Tidal Energy Systems: Design Guidance on

Grid Connection Configuration and

Infrastructure for Multiple Device Types

VERSION: 1.0 DATE: SEPTEMBER 2015

Author Ben Whitby (IT Power)

2

This project has been co-funded by ERDF under the INTERREG IVB NWE programme. The report reflects the author’s views and the Programme Authorities are not liable for any use that may be made of the information contained therein.

Technical Report: Tidal Energy Systems: Design guidance on grid

connection configuration and infrastructure for multiple device types

September 2015

Contractor: IT Power

IT Power

St. Brandon’s House

29 Great George Street

Bristol, BS1 5QT, UK

Tel: +44 117 214 0510

Fax: +44 117 214 0511

E-mail: itpower@itpower.co.uk

Website: www.itpower.co.uk

Document control

File path & name I:\Data\0WorkITP\0Projects\1191 PTEC Phase 1

Delivery FEED & EIA\2 Work

Author B. Whitby (IT Power)

Project Manager J. Hussey (IT Power)

Approved J. Hussey (IT Power)

Date September 2015

Distribution level -

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CONTENTS

Chapter 1 Introduction ............................................................................................ 4

Chapter 2 Grid Connecting Tidal Turbines .................................................................. 5

Electrical drivetrain options ................................................................................... 5

Asynchronous (Induction) Generator ................................................................... 6

Doubly-Fed Induction Generator ......................................................................... 7

Generator with full power converter interface ....................................................... 8

Voltage levels .................................................................................................... 10

Chapter 3 Design of the Electrical netwrok architecture ............................................. 13

Radial String Connection ..................................................................................... 13

Offshore hubs and platforms ............................................................................... 16

Topologies for Larger Arrays ................................................................................ 20

DC transmission connection ................................................................................ 22

Chapter 4 Conclusions ........................................................................................... 27

Chapter 5 References ............................................................................................ 28

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CHAPTER 1 INTRODUCTION

The tidal energy industry has seen little convergence on a particular electrical design and

developers use a variety of different design philosophies. This report provides guidance on

the design options for the electrical infrastructure required to connect arrays of tidal

energy devices to the grid. The report initially looks at the electrical options at the turbine

level in Section Error! Reference source not found., which includes the drive train and

power take-off options. The electrical architecture options for grid connecting the array

system, including inter-array and transmission connection options, is discussed in Section

Error! Reference source not found.. The feasibility of designing an electrical

infrastructure flexible enough to accommodate multiple device types with non-

standardised electrical outputs is also investigated.

5

CHAPTER 2 GRID CONNECTING TIDAL TURBINES

There are a number of different options to consider when connecting an array of tidal

turbines to the grid. This section covers the electrical drivetrain selection considerations

and the voltage selection at the turbine level. The options discussed within this section

are:

Electrical drivetrain options for the generator and convertor package distinguished

by rotational speed, power regulation and type of generator. The type of generators

available (synchronous or asynchronous) are discussed.

Voltage selection including generator voltage and transmission voltage, identifying

the options available for the location (i.e. onshore or offshore) and type of electrical

infrastructure (transformers, converters/rectifiers, power conditioning equipment

etc.).

ELECTRICAL DRIVETRAIN OPTIONS The electrical drivetrain – generator converter package - is an integral part of any tidal

turbine and is responsible for converting the kinetic energy into electric power for transfer

into the grid. The types of generator used by tidal turbine developers can be divided into

two groups:

Synchronous generators, where the rotor and magnetic field rotate with the same

speed, and are either permanently or separately excited.

Asynchronous (induction) generators which are either singly or doubly-fed.

These generators can be used in a number of configurations. Induction generators directly

coupled to the grid offer a simple low cost solution but have a number of disadvantages

and are unlikely to be widely adopted by developers. The remaining configurations, which

could be used by developers, can be summarised as follows.

Doubly-fed induction generators using partially rated converters.

Synchronous generators (either permanently or separately excited) use full power

converter interfaces.

These configurations are discussed in more detail in the section that follows.

