Continuously controllable apparatus
This group of apparatus will normally include a voltage source converter, controlled by various control strategies. The connection to the network grid is usually done by use of transformers. New power electronic devices such as Insulated Gate Bipolar Transistors IGBT , Insulated Gate Commutated Thyristors IGCT , and MOS controlled Thyristors MCT are used in the converters. Depending on the topology used for the converters they are split up into two groups The shunt converter injects current...
Advantages in the use of the Tandem Converter
The investigation of new converter topologies is commonly justified by the search for higher converter efficiency. Advantages of the tandem converter are the low switching frequency of the primary converter, and the low level of the switched current in the secondary converter. It is stated that the switching losses of a tandem inverter may be reduced by 70 , Trzynadlowski et al., 1998a in comparison with those of an equivalent VSI, and even though the conduction losses are higher for the tandem...
The Transverse Flux machine
The Transverse Flux TF machine topology is new, but appears to be interesting. The high values of torque per kilogram of active materials reported make it seem very attractive. The transverse flux principle may be applied to a range of machine types, e.g. both permanent magnet and reluctance machines could be made. The machine will inherently behave as the generic type applied, but will have characteristics, influenced by the TF design. Here, the Transverse Flux Permanent Magnet Machine will be...
Axial Flux PMG with salient poles
The hybrid axial flux PMG with salient rotor poles is described in Muljadi et al., 1998 . This is a modular construction, which should make it easy to maintain. Like the transverse flux machines, for this design a complete rotor stator unit is required for each phase. Poles may be added around the circumference to increase the torque. Figure 16. 3D representation of hybrid axial flux PMG with rotor saliency Muljadi et al., 1998 . Figure 16. 3D representation of hybrid axial flux PMG with rotor...
Wound rotor Induction Generator
It has been found useful to apply the wound rotor Induction Machine as a Wind Power Generator. In this case, the rotor is provided with a three-phase insulated winding very similar to that of the stator. The rotating winding may be connected to the stationary supply circuits via a set of slip rings and brushes. This is the traditional manner. An alternative is to use a power electronic converter, which may or may not require slip rings. The output of the generator may now be controlled, by...
State of the art
Since the start of modern wind turbine development in 1957, marked by the innovative Gedser wind turbine 200 kW , the main aerodynamic concept has been a horizontal axis, three bladed, downwind wind turbine, connected to a three phase AC-grid. Many different concepts have been developed and tested since. Activities in this field were encouraged by the oil crisis in 1973. Twenty years ago the concept of the Riisager wind turbine 22 kW initiated a new area. The concept was similar to the...
Description
The synchronous machine is usually built with a rotor carrying the field system, provided with a winding excited by DC. The stator carries a three-phase winding quite similar to that of the induction machine. The rotor may have salient poles, or may be cylindrical - Figure 27. Salient poles are more usual in slow speed machines, and may be the most useful version for application to wind turbine generators. The saliency gives added torque, and a stiffer machine operation. Figure 27....
Capacitor bank
For the power factor compensation of the reactive power in the generator, AC-capacitor banks are used, as shown in Figure 36. The generators are normally compensated into whole power range. The switching of capacitors is done as a function of the average value of measured reactive power during a certain period. Figure 40. Capacitor bank configuration for power factor compensation in a wind turbine. Figure 40. Capacitor bank configuration for power factor compensation in a wind turbine. The...
Multilevel Converter
Since the development of the neutral-point clamped three-level converter Nabae et al., 1981 , several alternative multilevel converter topologies have been reported in the literature. The general idea behind the multilevel converter technology is to create a sinusoidal voltage from several levels of voltages, typically obtained from capacitor voltage sources. The different proposed multilevel converter topologies can be classified in the following five categories Lai amp Peng, 1996 Lai amp...
The Asynchronous Machine Induction Machine
Induction machines may generally be set in two categories, those with squirrel cage, and those with wound rotor. Both categories were considered, as each has some special features to offer. The search term 'Induction Generator' produced an immediate list of some 303 articles, and standards dating from 1990 and later. This was reduced to 133, by rejecting the majority of articles concerning island operation, and studies of the power electronics converter, and control system. The list of 133 is...
