12
Offshore wind farm arrays
Abstract
This chapter presents some of the main configurations and technologies that are being used or considered for offshore wind farm electrical collection. It introduces the industry’s current practice for design and implementation. Wind turbine array layouts and electrical collectors are designed on a site-specific basis to achieve a good balance between cost, electrical losses and wake effects. The material presented in the chapter is oriented towards the electrical aspects once a layout has been identified to ensure maximising the energy yield from the wind. Issues addressed include electrical performance, wind farm capacity, wind turbine generator technology and voltage levels among others.
Keywords
Arrays; Offshore; Platforms; Subsea cables; Turbines; Wind farmThis chapter presents the reader with configurations and technologies used in an offshore wind farm electrical collector. It also introduces the industry's current practices for their design and implementation. Array layouts and electrical collectors are designed on a site-specific basis to achieve a good balance between electrical losses and wake effects. The material presented in the chapter is oriented towards the electrical aspects once a layout has been identified to ensure maximising the energy yield from the wind. Issues addressed include electrical performance, wind farm capacity, wind turbine generator technology and voltage levels among others.
12.1. Fundamentals of offshore wind farm arrays
An offshore collector system consists of underwater cabling connecting all wind turbines within the wind farm to other turbines in an array structure, allowing the power generated by each unit to be exported to the substation(s). It can also be referred to as an interturbine/interarray system.
This cabling – operating at a voltage of 33 kV in the UK and 20 kV across Europe – channels the energy generated by each turbine to the nearest substation. One important feature of wind farms is that wind turbines are required to be spaced a certain number of rotor diameters away from one another due to the wake effect phenomenon. This often leads to large sites spanning tens of kilometres in size; such large sites will require substantial cabling distances to connect all turbines to the farm substation(s). As collector system cabling lengths span across the entire site, the losses associated with the collector system result in the largest portion of total wind farm losses. Therefore, it is extremely important to minimize losses introduced by the collector system as reduction of these losses will significantly reduce the overall electrical power losses across the farm, ultimately leading to a more efficient farm and more power generated. This increase in generation will, in the long term, lead to an increase in returns produced for the site owners. These losses may be reduced by simply rearranging the interturbine cabling arrangement.
A wind farm collector system is designed after all turbines and substation(s) have been suitably located. When designing a wind farm structure, engineers must go through many stages, one of which includes the design of the collector system. This process is a complex optimization task in which engineers try to strike a balance between minimising the collector cable losses whilst also minimising the development costs involving the submarine cabling.
12.2. Design considerations
The electrical array system has to be designed to meet the following general requirements [1]:
• Health and Safety: the system must be designed in a way that minimizes the risk of adversely affecting the health and safety of personnel and the public, and must comply with all relevant H&S legislation.
• Compliance with all relevant codes – national grid codes, distribution codes, STC (System Operator-Transmission Owner Code) – and standards, eg, IEC, IEEE.
• Export capacity to be at least 100% of wind farm output.
• Minimise CAPEX: material and installation costs.
• Minimise OPEX: losses and maintenance requirements.
• Maximise availability and reliability.
• Low environmental impact (low life cycle carbon emissions, recyclable, decommissionable).
• Strong supply chain of components.
• Adaptable to different wind turbine generators (WTGs) and wind farm sizes.
• Redundancy against a single fault to provide power to auxiliary demand.
In addition, bilateral agreements may dictate project-specific requirements depending on the location of a wind farm or grid connection point.
12.3. Main electrical components
12.3.1. Wind turbines
Currently installed offshore wind turbines are adapted from standard onshore wind turbine designs with significant upgrades to account for sea conditions. These modifications include strengthening the tower to handle the added loading from waves, along with pressurized nacelles and environmental control to keep corrosive sea spray away from critical drive train and electrical components [2].
Offshore turbine power capacity is in general greater than standard onshore wind turbines, currently ranging from 2 to 5 MW (Fig. 12.1). The current generation of offshore wind turbines is typically three-bladed horizontal-axis, yaw-controlled, active blade-pitch-to-feather controlled, upwind rotors, which are nominally 80 m to approximately 130 m in diameter [3]. Offshore wind turbines are generally larger because there are fewer constraints on component and assembly equipment transportation, which limit land-based machine size. In addition, larger turbines can extract more total energy for a given project site area than smaller turbines [4]. A critical issue in developing very large wind turbines is that the physical scaling laws do not allow some of the components to be increased in size without a change in the fundamental technology.
In onshore wind turbines, the drive train is typically designed around a modular, fixed-ratio, three-stage, gearbox with planetary stages on the low-speed side and helical stages on the high-speed side. Offshore towers are shorter than onshore towers for the same output because wind shear is lower offshore, which reduces the energy capture potential of increasing tower height [4].
Figure 12.1 Offshore wind turbine development [3].
12.3.2. Offshore substations
The main purpose of a substation is to step up the voltage from the collector level (eg, 33 kV) to a higher level suitable to export the electricity produced offshore to the grid, normally 132 kV. The voltage is required to be stepped up to reduced losses that occur with large transmission distances as offshore farms are often located great distances from shore.
Substations are required for large offshore projects (>100 MW) or when farms are situated at distances greater than 15 km from shore. An offshore substation costs around 7% of the total cost of a wind farm. For a 500-MW offshore wind farm the offshore substation costs around £80 million and weighs up to 2000 tonnes [2,5].
