skenderija, Steven Coppens, Fernando Sada, Nassim Raoo and Oskar Björkbacka.

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 wind farm.

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

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

Marja-Liisa Marttila and Sakari Laine at ABB Vaasa with the knowledge about shunt reactors 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. Without his help with essential information about the Wind turbine equipment this master thesis study couldn'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. They are Jörg Haabermaas at MTU, Göran Österdahl at Wärtsilä, Peter Bojtas at MAN and Karl Stapelfeldt at the company CATUM.

(4)

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) uppstår kan vindkraftsparker till havs vara utan el/energi i era månader och om inte vindkraftverk underhålls (med värme, ventilation etc.) under denna tid kan de bli skadade. Samtidigt måste navigationsbelysning 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 ett antal 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 grundläggande nivå 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 och fullomriktar-vindturbiner i kombination med dieselgen-sets.

Utifrån en grundläggande studie av nödkraftsystemen påvisades ett koncept som ansågs vara 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 lastödesanalys validerade 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. Diesel motorn krävde ca 4 MW och den lägsta generator storleken var 4.2 MVA med shunt reaktorer inkopplade 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 är spänningsrampning av vindparksnätet utan att ha vindturbinlaster inkopplade och mjukstart av vindkraftspark i nödläge där sträng efter sträng av vindturbiner energiseras tills att alla laster i vindparken tagits i drift.

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

(5)

Abbrevations and clarications

A Ampere

AC Alternating Current

APS Auxiliary Power System

CS Collector System

DC Direct Current

DFIG Doubly Fed Induction Generator

Gen-set Generator set

HV High Voltage

HVDC High Voltage Direct Current

MV Medium Voltage

O&M Operation and Maintenance

OS Oshore Substation

OnS Onshore Substation

OS Oshore System

OWF Oshore Wind Farm

OWP Oshore Wind Power

PF Power factor

pu Per Unit

PV Photo Voltaic

RMS Root Mean Square

SG Synchronous Generator

TC Transmission Cable

Trafo Transformer

TS Transmission System

TSO Transmission System Operator

UPS Uninterrupted Power Supply

V Volt

VA VoltAmpere

WECS Wind Energy Conversion System

WT Wind Turbine

WTG Wind Turbine Generator

W Watt

WPP Wave Power Plant

(6)

Description of parameters

C Capacitance

cos φ Power Factor denition

δ Load angle of Generator

E Electromagnetic force

f Electrical Frequency of grid

ηmodule PV module Eciency

ηtot PV system Eciency

I Current k linear constant L Inductance m Mass ω Angular frequency Ω Ohm P Active Power Pf Packing factor

φ Numerical operator phi which determines the angle between P & Q

Q Reactive Power R Resistance ρ Density S Complex Power |S| Apparent Power U Voltage V Volume Z Impedance X Reactance

(7)

1 Introduction

Today, our society's energy need is mainly supplied by the use of fossil fuels. The age of the fossil fuels has led the humanity to an advanced development in the technological area and we have created the modern society. However, as the Stone Age did not end because of the lack of stone the Age of the fossil fuels will not end because we run out of oil, gas and coal. It will end sooner. The fact is that the fossil fuels are nite energy resources which eventually will be consumed. In the striving towards a more sustainable energy system, many countries in Europe are trying to increase the use of renewable electricity sources in the system. This has started 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 the installed power is expected to increase. For Europe, there are scenarios for the development both in medium term (2020) and in long term (2030) [2]. In these scenarios the largest nations that will produced most of the oshore wind farms (OWFs) are UK and Germany, followed by France, Netherlands, Belgium, Denmark and Sweden. In total the amount of installed power could grow from present values of approximately 5 GW to 42 GW in 2020 and 126 GW in 2030. However, these numbers should only be treated as indications of the increased installed power and not as exact values.

Figure 1: The development of the oshore wind farm industry in the future, in a perspective of the year 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 restrictions of the components in an oshore wind farm during emergencies. Under these, the park is cut-o from the external grid (i.e. a cable failure) and no power can be transferred to land through the export cables.

