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Due to wind energy's energy saving and environment-friendly, most countries, particularly some European countries, use it to generate electricity. However, the instability of wind becomes a barrel for electricity industry to spread it. Conversely, the complete calm periods of sea is hardly to occur or ending soon. Naturally, offshore wind is becoming an emerging productivity of global wind power market. Offshore wind farm, which accounts for more than one third of UK electricity consumption, contributes to UK's total electricity generation capacity . As can be seen from Figure 1, with the increasing annual wind turbine installation, the cumulative offshore capacity of UK tends to rise quickly in these few years than ever before. Obviously, developing offshore wind farm is an essential trend for expanding renewable generation, especially recognizing the negative impact of Japan's nuclear power plant explosion.
Figure 1. UK offshore wind capacity delivery .
Basically, offshore wind farm could be roughly separated into three elements, including wind turbine, transmission cables to shore and onshore AC grid. In the past time, turbines for offshore wind farm had to follow onshore wind turbine markets, while nowadays turbines are being designed specifically for offshore deployment with larger capacity (i.e. 5MW+ rather than up to around 3MW for onshore use) . The offshore transmission cables focus on delivering the electricity to the public network and still keep to be challenging since the high level transmission tends to be applied. After transporting, electricity generating from wind turbine could be allocated to different AC grids. Alternatively, the technical interaction exists in the parts of grid, wind turbines and wind farm electrical system.
Figure 2. Offshore wind farm .
This literature review gives a overview to each component of a offshore wind farm electricity system, constructed with offshore wind farm and power transmission system, and prepares for the following research of the impact on the main-line grid transient overvoltage due to offshore wind farm connections. According to those overcomes from researchers, this project could get effective theoretical support during the next phase.
Offshore Wind Turbine
2.1 Component and Technology
In general, a common offshore wind turbine includes four basic components, rotor, nacelle, tower, and the balance system. Nacelle component takes the mainly responsibility of producing electric power, which consists of several major sections . From Figure 3, there are many components inside the nacelle of the offshore wind turbine. And the generator component is a core element for offshore wind turbine to change the sufficient wind into electricity.
Figure 3. Component inside the nacelle of the Vestas V80 Turbine .
Like onshore wind turbine, air flow could be transformed to kinetic energy in the rotating blades driving the generator, which would ultimately transform the kinetic energy to electricity . Since the wind moves more quickly and almost blows from one direction than on land, more electricity generated per square meters. The bellowing Table 1 shows that offshore wind turbine mostly implements in sufficient winds area where wind turbine could generate more electricity through flowing at a relative higher speed than onshore wind turbine.
Table 1. Suitable application situation of offshore wind turbine .
Also, due to the special environment of offshore wind turbine, there are some different characteristics than onshore wind turbine. At present, all of newly produced wind turbines owns new characteristics than early offshore wind turbines, that is, sizes of them are larger and the variable speed concept . Takoudis indicates that those larger rotor diameters of wind turbine have the capability to capture more wind energy . Moreover, locating in the sea means tip speed of offshore wind turbine could reach more higher, without worrying about the noise limitation and expensive drive train costs. Although the size of offshore wind turbine tends to larger, the height of it just remains the same or even less than the diameter. Because high tower determines high spending on materials and installation, which is not cost effective for offshore use . Despite of this financial reason, blazes still need to keep clear of waves via some newly techniques, even under the extreme weather conditions. To withstand the extreme conditions or unpredictable weather, engineers design a system that could make blazes turn out the wind and slow down for safety reasons ( i.e. 50 miles per hour and above) . Besides, compared to onshore wind turbine, protection for turbine structural components' corrosion by salt from seawater is a special demand of offshore one. Some researchers estimate that wind turbines exploited for optimized performance offshore will be increasingly developed, in terms of the expecting market expanding speed .
2.2 Wind Turbine System
In terms of changeable ocean wind speed, the development of variable speed system exists in huge potential since its wide- range variable speed. Henderson et al. states that, " Wide- rang variable speed has a further advantage in the ability to avoid damaging resonances, important for offshore turbine structures, where the resonant frequencies have proved difficult to predict accurately and may also change over the lifetime of the structure" . Furthermore, variable speed system allows a higher electricity production provided the same wind energy and drops down the dynamic loads on the gearbox .