One substation can typically support up to 500 MW of output from a wind turbine array. With an increase in wind farm size, the number of substations may increase too. An offshore substation is typically delivered as one component after contract by a supplier and weighs generally from 1800 to 2200 tonnes (for a 500-MW wind farm). The platform level is generally 25–30 m above sea level and has an area of 800–1200 m2. These offshore substations are generally not service-based but still have a small workshop available inside.
The major components of an offshore substation are divided into three categories [5]:
• Components related to electrical systems;
• Components related to facilities;
• Components related to structure.
The main components related to the electrical systems are as follows:
• The major component of the offshore substation is the transformer. The transformer steps up the voltage according to requirements for optimum power transfer and minimization of losses.
• A back-up diesel generator is also made available on the substation in case of loss of power via the export cable.
• If the onwards transmission is HVDC then converters are also installed in the offshore substation.
• Reactive power compensation equipment is used to provide optimum reactive power compensation required for the maximum power transfer to the onshore grid (eg, bank of reactors, capacitors, SVCs or STATCOMs)
• The substation is properly earthed to provide power safety in case of safety hazard or short circuit.
A typical substation can support about 500 MW of input from numerous wind turbines. Many farms require more than one substation to export the power generated due to the scale of the farm. However, if it is financially viable, farms will often have more than one substation to increase the farm's export security. The world's current largest operational wind site, London Array Phase 1, has two identical substations due to the scale of the farm.
12.3.2.1. Transformers
As already mentioned, the main task of offshore substations is to convert the interarray voltage to a higher magnitude to reduce transmission losses. Gas (SF6)-insulated transformers are specifically designed to be non-flammable; this is an important feature as offshore substations are located great distances from shore and in normal circumstances unmanned, hence any signs of fire would not be able to be quickly dealt with, potentially leading to a catastrophic outcome. When maintenance is required on the substation's transformers this can be easily and safely carried out as all live parts are contained within metal structures that are grounded.
12.3.2.2. Switchgear
Switchgears are used to control, protect and isolate the array of wind turbines. Wind turbines are required to be isolated if they malfunction, in order to allow maintenance to be carried out whilst working turbines continue outputting power.
Gas (SF6)-insulated switchgears are popular for offshore sites as not only they are compact but they also provide an improved level of safety than their vacuum- or oil-insulated counterparts.
12.3.2.3. Protection equipment
When a fault occurs in the internal collection grid, the protection system needs to identify and to isolate the faulty component. AC protection systems consist of transducers, relays, circuit breakers, switches, auxiliary power and all wirings in between. DC breakers are still an immature technology. For instance, in high-voltage DC systems, protection systems exist currently without DC circuit breakers, all relying instead on circuit breakers on the AC side of the converters [7].
Medium voltage (MV) switchgears are used in offshore wind farms for protection and they are located inside each wind turbine and on the offshore substation. The number and type of protection used is different depending on the design, grid topology and required level of availability.
In each turbine, there are breakers for LV and MV circuit protection. The LV protection clears internal wind turbine faults, while the MV protection is used to clear faults in the LV/MV transformer and faults in the cable connected to the next turbine. The MV circuit breakers may have the functionality to be operated remotely from shore. Gas-insulated switchgears (GIS) are also used to manually disconnect and ground transformers and cables.
Protection elements on the offshore substation are used to protect against faults in the MV/HV transformer and disconnect feeders in the event of faults on the feeders. The offshore substation transformer will have additional protection for oil temperature, pressure and so on.
Depending upon the internal grid topology, the type of protection differs. For example, in radial layout wind farms, it could be sufficient to place switchgear only at the beginning of the feeders, so that in case of a fault somewhere on the feeder, the whole feeder will be disconnected. Fig. 12.3 shows examples of different protection schemes for different types of wind farm layouts and different levels of reliability. In case of a fault, it is the nearest circuit breakers that isolate and reduce the currents to zero in the fault area. Once this has been done, manually, remotely operated switches can possibly be used to isolate the fault even further, allowing other parts of the grid to be re-energized.
Figure 12.3 Examples of configurations with circuit breakers and manual switches with different grid topologies. (a) String topology. (b) Star topology. (c) Ring feeder topology; a fault on any cable can be isolated and power from all six turbines can be transmitted to the substation (provided cable capacity is sufficient everywhere). (d) Ring topology where cable capacity is sufficient to carry the maximum generation from four turbines only; a fault in one of the first two cable segments, power output from two turbines will be lost [7].
12.3.2.4. Optimum substation locations
The location of any offshore substation is crucial when designing a collector system. This location has a great deal of influence on the layout of the cabling, and hence has a big impact on the expenditure of the project. Ideally, substations would be located in the farm where the amount of interarray cabling connecting all turbines is minimized.
The number of substations is determined by [7]:
• Size of wind farm, hence the total length of cables.
• Voltage level, which affects maximum length of a feeder.
• Capacity of wind farm, and capacity of transformer and HV cable.
