EE 986 Modelling

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1 Modelling and Control of Offshore wind farm for Grid code compliance.

EE986 Assignment and Professional Studies

Group Assignment

Modeling and Control of Offshore Wind Turbine for

Grid Code Compliance

Final Report by

Ahmed Daniyal Siddiqui 201252772

Arshad 201276792

Lu Tong 201285026

Supervisor: Dr. Olimpo Anaya-Lara

Department of Electronic and Electrical EngineeringUniversity of Strathclyde Glasgow United Kingdom

April 2013


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Abstract

This document explains wind turbines and wind farm control techniques in o

shore sites. Wind turbines used in offshore, wind farm schemes and connection to

onshore network are explained. Wind turbines and wind farm mechanism used in offshore

areas varies from onshore conditions and connection to onshore has to be made through

minimum losses in lines. All the wind turbine basic components, working mechanism,

connection schemes of wind turbines including the type of connections between wind

turbines are also discussed. Furthermore, mathematical modelling required for the wind

turbine analysis and calculations along with spacing between wind turbines/wind farms

are performed to reach the desired results. Connection schemes for the dispatch of power

from off shore to onshore will be critically analyzed to reach conclusions. In this report,

review of literature related to wind energy projects in o shore areas was done and analysis

was made related to requirements of wind energy project offshore. Calculations are madebased on three 0.5 GW capacity wind farms and related parameters are included for

maximum power transfer to onshore.


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Table of Contents

Executive Summary...............................................................................6

Project Objective....................................................................................7

Project Specification..............................................................................8

1. Offshore Wind Turbine technology..............................................91.1Introduction................................................................................101.2Fixed Speed Wind Turbine........................................................12

1.2.1 Soft Starters......................................................................131.2.2 Capacitor Bank.................................................................13

1.3Variable Speed Wind Turbine Generator System.....................141.3.1 Doubly Fed Induction Generator......................................151.3.2 Back to Back Convertors.................................................16

1.4Wound Rotor Synchronous Generators.....................................161.5Permanent Magnet Synchronous Generators............................171.6Wind Turbine STW-3.6-120.....................................................17

2. Wind Turbine and Wind Farm Connections...........................202.1Introduction.............................................................................212.2Wind Farm Layout..................................................................212.3Connection Schemes...............................................................22

2.3.1 Radial Schemes.............................................................222.3.2 Single Sided Ring Collector.........................................232.3.3 Start Collector with single hub.....................................24

2.4Cable used for Offshore Network...........................................252.5Inter Turbine Array Cables.....................................................252.6Calculation of Total Number of Wind Turbine......................252.7Wind Turbine Distribution.....................................................262.8Offshore Collector Substation................................................27


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3. Offshore Wind Farm Electrical Connection to Grid................283.1Introduction.............................................................................293.2AC Connections of Offshore Wind Farm................................303.3DC Connections of Offshore Wind Farm...............................31

3.3.1 HVDC Line Commutated Convertors...........................313.3.2 HVDC Voltage Source Convertors...............................33

3.4Multi Terminal VSC HVDC...................................................333.4.1 Modular Multilevel Convertor.......................................343.4.2 Black Start Capability....................................................34

3.5HVDC PLUS (SIEMENS).......................................................353.6HVDC LIGHT (ABB).............................................................363.7DC Submarine Power Cables.................................................37

3.7.1 Mass Impregnated DC Cables.......................................383.7.2 Extruded Polymer DC Cables......................................39

4. Conclusion...................................................................................415. References..................................................................................42


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List of Figures

Fig 1-1 Wind Turbine parts...................................................................................................11Fig 1-2- Wind turbine generator system..................................................................................12

Fig 1-3- Fixed Speed Wind turbine.........................................................................................13

Fig 1-4 Soft starter................................................................................................................13Fig 1-5- Variable speed wind turbine generator system..........................................................14

Fig 1-6- Doubly Fed Induction Generator...............................................................................15

Fig 1-7- Back to back converter..............................................................................................16Fig 1-8 Wind Turbine main parts. .......................................................................................17

Fig 2.1: Block Diagram of proposed project. ........................................................................22

Fig 2.1: Radial Scheme Mechanism for offshore wind farms.................................................23

Fig 2.2: Single Sided Ring [15] ................................................................................................23

Fig 2.3: Star Collector with Single Hub [15] ...........................................................................24

Fig 2.4: Layout of Wind Farm .................................................................................................24

Fig. 2.5 distribution of wind turbines in wind farm..............................................................26

Fig 3.1: A typical offshore wind farm ..................................................................................29

Fig 3.2: A graph shows the comparison between AC and DC system. ...............................30 Fig 3.3: AC connection scheme. ...........................................................................................31

Fig 3.4: Block diagram of DC connection scheme. ..............................................................31

Fig 3.5: A typical HVDC LCC connection............................................................................32Fig 3.6: HVDC VSC connection scheme...............................................................................33

Fig 3.7: Diagram of Modular Multi level Convertor..............................................................34

Fig 3.8: Basic structure of HVDC PLUS ...............................................................................35

Fig 3.9: Mass Impregnated cable............................................................................................39


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Executive Summary

Wind power energy is considered to be the most environment friendly power generationtechnology prevailing in renewable sector. Although, wind turbines are more environment

friendly as compared to other power production plants but they occupy a relatively a large

space. Few issues with wind turbines include large amount of noise produced and also insome areas wind in unpredictable. Wind farms in offshore areas serve as a good

alternative to land problems and in such areas wind speeds are also very good. In offshoreareas, wind speed is predictable and regular which results in higher and regular energy

production.

