Tempus Energy: Generators in Wind Turbines

TEMPUS ENERGY: GENERATORS IN WIND TURBINES

1: INTRODUCTION

The last two or three decades, the use of wind turbines to generate electrical energy has become common practice in a lot of countries. Using these wind turbines, the kinetic energy in the wind is used to drive the electrical generator. This generator converts mechanical energy into electrical energy. This electrical energy is stored or injected into the (public) electricity grid. Wind turbines are equipped with different types of generators but mainly alternators generating AC voltages are used. The present text discusses the main types of alternators all have their own properties, advantages and disadvantages.

1.1: LARGE, MEDIUM SCALE AND SMALL WIND TURBINES

Although the bounderies between large, medium scale and small wind turbines are not fixed, one mainly speaks about large wind turbines when the nominal power exceeds 300 kW. In general, the generated power will be injected into the public high voltage grid. Medium scale wind turbines mainly inject their power into the public medium voltage grid. Small wind turbines have a tower height not exceeding 15 m (reglementation in Flanders – Belgium). It is possible to inject the produced power into the low voltage grid or it is possible to use the power to load batteries.

1.2: DIFFERENT TYPES OF GENERATORS

The most important types of generators, used in wind turbines, which will be discussed below are:

-asynchronous generators with cage rotor,

-asynchronous generators with wound rotor,

-doubly fed induction machines,

-synchronous generators having electromagnetic excitation,

-synchronous generators having permanent magnet excitation.

2: ASYNCHRONOUS GENERATOR WITH CAGE ROTOR

Figure 1: Structure of a wind turbine equipped with an asynchronous generator

Figure 1 visualizes the main structure of a wind turbine equipped with an asynchronous generator. Using a gearbox, the rotor blades are connected with the generator (see e.g. The speed of the rotor blades (e.g. 20 rpm) is much lower than the speed of the generator (e.g. 1000 or 1500 rpm) implying the gearbox is used to increase the speed of rotation (this situation differs from the situation in a traditional car where a gearbox is used to decrease the speed of rotation since a combustion engine has a high speed). By increasing the speed of rotation, the applied torque decreases.

When considering large and medium scale wind turbines, it is possible to use an asynchronous generator (induction machine) with cage rotor as visualized in Figure 2. Although asynchronous machines (also called induction machines) are mainly used as motors, they can also be used as generators in case the rotor speed is higher than the synchronous speed.

Figure 2: Asynchronous machine

The stator of the generator is connected with the 50 Hz grid (or the 60 Hz grid) using a transformer. In case a generator having four poles is used, the synchronous speed equals 1500 rpm in case of a 50 Hz grid. Although the asynchronous generator ASG has a super synchronous speed, the speed variations are limited implying the generator mainly has a fixed speed of approximately 1500 rpm.

2.1: THE USE OF A GEARBOX

Although a gearbox is used to give the generator a higher speed than the rotor blades, the are some disadvantages related with the use of such a gearbox:

-A gearbox mounted in the nacelle requires space.

-Buying and installing a gearbox requires a financial investment (but it can help to use a cheaper generator and to limit or avoid the use of power electronics which also requires a financial investment).

-A gearbox produces noise.

-A gearbox is connected with the rotor blades withstanding sudden changes of the wind speed (wind gusts), sudden changes in the direction of the wind speed, …

-Although a gearbox has a high efficiency, there are energy losses due to friction (e.g. a loss of 1% in case of a 2 MW wind turbine implies a loss of 20 kW).

A lot of research is going on to increase the performance of gearboxes used in wind turbine applications.

2.2: THE INDUCTION MACHINE

An induction machine is generally used as a motor but can also be used as a generator. Induction machines with cage rotor are cheap and do not require a lot of maintenance. For these reasons they are very frequently used as a motor in the industry and are called “the work horses of the industry”

An induction machine is frequently used as a motor in the industry since:

-They are cheap and they do not require a lot of maintenance.

-They are available in a very broad range of powers.

-They have a long life expectancy and they are robust.

Figure 3: Induction machine with cage rotor

2.3: THE TORQUE SPEED CHARACTERISTIC

The torque speed characteristic of an induction machine is visualized in Figure 4. The induction machine has a synchronous speed

(rpm) or (rps)

where f is the grid frequency and p is het number of pole pairs in the machine. In case of a 50 Hz grid, implies a high synchronous speed of 3000 rpm. By using an induction motor having a larger number of pole pairs, the synchronous speed decreases which is useful in order to limit the required speed ratio of the gearbox.

When the speed of rotation is lower than the synchronous speed, the machine operates as a motor. This motoring principle can be useful to accelerate the rotor blades from a standstill to (almost) the desired speed without using the aerodynamics of the rotor blades and the wind. Of course, the induction machine can only be used as a motor when there is a grid supplying the required active power (in case of island operation this is only possible when there is sufficient energy storage able to supply this active power).