In modern wind turbine systems, it is obvious that induction generators occupied the main generator market . However, the permanent magnet and electrically exited synchronous generator, another type of generator, is also used in several types of wind turbine, especially some large capacity turbines. This machine could directly connect without gearbox, which will make contribution to turbine's lifetime and maintenance . According to Mittal et al., there are many differences between the permanent magnet and electrically exited synchronous generator and induction generator, including: high power factors and efficiencies depending on self- excitation, magnetization coming from the permanent magnet pole system or a dc rotor . This part of paper will give an overview and comparison of Double Fed Induction Generator (DFIG) and Permanent Magnet Synchronous Generator (PMSG) in following sections.
2.2.1 Variable Speed System Using Double Fed Induction Generator (DFIG)
As Kayikci has reported in his paper that during the past few years, high capacity wind power plants tend to use Double Fed Induction Generator since it could control reactive power exchange and electrical torque . Based on these advantages, Double Fed Induction Generator could realize variable speed operation and maintain the stability of electrical system better, particularly under some large disturbances. Like a common induction machine, Double Fed Induction Generator also connects to the rotor via a gearbox. As illustrated in Figure 4, the rotor winding is connected to slip rings and back-to-back converter, whereas the stator winding connects the grid directly . The grid side converter could receive the slip power from the rotor-side converter via the middle DC-Link converter, which is regarded as a cascade . The back-to-back converter controls the current and voltage of this system and enables the variable speed operation of turbine .
Figure 4. Wind turbine system with DFIG .
As a result of the back-to-back converter, Double Fed Induction Generator could operate normally without worrying about overloaded under high wind speed conditions. Moreover, Stiebler states that this back-to-back voltage converter has the ability to change active power direction automatically, such as transmitting from grid to the turbine rather than grid to the turbine . The AC filter, around grid side converter, enables to avoid harmonic voltage disturbance together with guaranteeing rating voltage level during system operating.
2.2.2 Variable Speed System Using Permanent Magnetic Synchronous Generator (PMSG)
Unlike induction generator, the magnetization of Permanent Magnet Synchronous Generator comes from a Permanent Magnet Pole System on the rotor rather than obtains excitation current from the armature winding terminals . This type of generator could connect to wind turbine without generator and achieve self-excitation through applying permanent magnet rotor. Compared to Double Fed Induction Generator (DFIG), the impact of the grid disturbance on the generator is decreased since the back-to-back converter separates the Permanent Magnet Synchronous Generator (PMSG) from the grid .
Figure 5. PMSG and converter .
In terms of Mittal's summary, the advantages of Permanent Magnet Synchronous Generator contains following points :
Higher efficiency and energy yield,
In the modern wind turbine market, the PMSG based wind turbine is gradually attractive. Some researchers estimate that this kind of variable speed direct drive permanent magnet generator will be a new tendency for offshore wind power in the future, due to the less cost of power electronics .
Electrical systems are extremely significant components for connecting the offshore wind farm and onshore grid. Figure 6 is a briefly illustration of off shore wind farm system. The functions of electrical system contain four sections, integrating AC power out put from individual wind turbine, step-upping voltage for transmitting to onshore substation and converting receiving AC power to DC for onward transmission . In order to improve the reliability and stability of the offshore wind farm, many new technical developments have been exploited and utilized in the field of electrical systems. In this part of paper, the set-up of onshore and offshore substations, use of transmission technology and cable technology will be illustrated in following paragraphs.
Figure 6. Offshore wind farm system .
3.1 Transmission Technology
Between offshore wind farm and common AC network, transmission system is used to delivery the electricity step-upped by offshore substation to onshore substation, which will transform the electricity to an appropriate voltage level for the integrated grid. In some cases, the distance starting from offshore wind farm to onshore network might up to 100 km or even reach 200 km, so that the power losses of the long distance transmission cable could not be ignored . Hence, high voltage transmission technology, during long distance transmission, is used to reduce the power losses. In general, there are two mainly technologies depending on different capacities of wind farm and distances between wind farm and grid on land: High Voltage Alternating Current (HVAC) technology and High Voltage Direct Current (HVDC) technology. For the small-scale and short distance wind farm, HVAC technology is applied to connect offshore and onshore systems. In comparison with HVAC, HVDC technology is used to connect large-scale offshore wind farm to onshore grid with a relative long distance. And also, HVAC technology can be classified into two basic types: Line- commutated Converter (LCC)- based HVDC and Voltage Source Converter (VSC)-based HVDC technology. Figure. 7 show a general application of different transmission technology.
Figure 7. Choices of transmission technology for
different wind farm capabilities and distances .
