With wind power engineerings deriving rapid impulse particularly offshore wind farm engineering, assorted issues affecting internal and external electrical connexion of offshore air current turbines to the grid arise. This literature reappraisal presents a survey of past researches refering offshore wind farm engineering and high spots wind farm connexion issues. Section 1 presents the debut of the subject followed by an overview of offshore air current farms in Section 2. Following, types of generators used are discussed in Section 3. Section 4 and 5 nowadayss the nucleus of the research, detailing transmittal and interconnectedness of offshore air current farms utilizing both AC and DC severally. Finally the last subdivision concludes the reappraisal.
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Wind power coevals has been on the rise as of late [ 1 ] . USA late took over Germany as the current universe 's leader of air current power capacity [ 2 ] , using air current energy as an option of electricity coevals. European states are preponderantly inclined towards wind power in add-on to Asiatic states such as China and India, which are amongst the top air current power manufacturers [ 2 ] .
There is a bound to onshore air current farm connexion nevertheless, due to set down bounds and with the huge potency of air current harvest home from the seas and less obstructor compared to onshore air currents [ 3 ] , it is merely evident that the following logical measure is to travel into offshore wind farms. Furthermore, with changeless power produced from the changeless air current velocity of the sea [ 1 ] , and the possible combination of moving ridge and tidal power [ 4 ] , chances of offshore air current farms are promoting.
Installations of offshore air current farms are non fiddling, as many factors and concerns are taken into history over the substructure and layout. The unsmooth hostile environment of the sea gives rather a challenge for installing and transmittal to the grid and it is a critical concern. Overall, there are several electrical challenges sing offshore air current farms in contrast to the mechanical challenges that has well reduced over the past decennary [ 1 ] .
A major challenge sing offshore air current farms would be its transmittal from generator to the grid [ 5 ] . The conventional AC transmittals provide restrictions as offshore air current farms move farther off from the shore. These restrictions will be studied and possible solutions are to be recommended.
This literature reappraisal presents an overview of grid connexion of offshore air current turbines, diging into types of generators used, every bit good as air current farm interconnectedness, transmittals of power to onshore grid and reappraisals of jobs related.
General Overview of Offshore Wind Farms
Wind farms are considered offshore if they are at least 30km from the seashore [ 6 ] . Presently, the biggest offshore air current farms has a capacity of up to 200MW [ 7 ] , with each turbines bring forthing 2-3MW. Offshore air current farms are dearly-won ( 30-60 % higher [ 4 ] ) than onshore due to the larger, higher rated power of turbines, undersea connexions and overall cost of installing and care [ 3 ] .
Harmonizing to Reference [ 3 ] , air current farms have electrical subsystems which consist of:
- Internal medium-voltage ( MV ) grid for aggregation of power produced by single generators ;
- Offshore electrical substructure for transit of power to the shore ;
- Onshore system which link the farm to HV webs.
The power produced by each air current turbine is collected via the internal MV grids utilizing either radial or pealing connected design [ 3 ] . Radial design, being the cheapest pick, uses a individual overseas telegram feeder that is connected to the turbines in contrast to the ring connected web where cabling and exchanging output higher costs. Radial design suffer from low dependability, though it can be improved by forking the radial feeders [ 3 ] .
Depending on air current farm size and distance from shore, every bit good as environmental restriction, [ 3 ] nowadayss two strategies of electrical substructures for transporting power to the shore. Scheme 1 utilizations an seaward substation incorporating switchgears and transformers [ 8 ] , which step up electromotive forces and transmit it utilizing HV overseas telegrams, while Scheme 2 utilizations undersea MV overseas telegrams [ 3 ] .
The defined point of common yoke ( PCC ) is where the offshore air current farm connects to the grid [ 3 ] . If an offshore substation is present beforehand ( Scheme 1 ) , the HV overseas telegrams are connected straight to the grid but if MV overseas telegrams are used for transmittal ( Scheme 2 ) , so an onshore step-up transformer is required with HV overseas telegrams before being fed to the grid [ 3 ] . Figure 1 below shows the two different electrical system strategies.
Figure 1: Wind farm electrical system schemes [ 3 ]
Offshore substation is normally required for electromotive forces above 33kv [ 9 ] and is costlier due to dearly-won indoor equipment and environmental protection [ 9 ] but contributes to take down power losingss and simpler electromotive force control [ 3 ] .
Wind Turbines and Generators
Different types of air current generators have been designed and manufactured within the past two decennaries, to provide to the demands of planetary air current power capacity every bit good as the turning development of air current power engineering [ 10 ] . These wind generators are designed to back up and manage mistakes on the grid [ 7 ] . There are assorted types of generators for air current turbines and they are categorized harmonizing to synchronal or initiation type generators.
3.1 Synchronous Generators
The power convertor in Figure 2 refers to a rectifier, with constellation a being stator connected and the remainder being rotor connected [ 11 ] .
3.2 Initiation Generators
As for initiation generators, typical constellations for air current farms are shown in Figure 3 below
For constellation a, the power convertor is a soft starting motor whereas for constellation B and vitamin D, it refers to a frequence convertor. Configuration degree Celsius has an external variable rotor opposition [ 11 ] .