Figure 4: Torque speed characteristic of an induction machine

Notice the torque speed characteristic of Figure 4 uses the pulsation

in case is expressed in revolutions per second (rps). The characteristic also mentions the slip s defined as

which is positive in case of motoring and which is negative in case of generating. In case of motoring in a steady state condition, the slip s has a value between 0 and the breakdown slip . In case of generating in a steady state condition, the slip has a value between 0 and .

When considering a classical induction machine with cage rotor, is small. This implies the breakdown speed when motoring and the breakdown speed when generating are close to each other and close to the synchronous speed. In case of generating, the speed is larger than the synchronous speed and the speed increases as the torque increases. Notice however, due to the small , the speed variations are very limited.

Since these speed variations are very limited, the speed of the generator and the speed of the rotor blades are approximately constant. This explains the statement ‘fixed speed’ in Figure 1.

The fact there is still some speed variation possible, is useful to absorb the energy in wind gusts. In case of a wind gust, the additional energy provided by the wind is stored as kinetic energy as the speed of the rotor blades increase. This implies the torque applied to the generator and the active power injected into the grid by the generator remains almost constant (when neglecting the losses in the generator, the mechanical power is converted into electrical power ). In case of a sudden and momentary decrease of the wind speed, the speed of rotation will have a limited decrease. This decreases the stored kinetic energy and this kinetic energy is used to be able to inject active power into the grid during this momentary decrease of the wind speed. Injecting an (almost) constant power into the electrical grid is desirable in order to maintain the power equilibrium of the grid (the generated power equals the consumed power).

2.4: REACTIVE POWER

2.4.1: ACTIVE AND REACTIVE POWER

In case an electrical grid supplies a sinusoidal voltage u(t) (having an RMS value U) and a load consumes a sinusoidal current i(t) (having an RMS value I) lagging an angleφ, than the current can be split in

• an active current component in phase with u(t),
• a reactive current component having a phase difference of 90° with respect to u(t).

This active current component has an RMS value I cosφ and the reactive current component has an RMS value I sinφ. In such a situation, the load consumes an active power P = UI cosφ anda reactivepower Q = UI sinφ(here, single phase powers are considered). The consumer will convert this active power into for instance light, heat, mechanical power… It is impossible to convert the reactive power into light, heat, mechanical power… since reactive power is actually instantaneous power which is exchanged back and forth between the source/grid and load.

In case the same grid voltage feeds an ideal inductance, the inductance consumes a current i(t) which lags 90° with respect to u(t). This implies the inductance does not consume or generate active power. The inductance consumes reactive power. In case the same sinusoidal grid voltage feeds an ideal capacitor, the capacitor consumes a current i(t) which leads 90° with respect to u(t). This means the capacitor does not consume nor generate active power. The capacitor generates reactive power.

Contrary to a load, an electrical generator generates active power P. Depending on the circumstances, agenerator will consume or generate reactive power. In an electrical grid it is important the generated power is always equal to the consumed power; this is the case for the active power and for the reactive power.

2.4.2: SUPPLYING THE REACTIVE POWER

An asynchronous generator with a cage rotor consumes reactive power Q. When the asynchronous generator is connected with an existing grid, the grid will supply (or has to supply) the required reactive power. By connecting capacitors in parallel with the generator, these capacitors are able to supply the reactive power (the reactive current component) implying this reactive power must not be supplied by the grid. This (usually) reduces the reactive current component in the grid which reduces the copper losses in this grid. Connecting capacitors in parallel with the asynchronous generator has a positive effect on the grid although these capacitors are not really mandatory.

In case an asynchronous generator is used in island mode (there is no previously existing electrical grid, the generator needs to form the grid), than there is no grid available to supply the required reactive power. In such a situation, the capacitors in parallel with the grid are indispensable to supply the reactive power. Figure 5 visualizes a mechanically driven generator equipped with capacitors mounted in parallel with the generator. Notice also the three phase load of the local grid formed by the asynchronous generator.

Figure 5: Asynchronous generator in island mode

2.5: THE SPEED OF ROTATION OF THE ROTOR BLADES

The main restriction when considering the realization visualized in Figure 1, is the fact that the speed of rotation is almost fixed. Although the speed of rotation of the blades is almost fixed, the wind speed is varying with respect to time. Figure 6 visualizes the rotor efficiency CP as a function of the wind speed when considering three different wind speeds.