(10)

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

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

As WT manufacturer REpower has raised a question to the ABB division ABB / Power Systems / 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 of pointing 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 have the potential to be a future constructed APS-system. The system(s) that could be of special interest is determined and can be investigated more deeply in coming sections later on. From the results of this deeper study, conclusions regarding the APS-system concept(s) will be taken.

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 categories with all the necessary infrastructure and components required for the dierent designs. They are the transmission system (TS), the collector system (CS) and the wind energy conversion

(11)

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 overall electric 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

(12)

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 need of 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 with increased 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. The study is focused to investigate a APS-system placed on the AC OS or at the wind turbines in the OWF, see the scope presented in Figure 2. In Figure 3 the geographical layout of a OWF 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 of power even during emergency situations when the OWF is without a power supply. The total WT 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

(13)

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 scenario consisting of 82 WTs with a rated output of 500 MW was chosen. This is a common size of a OWF constructed today and is therefore used as a reference design, as this study doesn't investigate any real OWF structure. The OWF conguration in emergency mode is shown in 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 substation

HVAC platform

Array with 6 WT loads

To onshore

Figure 4: The investigated scenario of a OWF with POW F ≈ 500 M W.

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 is used 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

cos φ 0.925

Rated Complex power S 6.65 MVA

Rated Active power P 6.15 MW

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

(14)

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 REpower 6M WTs. It is built up by a variable speed constant-frequency induction generator [10]. The generator has 0.66 kV as maximal rotor voltage with a stator voltage of 6.6 kV. During emergencies the circuit breakers from the WT transformer to the excitation of the generator are open and only the loads are energised from the transformer, see Figure 6.

(15)

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-system which main component is a leadacid battery with a capacity of supplying the average WT load demand of 42 kW up to 12 hours [4].

The batteries have specic charge-rates (C-rates) which describes the batteries ability to be recharged (can also represent discharge) [11]. A high charge-rate means that a battery will be recharge in a short time and a low charge-rate represent the opposite where the batteries have a long recharge-time. The C-rate depends on the battery type, i.e. a lead-acid battery is 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 useful combination in order to utilize the available emergency systems in the OWF. In that way, it could maybe be possible to make a sustainable adaption to a APS-system which is a design of both old and new equipment in the OWF. The use of the UPS-system in the WTs together with 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 with simple reinforcement because of the simplicity of this cable type [12]. They have dierent current 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 mm}2 _{400 mm}2 _{630 mm}2

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

Z0[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 WT transformer(s). The OS transformer is a step-up transformer where the voltage is transformed from 33 kV to 132 kV, 150 kV or 220 kV (MVAC TCs) and is the connection point in the OWF where transmission cables to land are connected. As an OWF can have a rated output of a hundreds 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 by REpower [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 MVA

(16)

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 MVA

Rated power S2 6 MVA

Rated 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 total load demand which are previously described in Section 3.2. They consists mainly of dierent motors and pumps which supports the heat, ventilation, navigation lights, magnetisation

losses5 _{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 kW

Peak load demand 193 kVA

2_{HV side of trafo} 3_{MV side of trafo} 4_{LV side of trafo}

(17)

4.5 Grid stability of electrical oshore systems

The Oshore wind farm in auxiliary power mode is a very weak grid operating in island mode where the only generation is the APS-system supply. This impacts the system stability when the 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 system as the capacitance together with the inductance in the cables and in transformers give rise to resonance and voltage oscillations.

When the wind farm is energised, so called "transformer inrush currents" occurs which can damage the transformers or other OWF equipment. The currents can be as high as

9 · Inomof the WT transformer nominal current [3]. This phenomena is further described

in 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, the power ow in the connecting TCs are dierent depending on the position in the string. For

example a cable outermost doesn't have as high current rating Irated as a TC innermost in a

string. Thus the rating of the TCs can be chosen according to the required Irated. Figure 8

shows 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,

(18)

Equation 2 describes the apparent power in a string SStringN ormal and Equation 3 the reactive

power ow in a string QStringN ormal.