Wind Turbines are extremely large structures and are able to extract energy from air.

These turbines blades angle can be changed according to requirements. Wind turbines

which are located at offshore locations are able to produce electricity as much large scale.

When offshore wind farm produces electricity, it involves different connections of wires

and maximum of them are used for connections of ineld power connection. These powerconnections in return have a lot of impact on power produced annually and the security of

supply. The offshore wind turbine technology is relatively new and a lot of upcomingtechnologies are providing multiple options for maximum development of energy out of

wind turbines. Construction of these offshore wind turbines is a challenge and they are

connected with each other to constitute wind farm from which energy is transmitted to

onshore network via cables.

Offshore wind farms produce electricity which is comparatively reliable and predictableas compared to energy produced by on shore turbine. The prime reason of this difference

is due to wind speed being more regular in offshore areas. This project is modelled onthree wind farms of power production capacity of 0.5 GW each. All calculations have

been made to model a Wind farm on ideal parameters.


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Project Objectives

The main objectives of the project are following:

1. Thorough understanding of offshore Wind farm sites and their controlling mechanismsand best power transfer schemes.

2. Detail analysis of Wind turbines used offshore, wind farms and their orientations.3. Wind turbine controlling techniques, types and components best suitable for Offshore

Wind Farm.

4. Best connection schemes between Wind Turbines, distances and power transfermechanisms required for the project.

5. Transfer of power from offshore to onshore study and grid plan for best transfer of energyminimizing losses and economic constraints.

6. Find the cost effective cable length keeping in mind the different topologies in practiceand find the most suitable combination for reliability and cost effectiveness.

7. Analysis of future enhancement in the project keeping in mind further growth of windfarm.


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Project Specification

Specifically, project has following details for modelling:

1. Three Offshore Wind Farm located in same geographical area, distribution of windturbines in the area has been done without geographical and wind terrain analysis.

2. Each having power production capacity of 500 MW of energy.3. Total Capacity of wind farm is 1500 MW which is transmitted through collector and

then DC convertor substation with transformation of power at various voltage levels.

4. This wind farm is connected to National Grid and transfer of power is done viaHVDC VSC.

5. The onshore gird is located at 160 km from the offshore wind farm.


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Chapter 1

Offshore Wind Turbine Technology


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1.1IntroductionWind turbines are the component of generating electricity in an offshore wind plant, whichtransforming the wind energy into electrical energy. And it is installed on the top of a support

structure. According to the figure 1.1, there are two parts in the support structure, foundation

and tower. The appearance of the offshore wind turbine is almost the same as the onshorewind turbine. However, some modifications have to be designed due to the specific severe

offshore environment, for example, corrosion protection, internal climate control and high-grade exterior paint. Currently, the offshore wind turbines in operation basically consist of

three-bladed horizontal axis, yaw-controlled, active blade-pitch-to-feather controlled, upwind

rotors whose diameter can range from 65 to 130m and capacity is typically between 1.5 MWand 5 MW [5-7]. In addition, the standard design of turbine consists of gearbox, drive shaft,

generator, the hub and the blade-rotor assembly and the structure is vividly demonstrated in

the following figure 1.2 [5].

Offshore wind energy is environmental friendly which has simply minimal environmental

influence on the surroundings. And it is considered to be the best resource in terms of the

location which is well located related to the centres of electricity demanding. Meanwhile,

wind is highly variable, unpredictable and cannot be stored. Therefore the wind turbinegenerator technology needs to be developed by keeping in mind these features of wind. The

output from the wind turbine is connected to the grid operating with fixed frequency and any

changes in load or generation will give rise to system disturbance. The generator of a windturbine transforms the mechanical power into electrical power [12]. There are different

generator technologies available for wind turbine and the selection of an appropriate

technology depends on the wind speed, power output and the grid requirements. At present,

there are four types of installed generator technologies: Squirrel Cage Induction Generator,

Doubly Fed Induction Generator, Wound Field Synchronous Generator and PermanentMagnet Synchronous Generator. In addition, wind turbine generator system can be divided

into two major types depending on the rotational speed: Fixed Speed Wind TurbineGenerator System and Variable Speed Wind Turbine Generator System.


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Figure 1-1 Wind Turbine parts


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Figure 1-2- Wind turbine generator system

1.2Fixed Speed Wind Turbine Generator SystemA fixed speed wind turbine generator system is a conventional squirrel cage induction

generator directly coupled with grid. Brushless, rigid construction, low cost and simple

operation are the characteristics of the fixed speed wind turbine generator system. And the

slip and rotor speed of a fixed speed wind turbine generator system, for example, the squirrelcage induction generator varies with the amount of power being generated. The advantage of

a fixed speed wind turbine generator system is the low cost and simple operation [6].

However, when comparing with the variable speed wind turbine generator system, fixedspeed wind turbine generator system is more robust. In addition, the rotor speed cannot be

varied variation in wind speed directly effects the drive train torque fluctuations, whichcausing higher loads than variable operation.