The High Voltage Machine
High voltage machines are manufactured both as synchronous machines and as asynchronous machines. They have been used in applications where high power is needed. They have especially been used for power generation in large power stations, but also for large loads such as large pumps, cement mills and fans. The main motivations for using high voltage machines are the reduction in current, leading to lower copper losses and possible direct connection to the grid without the need for a...
Operational Characteristics Normal Duty Wqe
Generator operation is claimed to be characterised by single pulse operation, rather than chopping, the DC-link voltage being kept constant during operation. By arranging the switching timing to position current pulses where the inductance is decreasing, generating mode may be selected Husain et al., 1999 Cardenas et al., 1995 . The bulk of the generated current is carried by the diodes Cardenas et al., 1995 . The controlled switches mainly provide excitation current Cardenas et al., 1995 . SRG...
Operational Characteristics Normal Duty
In normal duty and directly connected to a stiff AC grid, the Induction Generator is very robust and stable. The slip varies, increasing with increasing load. The major problem is that, due to the magnetising current supplied from the grid to the stator winding, the full load power factor is relatively low. This is to be compared with the fact that most power distribution utilities penalise industrial loads with low power factors. Clearly generation at low power factor cannot be allowed either....
Operational Characteristics Normal Duty 1
The Permanent Magnet Generator is a form of synchronous machine. As such it will be very stiff in operation if directly connected to the grid. That is to say that all variations of load and power from the turbine must be accommodated at constant shaft speed. This situation is unlikely to produce robust stable operation. The alternative is to use a power electronic controller between the generator and the grid. To compensate for the cost of the controller, the gearbox may be omitted. It may then...
Doubly salient Permanent Magnet Generator DSPMG
The DSPMG in Figure 15 is described in Sarlioglu amp Lipo, 1998 . No claims are made as to improvements in performance. It is expected that the DSPMG will improve the performance of an SRMG, by the addition of permanent magnets to bias the magnetic circuit. Alternatively, the performance of a surface mounted PMG is improved by the addition of saliency. This paper focuses on the modelling of the DSPMG, without comparing its performance with other types of generator. Figure 15. Cross-section of...
Conclusion on PMG
The PMG seems attractive as a wind turbine generator. Advantages include self-excitation, high power factor, and high efficiency. Designs have been proposed in cylindrical and axial flux versions. In the PM machine, efficiency is higher than in the induction machine, as the excitation is provided without energy supply, by the Permanent Magnets. However, permanent magnet materials are expensive in initial purchase, and are difficult to handle in manufacture. Additionally, the use of permanent...
The Permanent Magnet Generator
Figure 11 shows the cross-section of a typical Permanent Magnet Generator PMG . The PMG differs from the Induction Generator in that the magnetisation is provided by a Permanent Magnet Pole System on the rotor, instead of taking excitation current from the armature winding terminals, as it is the case with the Induction Generator. This means that the mode of operation is synchronous, as opposed to asynchronous. That is to say, in the PMG, the output frequency bears a fixed relationship to the...
Voltage Regulation
The main disadvantage of the permanent magnet generator is that it does not readily provide a constant voltage, when the shaft speed and the load current vary, Mitcham amp Grum, 1998 . Voltage regulation is the term used to describe the variation of the terminal voltage, as a result of load variations, see Figure 12. In a synchronous machine with a wound field winding this regulation may be compensated by varying the magnetising current. In a 30 W generator, fitted with ferrite magnets, the...
Categorisation and characterisation of disturbances in the grid
According to Dugan et al., 1996 and Thomsen, 1999 the different disturbances may be classified as in Table 8. Table 8. Categories and typical characteristics Low-Voltage grid for variations in the supply voltage with given voltage quality parameters. EN for chapters in EN 50160 Thomsen, 1999 . Table 8. Categories and typical characteristics Low-Voltage grid for variations in the supply voltage with given voltage quality parameters. EN for chapters in EN 50160 Thomsen, 1999 . These categories...
TFPM basic design
The magnetic circuit of the TFPM is complex due to stray flux and the constantly changing saturation of the iron. This means that analytical analysis of the magnetic circuit is difficult. Therefore, numerical methods are attractive. A well-known and documented method is the Finite Element Method FEM , where the problem is defined inside a boundary and solved by division into small segments with knowledge of the surrounding boundary conditions. Figure 24. Sketch of the flux path in a transverse...