At a short distance from shore, small wind farms can be connected directly to shore. In such cases, the MV level is used to connect to shore. The feeders can be divided into different groups with each group having separate connection to an onshore substation. Switchgears are used at the shore end of transmission cables. Thus, there is no need for an offshore substation. As the capacity of the wind farm increases the need for having an offshore substation also increases. This is because the lifetime cost of power losses due to transmission of high power at the MV level becomes comparable to the capital cost of having an offshore substation. At a large distance from shore, offshore substations are required to transform voltage to HV level suitable for long-distance transmission. Besides greater installed power, large wind farms will also cover a larger area. This means that long cables are required within the wind farm area to connect each wind turbine (or group of wind turbine feeders) to the offshore substation. But with MV level, there is a limited length cable can have before power losses become too large and compensators for voltage regulation are needed. The solution is therefore to divide the total turbine area in two or more parts, depending on the total area coverage of the wind farm, and have an offshore substation within each area. Each offshore substation will have a separate transmission cable link to shore. Other constraints that affect the decision regarding the number of offshore substations include existing offshore industries, shipping, subsea cables, pipelines, etc., in the wind farm area.
A higher voltage level increases the transmission capacity and allowable length of cables. Wind farms that have higher MV collection can be directly connected to shore which otherwise would be unfeasible to connect with standard 33 kV. A higher MV level increases the possibility of connecting small wind farms at longer distances and large wind farms at shorter distances to shore without an offshore substation. For wind farms covering a large area, with higher MV, longer array cables can be realized which would lower the need for having more than one offshore substation.
Wind farms with large installed power, even if they are close to shore, would require an offshore substation because of the large power losses due to transmission at MV level. On the other hand, small wind farms can be directly connected to shore without the need for substation.
The size of the offshore transformer can be a limiting factor for the total amount of power that can be transmitted. For wind farms with a capacity of several hundreds of MW, it could be possible to have more than one substation transformer connected in parallel in order to support higher wind farm production capacity. This will also increase the availability of the wind power because, in the case of transformer disconnection due to a fault, at least some of the power can be transported. The cost of transformers and their corresponding physical sizes and weight increase with their ratings. This also implies that the cost of offshore platforms needed to support the transformers and switchgears also increases.
12.3.3. Subsea cables
XLPE-insulated submarine cables are the most common types of interarray collection cables used in offshore wind farms. They have low electrical losses. In addition, they have high reliability and are environmentally friendly. The conducting material can either be aluminium or copper. Aluminium conductors, which are less commonly used, have a lower current-carrying capacity, and thus require large diameter and hence a larger bending radius [7].
Submarine cables can be three separate cables, one cable for each phase, also known as 3 × 1-core or they can be three cables bundled up together in a common shield and armouring (1 × 3-core). Single-core (3 × 1-core) cables have a higher current-carrying capacity than 3-core (1 × 3-core) cables. But at the same time, single-core cables have higher losses than 3-core cables. This is because in single-core cables there is current flowing through the armour that creates additional losses. There exists a magnetic coupling between the phases in single-core cables for separation distances lower than 50 m [7]. Since the cables are not transposed, the magnetic coupling results in different impedance in the cables and unbalance in the three-phase system. In 3-core systems, there is little or no coupling between the phases and therefore, the system remains symmetrical. All three cables are laid separately for 1-core cables, while for 3-core cables the cable is laid in one instance which makes the installation cost cheaper. In general, 1-core cables are more expensive than 3-core cables. Both single and 3-core cables' shield has to be grounded to avoid overvoltages. To increase reliability an additional fourth cable can be laid in parallel to the other cables in single-core systems while for a 3-core system a second cable in parallel can be installed. There is a possibility of integrating a fibreoptic cable to be used for communication purposes with 3-core cables.
Interarray cables closest to the substation carry the total sum of power produced by all the wind turbines linked to them. This required higher current-carrying capacity of these cables. The total number of wind turbine generators that can be connected in series is therefore limited by the maximum current-carrying capacity of the submarine cables. As already mentioned, higher current-carrying capacity for wind farm cables can be achieved if three 1-core cables are used instead of one 3-core cable. But the additional costs have to be justified.
Figure 12.4 Different cable cross-sections in Lillgrund offshore wind farm [7].
Submarine cables are available in a wide range of cross-sections ranging from 95 mm2 to 1000 + mm2. One factor that determines collection system cable sizes is the maximum amount of current the cable segment is expected to carry. Depending on the wind farm layout, many different types of cable sizes can be used within a wind farm. For example, Lillgrund offshore wind farm in south Sweden has a radial layout. The cable sizes in one of the feeders are shown in Fig. 12.4. The cable sizes at the end of the feeder are smaller than cable sizes found at the beginning of the feeder closest to the substation transformer. Heating and losses are also factors that are considered when selecting the appropriate cable sizes.
Some of the major submarine cable manufacturers in the world include ABB, JDR Cable Systems, Nexans, Prysmian, NSW and Parker Scanrope.
12.4. Topologies
As the power capacity of offshore wind farms increases, the adequacy of the wind farm electrical system becomes critical. The efficiency, cost, reliability and performance of the overall wind farm will depend, to a great extent, on the electrical system design [2,8]. The overall function of the electrical collector system is to collect power from individual wind turbines and maximise the overall energy generation. An electrical collector can be designed using different layouts depending on the wind farm size and the desired level of collector reliability. There are various arrangements for wind farm collector systems employed in existing offshore wind farms, whilst others are in a conceptual stage. Four basic designs shown in Fig. 12.5 are discussed below.
a. Radial design
The most straightforward arrangement of a wind farm collector system is a radial design (Fig. 12.5(a)), in which a number of wind turbines are connected to a single cablefeeder within a string. The maximum number of wind turbines on each string feeder is determined by the capacity of the generators and the maximum rating of the subsea cable in the string. This design offers the benefits of being simple to control and also inexpensive because the total cable length is smaller with tapering of cable capacity away from the hub being possible. The major drawback of this design is its poor reliability as, in the case of a cable or switchgear fault at the hub end of the radial string, it has the potential to prevent all downstream turbines from exporting power.