PstringN ormal =

POW FN ormal

nStrings (1)

|S|_String_{N ormal} = PstringN ormal

cos φ (2) QStringN ormal = q |S|2 stringN ormal− P 2 StringN ormal (3) IT CString = |S|String_√ N ormal 3ULN (4)

The currents that ow through the TCs in a string IT CString are calculated from Equation 4.

Table 5: Powers P, Q, |S| and the currents owing in one string IT CString of 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 TC cable dimensions that match the currents in the string, seen in Table 6. The dierent TCs used 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 and simulation of electric systems and includes a large variation of functions and applications that are used to create a model of a power system which can be simulated with respect to load ow calculations (steady-state), dynamical electromagnetic transients (EMTs), continency

(19)

analyses, short circuit analyses etc. [16]. The model parameters that are important to know of 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 lines 33 kV, rated currents for dierent_{conductor mm}2_{, Z} 0, Z1and Z2

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

WT transformer (2windings), rated power, rated_{voltages, Z} 0, Z1and Z2

External grid Reference for the power system_{outside of the OWF}

WT loads Average_{42 kW/WT}power demand of

Circuit breakers

WTG Rated power, cos φ

Shunt reactor Rated power

(20)

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 in the software DiGSILENT PowerFactory, in order to simulate the "loading" of the TCs for verication of the oshore wind farm layout. As the strings in the OWF have the same layout it'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 of each 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].

(21)

7.2 Calculations of load ow in Emergency mode

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

The active power transmitted in the system answers to the load demand of the WTs during

emergencies, PW T GEmergency stands for 42 kW per WT including with the WT transformer

magnetisation 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 operation

compared to normal operation. The total load demand in the WTs is POW FEmergency and the

total load demand answers to POW FEmergency = 42 kW · 82W T s = 3.44 M W.

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

The TCs are perfect capacitors (basic assumption as there in reality are both resistive and 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 the capacitive part of the TCs in the OWF is the part which contributes to the load ow as

XC ∼ U2 with a voltage of 33 kV (see Appendix A.4). QT C produced in the transmission

lines is calculated with Equation 5 and Equation 6.

Z =U

2

|S| V A (5)

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

QT C = QCap =

U2

XC

V ar (6)

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

Cable number [µF/km] [µF] QT C [MVAr]

1 0.35 0.875 -0.299 2 0.29 0.203 -0.069 3 0.24 0.168 -0.057 4 −”− −”− −”− 5 0.19 0.133 -0.046 6 −”− −”− −”− QString -0.575 QT ot -7.86

(22)

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 operational mode. The simulated load ow is needed to be found in order to investigate any APS-system concepts 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 layout of strings nr 1-3 in the OWF is seen in Figure 11a. Figure 11b show a detailed layout of a part of string 1-3 in the OWF.

(23)

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

(b) More detailed view of the system design. A part of string 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 order to nd the power ow in the system. From the results of the load ow analysis, the values of active power P and reactive power Q in the OWF in emergency operation mode are presented in Figure 12.

(24)

The reactive power in the PowerFactory model is diering ≈ 0.235 MVAr or 3 % from the calculated values, (see Table 9). This might be an eect of some inductive reactive power reduction from TCs and a voltage level which isn't exactly 1.0 pu. The results, however validates 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 -8.09

Calculations 3.44 -7.86

8 Auxiliary power supply concepts

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

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 control system 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 several possible 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, depending on which conguration that is studied. The following section describe the concepts in detail.

(25)

8.1.1 Stationary diesel gen-set concepts

Basic descriptions are introduced in this section and the most important advantages and disadvantages of the system designs are pointed out. There are two possible solutions for placing a stationary diesel gen-set in the OWF, the diesel gen-set is either placed on the oshore substation or at the bottom of the WT tower in at every turbine. The system designs are 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 special designed generator which can operate under-excited and consume reactive power from the 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 WT loads.

Limited place at the bottom of the WT tower to place a diesel gen-set. The area also require 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 equipment used in many applications today.