3.3 Fixed Speed and Variable Speed Wind Turbines
In conventional systems, fixed velocity air current turbines are used in order to accomplish equal frequence of the grid and the electrical end product [ 6 ] . However, variable velocity air current turbines are has an advantage over fixed velocity air current turbines due to holding advanced power electronic constituents and good control for grid support. Fixed velocity turbines are by and large cheaper, simple and robust and Reference [ 7 ] presented a new control method to get the better of the restrictions and believes that the fixed velocity turbines will be popular once more if betterments are made over its grid support.
Fixed velocity or variable velocity air current turbine design relies to a great extent on an optimal rotational velocity expressed by a factor called tip-speed ratio [ 6, 12 ]
Wind turbines will work under certain tip-speed ratio for the coveted turbine efficiency. In Figure 4, a typical graph demoing the relation of air current turbine efficiency and tip-speed ratio is shown. Mentioning to the graph, accomplishing 47 % efficiency requires the air current turbine to hold a tip-speed ratio of 8 [ 13 ] .
Figure 4: Example relation between air current turbine efficiency and tip-speed ratio [ 13 ]
3.4 Fixed Speed Generators
Fixed Speed Synchronous Generators
Fixed velocity synchronal generators produce end product frequence based on the turbine 's rotational frequence ; which implies that it is synchronized with the generator 's shaft velocity [ 6 ] . These machines can bring forth electricity by itself without the grid 's power, so in instance of care or fix plants, isolation of the generators must be implemented [ 6 ] for safety grounds.
Fixed Speed Induction Generators
Initiation generators are normally used for fixed velocity turbines, particularly squirrel-cage type generators and they connect straight to the AC grid without the demand for frequence convertors [ 13 ] . Reference [ 6 ] provinces that the grid 's excitement controls the frequence of the generator 's end product.
When air current velocities go above the rated velocity threshold ( shutdown or roll uping wind velocity [ 5 ] ) of normally 25/ , the end product power is constrained by natural stall or active pitching of turbine blades and if that fails, the turbine is so stopped. For improved turbine efficiency, fixed wind generators can exchange between pole yokes to run at two different velocities [ 13 ] . This characteristic is to let low air currents velocity to hold better efficiency.
Initiation generators are favoured for their low cost and isolation for care plants is non required compared to the synchronal generators, due to its inability to bring forth electricity when the grid 's power is cut off. However, initiation generators have a disadvantage of that it consumes reactive power from the grid, though including capacitances between the generator and the grid could counterpoise the job [ 6 ] [ 13 ] . Initiation generators besides require a cut-off system to avoid it moving as a motor when air current speeds lessenings.
3.5 Variable Speed Generators
Through variable velocity generators, we can do the rotor to run at a fixed tip-speed ratio over changing air current velocities to obtain maximal efficiency [ 13 ] . This is achieved by utilizing frequence convertors or commanding the rotor 's faux pas [ 13 ] . The former method licenses wider velocity scope. Reference [ 10 ] has made a comparing survey on seven variable velocity changeless frequence ( VSCF ) air current generators dwelling of both synchronal and induction generators. The initiation generators studied specifically are double fed initiation generators ( DFIG ) and VSCF squirrel coop initiation generators ( SCIG ) , whereas the synchronal generator types were lasting magnet synchronal generators ( PMSG ) and electrically excited synchronal generator ( EESG ) [ 10 ] . Both of the synchronal generators are direct driven in contrast to the geared goaded initiation generators. Based on the comparing consequences, the geared driven double fed initiation generators ( DFIG ) emerged as the most appealing due to its high one-year energy production ( AEP ) per cost and lowest generator system cost and it is recommended for big power evaluation use [ 10 ] .
Doubly Fed Induction Generator ( DFIG )
This lesion rotor machine can work as a variable-speed generator through debut of variable electromotive forces into its rotor at slip frequence [ 14 ] . The variable velocity is determined by the two IGBT based electromotive force beginning convertors ' evaluations and it is through these convertors that the rotor get the variable electromotive forces [ 14 ] .
Offshore Wind Farm AC Transmission
AC transmittal is a good developed engineering [ 15 ] and soon used in most air current farms [ 16 ] . For offshore air current farms that are below 100km distance from shore and less than 150MW capacity, high electromotive force AC ( HVAC ) transmittal is a favoured pick for being the most economical method [ 17 ] .
AC transmittal has the advantages of [ 16 ] :
- Simpler installing, care and interconnectedness
- Cost effectual
- Operational consistence
AC links are non used for longer distances due to high electrical capacity and bring forth reactive currents [ 15 ] . This implies bigger reactive power due to the bear downing currents produced as overseas telegrams become longer and reactive power compensators are needed [ 5 ] [ 15 ] . AC transmittal loses its economic entreaty when active current is less than the bear downing current [ 15 ] . The transporting burden current capacity of the AC overseas telegrams is reduced because the overseas telegrams must physically transport both burden and bear downing currents at the same clip [ 16 ] . Reference [ 16 ] shows that bear downing current is expressed by =2F where F is frequence, is electrical capacity and is electromotive force ; and the relation of current capacity is 2= 2+2, where is the overseas telegram current capacity and is load current.