6

ASYNCHRONOUS (INDUCTION) GENERATOR

The use of induction generators (either singly or doubly-fed) in conjunction with a

gearbox is commonly proposed. The squirrel cage induction generator (SCIG) is popular

due to its cheap and robust design; nothing more than an induction motor driven above

its synchronous speed. In its simplest configuration, the stator windings are directly

connected to the grid, and a high speed low torque SCIG with a gearbox inserted between

the shaft of the turbine rotor and the shaft of the generator is used (Error! Reference

source not found.). This configuration is considered to be fixed speed; although, the

speed will vary slightly (due to the slip) as the operating power of the rotor fluctuates [1].

The gearbox is necessary to convert the low rotational speed of the turbine rotor to a

compatible frequency of the available network, which would be 1500 rotations per minute

(RPM) for a four pole machine connected to the European 50Hz network. The turbine and

gearbox ratio has to match the available generator speeds, which are for standard

induction generators 1500RPM, 1000RPM and 750RPM (4, 6 or 8 pole) [2].

An uncompensated Induction generator at rated output will normally have a power factor

in the region of 0.89 leading (i.e. consuming reactive power). For a 1 MW turbine this

implies a reactive power draw of 512Kvar. However, typically in such a system the

reactive power is compensated using shunt capacitor banks, as shown in Error!

Reference source not found., so that the power factor is closer to 1. Typically, a

compensated system will operate at 0.98 leading which implies a reactive power draw of

85Kvar.

Figure 2.1: Squirrel cage induction genrator in fixed speed configuration

7

The disadvantage of this configuration is the fixed speed operation; which results in less

than optimal power production because the rotor speed cannot be adapted to the flow

speed (see Error! Reference source not found.). The fact that the generator is directly

coupled to the grid also means that any fluctuations in rotor torque are translated directly

into output power variations which adversely affects the power quality. Furthermore, one

has no control over the reactive power flow with this topology. The lack of control makes

it difficult to comply with relevant grid codes.

Figure 2.2: Turbine power curves comparing fixed speed(black line) and variable speed

(grey line) generators

DOUBLY-FED INDUCTION GENERATOR

An improvement on this configuration, which allows a degree of variable speed operation

to be achieved, is to use a wound rotor induction generator in conjunction with a partially

rated power converter (Error! Reference source not found.). This is known as a

doubly-fed induction generator (DFIG) and is widely used in the wind industry. During

under-speed (sub-synchronous operation) the converter will borrow power from the

line, which it passes on to the stator. When the rotor over-speeds (super-synchronous

operation) the converter absorbs power from the rotor and feeds it to the power line.

In this way the rotor speed can be varied to optimise the hydrodynamic efficiency. With

this configuration it is also possible to independently control the flow of active and

8

reactive power which makes the system more flexible and able to comply with relevant

grid codes.

The power flowing through the converter is proportional to the speed variation e.g. for

a +/-30% speed variation, the rated power of the converter is only about 30% of the

rated power of the generator [3]). Therefore, the converter is cheaper and causes less

loss than systems employing fully rated power converters. However, the generator is

more expensive and maintenance requirements are high due to the use of slip rings. For

these reasons, although it has been widely adopted in the wind industry, tidal

developers are unlikely to adopt the DFIG. The increased complexity of the generator

and control, as well as the need to use a multistage gearbox means that it is unlikely

to be considered robust enough for use in a marine environment.

Figure 2.3: Doubly-Fed induction generator configuration

GENERATOR WITH FULL POWER CONVERTER INTERFACE

The alternative is to use a full-power converter in conjunction with either an induction

generator or a synchronous generator. This configuration allows for full variable speed

operation and one has full independent control of the active and reactive power flow.

9

Figure 2.4: Full-power converter configuration

Although it is possible to use either an induction generator or a synchronous generator, in

this scenario it is usually preferable to use a permanently excited synchronous generator

(PMSG). However, both types of generator are used. The PMSG is considered superior in

this scenario for various reasons. In comparison to induction generators a PMSG will

operate at a higher power factor and achieve a higher power density. Furthermore, due to

the way in which generators scale with power level it is easier to manufacture efficient

high power PMSGs than induction generators. This is demonstrated using the

relationship between generator power output and generator size as given by reference

[2]:

Equation 2-1

where is the generator rotor diameter, is the rotor length, is the rotational

speed and is a scaling constant. To achieve a given power rating a compromise

must be made between increasing the diameter of the generator and increasing the

length. The square relationship means it is desirable to increase the diameter.

