However, by incorporating appropriate resistors into the rotor circuit, the slip is intentionally enhanced, but heat losses are increased and efficiency is reduced. Direct incorporation of the resistors into the rotor circuit triggers airflow and thus provides cooling. Since the air sucked in is saline, particularly at coastal sites, this design is prone to corrosion of the winding insulation. Currently, research is being conducted on outer rotor resistors which allow for a closed design of the actual generator.
A further possibility to influence the slip of asynchronous generators is the so-called double loaded asynchronous generator. Slip capacity is fed into or obtained from the grid by a frequency converter, whereas the stator is directly connected to the grid and the rotor is connected via the frequency converter. A modern insulated gate bipolar transistor allows for dynamic slip control and thus enables variable numbers of revolutions and idle power generation. A hybrid solution is an asynchronous generator in the form of an oversynchronous static Kraemer system (cascade conversion system). Slip power is unidirectional and can only be fed into the grid (Table 7.1) /7-6/.
Wind direction yaw mechanism. This system component serves for adjusting the machine nacelle, and thus the rotor, as exactly as possible to the respective wind direction. The wind direction yaw mechanism joins the machine house (na celle) and the tower head, as its components are incorporated into both system components (Fig. 7.11).
The nacelle is usually adjusted to the respective wind direction by a gear wheel mounted on top of the tower and operated by mechanical, hydraulic or electromechanical adjustment mechanisms. Small wind energy converters, rarely built nowadays, are provided with mechanical yaw mechanisms driven by wind vanes, servomotors or small size windmills. Bigger converters are usually provided with hydraulic, electromotive or electro-mechanical servo drives and are characterised by lower costs, smaller size and bigger torque at comparable construction costs.
All converters are additionally equipped with a stopping brake to lock the respective rotating mechanism. The brake compensates for low fluctuations in wind direction that may exert strain on the rotating mechanism and thus reduce its technical service life. It also permits to lock the nacelle during prolonged downtimes (e.g. during maintenance).
For bigger converters, the azimuth or tower head bearing is designed as antifriction bearing, whereas small converters are provided with friction bearings with sliding (e.g. plastic) elements. The entire wind direction yaw mechanism is controlled by a special control system that receives all relevant data from a wind direction measuring device mounted at the nacelle shell.
Tower. The main function of the tower of a horizontal axis converter is to enable wind energy utilisation at sufficient heights above ground, to absorb and securely discharge static and dynamic stress exerted on the rotor, the power train and the nacelle into the ground (Fig. 7.11). Another key factor regarding tower dimensions and design is the natural vibration of the tower-nacelle-rotor overall system in view of the prevention of dangerous resonance, particularly during rotor startup. Further influencing factors are dimensions and weight regarding transport requirements and thus available roads, erection methods, cranes and accessibility of the nacelle as well as long-term properties such as weathering resistance and material fatigue.
Most towers are made of steel and/or concrete. As far as steel constructions are concerned, besides the lattice towers usually observed for dated converters, there are also anchored and self-supporting tubular steel towers in closed, commonly conic design; the latter being the most common tower type applied nowadays.
The minimum tower height is determined by the rotor radius. Any additional tower height is a(n economic) compromise between the increased costs at enhanced heights and the increased mean wind speeds and thus increased power yield. Hence, the optimum between maximum energy yield and acceptable tower costs needs to be determined. This is why currently tower heights vary considerably with regard to site conditions; common tower heights vary between 40 and 80 m. On the mainland, due to generally lower wind speed increase at enhanced heights, when compared to coastal sites, usually higher towers (e.g. of heights of 90 or even 100 m or above) are built.
At equal power yield offshore installation of wind energy converters allows for a reduction of hub heights by approximately 25 %, when compared to onshore installation, due to different wind conditions at increased heights above ground (i.e. the mean wind speed at sea increases faster at enhanced heights above ground than on the mainland). Decreased hub heights correspondingly reduce tower costs.
Foundations. The type of foundation used to anchor towers, and thus wind energy converters, into the ground depends on the plant size, meteorological and operational stress and local soil conditions. On principle, support structures are subdivided into shallow and deep foundations. Both are state-of-the-art technologies but differ considerably with regard to costs. The optimum foundations design is determined by appropriate soil investigations.
Anchoring wind energy converters on the coastline is much more costly (see also /7-7/, /7-8/). There are various technologies that ensure stability. Foundation technologies include the support structures (i.e. foundation structure plus tower) and the technology required for anchoring the converters on the ocean floor. Installation is generally aimed at low manufacturing costs (e.g. by series production, material selection), low assembly costs (with regard to logistics, fast installation) and a long service-life (considering factors such as corrosion and fatigue).
Currently, floor-mounted support structures are preferred for water depths below 50 m. Floating support structures are also technically feasible, but will most probably be applied in areas with water depths exceeding by far 50 m. The exact dimensions of such floor-mounted support structures depend, among other factors, on the expected wind, wave and ice charges and geographical site conditions (e.g. water depth, soil conditions).
