**5. Design Considerations and Challenges**

Generally speaking, wind turbine generators can be selected from commercially available electrical machines with or without minor modifications. If a wind turbine design is re‐ quired to match a specific site, some key issues should be taken into account. These include:


Among these design considerations, the choice of operating speed, drive type, brush topolo‐ gy, and power converter are focused and further analyzed in details.

#### *(a) Fixed or Variable Speed?*

Clearly, it is beneficial to operate WTGs at variable speed. The reasons are several. When the wind speed is below rated, running the rotor speed with the wind speed and keeping the tip speed ratio constant ensure that the wind turbine will extract the maximum energy. Variable speed operation helps reduce fluctuating mechanical stresses on the drive train and machine shaft, the likelihood of fatigue and damage as well as aerodynamically generated acoustic noise. The rotor can act as a regenerative storage unit (e.g. flywheel), smoothing out torque and power fluctuations prior to entering the drive train. Direct control of the air-gap torque also aids in minimizing gearbox torque fluctuations. Since there is a frequency converter be‐ tween the wind turbine generator and the power grid, it becomes possible to decouple the network frequency and the rotor rotational speed. This permits variable speed operation of the rotor and controllability of air-gap torque of the machine. Furthermore, variable speed operation enables separate control of active and reactive power, as well as power factor. In theory, some wind turbine generators may be used to compensate the low power factor caused by neighboring consumers. In economic terms, variable speed wind turbine can pro‐ duce 8-15% more power than fixed speed counterparts [45]. Nonetheless, the capital costs will be increased arising from the variable speed drive and power converters, as well as in‐ creased complicity and control requirements.

**Figure 15.** Variable speed control system [35].

harsh or high-temperature environments. Because the reluctance torque is only a fraction of electrical torque, the rotor of switched reluctance is generally large than other with electrical excitations for a given rated torque. If reluctance machines are combined with direct drive features, the machine would be extremely large and heavy, making them less favorable in

Generally speaking, wind turbine generators can be selected from commercially available electrical machines with or without minor modifications. If a wind turbine design is re‐ quired to match a specific site, some key issues should be taken into account. These include:

wind power applications.

192 Advances in Wind Power

**•** Choice of machines **•** Type of drive train

**•** Rated and operating speeds **•** Rated and operating torques

**•** Voltage regulation (synchronous generators)

**•** Power factor and reactive power compensation (induction generators)

gy, and power converter are focused and further analyzed in details.

Among these design considerations, the choice of operating speed, drive type, brush topolo‐

Clearly, it is beneficial to operate WTGs at variable speed. The reasons are several. When the wind speed is below rated, running the rotor speed with the wind speed and keeping the tip

**•** Starting current (induction generators) **•** Synchronizing (synchronous generators)

**•** Brush topology

**•** Tip speed ratio

**•** Power and current

**•** Methods of starting

**•** Cooling arrangement

**•** Weight and size

**•** Power converter topology

**•** Protection (offshore environment)

**•** Capital cost and maintenance.

*(a) Fixed or Variable Speed?*

**5. Design Considerations and Challenges**

In principle, variable speed operation can be achieved mechanically by the use of differen‐ tial gearboxes or continuously-variable transmission systems [8], based on the control of speed and angular speed of gyroscopes. But the general practice is to achieve this goal by electrical means. There are two major methods in use: broad range and narrow range varia‐ ble speed [8]. The former refers to a wide operational range from zero to the full rated speed where the latter refers to a narrow operational range between a fraction (up to ±50%) of syn‐ chronous speed. In reality, this latter range is practically sufficient and can saving significant costs on power electronic converters. A closed loop speed control of such a method is dem‐ onstrated in Fig. 15.

In the design of variable-speed wind turbines, three control aspects in association with the wind speed need to consider. First, a constant optimized tip speed should be maintained to achieve maximum aerodynamic efficiency by varying the rotor speed with the actual wind speed. Second, the rotor speed should be maintained constant after the rotor has reached its rated speed but the power has not, in the case of moderate winds. When the wind speed is higher, the control is to maintain a constant rated power via the pitch angle control or stall control. Whilst using the pitch angle control, the blade pitch is varied to control the rotor speed together with the generator torque.

#### *(b) Direct or Geared Drive?*

In a geared wind turbine, the generator speed increases with the gear ratio so that the reduc‐ tion in machine weight is offset by the gain in gearbox weight. For instance, the wind tur‐ bine operates at a speed of 15 rpm and the generator is designed to operate 1200 rpm (for 60 Hz) [2]. An up-speed gearbox of 1:80 is required to match the speed/torque of the turbine with these of the generator.

However, historically, gearbox failures are major challenges to the operation of wind farms. This is especially true for offshore wind turbines which are situated in harsh and less-acces‐ sible environments. Because of this, direct drive systems are increasingly desired in new wind turbine systems. One example is the excited synchronous generator with wound field rotor is a well-established design in the marketplace; and another may be a popular neody‐ mium magnet generator design which also attracts much attention in the marketplace.