The total power demand in the studied OWF during to be supplied of the dierent diesel gen-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 ≈9

(26)

8.1.2 Mobile diesel gen-set concept

A mobile diesel gen-set consists of a standby marine vessel which in emergencies travels out to the OWF. The system design is similar to the stationary concept of having a diesel gen-set placed on a OS or at the WT towers, the only dierence is that the diesel gen-set are placed in the OWF when an emergency has happened. A stationary concept have the diesel gen-set placed 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 at the WT towers.

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

To fulll the TSO TenneT criteria and supply the WT loads in twelve hours, there are an uncertainty whether the marine vessel will have time to place diesel gen-set(s) at the OWF when 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 are needed to stationary be placed at a OWF, to handle the WT load demand until the mobile vessel 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 stationary diesel gen-set concepts, see Table 10 in Section 8.1.1.

8.2 Gas/diesel gen-set(s)

A similar concept to the diesel 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-gen-set (see Section 8.1.1), but can also run on gas and not only diesel. The advantage is that there could be less vibrations and less pollution of the environment [19]. The exhaust gases have reduced concentrations of nitrogen-oxides (NOx) and sulfur-oxides gases. A major disadvantage is that an additional fuel 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 utilize the energy from the sun by using PV's. The solar cells produces electric energy for free and

support 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 solar

cells) Pf and on the eciency of the converter system (power conditioner) that deliver a good

voltage quality to load equipment ηpc. The resulting eciency of a PV module ηtot is found

from the relationship between these factors and is shown in Equation 7 [20, 21].

(27)

From [20] a specic site consisting of 329 m2 _{had a total η}

tot = 8 % which are used as a

reference for a low total eciency of the the PV system in this study. To examine the power

production, a study of the power production in W/m2 _{with dierent η}

tot and weather

condi-tions (sunny, partly cloudy and cloudy condicondi-tions) were examined. For simplifying reasons, the PV's are placed on a at area pointing directly vertical to the sky. Then the need of taking care of the tilt angle and the compass directions relative to the sun that otherwise will aect the 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 η} tot

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 length and 20 m in width. The total area AreaOf f S= 800 m2 (Manipulated picture) [7].

There is assumed that half of a typical AC OS roof area is available for installing PV's AreaOf f S = 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 8 and the results are shown in Table 12 and Figure 16.

PP VOf f S = PP V · AOf f S (8)

From the results it's visible that the PV's are not producing enough of power for supplying the WT load demand with only a small portion of the load demand supported. In addition the 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 energy

(28)

Table 12: The total power POW FAP S with dierent ηtotof 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 kW

11 % 44 kW 22 kW 4 kW

13 % 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 the WT nacelles is utilized for power production by PV's. The PVs have been assumed to occupy the roof of the WT nacelle with a width of 6 m and a length of 15 m. The total area of the

roof of one WT nacelle is AW Tnacelle = 90 m

2 _{[23]. The total area of all WT nacelles in the}

OWF that consists of 82 WTs is AP VW T s = 90 m

2_{· 82 W T s = 7380 m}2_.

Similar to the previous Section 8.3.1, the total power produced by the PV's PP VW T s is

related to the total eciency of the PV system ηtot and the total area available for installing

PVs, see Equation 9, the total power production is shown in Table 13 and Figure 17.

PP VW T s = PP VAW TnacellenW T s (9)

The power production of PV's placed on WT nacelles consists of a larger total area relative

to placing the PV's on the OS (7380 m2 _⇔ _{400 m}2_{). This increases the maximum power}

production from 53 kW to 492 kW, please see Figur 16 and Figure 17. The results shows that the power demand from the WTs isn't supported by only using PVs placed at the WT nacelles.

(29)

Table 13: The total power PP VW T Gs with dierent ηtotof the PVs placed on the WT nacelles.

Power production of PV's

ηtot Sunny Partly cloudy Cloudy

8 % 590 kW 295 kW 59 kW

11 % 820 kW 410 kW 82 kW

13 % 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 the concept design.

8.4 Batteries combined with PV's

Another APS-system can be to use batteries in combination with PVs placed on the OS or at the WTs, then the WT loads might be supplied with enough power. The reference battery used in the study are assumed to be Lead Acid batteries which are used in the DolWin1 OS 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 and they are combined one battery. Such batteries have the following characteristics, shown in Table 14 [24].