Floor-mounted support structures (Fig. 7.13) are subdivided further into gravity, monopole, and tripod foundations, outlined as follows. A forth type, currently of less importance, consists of a four-pile trelliswork construction, similar to the lattice towers applied for onshore wind energy converters.
Gravity foundation (Fig. 7.13, left). The gravity foundation principle is based on gravity force utilisation. Gravity foundations consist of concrete or steel frames which are filled with ballast on site. Foundations are put on the seabed on a levelled surface provided with a compensating layer to prevent transmittance of tensile forces onto the seabed and to make the system sensitive for extreme hydrody-namic loads. Since the maximum wave height, which partly influences the forces effective on the foundation, depends on the water depth, among other factors, larger foundations are required with increasing depths. According to current economic knowledge, this technology should only be applied up to a sea depth of 10 m. From a physical point of view, it should only be applied up to 20 m. Gravity foundations have, for instance, been applied for the offshore wind parks of Vin-deby and Middelgrunden (both located in Denmark); the mass of such a foundation amounts to approximately 1,500 t at a water depth of about 5 m and an installed wind energy converter capacity of approximately 1.5 MW.
Monopile foundation (Fig. 7.13, centre). This foundations type consists of a single foundation pile (monopole), which quasi extends the wind energy converter tower up to the seabed. Depending on the plant size and support structure design its diameter varies between 3 and 4.5 m and its mass between 100 to 400 t. Installation on the seabed is performed by pile-driving, vibrations, and drilling, whereby the penetration depth varies between 18 and 25 m. The actual tower of the wind energy converter and the foundation pile are subsequently connected by a joining element to compensate for a possibly oblique position of the foundation pile. From a current viewpoint, the maximum water depth, up to which the installation of monopile foundation is sensible, is reported to be 25 m. Monopile foundation technology seams comparatively economic for water depths up to 20 m and favourable ocean floor conditions and has, for instance, been applied for the offshore wind parks of Bockstigen and Utgrunden (both located in Sweden).
Tripod foundation (Fig. 7.13, right). Tripod foundations consist of a central column element connected to the tower of the wind energy converter and a three-legged three-dimensional steel truss, which transmits the forces and the torque effective onto the construction corners. The diameter of the foundation piles anchored into the seabed by pile-driving, drilling and vibrations, amounts to about 0.9 m. Depending on the respective soil conditions, the desired seabed penetration depth varies between 10 and 20 m. Tripod foundations are most suitable for water depths exceeding 20 m, as landing water vehicles may collide with the steel truss submerged into the water at water depths below 7 m. To date, the described tripod foundations have not yet been applied for offshore wind energy converters. However, their application at a water depth of about 30 m is planned for the first German offshore wind park "Borkum West", currently being planned for the North Sea. The mass of such a foundation designed for the planned multi-megawatt converters amounts to approximately 800 t.
Fig. 7.13 Floor-mounted support structure of offshore wind energy converters (gravity foundation (left), monopile (centre), tripod (right); see /7-7/, /7-8/)
Grid connection. With regard to the connection of wind energy converters to the public power grid or any isolated power grid, direct and indirect grid connections are distinguished /7-6/; for both types asynchronous and synchronous generators are suitable (Table 7.1).
- For direct connection to an invariable frequency power grid, as it is the case for public European supply, synchronous generators turn at a constant number of revolutions and asynchronous generators at an almost constant number of revolutions according to the grid frequency (Fig. 7.12). Due to the inevitable "hard" connection, particularly in case of synchronous generators, high dynamic stress may be exerted onto the power train (hub, shaft, gearbox and generator rotor). This is why in most cases asynchronous generators are applied for direct grid connection.
Table 7.1 Comparison of direct and indirect grid connection with regard to the applied generator type (see /7-6/)
_Synchronous generator_Asynchronous generator_
Direct nG = f grid constant number of revolu-
connec- tions; hard grid connection tion
Indirect 0.5 f<nG <1,2f grid variable number of revolu-
connec- tions; grid connection via a tion rectifier with connected in verted rectifier (i.e. intermediate direct current circuit or direct converter); soft grid connection nG = (1 - s) f; 0 >s >-0.01 slightly declining number of revolutions that decreases with increasing converter size; simple grid synchronisation; reactive power consumer; relatively hard grid connection
0.8 f <nG <1.2f variable number of revolutions; squirrel cage induction machines via direct current intermediate circuit or direct converter (reactive power consumer); slip ring induction machine via dynamic slip control, oversynchronous static Kraemer system (both reactive power consumers) or double loaded asynchronous generator with direct stator and indirect rotor connections (e.g. via direct current intermediate circuit) (reactive power generation); soft grid connection nG generator number of revolutions; s slip (deviation from nominal number of revolutions); f grid frequency.