Figure 12.5 Wind farm electrical collector basic designs [8].
b. Single-sided ring design
With some additional cabling, ringed layouts can address some of the security of supply issues of the radial design by incorporating a redundant path for the power flow with a string. The additional security comes at the expense of longer cable runs for a given number of wind turbines and higher cable rating requirements throughout the string circuit. A single-sided ring design, illustrated in Fig. 12.5(b), requires an additional cable run from the last wind turbine (ie, G7) to the hub. This cable must be able to handle the full power flow of the string (eg, 35 MW in a 5-MW seven-turbine string) in the event of a fault in the primary link to the hub end (denoted by the open breaker B1 in Fig. 12.5(b)).
c. Double-sided ring design
Fig. 12.5(c) illustrates a double-sided ring design [8,9]. In this configuration the last wind turbine in one string is interconnected to the last wind turbine in the next string (eg, G7 to G8 as shown in Fig. 12.5(c)). If the full output power of the wind turbines in one of the strings were to be delivered through the other string, then the cable at the hub end of the latter needs to be sized for the power output of double the number of wind turbines.
d. Star design
The star design shown in Fig. 12.5(d) aims to reduce cable ratings and to provide a high level of security for the wind farm as a whole, since one cable outage only affects one wind turbine in general. A cost implication of this design is the more complex requirements at the wind turbine in the centre of the star (that is, turbine G5 in Fig. 12.5(d)).
12.5. Converter interface arrangements and collector design
At present, offshore wind turbines are based on designs for onshore use, producing an AC output for direct connection to the electricity grid, and complying with the relevant grid codes for power quality and fault response. To gain an understanding on how wind turbine generator technology may influence control approaches, it is necessary to consider the electrical system as a whole, that is, the turbine topology (converter interface arrangement), wind farm collector, and the offshore transmission type (eg, AC or DC). Two cases are discussed next in the context of the converter interface arrangement and location [2,10].
12.5.1. Converters on turbine
12.5.1.1. AC string
The conventional AC string arrangement is shown in Fig. 12.6. Turbines feature a squirrel-cage induction generator (SCIG), or a permanent magnet generator (PMG) connected to a fully rated converter, or alternatively a doubly fed induction generator (DFIG) and partially rated converter can be used. The output of the converter is stepped up to the collection network voltage, and the turbines are connected together in strings. The number of turbines on a string is determined by the current and voltage rating of the available cables, and the rated power of the turbines. Voltage is limited by water-treeing effect with wet insulation cables, while dry-insulation cables with a lead sheath around the insulation would be too expensive. A higher voltage also requires a higher voltage rating for the transformer, which increases the cost and size. Available current ratings are also limited, as the skin depth of the AC current means that conductors with larger areas will be less effective, as the current will not flow in the centre of the cable. For this reason, the cost of AC cables tends to increase quickly with current capacity.
Figure 12.6 Conventional AC strings [10].
12.5.1.2. DC string
An arrangement using DC in the collection strings is shown in Fig. 12.7. In this system, the turbines output a DC voltage, which is then stepped up to the transmission voltage at the collection platform. In most studies, the turbine produces a voltage of around 40–50 kV DC, which requires an AC–DC converter capable of producing such a voltage, featuring many switching devices in series, or a lower voltage AC–DC converter and step-up DC–DC converter. A solution involving a lower-voltage converter and a DC voltage of 5 kV is also possible, which has the advantage of eliminating the turbine transformer and using a conventional 3.3-kV three-level converter. However, the currents in the strings will be extremely high, requiring thick cable and leading to high losses. DC systems are attractive as they could reduce the number of conversion steps between AC and DC, but converters with a voltage boost ratio will require a transformer, requiring conversion to AC and back.
As DC cables do not suffer from water-treeing degradation, higher voltages could be used without needing dry-insulation cables, while the current in a DC cable can use the entire surface area of the conductor, so the cable cost will increase linearly with current capacity rather than exponentially as with AC. Because of these factors, it could be possible to implement longer turbine strings much more cheaply with DC than with AC collection. However, this is difficult to quantify as there are no commercially available cables with the required configuration and voltage rating, and previous studies of the cable cost have extrapolated the cost for multicore DC collection cables from the costs for single-core HVDC transmission cables with a significantly higher voltage rating.
Another issue with DC collection networks is with fault protection, as the fact that the current does not continually reverse as with AC means that when a circuit breaker opens, the switching arc will not be automatically extinguished when the current reverses. Various DC circuit breaker designs have been proposed, but these become increasingly expensive at higher voltage ratings. DC collection and transmission networks have been designed considering the use of power converters which are capable of stepping down the voltage as well as stepping up, and these can be used to limit the fault current, but at the cost of extra complexity.