Based on look, utilizing DC outputs =0 at steady province [ 16 ] , which shows the advantage of DC holding higher transporting load current capacity. Besides bear downing currents, AC cables besides experience power losingss ( 2 ) [ 16 ] and there are bounds to a figure of AC overseas telegrams that can be installed due to the environment [ 18 ] .
Offshore Wind Farm DC Transmission
5.1 HVDC Links
Conventionally, offshore wind farms are connected to the grid via AC transmittal as antecedently stated. Latest developments nevertheless, have suggested that AC links are to be substituted with a District of Columbia transmittal system. Typically used for far wind farms of more than 60km, HVDC links are non limited by the length of transmittal and therefore will be a executable and economical option when air current farms are larger and further off from shore compared to AC links [ 1 ] . HVDC transmittal is associated with variable velocity air current turbines due to the ability to command the frequence of air current turbine grids and is independent of the power system frequence [ 11 ] .
Reference [ 19 ] has stated the list of advantages of HVDC compared to HVAC, based on Reference [ 8 ] , which are:
- Independence of directing and having frequences every bit good as isolation of system from other webs.
- The distance of HVDC transmittal are unaffected by overseas telegram bear downing current.
- HVDC overseas telegrams are capable of higher transmittal capacity.
- Power losingss on overseas telegram are low.
- Controllability of power flow.
5.2 Wind Farm AC Grid Configuration
The simplest DC grid connexion for air current farms is by feeding the AC grid air current turbines into a power convertor and linking it to the AC grid utilizing HVDC links. The figure below illustrates this.
Figure 5: Group connexion of a air current farm to an HVDC nexus [ 11 ]
The benefits of this constellation is that sum of power convertors are lessened compared to DC grid constellation, but they supply reactive power to the air current farm, enabling it to defy grid mistakes [ 11 ] .
5.3 Wind Farm DC Grid Configuration
An alternate method is to hold each turbines have their ain rectifier such as Figure 6.
Figure 6: Individual connexion of air current turbines to HVDC links [ 11 ]
Compared to AC grid constellation, DC grid enables control of frequence and velocity of each single turbines [ 11 ] . Reference [ 1 ] explains the working of an DC grid air current farm in more item.
Figure 7: Example of DC grid [ 1 ]
Mentioning to Figure 7, the electromotive force from the air current generators is first rectified, gathered and so transferred to an offshore platform where the electromotive force is stepped up well by a DC/DC convertor, or else the system will endure high losingss during transmittal [ 1 ] . Power is so transmitted through a District of Columbia overseas telegram, connected to an inverter onshore and finally fed into the grid.
5.4 Line Commutated Control ( LCC ) HVDC utilizing Thyristors
LCC transmittal require commuting electromotive force which is conventionally supplied through a normal or a inactive ( STATCOM ) type synchronal compensator [ 19 ] . The advantages of LCC systems in seaward air current farms are [ 19 ] :
- LCC transmittal can be used for high capacity power, making 1600MW nexus compared to VSC transmittals, which reach 300MW [ 17 ] .
- Over 30 old ages of development in LCC engineering
- The convertor station for an LCC system is twice the size in contrast to a VSC system due to the switchgears and breaker- switched AC harmonic filters necessitating much infinite.
- LCC systems have lower power losingss compared to VSC systems.
5.5 Voltage Source Converter ( VSC ) HVDC utilizing IGBTs
Besides conventional LCC transmittal, another engineering is the electromotive force beginning convertor ( VSC ) transmittal utilizing IGBTs. The electromotive forces beginning comes from capacitances [ 20 ] . The advantages of VSC transmittals used in air current farms are [ 19 ] :
- VSC systems are self-commutating and external electromotive force beginning is non required for operation.
- Reactive power control does non necessitate switchable AC harmonic filters and is independent of active power control.
- The power reactive power flow is independently controlled, doing AC electromotive forces from each terminal controllable.
5.6 HVDC and HVAC Combination
Not all offshore air current farms are built in it 's wholly. They are normally built in stages. For such state of affairss, Reference [ 21 ] recommends transmission combination of HVAC and HVDC. Preliminary stages of building of air current farms can use AC connexion due to the initial little graduated table and subsequently utilizing HVDC connexion as the air current farm 's scale expands [ 21 ] .
Overview of offshore air current farms has been presented in this literature reappraisal. Different topologies and internal grid connexions are outlined. Furthermore, we have looked into applications of synchronal and initiation generators used in air current turbines and explored the difference between fixed velocity and variable velocity generators, which are indispensable in air current turbines in regard to generated end product power. Finally, a general sum-up of the transmittal methods for offshore wind farms were documented, traveling into specific AC transmittal drawbacks and the emerging tendency of HVDC transmittals.
Based on the generalised apprehension of the air current farms, we aim to further analyze the jobs of its connexion, with respects to the air current farm 's internal grid and AC transmittal to the chief grid. Comparisons between internal DC and AC grid of air current farms will besides be studied. Power flow surveies and simulations will be conducted and aimed at bring forthing new recommendations and solutions.
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