A fixed speed wind turbine generator system is shown in the following Figure1.3,


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Figure 1-3- Fixed Speed Wind turbine

1.2.1 Soft-starterThe soft-starter is used to decrease the starting current value of the wind turbine and isactually a power electronic converter device. The soft starters are able to bring wind turbine

generators online smoothly. And they can make the output of the generator to be

synchronised to the grid without incurring problems. For example, giving raise to massive

shock currents and mechanical shocks. In addition, the soft starters are not simply applied in

wind turbine, it is widely used in relative industrial area where it is necessary to operate withinduction machines because it can control the starter current in an effective way [9].

The soft starters are in fact a parallel-inverse connection of two thyristors; the structure of thesoft starter is demonstrated in the following figure 1.4. And the angle of the soft-starter

decreases by a constant value in each stage until it reaches to zero and then being shortcircuited by a power switch.

Figure 1-4 Soft starter

1.2.2 Capacitor BankA capacitor bank is a form of several identical capacitors interconnected in parallel or in

series with one another. Capacitor bank is basically used to deal with the issues based on

different operation conditions. For example, under the condition of direct current powersupplies, the capacitor bank is used to improve the ripple current capacity of the power


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supply and increase stored energy. And under the condition of alternating current powersupplies, it is typically applied to improve the power factor lagging or phase shifts inherent.

Basically, the aim to use the capacitor bank in wind turbine is to correct AC power supply

anomalies which are typically raised by heavy load, for example, the use of electric motors

and transformers. From the perspective of power supply, this type of working load is not

reasonable, because both of transformers and motors are inductive load which directly leadsto problems about phase shift or power factor lagging [9].

Additionally, in order to deal with the problem about correcting power lagging or maintaining

the power factor, installation of a capacitor bank is a better and the cheapest way to meet the

demand. However, when dealing with the issues about stored energy, if discharging

incorrectly it will cause serious electric problems. That is the most important thing shouldalways be kept in mind when working with capacitor bank

1.3Variable Speed Wind Turbine Generator SystemThere are three types of generator for variable speed wind turbine generator system,

Doubly Fed Induction Generator Wound Rotor Synchronous Generator Permanent Magnet Synchronous Generator

The principle advantage of variable speed wind turbine generator system is that it can be

adjusted to the specific wind regime. The efficiency of variable speed wind turbine generator

system decreases due to the use of power electronic converters necessary for variable speed

operation, however, the aerodynamic efficiency increases. The major disadvantages ofvariable speed wind turbine generator system are high cost but using variable speed

technology other cost such as foundation cost of offshore turbine are less than the fixed speed

turbine. A variable speed wind turbine generator system is shown in the following Figure 1.5.

Figure 1-5- Variable speed wind turbine generator system


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1.3.2 Back-to-back converter

The back-to-back consists of a force-commutated rectifier and a force-commutated inverterand both of them are designed to connect to a common DC-link, the structure of the converter

is demonstrated in the following figure 1-7. The properties of this combination are practical.

The line side converter is designed to be operated to output sinusoidal line currents, and the

DC-link voltage will be higher than the peak voltage due to the sinusoidal current.Additionally, the DC-link voltage is regulated by controlling the power to the AC grid, and

the inverter operates on the boosted DC-link finally which makes it possible to increase the

output power from a connected machine whose power is already rated. Besides, the brakingenergy can be fed back to the power grid rather than simply wasting it a braking resistor.

In addition, controlling the power flow immediately is the important characteristic of the

back-to-back converter. The DC-link voltage can be remained constant through controllingthe power flower to the grid. And it is possible to make the size of the DC-link capacitor

smaller without having influence on the performance of the inverter due to the presence of an

immediate control loop for the DC-link voltage [9, 13].

Figure 1-7- Back to back converter

1.4Wound Rotor Synchronous GeneratorThe wound rotor synchronous machine is one kind of the doubly fed electrical machinewhich can be fed simultaneously via the stator and the rotor side. In addition, this kind of

machine can be applied as driving machines and mainly used for the purpose of electricitygenerating.

When the machine is used as a generator, the stator voltage can be regulated due to the field

voltage. Beside, the wound rotor synchronous generator is able to compensate the reactivepower which is consumed by the electrical machine.


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1.5Permanent Magnet Synchronous GeneratorThe excitation field of a permanent magnet synchronous generator is provided by a

permanent magnet rather than a coil. The synchronous generators are widely used in the areaof electricity generating. And the rotor speed must always match the supply frequency.

Additionally, the magnetic field of the rotor of a permanent magnet generator is supplied by

the permanent magnets. However, other kinds of generator choose electromagnets to produce

a magnetic field in a rotor winding and the direct current in the rotor field winding isdesigned to be provided by a brushless exciter on the same shaft or a slip-ring.

A DC supply is not required for the excitation circuit in a permanent magnet generator.

However, the economic rating of the machine is restricted due to the expensive large

permanent magnets. Besides, it is difficulty in regulating the voltage of the machine for thereason that the air gap flux is not controllable. In addition, due to the structural and thermal

issues of high performance permanent magnets, safety issues occurred in the period ofassembly, field service or repair for a persistent magnetic field. And the speed is directly

proportional to the output voltage of the alternator in permanent magnet alternators.