12.5.1.3. DC series
An alternative DC collection architecture is to use series DC connection of the turbines as shown in Fig. 12.8. Here the DC outputs of the turbines are connected in series, and the turbines connected in a loop. This allows the high collection voltage to be achieved without using high-voltage converters, although the wind generator converter would need to be isolated with respect to ground. An isolation transformer would need to be used, or a generator capable of handling a high-voltage offset. Another option is to use a transformer-isolated converter in the turbine, where the high-voltage side of the converter only consists of a passive diode rectifier, which is much easier to isolate.
This arrangement could reduce the cable costs, as it only uses a single-core cable loop, although there is no scope to taper the current rating of the cable. In the event of a turbine fault, the faulty turbine could be bypassed using a mechanical switch, but any cable faults will mean that none of the turbines on the loop would be able to export power.
A related idea is to increase the turbine output voltage and the length of the strings, so that the full transmission voltage is produced, eliminating the need for the collection platform. This system has been shown to have the lowest losses due to the high collection network voltage, and the lowest cost due to the elimination of the collection platform. Several strings could be used in parallel to increase fault tolerance. The disadvantage of this system is that the transformer and converter in the turbine must be capable of isolating the full transmission voltage, and high-voltage transformers with a low enough power rating are not commercially available.
12.5.2. Converters on platform
12.5.2.1. AC cluster
Research is being conducted on a connection arrangement where turbines with fixed-speed induction generators are connected to a variable-frequency AC collection grid, with strings of turbines being connected through a single converter. This places the converters on the collection platform, allowing them to be more easily repaired in the event of a fault, and a single large converter could potentially be cheaper than several small ones. An AC or DC collection system could be used within the collection platform as shown in Fig. 12.9.
The speed of all the turbines in the string can be varied together to track the maximum power point for the current wind speed, but speed control over the individual turbines is lost. The speed of each turbine will be able to vary by a small amount relative to the others, due to the slip of the induction generator, with an increase in turbine speed leading to an increase in slip and an increase in torque. Depending on the number of turbines connected to each converter, this will result in a reduction in the amount of power extracted.
This system could also have an impact on the drive-train loads experienced by the turbines, as a turbine experiencing a gust would not be able to speed up to absorb the excess power, leading to a high transient torque, putting strain on the drive train and blade roots. Research on the reliability of turbines in service has shown that the move to variable-speed turbines has reduced the level of blade failure compared with fixed-speed turbines.
12.5.2.2. Parallel DC cluster
This method, shown in Fig. 12.10, uses a permanent magnet generator and passive rectifier in the turbine, with a DC–DC converter for each string of turbines. The speed of the turbine will be determined by the DC voltage of the string, so the system will behave in a similar way to the AC cluster connection system described previously, with similar issues of drive-train torque transients during gusts. It is considered that the passive rectifier will have considerably greater reliability than an active converter.
For a given DC voltage, the amount of possible speed variation of the turbine will depend on the generator inductance, with a higher inductance giving a greater variation in speed. The passive rectifier is unable to supply the generator with reactive power, and if the generator inductance is too high then the maximum torque will be reduced. Inductance is typically much higher in low-speed machines, used in direct-drive turbines, and in these cases capacitors can be used between the generator and rectifier to supply the reactive power requirements. The main advantage of DC over AC clustering is the greater efficiency of the permanent-magnet generator, compared with the induction generator used in the AC system. The greater current and voltage capability of the DC cables could also lead to larger cluster sizes, and a reduction in cable cost, but this could also reduce the power capture. A DC system could also reduce the number of conversion steps and associated losses, increasing efficiency.
12.5.2.3. Series DC cluster
A variation of the parallel cluster arrangement is to connect the turbines in series, in a loop, with each loop controlled by a single converter, as shown in Fig. 12.11. In this case, the converter will control the current within the loop, which will determine the generator torque within the turbine, and will be much more analogous to the conventional turbine control method. As the turbine speeds will be capable of varying individually, transient torque spikes should not be a problem. This emerging connection method is in its initial stage of investigation.
12.5.3. AC collection options: fixed or variable frequency
Fixed-frequency AC operation of the offshore network is normal practice and possible with both synchronous (HVAC) and asynchronous (HVDC) connection of the offshore wind farm. AC variable frequency operation at the collection network would be cost-effective only when the wind farm is connected to the grid through an HVDC transmission system [11–14]. This is because the offshore HVDC rectifier can control the offshore frequency independently from the onshore grid, whereas for a synchronous AC transmission link an additional AC–AC or AC–DC–AC conversion system would need to be installed. Examples of wind farm configurations using DC collection as proposed in the literature are described next [15,16].
12.5.3.1. Examples of variable-frequency collection configurations
A concept, currently on early research stages, is the use of variable-frequency operation in the collection network with DFIGs and an HVDC transmission link pursuing one of the following objectives [12]:
• Reduce the rating of DFIG converters;
• Extend the speed range to maximise the power capture without increasing the rating of the DFIG converters.
A multiterminal configuration based on VSC-HVDC transmission that allows variable-frequency operation in the offshore collection network is under investigation [13]. The proposal in this reference is similar to the collecting network configuration presented in Jovcic and Milanovic [14] but with a multiterminal HVDC link based on current source converters that use force-commutated devices such as IGBTs. In the latter arrangement, the wind turbine generator transformers are not needed to step up the voltage to transmission level, because the current source converters are connected in series. In both schemes no additional converters are required at each wind turbine generator (WTG) terminal, and 2-MW permanent magnet generators are synchronized together in a group connected to a centralized converter and all are operating at the same speed.