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

(30)

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

(a) Cell characteristics

Battery cell Height 0.824 m Length 0.225 m Width 0.580 m Area 0.130 m2 Volume 0.108 m3 (b) Battery characteristics Battery Cell voltage 2.00 V Battery voltage 110 V

Number of cells 53 cells

Area of battery 6.917 m2

Volume of battery 5.700 m3

where Ptot is the resulting required power supply from the PV/battery system, PW T s is the

load demand of the WTGs and PP V 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 3441

Placed in WT nacelles 2237

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

WDemandOW F = WBatteryDAY + WBatteryN IGHT = Ptottday+ PW T stnight (11)

The length of the day is changing on yearly basis depending on the geographical location where the site is placed the total energy demand will dier. A OWF placed in the North Sea will 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 calculated with Equation 12 and presented in Table 16.

WBattery =

WDemandOW F

UBusbar (12)

where WBattery is the required battery capacity in [Ah] and UBusbar is the voltage on the

busbar the batteries are connected to.

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

(31)

Table 16: Required capacity in [kWh], [kAh] and numbers of batteries needed to fulll the required energy 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 48

Placed 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 3.6

Specic energy [Wh/kg] 30 50 48 80 60 120 90 190

Maintenance 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 examined here in a technical review of a point absorber wave power pland (WPP) concept created by the 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 power in the world are pointed out in Figure 18.

Pwave= 0.5Hs2Tp (13)

where Pwave is the power in the wave, Hs is the signicant wave height and Tp is the period

time of the wave.

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

(32)

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

(a) A sketch of the point absorber WPP

concept Seabased [30]. (b) [A schematic picture of a wave power farm (WPF) wherethe generator structure are visible standing on the seabed, the TCs that transfer the power to an central hub/converter sta-tion and the wires with connected buoys at the surface of the sea [31].

Figure 19: The Seabased concept with substructure.

The wave climate outside of the Danish coast in the North Sea, were OWFs often are

constructed is presented in (kW/m) of signicant wave height Hs in Table 18 [27, 32]. The

wave climate at the Swedish west coast is also included in order to understand the potential for dierent locations.

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

Location Wave climate [kW/m]

North Sea 9.8

Skagerrak 5.2

Kattegatt 2.4

Apart from the wave climate the utility factor of the WPPs is an important economic factor to take account for. Dierent utility factors will aect the amount of WPPs needed for supporting the WT loads. Therefore the calculations in this report has taken this into consideration 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 WT loads. This is a large quantity that require an extensive installation and maintenance

(33)

proce-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 as an APS-system today for the studied OWF. There is a possibility that this may change in the future 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 the OWF by restart some of the wind turbines in the OWF with a few small diesel gen-sets placed at a handful number of WTs [33]. These diesel gen-sets energise the WTGs (full converter wind turbines) after a grid blackout if needed and in that way restart the WTs to normal operation 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 can control 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 always rotates with optimal speed in order to produce the highest possible power [34]. The converter closest to the grid can fast change the active/reactive power transferred from the WT to the system which can contribute to maintaining stability. When an emergency occur and a system fault is present the WT have inertial energy preserved from normal operation which may help restarting the system in emergency mode (diesel gen-set's available with extra power if needed) when its operating as an APS-system [35]. Figure 20 show a schematic picture of a full converter WT.

(34)

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 be solved. 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 summarized here in order to focus the remaining part of the study to the most feasible concept(s). A summary of each APS-concept studied is described in the list that follows. The Diesel gen-set(s), Gas gen-gen-set(s), Photo voltaic's (PV's), Batteries combined with PV's and nally full converter 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 and mobile diesel gen-set on marine vessel are summarised here with the dierent advantages and disadvantages.

9.1.1 On oshore substation

The diesel gen-set solution placed on the OS includes a possible concept with a motor rated approximately 3.5 MW and a generator that can handle the active and reactive load ow in the 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 reactive power compensation equipment.