However, an induction generator supplies the excitation field to the rotor via the

stator winding [4]; therefore, to keep copper losses in the stator coils down and

maintain a high power factor it is necessary to have a small air gap. Manufacturing

large diameter induction machines with small uniform air gaps is hard to achieve. For

these reasons synchronous generators (either permanently excited or separately

excited) are preferable, particularly in direct-drive applications where generators with

a large number of poles are required. This is because larger air gaps can be used and

the stator coils only have to carry active power produced in the conversion, which

reduces copper losses in the stator.

10

Separately excited synchronous generators can also be used. The main advantage of

this configuration over the permanently excited machine is that one has the ability to

control the excitation current and therefore the no load voltage can be controlled. The

disadvantage is the high complexity of this type of machine for excitation and voltage

control and the use of brushes and slip rings which will increase cost and

maintenance. In the wind industry these were initially favoured over PMSGs due to

the high cost and limited availability of the materials needed for permanent magnets;

however, availability is no longer a significant issue and prices have dropped over the

last ten years.

Based on this review, it can be argued that the most attractive solution for tidal

turbine developers is the PMSG with full-power converter interface. However, it is

acknowledged that some developers may also choose to use an induction generator in

combination with a full-power converter. Fixed speed induction generators and DFIG

configurations are highly unlikely to be used in large numbers by tidal developers;

therefore, for the purposes of this report these topologies will not be considered when

designing the electrical network.

VOLTAGE LEVELS

Another differentiator between tidal devices is the voltage level at which the devices

operate. In the wind industry low voltage generators ranging from 690V to 1000V are

very common. This voltage is usually stepped–up using a transformer located in the

base of the turbine tower. More recently medium voltage generators, ranging between

3kV and 6.6kV, are starting to be used on larger multi-megawatt wind turbines. This

is because the higher voltages lead to lower currents, reducing resistive losses and

enabling the use of smaller diameter cables within the turbine [5].

In the tidal sector there has been little convergence and device developers are

generating at a range of voltages from 0.4kV to 6.6kV. This is further complicated by

the fact that some developers have on-board transformers which step up the voltage

whereas others simply transmit to shore at a variable AC voltage direct from the

generator. Others have proposed using a DC transmission option. Developers such as

Tidal Generation Ltd [6] and Marine Current Turbines [7] have on-board transformers

which step up the voltage from the low voltage generators to 6.6kV and 11kV

respectively. Both these devices also have on-board power converters. Therefore,

these devices are configured exactly as shown in Error! Reference source not

found. and are very similar to modern wind turbines. Both these companies also

state that they would be capable of generating higher voltages such as 33kV,

11

presumably by simply changing the transformer or altering the tapping ratio. The

developer Tidal Energy Ltd [8] is believed to be using a medium voltage 6.6kV

generator; however, there is not believed to be a transformer or additional conversion

equipment on board. The turbine simply transmits variable AC to shore at which point

it connects to a power converter and transformer as shown in Error! Reference

source not found..

Figure 2.5: Turbine with medium voltage generator and all power conversion equipemnt

onshore

Another option is to split the power converter so that the rectifier is located offshore,

either on-board the turbine or in a separate hub, and the inverter is located in an onshore

converter station. The power is transmitted to the shore-side converter via a medium

voltage DC link as shown in Error! Reference source not found.. The necessary DC

voltage level will be determined by the length of the link and will also depend on the AC

voltage from the generator [9].

12

Figure 2.6: Split Converter topology using DC transmission link

The range of configurations being adopted by device developers makes it difficult to

design a standardised electrical infrastructure able to cater to all current device types.

However, this report will aim to propose systems flexible enough to accommodate the

range of configurations detailed in Error! Reference source not found., Error!

Reference source not found. and Error! Reference source not found..