1.6Wind Turbine: STW-3.6-120This project is related to Wind Turbine and Wind Farm Control Techniques in offshore sitesspecifically at Dogger Bank. Therefore, in the first section the wind turbine STW-3.6-120

manufactured in SIEMENS is selected to be applied in the project. The wind turbine SWT-3.6-120 applied in the project is based on the proven technology of the SWT-3.6-107, whichis currently the most popular offshore wind turbine around the world.

Typically, the main difference between the two types of the machine is the rotor. Take the

SWT-3.6-120 for example; the length of the rotor blade is 58.5 meters. Therefore, the sweptarea is up to 11,300 which is equivalent to approximately to two football fields. In

addition, according to the tests that the new machine SWT-3.6-120 will generate about 10percent more energy when compared with the other wind turbines.

The structure of the selected wind turbine is visually described in the following [10].


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Fig 1-8 Wind Turbine main parts.

1. Spinner 9. Brake disc2. Blade 10. Coupling3. Pitch 11. Generator4. Rotor hub 12. Yaw gear5. Main bearing 13. Tower6. Main shaft 14. Yaw ring7. Gearbox 15. Generator fan8. Service crane 16. Canopy

Technical Specifications of SWT-3.6-120

RotorType 3-bladed,horizontal axisPosition UpwindDiameter 120mSwept area 11,300

Nominal rotor speed 5-13 rpm

Power regulation Pitch regulation with variable speedRotor tilt 6 degrees

BladesType B58Blade length 58.5 m


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Root chord 4.2 mAerodynamic profile NACA63.xxx, FFAxxxMaterial GRESurface gloss Semi-matte,


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Chapter 2

Wind Turbines & Wind Farm Connection Analysis


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2.1 Introduction

Wind Turbines are connected in a cluster together called wind farms. These wind turbines are

connected in different orientations and connection schemes to produce electric power. Wind

farms may consist of few wind turbines to hundreds of wind turbines depending on the natureof wind farm, generation capacity and location offshore.

Generally, wind turbines are settled in perpendicular rows to the direction of the wind. Thespacing between the wind turbines may vary according to the type of wind turbines used.

Normally 2 to 4 rotor spaces are kept in arranging the wind turbines in different dimensions.

Wind turbines are also placed sometimes behind each other in a mechanism to support thewind directions as well. If the second wind turbine gets the strike of wind coming from the

first wind turbine then it is expected that the second wind turbine will have less powerproduced if composed to the power produced by unshielded wind turbine. [14]

Wind turbines might be placed at a far distance from each other and by this way they might

be able to produce more power but this will be at the expense of much more longer electrical

wires, area occupied and distances of travel between wind turbines. Therefore an optimumdistance and arrangement mechanism between wind turbines is maintained.

2.2 Wind Farm layout

There are many factors considered when the layout of wind farm is planned. The exact

placement of wind turbine is based upon size of space available and the effects of wind. In

our case of offshore wind farm, the distance to shore is also taken into account. Arrangementof lay out depends on no. of wind turbines and the power rating of each turbine. Each wind

farm is connected to electrical collector station; this may be one or more depending on the

wind farm sizeThis collector station is then connected to DC convertor station through AC power cables.

One DC convertor station may be connected to one or more collector stations depending onthe power capacity of wind farm and convertor station

Wind farms collect power from individual wind turbines and transmit it to shore. There aredifferent types of wind farms on basis of connections.


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Fig 2.0: Block Diagram of proposed project.

2.3 Connection schemes

Different wind farm schemes are in practice depending upon the requirements of locationgeographically, maximum power transfer and minimizing the losses. Different connectionscheme in practice are:

2.3.1 Radial Scheme

Radial scheme has the benefit of shorter cable schemes and the cable capacity away from hub

can be tapered. This results in decrease in cable cost.


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Fig 2.1: Radial Scheme Mechanism for offshore wind farms. [15]

2.3.2 Single-Sided Ring Collector with single hub

This arrangement is best suited in case of a fault in cable or switchgear in distance which is

quite near to hub. Its installation cost is quite high as it requires a cable with more length andof similar current rating throughout the length of string.

Figure 2.2: Single Sided Ring [15]


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2.3.3 Star Collector with Single Hub

Star Collector with single hub scheme has very high security of supply but provides choice

between both longer and shorter cable lengths with different current ratings. This scheme also

provides voltage regulation but requires expensive switch gear as it has to be placed in centreof star.

Figure 2.3: Star Collector with Single Hub [15]

For this project, scheme which has been followed is Radial scheme with circuit breakers witheach turbine to avoid any power loss in case of one turbine not working.

Fig 2.4: Layout of Wind Farm


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2.4 CABLES USED FOR OFFSHORE NETWORK

Turbines in wind farm normally have one km of cable for array arrangement with it. Its

actually length varies with the exact arrangement of WTs in the wind farm. The array cable

for connection between wind turbines is normally having a rating of 36 kV [16] but in somenew wind turbines having array cable of 66 kV rating [16]. These cables normally have

weight of 20 kg/m. These cables are also provided with cable stiffeners to make these cables

more resistant to harmful effects of water. In some places cable mats are also used where

cables cannot be buried inside the sea especially at the places where they are exposed towater.