Limited fuel capabilities.

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

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 the diesel 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 connected via a busbar and with circuit breakers to the WT loads. The magnetisation losses in the WT transformer are 14 kW which in this case have lowered the total load demand to 28 kW.

(35)

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] this concept is used today for 5-10 kW loads in times when the erection of the WTs are taking 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 wind farm in case of an emergency could be benecial if a standby crew with a vessel for example is stationed near the oshore wind farm today. The concept however has some other aspects which 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 on the marine vessel. Then the reactive power in the TCs is less of an issue for the system design.

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

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

9.2 Gas/diesel gen-set

Ägas/diesel gen-set placed on a AC OS as a APS-system has almost the same conguration as a diesel gen-set alone, see Section 9.1.3. The gas gen-set major disadvantage compared to a diesel gen-set is related to the gas fuel. A gas/diesel gen-set require an gas fuel tank in addition to the existing diesel tank(s) and this demand valuable space on the OS [19]. There are also some logistical problems when handling gas which are not fully developed for oshore substations 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 positive environmental impact.

A gas turbine can be used instead of a gas engine. The turbine will have a quicker starting 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 tanks in addition to diesel tanks and more regulation equipment.

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

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

(36)

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 WT nacelle roof aren't enough to support the WT loads during emergencies. The oscillations in

power productions due to weather conditions and dierent ecencies ηtot of the PV systems

will make it an unreliable power supply, it is therefore not recommended that a PV system is used 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 is

only 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 load demand 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 security

of the OWF.

9.3.2 On WT nacelle roof

A larger total area compared to the available AC OS area increases the power production relative 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 comparedto the load demand of 2296 kW).

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

No power supply during night-time when navigation lights are essential for the security of 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 be impossible with the use of Lead-Acid batteries. The requirement of 216 batteries with a total

volume and weight of approximately 1230 m3 _{respectively ≈ 48 ton are nether possible nor}

advantageous. The case when batteries were placed in WT nacelles was a better solution with

less requirements of 144 batteries,a volume of 821 m2 _{and a weight of 32 ton. This is still not}

a realistic scenario to investigate as the WT nacelles would require ≈ 2 batteries/WT which not is likely to t in the nacelle or at the WT tower structure. The use of more advanced and suitable batteries may reduce the total numbers of batteries needed, but will not enable batteries 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 limited AC OS area, the total weight is very high and the system is ineective.

(37)

In cases when the load demand is very small (a couple of kWs) the system may be theoretical possible, however the need of reactive power compensation equipment will be present as well as the need of recharging the batteries after ≈ 24 hours when they are 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 any conclusions. 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 others to perform.

9.6 Chosen APS-system for further investigation

To simplify the review of the dierent APS-system concepts, a short summary of each concept is seen in Table 20 where the most feasible concept was selected with respect to the technical possibility of constructing the APS-system together with the weighted feasibility (expected economical 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 ranked in order to visualise the most feasible concept.

Summary of APS systems

Concept Technical possible Feasible Rating7

Stationary diesel gen-set on OS Yes Yes8 ₁

Stationary diesel gen-sets at WT towers Yes Yes9 ₂

Stationary gas/diesel gen-set on OS Yes Yes10 ₃

Mobile diesel gen-set on marine vessel Yes Yes11 ₄

Wave power farm No No 5

PVs combined with batteries No No 6

PVs on WT nacelles No No 7

PVs on OS No No 8

Batteries 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. The summary should therefore be treated as a guideline. A deeper analyse of the the diesel gen-set placed on the OS concept is followed in coming sections.

7_{Weighted feasibility (expected economical costs & technical functionality.}

8_{Reliable energy source, relatively ecient, manageable construction, limited space requirement etc.} 9_{Requirement of a low WT load demand to be feasible (≈ 5-10 kW) [4].}

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

(38)

10 Sizing of diesel gen-set: Calculations

The diesel gen-set sizing procedure is described here and the validated power ow calculated in Section 7.2 is used as a reference to nd the right dimensions and rating of a diesel gen-set placed on an OS. The size of the Diesel gen-set system depends on both steady-state load demand 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 in APS-system mode.