13

CHAPTER 3 DESIGN OF THE ELECTRICAL NETWROK ARCHITECTURE There are a number of options when considering connecting an array of multiple tidal

turbines to the grid. Within this chapter the following design options are reviewed:

Individual radial strings to shore for devices operating at the same voltage or

different voltage levels.

Offshore collection hubs for the use of grid connecting tidal turbine arrays consisting of devices operating at the same or different voltage levels. The options

for onshore and offshore transformers are discussed for the hub layout.

The considerations for hub connections with dry mate or wet mate connectors.

Subsea hubs versus surface-piercing offshore platforms.

Options for larger turbine arrays considering a multiple hub layout.

DC transmission connections (split converter topology).

RADIAL STRING CONNECTION

For full-power converter devices, like that shown in Error! Reference source not

found., the connection can be done in a very similar way to a conventional offshore wind

farm; whereby, individual devices are connected in radial strings as shown in Error!

Reference source not found.. Switchgear devices enable individual turbines to be

isolated.

14

15

Figure 3.1: Turbines connected in radial strings with individual AC cables to shore: (a)

onshore substation contains a couple of two-winding transformers to accommodate

multiple voltage levels, (b) three winding transformer is used in the onshore substation

to accommodate multiple voltage levels.

In Error! Reference source not found., a scenario where two voltage levels are used is

shown. In this scenario separate two winding transformers can be used as shown in

Error! Reference source not found.(a) or alternatively a single three winding

transformer can be used as shown in Error! Reference source not found.(b). The

advantage of the latter option is that a three winding transformer will have a smaller

footprint than two separate transformers. It also means that only one set of switchgear

needs to be used on the high voltage side. The disadvantage of the three winding

transformer are the higher power losses which is significant when one considers that the

lifetime of the plant could be between 20-30 years. The three winding transformer is also

heavier, as a single component, than separate two winding transformers which may have

implications on the installation.

This is a flexible solution as it allows multiple device types operating at different voltage

levels to be used. In theory each radial string could be operated at a separate voltage

level. The use of multiple cables to shore also provides a high level of redundancy. This

system could be used with devices that do not have an on-board transformer. However,

for devices using low voltage (<1kV) generators the transmission distance to shore would

be limited. Devices using medium voltage generators (3kV-6.6kV) would be able to

16

achieve longer transmission distances. The voltage level of each string will also affect the

number of devices that can be connected in a given string. Higher voltages will reduce

current flow in the radial cable and increase the number of devices that can be connected.

The high number of cables is a disadvantage of this system. Furthermore, in order to

accommodate multiple voltage levels the onshore substation needs to be equipped with

multiple transformers and switchgear devices to accommodate each voltage level. This

will increase the cost and footprint of the substation.

Another issue with the radial string design is how to loop the cable in and out of each

turbine in order to create the string. This could be achieved using a double set of

connectors but this will add cost. Furthermore most tidal developers seem to have

designed their devices to accommodate just one cable connection due to space

constraints. A solution could be to use a three-way (or T-connector). However, the

commercial availability of such connectors is not known and this is an area that may

require development before such systems can be realised in reality.

Figure 3.2: Four turbine radial string with three-way connectors

OFFSHORE HUBS AND PLATFORMS

The electrical topologies proposed above are based on the use of radial array networks.

Other solutions which fall into the category of hubs utilising star cluster type network

configurations offer an alternative solution. Offshore collection hubs are useful for

reducing the number of cable runs to shore. As illustrated in Error! Reference source

not found., to accommodate multiple voltage levels, multiple cable runs to shore would

still be required; however, if several radial strings operate at the same voltage level then

the number of cables to shore will be reduced.

17

Figure 3.3: AC radial design with offshore collection Hub

Error! Reference source not found. shows an example of a farm consisting of four

radial strings where each pair of two operates at the same voltage level. The number of

cables to shore is halved. In this design the offshore hub is simply a bus-bar and contains

no transformer or power electronics. For these reasons it would be relatively

straightforward to design this as a subsea unit for deployment on the seabed. A similar

subsea collection hub has already been used in the industry at the Wave Hub test site

[10]. The bus-bar would probably be oil filled and pressure compensated to avoid a

pressure gradient forming that would force seawater into the hub. It is also likely that a

communication link to the turbines will be required. This will usually be in the form of a

fibre link bundled within the power cables. A separate connector into the hub for the fibre

link may be required.