2.5 Inter- Turbine Array Cables

The cables which are used to connect turbines with each other are called inter turbine array

cables. These cables also connect different arrays to each other in a way that each cable

provides a single link between two cables. Normally these cables are connected at end to a

switch gear so these inter-turbine cables are limited to 33 KV normally. These cables alsoconnect Wind turbines to offshore substation. The length of the cable depends on the windfarm design parameters and may vary from 500 meters to 3000 meters.

In general, the inter turbine array cables are 33 KV and three copper core conductors and

have steel wire armoured. There are different cable sizes available depending on the currentload which has to be carried.

Three types of inter array cables used in this project are:

18 MW, 33 KV, 89 mm diameter, 600 Amperes. 44 MW, 33 KV, 143 mm diameter, 1.4 Kilo Amperes 48 MW, 33 KV, 153 mm diameter, 1.5 Kilo Amperes

All the cables used are extruded polymer.

2.6 CALCULATIONS OF TOTAL NO. OF WIND TURBINES

Total no. of Wind Farms: 3

Max. Power output of each Wind farm: 500MWSTT-3.6-120 Wind Turbine Rated Power: 3600KW

No. of Wind turbines for one Wind farm: 500/3.6=138Total of Wind turbine: 415Rotor Diameter of Wind turbine: 120metersTotal distance between wind turbines= 5 times Rotor Diameter: 600 Meters.


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2.7 WIND TURBINES DISTRIBUTION

In offshore distribution of wind turbines depends on wind terrain and cables used for thenetwork. Here the proposed system includes four different types of cables depending on the

distance to and from collection point and substation. As radial scheme is used here in thisproject, it allows the usage of much shorter cables and cables can be tapered as we go awayfrom the hub or meet point of cables. Resultantly, the cables cost is considerably low. A hub

fault may result in considerable loss of power. Therefore circuit breakers are used with each

wind turbine to avoid loss of power for whole array if one of the turbines goes faulty. Fig2.5explains the arrangement of wind turbines without considering the geographical location.

Fig. 2.5 distribution of wind turbines in wind farm


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2.8 OFFSHORE COLLECTOR SUBSTATION

On shore substations can typically combine power of 500- 800 MW [16] of wind energy from

different wind turbines. Normally wind farms have one on shore substation but in some latest

wind farms have more than one substation to increased security of power system. Thesubstation platform is raised to approximately 25 meters above sea and generally has area of700-800m. [16]. Substation that is used in this system has capacity of 800 MW.


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Chapter 3

Offshore Wind Farm Electrical Connection to Grid


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3.1 Introduction

The connection of an offshore wind farm having large power production capacity to the

onshore grid is a challenge for scientists and engineers. The main purpose of the gird

connection planning ahead of the wind farm installation that the electrical system of windfarm operates in a safe and reliable manner during normal as well as during fault conditions.

The energy produced by an offshore wind turbine or wind farm has to be transferred to

onshore and connecting it to local grid. The connection strategy for onshore and offshore

wind farms are different, the onshore wind farm using AC for production as well as

transmission while in the case of offshore there are different options available which needs tobe synthesized according to the power capacity, distance from shore ,and reliability of the

wind farm. For offshore connection the use of AC technology is limited by the distance of

wind farm from coast line and the production capacity of the wind farm, DC has some

advantages over AC in terms of power rating, transmission length and the available DC

technology like voltage source convertor (VSC), line commutated converter (LCC) [17]. Forlarge wind farms having rated capacity of hundreds of MW and long distances from shore the

need for offshore substation for stepping up the voltage level or converting it to DC arises[18]. The purpose of this study is to analyze the different offshore transmission schemes,

there pros and cons and at the end suggest a feasible solution for doggar bank wind farm. A

diagram showing the connection scheme of a wind farm power output to public grid is shownin fig 3.1.

Fig 3.1: A typical offshore wind farm connection [18]

Wind power is an emerging renewable technology and the due to its unique nature offshore

wind farms are promising solution to the growing demand of renewable energy source. Alongwith the increase in the capacity of offshore wind farms and the distance between offshorewind farms and coastline, the high-voltage direct current (HVDC) is attractive technology.

There exist fundamentally two HVDC technologies: Conventional thyristor-based line

commutated converter (LCC) HVDC, which is a well developed and old technology, with the


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first application in 1954 in Gotland, and VSC-HVDC, which is a relatively new technology,which is under rapid development and available for low power production wind farms.

3.2 AC Connection of Offshore Wind Farm

AC connections are used when the wind farms are located near the coast line and the grid

connection point is also close to the coast line on shore side. [17] Suggest a cut off length forthe HVAC connection to the offshore wind farm to be between 50-100 km for underseacables and 400-700 km for overhead lines. AC connections to the grid may be single link or

multi link depending on the project budget and MW rating, although multilink is costly but

has advantages over single link. In the case of cable failure the multi link has the ability toswitch load between cables and prevent connection loss to the grid [19]. The major limiting

factor in the use of HVAC connections for offshore wind farm is the increase in line losseswith distance; this can be overcome by stepping up the voltage at the wind farm to a higher

level by using a step up transformer, but again this require an offshore substation setup which

is very expensive and having many technical limitations as well. In the case of HVAC the

distance is not the limiting factor due to decrease current but the properties of insulatingcables remains the problem. The dielectric insulation of HVAC cables acts as a capacitor,

when the AC voltage change direction during each cycle the electric dipoles need to berealigned. This realignment required current which produces heat and the end result is the

loss of active power in the transmission lines. To eliminate this effect the transmission needto be carried out at zero frequency which is equivalent to DC transmission [20]. Fig 3.2

shows the comparison of AC and DC transmission schemes in terms of cost and transmissiondistance [23].