3.1.1 Comparison between HVAC and HVDC
As AC power is common form of electricity, most operational wind farm use HVAC connection to transmit electric power. Because of the special location, submarine cable is the best material to deliver power. However, the excessive reactive current would have influences on transmission cable, which will both increase power losses and decrease the power transfer capability of HVAC cables . On the other words, if the distance increases, the power losses during the procedure of transmitting will rise at the same time. Furthermore, the excessive reactive power from long distance cable may probably lead to over voltage. As a result, HVAC links is a popular connection method for small- scale wind farm over a short distance because its cheap cost and simple structure. To meet the demand of integration requirement, some extra electronic based FACTS devices, including Static var compensator (SVC) and Static synchronous compensator (STATCOM), are applied in HVAC connection . Commonly, the function of SVC and STATCOM is providing reactive power to compensate system.
On the contrary, According to Henderson et al., HVDC links and controllable reactive power compensator will possibly offer many opportunities, which permit large-scale offshore wind power plants to meet grid connecting demands more easily, to encourage the grid stability to a higher level . Alternatively, there is no need to worry about the excessive reactive power, which would effects power quality. Thus, HVDC transmission technology is more attractive in long distance transmission, such as offshore wind farm system. In accordance to Yao, there are several advantages of HVDC transmission as follows :
Power flow is controllable;
The power losses of submarine cable is less than normal HVAC transmission cable;
Isolate faults between offshore and onshore AC grids through utilizing asynchronous connection;
Cable charging has no influence on HVDC transmission.
Similarly, disadvantages also exist in HVDC transmission :
High expenditure than HVAC transmission, such as electronic devices;
Complex control system
As shown in Figure 8, they are two basic model of single line HVAC and HVDC interconnection of offshore wind farm to the grid. HVAC model uses three conductor cables, while HVDC model uses one pair of single return conductor cables. And clearly, two Voltage Source Converters are equipped in the two sides' bus bar. This VSC-based HVDC links make it possible to connect weak networks since VSC has the capability of controlling active and reactive power quickly and independently at the bus bar . For large transmission power capacity, installing this converter system is necessary. In the comparison of HVDC and HVAC transmission, there are many distinctions. The main merits of utilizing HVDC transmission than HVAC transmission are  :
HVDC subsea cables are less costly than HVAC subsea cables
Improve system stability via decoupled connection
Support large-scale wind farm transmitting power
Power transmission capability is independent from distance
High efficiency during long distance transmitting
Full voltage control
Figure 8. Single line diagram of HVAC and HVDC interconnection of
offshore wind farm to the grid .
3.1.2 Two Types of HVDC Transmission
In terms of different kinds of converters, HVDC transmission can be primarily separated into LCC- based HVDC and SVC-based HVDC. LCC-based HVDC technology has been used for decades of year, and adoptable for high voltage level. Based on the thyristor bridge converters, which could draw reactive power, an alternating voltage needs to be provided. On the contrary, VCS-based HVDC consists of IGBT, which has the ability to flow high currents. Compared to LCC- based HVDC, some special features of SVC-based HVDC could be summarized as bellowing three points :
Separate active power control from reactive power control;
Maintain reactive power flowing with out influences from the other sides' AC network;
There is no need to provide another voltage source.
Although above characteristics make SVC-based HVDC transmission popular than conventional one, the power loss is much higher. Therefore, it is rather important that different type of connection method for offshore wind farm will affect system's reliability under normal situation and fault conditions. Table 2 is a summary of four transmission technologies compared with three points: gear size, coverable distance and cost of transmission system. Take conventional HVDC transmission as an example, it is more suitable for long distance transmission even though its high cost, such as offshore wind farm.
Table 2. Comparison of shore connection options .
3.2 Cable Technology
Currently, the commonly used cable is called XLPE cable, which requires an impermeable moisture barrier . And XLPE cable has lower capacitance and the ability to hold reactive power. From researcher's studies, XLPE insulating cable is much suitable than paper insulating one for long distance transmission . Hence, this kind of cable is widely used to connect each section and transmit the electric power from wind turbine to shore in the whole offshore wind farm system.
They can be classified into three main cables, including: export cables and array cables. Firstly, export cable is used to transmit electricity from offshore and onshore substations. In fact, most operational wind farm use AC three-phase cable to output power form offshore wind farms. As mentioned before, the HVAC transmission cable is 3-core while HVDC just contains two single-core higher-voltage cables . And Volzke states, "authorities are not planned to support single-pole transmission systems with one cable route and returning of the current through subsea electrodes " . Thus, the layout to DC cable is simpler than AC cable, because there is only one path between those two AC networks. Also, it is clear that the weight and cost of HVDC cables is less than conventional AC submarine cables. Secondly, array cables connects offshore wind farm to offshore substation. This cable is equipped to individual wind turbine.