In case(when considering the situation visualized in Figure 6)the wind speed is varying between 8 m/s and 11 m/s, a speed of rotation of 30 rpm is well chosen. In case of lower wind speeds, the efficiency increase when the speed of rotation is lower. In case of higher wind speeds, the efficiency increases when the speed of rotation is higher. However, in case the gearbox has a fixed speed ratio, the speed of rotation of the blades is fixed when considering Figure 1.

Figure 6: The rotor efficiency

In case an induction machine is used where the synchronous speed

(rpm) or (rps)

can be changed by changing the number of pole pairs, not only the speed of the generator but also the speed of the rotor blades can be changed. In case of a low wind speed, for instance a stator winding having eight poles will be used (the generator has a speed of 750 rpm and the rotor blades have a speed of 30 rpm).In case of a higher wind speed, for instance a stator winding of six poles will be used (the generator has a speed of 1000 rpm and the rotor blades have a speed of 40 rpm).Notice however, only a stepwise speed control is possible.

Figure 7 visualizes the generated power of the induction generator as a function of the wind speed when considering the same speeds of rotation visualized in Figure 6. Also Figure 7 shows the generated power is larger in case the speed of rotation is higher in case of a higher wind speed.

Figure 7: Generated power depending of the speed of rotation of the rotor blades

3: ASYNCHRONOUS GENERATOR WITH WOUND ROTOR

3.1: GENERAL PRINCIPLE

Figure 9 visualizes a wind turbine equipped with an asynchronous generator with wound rotor (see Figure 8). Using a gearbox, the rotor blades drive the generator and using a transformer the generated electrical power is injected into the grid. In case an asynchronous generator with wound rotoris used, it is possible to connect variable resistors in series with the rotor windings implying the total rotor resistance increases (and can be changed).

When the total rotor resistance is small, mainly the behavior of a classical induction machine with cage rotor is obtained. The speed of rotation will be close to the synchronous speed (the torque speed characteristic is visualized in Figure 10 by the solid line). When the total rotor resistance is larger, the difference between the actual speed and the synchronous speed will be larger (the torque speed characteristic is visualized in Figure 10 by the dashed line).

Figure 8: Wound rotor

Due to the variable rotor resistances, in Figure 9 the speed of the generator is able to vary over a range of 10%. This implies also the speed of the rotor blades is able to vary over a range of 10%. This not only means that the rotor speed can be adapted to the wind speed in order to maximize CP, it also means it is possible to store additional kinetic energy in case of wind gusts. By storing the additional energy originating from wind gusts, the torque variations and the variations of the power injected into the grid will be smaller. This is important since the grid operators do not like sudden and large variations of the power injected into the grid.

Figure 9: Wind turbine equipped with a generator with wound rotor

Figure 10: The torque speed characteristics

3.2: EVALUATION

In reality, an asynchronous generator with wound rotor and additional rotor resistances will not be used that often. A wound rotor is much more expensive than a cage rotor and also the rotor resistances require a considerable investment. Moreover, the fixed rotor resistances are connected with the rotor using slip rings and carbon brushes. The use of these slip rings and brushes requires additional maintenance.

The higher the rotor resistance, the higher the obtained speed variation and the higher the realizable slip. However, as the slip increases also the heat dissipation in the rotor resistances increases which reduces the efficiency. Mainly to avoid this heat dissipation, a doubly fed induction generator can be used.

4: DOUBLY FED INDUCTION GENERATOR

4.1: GENERAL PRINCIPLE

When using an induction machine with wound rotor and external rotor resistances, the speed is varying between the synchronous speed and the breakdown speed of the generator (which is for instance 10% higher than the synchronous speed). By using a doubly fed induction generator, the speed can vary in a range between for instance 90% and 110% of the synchronous speed. A total speed variation of 20% (or more) can be obtained using the configuration of Figure 11.

Figure 11: Wind turbine with doubly fed induction machine

Figure 11 visualizes a wind turbine where, using a gearbox, the rotor blades drive an asynchronous machine with wound rotor. The power generated in the stator is injected into the grid using a transformer. In Figure 9, the rotor resistances extract power from the rotor windings which is converted into heat (heat losses).Using a frequency converter, as visualized in Figure 11, it is possible to extract power from the rotor windings and it is also possible to inject power into the rotor windings.

4.2: BEHAVIOR OF AN INDUCTION MACHINE

The active power PS is the power consumed by the stator of the induction machine. In case , the machine consumes active power and the machine is motoring. In case , the machine generates power and a power is injected into the grid.

When neglecting the losses in the stator, a power is transferred from the stator to the rotor. In case , motoring occurs. In case , the generated power is transferred from the rotor to the stator.

When considering a classical induction machine (with cage rotor or with wound rotor), the rotor power

is positive and dissipated into the rotor resistances. When the speed is subsynchronous, the slip and also (motoring) implying . When the speed is super synchronous, the slip and also (generating) implying .