A wind turbine topology that allows variable frequency operation with a squirrel-cage induction generator over a wide range of operating conditions is also under investigation [15]. The topology replaces the fully rated back-to-back converter with a single-stage cyclo-converter to decouple the frequency at the offshore substation from the wind turbine side. The scheme has the following features and potential advantages:
• The 50-Hz three-phase transformers within each wind turbine generator and at an offshore substation are replaced by medium-frequency (400–500 Hz) single-phase transformers. The transformer insulation must be designed to cope with increased voltage stress, and high dv/dt resulting from step changes in the voltage due to the snubber capacitors of the converter.
• Since only a single-phase converter is required at the offshore substation instead of a three-phase converter, the number of series-connected devices required decreases significantly resulting in significant reduction in cost, conduction and switching losses. However, as the single-phase converter handles the full power this will also have an impact on cost and losses.
• Soft switching of the cyclo-converter and offshore converter of the VSC-HVDC reduces the switching losses significantly.
• The use of thyristors rather than IGBTs in the cyclo-converter reduces the power loss and cost.
12.5.3.2. AC variable-frequency collection evaluation
Variable-frequency operation at the offshore collection network in conjunction with an HVDC transmission link and WTGs of the DFIG type can maximise the power extraction and reduce the overall wind farm cost according to reference [12]. However, the consequences of having variable frequency at the offshore network regarding switchgear and protection, transformer operation, voltage and current rating of the equipment located at the offshore network need to be thoroughly investigated. Standard power transformers are designed for a specific frequency of operation (50 Hz or 60 Hz), and the normal tolerance of frequency variation is around ±5%, ie, for a 50-Hz unit, limits of 47.5–52.5 Hz. For a lower-frequency design of transformer, a larger core would be needed in order to maintain the required voltage ratio, for a specific current rating and a reasonable flux density avoiding saturation. However, this can be overcome by reducing the voltage in proportion to a reduction in frequency. Another aspect to be investigated is how the reactive power flow through the transformers and cables changes with variable frequency.
Operating VSC-HVDC at variable frequency has been demonstrated practically in the gas platform Troll [11]. In that application, the need for variable-frequency operation by the VSC-HVDC inverter is clear, namely to control the speed of an induction motor. For an offshore wind farm on the other hand, variable frequency can be beneficial for the generator rotor only, but further research is required to assess whether it is more economical to achieve this locally at each individual generator rather than at a collection network level.
12.5.4. Evaluation of higher (>33 kV) collection voltage
As wind farms and turbines have increased in power capacity, the collection voltage levels have increased from typically 11 to –33 kV nowadays. The use of 48-kV or 66-kV cables to connect wind turbines to the onshore grid via a 48/132-kV or 66/132-kV transformer onshore, instead of using a 132-kV submarine cable and a 33/132-kV transformer offshore is being investigated [17]. The study shows that stepping up the voltage from 33 to 132 kV offshore is most economical for greater distance (>25 km), because the cost of the offshore substation is then less significant compared to the cable costs, and the losses are much more reduced.
Another reason for using higher-voltage collection cables is that they can bring the benefit of needing fewer cable strings in collection networks for large offshore wind farms (>300 MW), because each cable has a higher capacity. Especially for ever-increasing wind turbine sizes, this may become an attractive solution. For example, with present designs of 33-kV cables it would only be possible to connect up to four 8-MW wind turbines per cable string, whereas in the London Array up to nine 3.6-MW wind turbines are connected to one string. Higher voltages also reduce fault levels for a given MVA generation and ohmic losses would be less, although the overall losses in a particular design must be considered. In addition, since higher voltages offer a longer transmission distance, the collection cables can be longer, so fewer offshore ‘sub-transmission’ platforms may be needed. These were introduced for example in the 400-MW VSC-HVDC linking the BARD Offshore 1 wind farm to the transmission grid in north Germany, where the collection voltage is stepped up from 30 to 155 kV on two separate ‘sub-transmission’ platforms. These are linked via 155-kV cables to the offshore 400-MW HVDC substation [18] to transmit the power via DC over a distance of 203 km to the onshore grid.
The challenges of using voltages higher than 33 kV for the collection network presently are:
• There is a limited supply of commercially available dry-type transformers rated above 33 kV and also capable of stepping up from a suitable generator voltage, for example 3.3 kV. They are also more expensive than the more widely available 33-kV transformers. Oil-filled transformers at the wind turbines are undesirable because of the risk of a spillage at sea, which poses an environmental threat.
• Collection cables at 33 kV and below can be of the ‘wet design’, which do not require metallic moisture barriers surrounding the cable as an outer sheathing layer or around the insulated core(s). The sheathing/bedding layer(s) are made from polypropylene (or jute) string/rope. Seawater fills the empty spaces inside the cable making direct contact with the outside of the insulated core(s). Higher-voltage submarine cables presently available are of the ‘dry design’, which use a lead sheath as a water barrier. Their drawbacks are the increased cable capital cost and perhaps somewhat higher installation cost due to the additional weight, and the lead sheath is susceptible to fatigue failure if movement or vibration occur.