Connection from the individual strings to the hub could be made using either wet mate or

dry mate connectors. The wet mate connectors allow for a quick connection philosophy

whereby the subsea cable can be plugged into sockets or receptacles (with pin contacts)

on the hub. This can be done without having to raise the entire hub to the surface which

would be the case if dry mate connectors were used. Furthermore, to allow the hub to be

lifted a cable service loop that is a minimum of twice the water depth would be required.

This excess cable would have to be stored and protected on the seabed during periods of

operation. Due to the high currents at tidal sites concrete mattresses or rock dumping

would be required to prevent cable movement. This protection would have to be removed

prior to recovery and reinstated after the device is installed increasing the logistic

requirements and cost. The disadvantage of wet mate connectors is their higher cost.

They are also currently limited to relatively low voltages (≈30kV) and do not have the

power handling capability of dry mate connectors.

18

The design shown in Error! Reference source not found. could be altered by moving

the transformers offshore as shown in Error! Reference source not found.. This

reduces the number of cable runs to shore. The decision to move the transformers

offshore would primarily depend on the power rating of the array and the distance to the

onshore point of common coupling. Increasing the transmission voltage to shore will

reduce the resistive losses in the cable which will, to a certain extent, offset the extra cost

of placing transformers offshore. However, at very long distances (>50km [11]) reactive

power losses will dominate and the cable will need to be compensated. Compensation can

be done only on the onshore side, but is most effective when done at both ends of the

cable [12]. In this design the offshore hub would contain the following components:

Step-up transformers

Circuit breakers

Offline switches to isolate incoming lines on each bus bar

Wet mate connectors for connection of turbines

Reactive compensation equipment (dependent on cable length and voltage level)

Figure 3.4: AC radial design with offshore substation

19

In Error! Reference source not found. the hub contains multiple transformers. This is

to accommodate multiple voltage levels within the array. As explained previously the use

of two separate transformers could be replaced by a single three winding transformer. In

either case the size and mass of the equipment may mean that it cannot sensibly be

housed in a single subsea hub. It may be sensible to adopt the approach shown in Error!

Reference source not found. whereby external subsea transformers are used to step

up the voltage from certain strings before connection to the main hub. Error! Reference

source not found. shows an example where strings 1 and 2 operate at a lower voltage

than strings 3 and 4. Subsea transformers designed as standalone systems are now

available from a number of vendors ( [13], [14]), having been primarily developed for the

oil and gas market. These transformers can be fully submerged at depth and do not need

to be housed in a barometric chamber.

An alternative to the subsea approaches would be to use a fixed platform. Platforms are

better suited to coping with complex cable landings (making it easier to incorporate large

numbers of turbines), and with large volumes of equipment weighing significant amounts.

Platforms would also provide for more space and additional services. For prototype

farms, using new power collection concepts, deploying equipment above the surface

would provide easier access for maintenance and monitoring. Furthermore, electrical

equipment for use on platforms is more widely available and has been tried and tested.

Once the power system has been proven and understood, the riskier step of repackaging

into a subsea module could potentially be undertaken.

The disadvantage of platforms is the increased visual and environmental impact that they

may have. Particularly as most early tidal array developments are likely to be relatively

close to shore at distances between 5 and 20km. The cost of the platform will be a big

factor and is one that is hard to quantify as it will be influenced by the strength of the

tidal flow and the specific conditions at each site. A platform is likely to become an option

for larger farms (>50MW) further from shore where the size of the equipment and the

number of cables make it a more viable solution. However, as stated already, they may

also be viable in smaller prototype farms where proof of concept and reliability are

considered more important than other factors.