Fig 3.2: A graph shows the comparison between AC and DC system.


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Fig 3.3: AC connection scheme.

3.3 DC Connection of Offshore Wind Farms:Due to the limitation of AC technology for longer distances DC cables can be used to transfer

power from offshore wind turbine to onshore grid, as the power generated by wind turbine isAC and the power connected to the grid is also AC therefore it has to be converted from AC

to DC at an offshore convertor and transformer station and then from DC to AC at the

onshore grid connection point. Figure 3.4 shows a typical DC link configuration [20]. Two

different DC technologies are currently in use for offshore wind farm power transmission.

Fig 3.4: Block diagram of DC connection scheme.

3.3.1 HVDC Line Commutated Converter (LCC):HVDC LCC is line commutation technology and uses thyristors as switching device with a

frequency in the range of 50 to 60 Hz [21]. The name of commutation originates from the fact

that the applied thyristors need an AC voltage support in order to commutate and thus onlycan transfer power between two active AC grids. There is no reactive power support available

so an auxiliary start up system is necessary at the offshore side. HVDC LCC has no

independent control of active and reactive power. It also needs large filters due to the largeamounts of harmonics production. To overcome these challenges different solution has been

proposed and implemented, one of these is the use of static compensator (STATCOM) [22].

STATCOM provides commutation voltages and reactive power support during steady state aswell as during dynamic transients to the HVDC convertor. STATCOM also provides a

limited active power support during transient condition. A schematic diagram of HVDC LCCbased transmission scheme with STATCOM is shown in the figure 3.5 [26].


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Fig 3.5: A typical HVDC LCC connection

HVDC LCC is a mature technology and provides high reliability, lower power losses and

requires very little maintenance but it require a large space for convertor station which is atechnical and well as economical challenge in the case of offshore wind farm located far

away from the coast line. There are some shipping limitations and the installation ofSTATCOM and other reactive power compensation equipment at offshore is also a challenge.

Examples of HVDC LCC projects [23-24]:

Project Name Company Year ofCommissioning

Power Rating

GOTLAND* ABB 1970 20 MW100 KV

Western HVDC Link** Siemens 2015 2200 MW600 KV DC

EstLink 2 Siemens 2014 670 MW450 KV DC

Back-to-Back Bangladesh Siemens 2013 500 MW158 KV DC

HIGHGATE ABB 1985 200 MW57 KV DC

NORTH-EAST AGRA** ABB 2015 6000 MW(4*2000)800 KV DC

Storeblt Siemes 2010 600 MW

400 KV DCNORNED ABB 2008 700 MW450 KV DC

Table 3.1: A few examples of HVDC LCC projects

*. The first HVDC LCC project **. Thelatest planned HVDC LCC project


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3.3.2 HVDC Voltage Source Convertor (VSC):HVDC VSC uses high power IGBT technology which has a switching frequency in the rangeof 1-2 KHz [25]. HVDC VSC has high flexibility and control over power transmission, low

harmonic, active and reactive power control at both ends but higher power losses. The active

power control capability of VSC can be used to regulate the grid frequency, while in the

absence of active power from the wind farm the reactive power can sustain the onshore gridvoltage. HVDC VSC is compact and the offshore platform size is smaller and less expensivethan HVDC line commutated convertor.

Voltage source convertor offers several advantages over line commutated convertor; the mainreason is the use of IGBT in the VSC technology which can be turned on/off using a gate

signal. Due to this feature VSC offers insensitivity to the strength of AC networks, black start

capability, and bi-directional control of active and reactive power flow. In the case of powerreversal VSC does not need to invert voltage polarity as in the case of LCC, which makes

extruded polymers compatible with VSC which offers the advantage of lower weight and cost

compared to the mass impregnated cables. Figure 3.6 shows a schematic diagram of a 300

MW, 150 KV HVDC VSC wind farm connection scheme [26].

Fig 3.6: HVDC VSC connection scheme

3.4 Multi-Terminal VSC HVDC:

Multiterminal HVDC voltage source convertors are composed of different level of convertorswhich can be connected to a common HVDC convertor. The wind turbine should be an

induction generator and it may be doubly fed induction generator (DFIG), squirrel cageinduction generator (SCIG) or full convertor generator (FCG).

There are different topologies used for Multiterminal HVDC VSC convertor system. In the

simplest topology the IGBTs are connected in series and it has the advantage of highblocking voltage for VSC. The multi level topologies are neutral point clamped (NPC) and

flying capacitor (FC) convertor. To achieve different number of voltage level the modular

multilevel convertor (MMC) are used , MMC is the latest class of Multiterminal HVDC VSCand it uses half bridge cascaded connections (sub module) [27].