For a specific project, a thorough cost–benefit study (ie, cost, supply chain, certification, insurance, etc.), is required to make an informed decision on whether to opt for 33-kV or higher-voltage collection cables because the impact on the overall wind farm design may be profound.
12.6. Wake farm arrangement – wake effects
The arrangements of the turbines within a wind farm depend on the site terrain, wind conditions (velocity and direction) and the size of the turbines. In order to maximize the power output from a wind farm, the wind farm layout needs to be designed in such a way that wake effects will be minimized. To reduce the wake effects and, hence, increase power output from the wind farm, the simplest option is to space the turbines far apart until wake effects are completely negligible. However, this approach will lead to increased interturbine cable cost and land wasting [19,20]. It is therefore important that the turbines are not distributed at unnecessary separations and the economical aspects of site development must balance wake effects and possible loss in energy production. In addition, to the common rectangular layout, some wind farm layout optimization studies have suggested that wind turbines should be arranged in a scattered pattern [21]. In general, in a flat terrain (eg, offshore sites), wind farm layout is mainly based on the prevalent wind direction. If the wind speed is uniform with no dominant wind direction, the distance between wind turbines in rows and columns could be about 5D (where D is the turbine rotor diameter) [21]. However, if there is a predominant wind direction, turbines are generally spaced about 1.5D and 4D apart in the cross-wind direction to the prevailing wind direction, and between 5D and 12D apart in the direction of the prevailing wind.
The turbine wake, in general, is characterized by streamwise (axial) velocity deficit, which leads to less power being available for the downstream turbines. It also causes high turbulence levels which can give rise to high fatigue loads. The wake could have significant effects up to a distance 15D downstream of the upstream turbine [22]. The effect of these interactions will have severe implications on the downstream turbines which are located in the wake of the upstream ones. Depending on the distance between the turbines and the arrangement pattern in a wind farm, the power losses due to wake effects can be up to 23% [23] compared to a farm consisting of unobstructed turbines. In fact, these losses can be considerably higher for the first turbines immediately downstream of the most upstream turbine that is exposed to the undisturbed free-stream conditions. Similar effect is experienced on the subsequent downstream turbines but the effect decreases slowly downstream. The increase in fatigue loads on the downstream turbine due to wake interference effects can up to 80% [24] and this may severely shorten the life span of the rotor blades. Turbine wake properties and development depend on many factors which include the wind conditions (speed, direction and turbulence intensity), site topology and surface roughness and, upstream turbines operating conditions. The performance of any turbine operation within the wake of another turbine depends on these parameters as well as the distance between them. The wake of the turbine and some of the factors that can affect its properties are schematically shown in Fig. 12.12.
12.7. Control objectives
The WTG interarrays inside a wind farm (WF) are controlled to fulfil three objectives; (1) high energy efficiency, (2) compliance with grid codes, and (3) high reliability. The control methodology of WTG increases the amount of harvested wind energy as the case of maximum power tracking (MPT). However, controlling the power transmission through the WF interarrays is also significant. As an illustration, the voltage level, losses factor and connection topology have an impact on the WF capacity factor. For example, in some cases, the output power at the default cutoff wind speed of the WTG might be equivalent to the transmission and conversion losses from the turbine to the collection platform particularly in the case of radial topologies. Conversely, star (ie, ring) topologies offer a flexible power transmission, so that in case of poor wind conditions all the power of an array is transmitted through a shorter circuit to mitigate losses. The voltage source controlled DC links offer a wider range of power transfer control [25], such that the voltage level can be tuned within limited ranges (eg, ±10% from nominal voltage) to curtail the losses according to WTG production.
The second task for controlling the switching and reference values for interarrays is to fulfil the grid code requirements [26,27]. Modern grid codes imply strict requirements on WFs, especially during voltage and frequency events. Voltage events include solid line to line faults where the WTG must ride-through the fault with minimum possible duration of disconnection. In addition it has to provide post-fault reactive power/current support to retain the nominal voltage fast enough.
The control and manoeuvre of interarrays can play an essential role in limiting the fault currents, and making the WTG sense the fault in an earlier stage, instead of depending on the measurements at the point of common coupling (PCC). In the case of DC links, the stored energy in the cable capacitance could be utilized within a very short duration to provide the reactive power support when the appropriate controllers are integrated. Frequency support is also an important ancillary service to be provided by WFs. Similar to reactive power support, stored energy in cable capacitance can provide an active power surge to mitigate frequency drops [28]. In the case of over-frequency events, suitable manoeuvres (flexibility in interarray disconnection) will reduce the WF production so that the generation–load balance is reached faster. The control of interarrays can also alleviate the wind power fluctuations, especially when the WTGs, inside a WF, are located far from each other.
The reliability of WFs is highly related to the connection topology of WTGs. Flexible and expanded control methods provide several alternatives for power transmission which improve the overall WF reliability and curtail the amounts of non-supplied energy. In other words, when the connection topology is changeable through controlling the switchgears; power rerouting is possible when some components fail. This also helps in maintenance procedures, namely for collection platforms so that the platform can split into more than one sector (instead of making the whole platform/WF out of service) to improve the availability of the connected WF.