20

Figure 3.5: AC radial design with offshore substation and standalone subsea

transformers catering for multiple voltage levels

TOPOLOGIES FOR LARGER ARRAYS

For larger arrays a solution where multiple hubs are used can be implemented. The

proposed design in Error! Reference source not found. shows multiple hubs connected

in a ring design. Connecting the hubs in this way provides redundancy should a cable fault

or component failure occur with the collection network. How the turbines are arranged

and connected to the hubs will largely depend on the size of the farm and the number of

turbines being connected. The turbines could be connected in a star arrangement as

illustrated in Error! Reference source not found.. The number of turbines that can be

connected to each hub will be limited by the power handling ability of the connectors. It

will also be affected by the available space around the hub for routing cables, the seafloor

topography and the size of the cables being used.

A way to increase the number of turbines without increasing the number of terminations

on the hub would be to daisy chain several turbines together in a radial string as

illustrated in the previously introduced connection examples. However, doing this would

21

increase the size of the cables needed and one would still ultimately be limited by the

power handling capabilities of the connectors going into the hub.

Figure 3.6: Multi-hub collection network

22

Figure 3.7: Turbines connected to hub in a star configuration

DC TRANSMISSION CONNECTION

This type of connection was introduced in Error! Reference source not found. and is

often referred to as the split converter topology. The rectifier is located offshore either on

board the turbine or in a separate hub, and the inverter is onshore. This topology

minimises the amount of equipment located offshore; however, the transmission distance

is limited by the current carrying ability of the DC cable which in turn is affected by the

chosen voltage level. Selecting a high DC voltage level will reduce the current flow but will

increase the size and cost of the converters. The minimum DC voltage depends on the AC

input voltage from the turbines given by [15]:

Equation 3-1

is the DC link voltage, is the line to line AC input voltage and is the modulation

ratio of the converter.

The design of the inter-array network can be done in a similar way to the AC designs

already discussed where individual devices are connected in radial strings or in a star

configuration. Error! Reference source not found. shows a scenario where multiple

devices are connected in parallel to a hub in a star configuration.

23

Figure 3.8: Split converter topology with turbines connected in star configuration using

DC transmission link to shore

This system could be expanded to accommodate larger farms in a ring topology as shown

in Error! Reference source not found.. DC switchgear devices would be required to

maintain the integrity of the ring.

24

Figure 3.9:DC RING SYSTEM

An alternative to the system of Error! Reference source not found., which could also

accommodate larger farms, is shown in Error! Reference source not found.. In this

system two collection hubs are combined to form a bipolar DC transmission system. One

of the collection hubs produces negative valued DC output while the other produces

positive valued DC output. This leads to a bipolar DC transmission system where one pole

is positive with respect to earth and the other is negative. Under normal operation

current will flow in a loop and there will be no current flow through the ground return.

Crucially each pole is able to function independently as a monopolar link, by using the

earth as a return path. If the return path is designed with suitable capacity to carry the

load of both poles then power transmission can continue should one of the poles be out of

service due to a fault or maintenance work. This provides useful redundancy and is the

main advantage of this topology.

25

Figure 3.10: Bipolar DC transmission topology

As previously stated, it would also be possible to design a DC connection system using

radial strings, which would look very similar to the AC topology presented in Error!

Reference source not found.. With the radial design the turbines would be connected in

parallel to a common DC voltage bus forming a string that is then connected either to an

onshore substation or an offshore collection hub. Error! Reference source not found.

shows a scenario where the devices are connected directly to an onshore substation. The

current flowing in the cable depends on the location in the string. The end of the cable

closest to the hub/substation will experience the maximum load current; therefore, with

this type of connection it is possible to taper the cables which may significantly reduce

cost. Each turbine can also be isolated from the string by using switchgear components as

shown in Error! Reference source not found.. The main advantage of the system is

that it removes the need for an offshore hub.

The topology in Error! Reference source not found. uses DC aggregation, whereby

each feeder is connected to a common DC bus in the substation and then routed through

a single inverter for conversion to AC. The advantage is that only a single inverter is

required. A potential disadvantage is the need to use DC switchgear components to

isolate faulty strings from the DC bus. DC switchgear components able to deal with

problems such as no zero crossing of the current are now available; however, the cost

and availability (particularly at higher power levels) may be an issue. The alternative is to

use AC aggregation in the substation as shown in Error! Reference source not found..