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3.4.1 Modular Multilevel Convertor (MMC):

MMC is mainly intended for high voltage and high power application and is suggested in

[28]. MMC has the advantage of low harmonics, low switching losses and robustness as

compared to the conventional two level convertors. It consists of sub modules and offer theadvantage that when one sub module fails the system continue to work. The circuit

configuration of a three phase MMC is shown in figure 3.7. To obtained desired power and

voltage levels these sub modules can be connected in series, where the number of series

connected modules depends on the desired power and voltage ratings. Each sub module act as

half bridge rectifier and consist of a DC capacitor and two IGBTs. A bypass thyristor is usedto prevent the system from malfunction in case of one module failure.

Fig 3.7: Diagram of Modular Multi level Convertor

3.4.2 Black Start Capability:

Black start capability of a wind farm is a very important characteristic which become easy to

achieve through the use of HVDC VSC technology. When a fault arises in the offshore windfarm or the onshore grid the offshore convertor need to be disconnected from the onshore

grid. When the fault is removed there is a need to start the offshore wind farm again to

function, by using the HVDC VSC you dont need any supply at offshore for this process, it

can be achieved by energizing the onshore convertor from the AC grid. The DC capacitorsand cables are charged through the onshore convertor diodes, the DC voltage need to beregulated to its nominal value through the onshore VSC. When the DC voltage reaches its

nominal value the offshore convertor deblocked and starts ramping up the offshore AC

voltage. A small amount of power is required to cover up the losses in the offshore equipmenttherefore the onshore VSC acts as rectifier while the offshore VSC acts as invertors. The DC


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capacitors in the wind turbine power electronic convertor are charged and generators are

synchronised to the network through the controller. With the increase in power production

from the wind turbine the VSC changes the power flow and transfers power to the onshoregrid.

3.5 HVDC PLUS (Siemens):

After the introduction of multilevel modular convertor the manufacturers start developing this

technology and to convert this idea into real life products. Siemens, ABB, ALSTOM were the

leading companies which start manufacturing HVDC VSC based on MMC. Siemens name itas HVDC PLUS, which provides significant benefits for high voltage application including

independent control of active and reactive power, lower space requirements, and support for

weak or passive networks. It will find large applications in transmission and distribution

system in future due to its capability of self commutation. HVDC PLUS doesnt require any

driving system voltage and can build 3 phase AC network using the DC voltage. HVDC

PLUS has the ability to reduce the size of voltage steps and voltage gradient due to multilevel

conversion capability. It offers the advantage that the AC voltage can be selected in smallerstep size and the harmonics contents can be minimized. The high frequency noise, switching

losses and switching frequency of individual semiconductor can also be reduced using MMC.

Siemens secured the first order to supply HVDC PLUS in September 2007 for the submarinetransmission link in the Bay of San Francisco. This system is transmitting up to 400 MW at avoltage of 200 KV DC. A basic module of HVDC PLUS is shown in figure 3.8 [29].

Fig 3.8: Basic structure of HVDC PLUS [29]


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Examples of HVDC PLUS:

Project Name Capacity Year of Installation

Trans Bay USA 400 MW 2010

HelWin1 576 MW 2013

SylWin1 864 MW 2014

INELFE 2 x 1000 MW 2014

BorWIn2 800 MW 2015

3.6 HVDC LIGHT (ABB):

HVDC LIGHT is also an MMC developed by ABB after the introduction of multilevelconvertor. HVDC LIGHT includes a complete convertor station, converting high voltage ACto high voltage DC and on the other hand from DC to AC to comply with the local grid. The

most important characteristics of HVDC LIGHT is that it stabilize the AC voltages at the

terminal of the grid and it insured the grid code compliance. As it is still in developing phaseit cannot offer high capacity power transmission at the moment like HVDC LCC, but it is

expected that this technology will reach up to 1200 MW in near future [30]. HVDC light has

the advantage of low losses in the case of undersea DC cables. It also increases the security of

supply by controlling the power flow. HVDC LIGHT cannot be overloaded and in effect willnot contribute to the cascaded tripping. The following chart shows some of the HVDC

LIGHT projects completed or still in commissioning phase by ABB [24].


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Examples of HVDC LIGHT:

Project Name Power Rating Year of Installation

BorWin1 400 MW 2009

Dolwin1 800 MW 2013

Dolwin2 900 MW 2015

Nor Ned 700 MW 2008

3.7 DC Submarine Cables:

To transfer power from an offshore wind farm to onshore grid using HVDC VSC DC powercables can be used. There are two major types of HVDC cables, one is mass impregnated and


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the other is extruded polymer DC cables. Both has its own advantages and draw backsdepending on the HVDC technology, wind farm capacity and distance from shore line. Thereare many companies manufacturing DC submarine power cables but the major competitorsare ABB, Prysmian, Nexans, Sumitomo, and Fujikura. The following table shows availablemanufacturers of DC cables.

3.7.1 Mass Impregnated DC Cables:

A mass impregnated cable is shown in figure 3.9. Stranding copper layers segments are madearound a central circular rod. The copper conduction is fully covered by resin impregnated

papers. The inner and outer layer of an MI cables consist of carbon loaded papers and copper

fabrics respectively. To prevent the cable from permanent deformation a galvanized steel taps

are applied during cable loading. Over the steel tapes a polypropylene string is appliedfollowed by galvanized steel wire armour. This technology has the capability to transmit

power in the voltage ranges of up to 500 KV and 800 MW while the installation depth is

nearly 1000 m under sea level and the transmission length is ideally unlimited. The power

carrying capacity of MI cables is limited by the conductor temperature and low overloadcapabilities. MI cable has the disadvantage of high cost and high weight.