TFPM technology
The TFPM machine has its origins in the SR machine, and it is found to be a promising new design of a linear SR machine. The SR machine is named after its ability to exploit the change in air gap reluctance. This means it produces force by moving according to the maximum inductance of the excited winding. The motion may be rotary or linear, and the rotor may be interior or exterior Miller, 1989 . In a TFPM machine the flux path in the core back iron is perpendicular transverse to the direction...
Disadvantages of applying the backtoback PWMVSI
This section highlights some of the reported disadvantages of the back-to-back PWM-VSI which justify the search for a more suitable alternative converter In several papers concerning adjustable speed drives, the presence of the DC-link capacitor is mentioned as a drawback, since it is heavy and bulky, it increases the costs and maybe of most importance, - it reduces the overall lifetime of the system. Wen-Song amp Ying-Yu, 1998 Kim amp Sul, 1993 Siyoung Kim et al., 1998 . Another important...
Soft starter
The soft starter is a power converter, which has been introduced to fixed speed wind turbines to reduce the transient current during connection or disconnection of the generator to the grid. When the generator speed exceeds the synchronous speed, the soft-starter is connected. Using firing angle control of the thyristors in the soft starter the generator is smoothly connected to the grid over a predefined number of grid periods. An example of connection diagram for the soft-starter with a...
Stiff or weak grid
The properties stiff and weak are more difficult to handle in a short and precise definition. A stiff or a weak grid is not just a stiff or a weak grid, independently, since a grid could be characterised e.g. as frequency stiff and voltage weak. A frequency weak grid would influence the energy production of a fixed speed wind turbine in a less desirable manner. This has been studied in e.g. S0rensen et al., 2000 . Another problem related to the stiff weak property is that, while a grid with a...
Disadvantages of the NCC
Although the complexity is less than the pole commutated resonant converter Pickert amp Johnson, 1998 Teichmann amp Bernet, 1998 , the converter is still more complex hardware and control than the back-to-back PWM-VSI and several additional components are needed. Furthermore sensors are needed to detect the zero voltage events and to determine the energy needed to maintain the resonance. As mentioned above, the use of the DPM technique yields fast response time, but a consequence of the DPM is...
Torus Generator
The Torus machine, see Figure 14, with a toroidal stator, wound without slots and employing an axial air-gap, has been proposed as a wind turbine generator Chalmers et al., 1996 Spooner amp Chalmers, 1992 Wu et al., 1995 . Difficulty with assembly was overcome on a Torus machine. An axial force of 10 kN was experienced, with a 1.5 mm physical air-gap on a 5 kW generator Chalmers et al., 1996 . Use of a machine with a large diameter, a thick winding, and a low, air-gap flux density will reduce...
The Switched Reluctance Generator
The Switched Reluctance Generator has in the last decades been considered for Wind Power applications, but much of the available literature is focussed on aircraft generators. However, the two applications should have similar requirements, the main differences being a matter of scale. The rotor construction is particularly robust, being without windings. Excitation of the magnetic field is provided by the stator current in the same way as for the Induction Machine. The Switched Reluctance...
Performance Of Voltage Control Unit Vcu At Cronalaght Wind Farm Ireland
Abdin,E.S., amp Xu,W. 1998 . Control design and dynamic performance analysis of a wind turbine induction generator unit. In POWERCON '98, International Conference on Power System Technology Vol. 2. 1198-1202 Abou-Zaid,M., El-Attar,M., amp Moussa,M. 1999 . Analysis and Performance of axial field switched reluctance generator. In International Conference on Electric Machines and Drives. 141-143 Abouzeid,M. 1998 . Load effect on the output current generated from the switched reluctance generator....
Concepts and wind turbine configurations
The generator and power electronic configurations most commonly applied in wind turbines are displayed in Figure 2. Concepts using the Asynchronous Generator Concepts using the Synchronous Generator Figure 2. Standard wind turbine configurations using the asynchronous generator AG and the synchronous generator SG . Depending on the particular configuration, the term Power Converter covers different types of power electronic components such as a soft starter part a , an external variable rotor...