- Indirect grid connection converters may be connected via a direct current intermediate circuit, which allows for the operation of wind energy converters at a variable number of revolutions and generates alternate current at variable frequencies. Current is first converted to direct current by a rectifier and subsequently reconverted into alternate current by an inverted rectifier to match the voltage and frequency specifications of the power grid. This allows for optimum aerodynamic operation of the rotor within a revolution range from 50 to 120 % of the nominal number of revolutions (Fig. 7.12). The variable number of revolutions reduces the dynamic stress exerted on the converter. However, the direct current intermediate circuit incurs in additional costs and increases electrical losses. Grid connection via direct current intermediate circuits is common practice for medium to large converters, preferably in conjunction with synchronous generators.
For older converters equipped with direct current intermediate circuits, frequently inverted rectifiers have been used, which in some cases produced considerable overtones. They may interfere with the operation of other equipments in weak power grids. However, recent investigations in terms of power semiconductors have led to the use of inverted rectifiers, which supply current with relatively little distortion and are also partly suitable for idle power provision (e.g. IGBT inverted rectifiers with pulse-width modulation (PWM)).
Provided that the required network specifications are strictly adhered to, wind power converters may also be indirectly connected to the grid via direct converters.
Wind power converters can either be connected to the grid as individual converters or in the form of a wind park. The connection grid interference caused by the converter or the wind park has to be determined at the respective grid connection point. Short-term power fluctuations, sensitively perceived as flickering lights by the human eye, but also continuous voltage changes and possible overtones need to be considered. Fluctuations may be measured by the ratio of converter power to grid short-circuit power at the grid connection point. If certain values are exceeded, connection is only possible at a connection point of superior grid short-circuit power (e.g. at the bus bar of a substation), in order not to interfere with other consumers connected to the grid.
The main components of a grid connection are the electric coupling of the wind power converter or the wind park to the transformer, a transformer (if required) including a distributing substation equipped with medium voltage substation as well as a medium voltage connecting line up to the grid connection point.
The design of every wind power converter including control and protection equipment has to rule out all potential damage to the grid due to a failure of the wind power converter (such as power failure or short-circuit). Complete disconnection also has to be ensured for the safe performance of operation and maintenance.
Losses occur when power generated by wind energy converters is fed into the grid. They are mainly caused inside the transformer when power is converted into heat. However, they are comparatively low and at the most amount to a few percent.
System aspects of offshore installation. The available technologies need to be adapted to suit the changed framework conditions of offshore operation, when compared to onshore installation. Technologies may also be optimised accordingly.
Since onshore wind energy converters are in most cases optimised with regard to minimum noise creation, and as blade tip speed is the deciding parameter regarding noise emissions, the maximum number of rotor revolutions is subject to certain restrictions with increasing rotor diameters. If noise characteristics are less important, as it is likely for offshore wind energy converters, a higher numbers of rotor revolutions should be admissible. These would result in decreased tower head mass (i.e. power train mass) due to the reduced driving torque and would also help cut costs. However, considerably enhanced blade tip speeds may damage rotor blades due to erosion at high concentrations of particulate matter in the air. A high content of water drops and salt particles in conjunction with a relatively high air humidity and throw of spray, as it can be expected for offshore installations, additionally require effective protection of all offshore wind power converter components against corrosion and detrimental deposits.
Electric and electronic system components (such as operating control, sensors, generator, and transformer) require special protection against spray and deposits. For this purpose hermetic protection against ambient air or an installation under slight excess pressure, ensuring appropriate air-conditioning, appear suitable. However, the climate has to be controlled with regard to temperature and humidity to prevent overheating and moisture condensation.
The design of offshore wind energy converters includes the installation of hoisting equipment on board to remove and insert all major components, such as generator, gearbox, if applicable, etc. without expensive external hoisting equipment, from and into the nacelle. Furthermore, each offshore wind energy converter needs to be provided with a landing platform to enable personnel landing and material provision.
To ensure plant safety and a safe start-up in the case of power failure, an emergency power generation unit must be provided. Available options include batteries or power stand-by units for electro-mechanical systems, hydraulic accumulators for hydro-mechanical systems and spring brakes for exclusively mechanical systems. However, such short-term accumulator units only secure shut-off of wind energy converters. Special stand-by units are required for prolonged power failures.
To achieve the same plant availabilities as for onshore converters, in spite of the meteorologically more difficult plant accessibility, particularly during winter months, quality and robustness of all plant components need to be ensured over long periods under hard environmental conditions. For this purpose a highly sophisticated operating control system is required. The system should allow for an early detection of damages, start itself after possible grid damages and revert the wind energy converter to normal operation. Remote complete reprogramming and re-initialisation should also be possible from the onshore control panel. For this purpose, effective tele-monitoring and reliable communication technologies are required. Furthermore, the converter should be adequately equipped with all major sensors (such as vibration monitors and temperature sensors) and all indispensable system components.
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