Transients as (inrush currents) in equipment when switching in WT transformers after the 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 through the system which are: P = 3.44 MW and Q = -8.09 MVAr, see Section 7.2. The sign before the powers refer to the WT loads which require active power (+), and TCs that produce reactive power 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 require a special diesel gen-set solution with the ability to work well in under-excited mode (see Appendix A.2.1). The generator is producing active power P and consuming reactive power Q 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 require a production of reactive power, but as the emergency layout of the OWF symbolises a "weak grid" with reactive power production in TCs and no major reactive power consumption, the generator instead needs to produce active power and consume reactive power. To handle the reactive power, the standard diesel gen-set needs to be overrated. Manufacturers visualises the 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 diesel gen-sets are a limit of the possibility to operate under-excited, see Appendix A.2.1 for more information regarding the issue.

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

(39)

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

Figure 22: The determination of a linear function.

generator will produce enough amount of active power that is required. Figure 22 shows the procedure 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 EquaEqua-tion 15.

QRated= QSystem= −8.09 M V Ar (14)

SRated =

QRated

(40)

(a) The capability curve of an ABB syn-chronous generator where the linear function is 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-Somer synchronous generator where the linear func-tion is intersecting the stability limit of the capability curve [37]. P/S=0.19 pu Q/S = −0.43 puat 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 needs to be overrated to handle the reactive power demand.

(41)

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 can be downsized. The gen-set is constructed to work in under-excited mode and the system isn't needed to be overrated [39]. The system design have been validated by ABB - Helsinki and is shown 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 operate under-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 eect of increased air-gap between stator and rotor (25 mm-35 mm) [39]. It usually decreases the magnetic ux in the stator (and reduce eciency), however in this case the magnetisation instead is coming from the stator side of the generator. It should be clear that the generator

(42)

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 of 12 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 generator and possibly reduced fuel use, as the gen-set is less frequently in operation. By altering some of the WT loads to be supplied from diesel gen-set or batteries it could be possible to use special 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 all scenarios are done from Equation 16, Equation 17, Equation 18 and Equation 19.

WLoad.Gen−Set= PW T.load· (nW T s· Seq) · tSequence (16)

Where WLoad.Gen−Set is the energy needed for the gen-set, PW T.load is the WT load demand,

nW T s is the number of WTs, Seq is the sequence scenario used (i.e. 1/2) and tSequence is the 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).

(43)

WSupply.U P Sbattery 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.U P Sbattery (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 time

for all UPS batteries in the OWF to be recharged.

Figure 27 shows the active power required to be handled by the generator in sequential operation schemes. The results showed that the sizing in all case have a total active power demand 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-C-rates depending on the Sequence operational procedure 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 energising the WT loads. The smaller sequence, the lower the reactive power demand, fewer TCs in the OWF simply doesn't have to be operation at the same time. The disadvantage is that the UPS normally doesn't operate in on/o mode. The deep discharge of the batteries may reduce the 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-system batteries and 1/2 OWF on diesel gen-set savings on the fuel is done, the reactive power in the system is reduced from -8.09 MVAr ⇒ -4.045 MVAr and the C-rate of the batteries aren't

(44)

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 overrating generator procedure used as in Section 10.1.2, calculated from the two dierent capability curves, 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 19.04

1/2 wind farm 11.56 9.63 10.60

1/3 wind farm 8.70 6.58 7.64

1/4 wind farm 7.49 5.06 6.28

1/5 wind farm 6.47 4.15 5.31

(45)

10.2.2 Rating of special generator

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

Figure 29: Special generator sizing.

Apparent power |S|

Sequence Size [MVA] ≈ [MVA]

Whole wind farm 8.79 8.8

1/2 wind farm 5.31 5.3

1/3 wind farm 4.37 4.4

1/4 wind farm 3.99 4.0

1/5 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 sequential loading. This is the eect from the less reactive power in the system (i.e. it is assumed that a 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 unit in form of shunt reactors can be used. A shunt reactor is a piece of equipment that basically