Since the particular environment of wind farm, it is necessary to provide protection to keep away from the wave and tidal action, such as J-tube seals, Bend restrictors, Stiffeners and Cable mats . Take J-tube seals as an example, it could prevent the water enter into the J-tube. And also, choosing a functional cover material to prevent seawater corrosion is needed.
4. Offshore Wind Farm Integration
For an offshore wind farm, if one part of power system encounters faults, this section would also face interruption from power system. During integration process, the reactive power control and voltage support must be guarantee in transmission system . Therefore, continuous uninterrupted operation is required even under faults situations for offshore wind farm integration, for instance, short circuit fault. Typically, ride-through capability is used be a form of the time, which needed to distribute between 100-200 ms in the high voltage system .
4.1 Requirements for Integration
Figure 9. Classification of power system stability .
A successful integration between offshore wind farm and on land main grid must keep the stability of power system. On the other words, power system stability is the main requirement of wind farm integration. As illustrated in Figure 9, power system stability could be classified in three aspects :
Rotor angle stability;
When connecting, the rotor angle of generator must keep the same under disturbances. And also, steady frequency needs to be maintained even though there exist in imbalance between two AC networks . As for voltage stability, power system is required to keep stead voltages in whole system despite of different kinds of disturbances.
Under a normal operational situation, wind turbine would runs at an acceptable wind speed. However, some extreme weather conditions- typhoon- may lead to massive tripping of the induction generator because of exceeding the prescriptive wind tripping speed of turbine. This sudden change of wind turbine operation brings in rather huge impacts to network and voltage fluctuation.
4.2.1 Overvoltage causes and impacts
As the whole system must operate under a stable situation, the influences of overvoltage could not be ignored. Based on Akhmatov's studies the reason of occurring overvoltage can be presented as following reasons :
Excessive reactive power in the transmission system on hand;
The distribution transformers' tap changers and reactive shunts could not quickly move
Connected grid is not strong enough to meet the requirement of fast control
The absorption of Reactive power decreases together with active power generation
To be more specific, for HVAC technology, many researchers have studied the stabilities and reliabilities for decades of years. Since the implementation of HVAC for offshore wind farm needs long distance transmission, the overvoltage issue requires taken into consideration carefully. Generally, the cable owns large shun capacitance. As mentioned in Guo al et. ' paper, the HVAC cables would be charged and discharged in different wave period and as a result of it, large reactive power is produced continuously . In accordance to the power balance equation:, when the reactive power rises, the active power falls. Moreover, large reactive power causes overvoltage at the end of cable . Thus, Hanson divided this overvoltage into three types :
Power frequency and transient overvoltage;
In this literature review paper, we focus on transient overvoltage. The reason of transient overvoltage usually comes from switching operations for faults .Clearly, transient overvoltage exceeds the voltage rating of power equipment, which will lead to destructions. And in some cases, mistaken relay action will be caused by overvoltage. To certain extent, overvoltage affects voltage quality and stability in the gird. Theoretically, transient overvoltage of higher frequency may also come from resonant conditions of gird, and owing to harmonic overvoltage, high voltage pressures may possibly appear .
4.2.2 Overvoltage Solutions
In order to solve the overvoltage issue, Akhmatov also proposed several solutions respectively. Via installing switched capacitor banks to offshore wind turbine, the excessive reactive power could be drawn, so that transient overvoltage issue resolved . It must be emphasized that the function of regulating capacitors numbers of capacitor banks plays a vital role in this process. Although installing capacitors can reduce reactive power, onshore transmission network requires solve the overvoltage completely under some disturbances . Dynamic compensators, which demand sufficient voltage support, are therefore recommended to applied to avoid overvoltage of on land transmission network . For instance, if the established SVC compensator meets the ratings of requiring values, both reactive power controlling and voltage support could be performed well. To weak grid, dynamic compensators perform better if given efficient voltage.
Apart from this, the grid code of some European countries has been extended to deal with problems, such as transient overvoltage, in offshore wind farm integration. The grid codes give new definitions for some requirements, containing: normal steady state stability and dynamic stability under faults conditions .
This literature review gives an overview of an offshore wind farm model, containing: offshore wind turbine, onshore and offshore substation and transmission system. Through the study of this period, different types of elements will make different influence on power system. And also, comparison of power devices in this paper provides clear characteristics and helps to build up simulation model in next step. After theoretical studies, however, there are still a lot of things needing to study. In next stage, an offshore wind farm model requires to establish and analyzing the impacts of transient overvoltage from wind farm integration under different faults or disturbances.