Power Electronic Concepts

Power electronics is a rapidly developing technology. Components are getting higher current and voltage ratings, the power losses decrease and the devices become more reliable. The devices are also very easy to control with a mega scale power amplification. The prices are still going down pr. kVA and power converters are becoming attractive as a mean to improve the performance of a wind turbine. This chapter will discuss the standard power converter topologies from the simplest converters for...

Trends in wind turbine design

Based on the previous subsection the present state of the art large wind turbine may be summarised as a 3-bladed upwind turbine (with tubular tower) using Active stall with a two speed asynchronous generator or pitch control combined with variable speed. Moreover, the variable speed concept is mainly realised using configuration d, i.e. a doubly-fed induction generator with a rotor connected IGBT based frequency converter. Only one of the top-10 manufacturers offers a gearless (variable speed)...

Offshore wind farms on the transmission grid

As mentioned in Section 3.3.3, energy planning of the Danish Government has scheduled a number of offshore wind farms. According to the plan, the first two will be ready for operation in 2002 - each rated at 150 MW. By the year 2030, 50 of the electrical energy consumption in Denmark will be supplied by wind energy. The Danish power system operators Elkraft System and Eltra are responsible for the integration of these wind farms. Due to their rated power, they will be connected directly to the...

Comparison of the five frequency converters

The five converters presented above the back-to-back PWM VSI, the tandem converter, the matrix converter, the multilevel converter and the NCC, may be evaluated in terms of their applicability to wind turbine systems. For each converter, a presentation of the topology and the working principles has been presented, combined with a discussion of advantages and disadvantages. It is evident that the back-to-back converter is highly relevant, as this converter is the one used in wind turbines today....

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...

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...

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...

The Asynchronous Machine Induction Machine

Exploded View Squirrel Cage

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

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...

Doubly salient Permanent Magnet Generator DSPMG

Permanent Magnet Generator Schematic

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

Permanent Magnet Machines

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 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...

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...

Torus Generator

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

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...