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Fig 3.9: Mass Impregnated cable

3.7.2 Extruded Polymer DC Cables:

The ability to reverse power flow without changing the voltage polarity allows VSC HVDCtransmission systems to use extruded cables which are lower in cost than the alternative massimpregnated cables. However, where extruded cables are used, the achievable transmissioncapacity may be limited by the ratings of the cable rather than the converter. More recently,as the interest in Voltage Source Converter technology has grown, DC cables have also beendeveloped that rely on extruded poly-ethylene as the insulation medium for the conductors.These cables are easier to manufacture and correspondingly cheaper than their MI equivalent,however currently can only operate at voltages up to 300kV which limits possible powerflow.The new generation of converters (VSC Voltage Source Converters) use IGBT (InsulatedGate Bipolar Transistors) which allow the power to be transmitted as it is in both directions

without requiring polarity reversal. This has allowed re-introducing the use of extrudedcables in DC power transmission as, with the polarity reversal being no longer required, theproblem of space charges that can arise with an extruded insulation and create excessivedielectric stress within the cable in the case of sudden polarity reversal does not exist anylonger.

Examples of Extruded Polymer DC Cables Projects:Project Name Rating Distance(Km) Diameter(mm2)

Trans Bay Cable 400MW

200KV

88 1100 Bipole

NordE.ON1 400MW

150KV

128 1600 Bipole

DolWin1 800MW 165


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320KV

DolWin2 900MW

320KV

45

BorWin1 400MW

150KV

125 1200 bipole


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Conclusion

This project addressed maximum issues related to offshore wind farm and

related parameters. It undertakes all aspects of wind turbines selection keeping

maintenance and operation in mind. In this report issues of wind farm

orientation types along with connection schemes were also discussed. Thetransfer of power to on shore from offshore is also highlighted with the best

possible schemes prevailing the wind power technology. This report vividly

helps to nurture conceptual knowledge based on review of literature and

drawing outcomes from it. It is extracted from this project that offshore wind

farms have more predictable and regular output and are able to produce power

on regular patterns. This project helped us to understand the offshore wind farm

analysis and its operation parameters in detail.


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[13] Shixiong Fan "Current Source DC/DC Converter Based Multi-terminal DC WindEnergy Conversion System"(Ph.D. Dissertation, University of Strathclyde, April 2012 ).

[14] Wind Energy Systems by Gary l. Johnson.

[15] Lecture Slides Dr. Olimpo Anayalara

[16] Josef Schachner , Power Connections for offshore wind Farms.

[17] Hennann Koch, "Connecting Large Offshore Wind Farms to the Transmission

Network," IEEE Dietmar Retzmann, 2010 T and D Conference, New Orleans.

[18] R. Gasch, J. Twele, "Wind Power Plants: Fundamentals, Design and Operation", Solar

praxis AG, Germany, 2001.

[19] Gardner, Craig, Smith, "Electrical Systems for Offshore Wind Farms", Garrad Hassan

and Partners Glasgow UK June 2003.

[20] Josef Schachner, "Power connections for o_shore wind farms,"Diploma Thesis January

2004, Department of Electrical Engineering, University of Leoben, Austria.

[21] Cartwright P, Xu L, Sasse C. "Grid integration of large offshore wind farms using hybrid

HVDC transmission",In Proceedings of the Nordic wind power conference; 2004.

[22] Stephan Meier, Novel Voltage Source Converter based HVDC Transmission System

for Offshore Wind Farms, Royal Institute of Technology Department of Electrical

Engineering Electrical Machines and Power Electronics, Stockholm 2005

[23]http://www.energy.siemens.com/mx/en/power-transmission/hvdc/hvdc-

classic/references.htm#content=2010%20Storeb%C3%A6lt%2C%20Denmark.

[24]http://www05.abb.com/global/scot/scot221.nsf/veritydisplay/35687be99f61ae42c1257b2

7004315ef/$file/POW0013.pdf

[25] Meier S. "Novel voltage source converter based HVDC transmission system for offshore

wind farms".Doctoral Thesis. Stockholm, Sweden: Royal Institute of Technology; 2005.

[26] Lie Xu, S. Mathew and G. S. Philip, Grid Integration of Offshore Wind Farms, DOI:10.1007/978-3-540-88258-9_7.

[27] Xiguo Gong, A 3.3kV IGBT module and application in Modular Multilevel converterfor HVDC, Semiconductor Division Mitsubishi Electric & Electronics (Shanghai)Shanghai,China.


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[28] R. Marquardt and A.Lesnicar, A new modular voltage source inverter topology,, 3rded.,in Conf.Rec.EPE,2003.[29] M. Davies, M. Dommaschk, J. Dorn, J. Lang, D. Retzmann, and D. Soerangr, HVDCPLUS- Basics and Principle of Operation, technical article.

[30] Alf Persson, Lennart Carlsson, Mikael berg, NEW TECHNOLOGIES IN HVDC

CONVERTER DESIGNABB Power Systems, Sweden.

EE 986 Modelling

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