The applied control methods should be not so complicated and accurate at a reasonable sampling rate for the required signals, and average switching frequency for the installed power electronic converters (PECs). The complication of control methods implies longer time delays between setting the reference signals, and the response of controlled components. Moreover, the costs of integrated CPUs and FPGAs will increase, and the equivalent simulation models will require higher computational efforts (and memory storage), and their accuracy will be questionable. Likewise, high switching frequency is a requisite for some complicated control algorithms, which increases the losses and heating problems of PECs. These problems are clearer in the case of DC links rather than AC connections.
12.8. Collector design procedure
The high-level requirements for the electrical array were introduced in Section 12.2. The design procedure has to take into account and address some technical issues for the transmission and collection networks including the following [1]:
• Connection distance to the onshore grid. This will determine whether AC or DC transmission should be used (subject to a detailed site-specific cost-benefit analysis).
• The WTG size and WF layout. The WTG size is required in order to be able to design the wind farm array layout. The location of the WTGs and the offshore substation within a defined area must be known in order to be able to design a collection network that is optimized for lowest life cycle costs, determined by cable lengths, losses and availability. Some research suggests that a larger WTG could reduce the cost of the collection network somewhat, however the collection network with its relatively short lengths of cables bears only a relatively small proportion of the overall cost, and a larger WTG size may not be optimum for the entire wind farm.
• The optimum WTG output voltage. Whereas for the collection network itself there is no convincing benefit to change from the present standard AC fixed-frequency 50 Hz, a change in this may be beneficial for the WTG. Regarding DC, research has identified that it is currently an unfeasible option for the collection network because of the difficulties in stepping up a DC voltage to a required transmission level, and the market unavailability of DC circuit breakers. Variable frequency may allow omission of power electronic converters at the WTGs, thus reducing its costs, if the required converter control functionalities can be taken up by a VSC-HVDC converter offshore. Evaluating the use of 66 kV instead of 33 kV required an optimum design to be carried out for both voltage levels for a fair cost comparison.
• The optimal number of transmission links and offshore substations. This will partly depend on whether AC or DC transmission is used. Since high-voltage AC cables are more limited in their maximum power ratings than DC cables, they will naturally require more parallel links to connect a large offshore wind farm and thus have more built-in redundancy.
For a specific project the optimum collection and transmission networks can be designed, after the above issues are resolved.
A step-wise procedure for designing the electrical system for the internal collection grid of an offshore wind farm is now introduced [7]. Since the design choices are very dependent on the specific circumstances, including cost data that are difficult to obtain on a generic level, this outline does not intend to provide a recipe for making “best” choices. Instead, it aims to describe which electrical engineering considerations need to be addressed, in which order to proceed, and what the options are. This procedure is intended for present and near-future wind farms, such that the solutions which are considered are either presently available or expected to be available in the near future. Fig. 12.13 shows the flowchart for the design procedure for the electrical design of the interarray of an offshore wind farm. The flow chart is composed by selection, calculation and decision processes. These are represented in the chart by a block with blue line, green line and red line, respectively.
The procedure assumes that the capacity and physical layout of the wind farm have been defined previously. This means that wind patterns, wake effects, structural constraints, and all relevant studies concerned with the location are assumed to be available data.
In the flowchart, the RAMS stands for reliability, availability, maintainability, and serviceability. In general, high reliability is required from offshore wind farm components due to difficult and expensive access to perform repairs. Overall reliability is usually calculated based on a number of reliability indices specified at the component level. These indices include, for example, failure rate and time-to-repair. Availability refers to the ability of the wind farm to produce the maximum possible amount of power. It is dependent on the reliability, but can be increased by including redundancy.
The level of reliability/availability within the wind farm is a trade-off between the extra investment cost and the cost savings from reduced loss of production. The choice affects the grid topology and switchgear and protection system in the wind farm.
Figure 12.13 Flowchart of procedure for designing the electrical collector of an offshore wind farm [7].
Once the reliability indices have been defined, the RAMS assessment is done. Based on this information, the topology can be modified to improve wind farm availability or to make the system less costly. If the topology is modified all the power flow and short-circuit analyses must be done again to ensure the cables and protection systems are correctly calculated.
Abbreviations
| CAPEX | Capital expenditure |
| CPU | Central processing unit |
| D | Diameter |
| DC | Direct current |
| DFIG | Doubly fed induction generator |
| FPGA | Field programmable gate array |
| GIS | Gas-insulated switchgears |
| HVAC | High-voltage alternating current |
| HVDC | High-voltage direct current |
| IEC | International Electrotechnical Commission |
| IEEE | Institute of Electrical and Electronics Engineers |
| LV | Low voltage |
| MV | Medium voltage |
| OPEX | Operational expenditure |
| PCC | Point of common coupling |
| PEC | Power electronic converter |
| PMG | Permanent magnet generator |
| RAMS | Reliability, availability, maintainability and serviceability |
| SCIG | Squirrel-cage induction generator |
| SCT | System Operator-Transmission Owner Code |
| STATCOM | Static compensator |
| SVC | Static var compensator |
| VSC | Voltage source converter |
| WF | Wind farm |
| WTG | Wind turbine generators |
| XLPE | Cross-linked polyethylene |
Acknowledgements
Some content in Sections 12.3.1, 12.3.2, 12.4 and 12.5 is adopted from that originally published in [2]. The author of this chapter also acknowledges Harald Svendsen, Atsede Endegnanew and Raymundo Torres-Olguin for the material in Sections 12.3.2 and 12.8, which was adapted from their report in Endegnanew et al. [7].
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