26

Individual inverters are used for each string and the AC switchgear can be used to

disconnect a faulty feeder.

Figure 3.11: Parallel connection of turbines to form DC network with DC aggregation of

strings at the onshore substation

27

Figure 3.12: Parallel connection of wind turbines to form DC network with AC

aggregation of strings at the offshore substation

CHAPTER 4 CONCLUSIONS

It is difficult to recommend a particular electrical network from those introduced as the

most suitable network will very much depend on influencing factors; particularly, the type

of devices used, size of the array and distance from shore. Accommodating wide variance

in machine types and voltage levels is impossible to do while still maintaining a

standardised electrical collection network. However, different voltage levels can be

accommodated albeit at an extra cost.

The radial network configurations introduced in Error! Reference source not found. will

likely be the lowest cost options and offer flexibility in terms of voltage levels as each

string can be operated at a different voltage level. The use of subsea hubs has drawbacks

and presents a number of challenges. There are practical difficulties with connecting

multiple cables to a submarine hub. The operation requires the use of expensive mate-

able connectors and potentially the use of ROVs and divers. There are also limitations on

the number of devices that can practically be connected to a hub. Finally, the construction

and installation costs of submarine hubs is likely to be high and there is little experience

in this area apart from that gained by the oil and gas industry. Despite this hubs will still

be a viable solution in situations where arrays are deployed far from shore and stepping

up the transmission voltage offshore is necessary. It can also be justified in situations

where a large number of devices are deployed and the use of a hub will significantly

28

reduce the number of export cables to shore, such as in the topology shown in Error!

Reference source not found.. In this scenario the cost saving by reducing the cabling

will offset the price of the hub.

The DC solution proposed has the potential to be one of the lowest cost solutions with

lower electrical losses than many of the proposed AC solutions. The DC solutions proposed

in Error! Reference source not found. and Error! Reference source not found.

minimise the offshore equipment and are particularly suitable in situations where placing

full power converters within devices or in separate offshore hubs is not feasible. A study

conducted by ABB [16] into electrical infrastructure for connecting commercial tidal

power arrays of between 30MW-200MW in size concluded that a DC collection network

was their preferred solution as it offered the lowest component count of subsea elements

and the lowest electrical losses. There is also potential to reduce the cost of DC topologies

further by allowing multiple devices to share common converters on the generator side.

There is much research being done in this area [17] [18]. However, the control aspects of

such topologies are onerous and the sharing of converters will likely result in a reduction

in energy yield from an array as individual devices will not be able to operate at optimal

efficiency.

CHAPTER 5 REFERENCES

[1] O. Anaya Lara, N. Jenkins, J. Ekanayake, P. Cartwright en M. Hughes, Wind Energy

Generator Modelling and Control, Chichester: John Wiley &Sons Ltd , 2009.

[2] T. Burton, D. Sharpe, N. Jenkins en E. Bossanyi, Wind Energy Handbook, Chichester :

John Wiley & Sons , 2001.

[3] E. Muljadi, M. Singh en V. Gevorgian, „Doubly Fed Induction Generator in an Offshore

Wind Power Plant Operated at Rated V/Hz,” in IEEE Energy Conversion Congress and

Exhibition, Raleigh, 2012.

[4] P. Pillay en K. R., „Modelling, Simulation, and Analysis of Permanent Magnet Motor

Drives, Part 1: The Permanent Magnet Synchronous Motor Drive,” IEEE Transactions

on Industry Applications, vol. 25, nr. 2, pp. 265-273, 1989.

[5] ABB , „ABB generators and converters help AREVA wind turbines optimise energy yield

at alpha ventus,” [Online]. Available:

29

https://library.e.abb.com/public/e6ede065616409e6c1257d89002a6875/Case%20not

e_alpha%20ventus_lowres.pdf. [Geopend 6th August 2015].

[6] ALSTOM, „Tidal Generation Ltd,” [Online]. Available:

http://www.alstom.com/products-services/product-catalogue/power-

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