Obviously, direct drive configuration removes the necessity for gears and the related relia‐ bility problems [46]. Therefore, some wind turbine manufacturers are now moving toward direct-drive generators to improve system reliability. Since wind turbine generators are op‐ erated with power electronic converters, direct drive topology can provide some flexibility in the voltage and power requirements of the machines. Nonetheless, a drawback of the di‐ rect drive is associated with the low operating speed of the turbine generator. As the nomi‐ nal speed of the machine reduces, the volume and weight of its rotor would increase approximately in inverse proportion for a given power output. This can be explained in the following equation governing the power output of any rotating electrical machine [28],

$$P = k \times (D^2 L) \times \mathfrak{n} \tag{11}$$

where *k* is a constant, *n* is the rotor rotational speed, *D* is the rotor diameter and *L* is the rotor length, in arbitrary units.

Direct drive increases the size of electrical generators which effectively offsets some of the weight savings from removing gearboxes. See Fig. 16 for a direct drive wind turbine genera‐ tor, which is more than 10 times larger than its equivalent geared machine. Moreover, it typ‐ ically requires the full rated power converters for grid connection. As a consequence, it is always needed to strike a balance between the weight of machines and the weight of gear‐ boxes. Hybrid systems use one or two stages of gears rather than three or four required by conventional MW generators. Sometimes, hybrid systems can offer a better compromise in terms of the overall performance of the wind turbine system.

**Figure 16.** Example of a direct drive MW wind turbine generator.

costs on power electronic converters. A closed loop speed control of such a method is dem‐

In the design of variable-speed wind turbines, three control aspects in association with the wind speed need to consider. First, a constant optimized tip speed should be maintained to achieve maximum aerodynamic efficiency by varying the rotor speed with the actual wind speed. Second, the rotor speed should be maintained constant after the rotor has reached its rated speed but the power has not, in the case of moderate winds. When the wind speed is higher, the control is to maintain a constant rated power via the pitch angle control or stall control. Whilst using the pitch angle control, the blade pitch is varied to control the rotor

In a geared wind turbine, the generator speed increases with the gear ratio so that the reduc‐ tion in machine weight is offset by the gain in gearbox weight. For instance, the wind tur‐ bine operates at a speed of 15 rpm and the generator is designed to operate 1200 rpm (for 60 Hz) [2]. An up-speed gearbox of 1:80 is required to match the speed/torque of the turbine

However, historically, gearbox failures are major challenges to the operation of wind farms. This is especially true for offshore wind turbines which are situated in harsh and less-acces‐ sible environments. Because of this, direct drive systems are increasingly desired in new wind turbine systems. One example is the excited synchronous generator with wound field rotor is a well-established design in the marketplace; and another may be a popular neody‐ mium magnet generator design which also attracts much attention in the marketplace.

Obviously, direct drive configuration removes the necessity for gears and the related relia‐ bility problems [46]. Therefore, some wind turbine manufacturers are now moving toward direct-drive generators to improve system reliability. Since wind turbine generators are op‐ erated with power electronic converters, direct drive topology can provide some flexibility in the voltage and power requirements of the machines. Nonetheless, a drawback of the di‐ rect drive is associated with the low operating speed of the turbine generator. As the nomi‐ nal speed of the machine reduces, the volume and weight of its rotor would increase approximately in inverse proportion for a given power output. This can be explained in the following equation governing the power output of any rotating electrical machine [28],

where *k* is a constant, *n* is the rotor rotational speed, *D* is the rotor diameter and *L* is the

Direct drive increases the size of electrical generators which effectively offsets some of the weight savings from removing gearboxes. See Fig. 16 for a direct drive wind turbine genera‐ tor, which is more than 10 times larger than its equivalent geared machine. Moreover, it typ‐ ically requires the full rated power converters for grid connection. As a consequence, it is

<sup>2</sup> *P k DL n* =´ ´ ( ) (11)

onstrated in Fig. 15.

194 Advances in Wind Power

*(b) Direct or Geared Drive?*

with these of the generator.

rotor length, in arbitrary units.

speed together with the generator torque.

For direct drive, the popular machine option is the PM synchronous machines. Although considerable effort and investment have been spent on improving reluctance machines [10; 15], they are still not commercially competitive to date. Direct drive brings about some de‐ sign challenges on the generator and the power converters. For PM direct drive generators, they require a significant amount of costly rare-earth permanent magnets [51; 53; 44]. In ad‐ dition, it needs to increase the rating of IGBTs in the back-to-back converter, or to integrate machine side converter components with the stator windings. Obviously, the advantage of direct drive is the removal of gearbox at the expense of increased size and weight of the wind turbine generator. As a rule of thumb, the machine volume is proportional to the tor‐ que required and inversely proportional to the operational speed for a given power. The in‐ creased mass of the generator can be a limiting factor for offshore installations because the shipping carrying capacity is generally limited to 100 tons so that the direct drive generator may not be greater than 10 MW.

With the hybrid option, the generator size and speed lie in between direct and geared drives. In this case, synchronous machines are more popular than induction machines. It generally involves medium-speed, multi-pole generators which are almost exclusively per‐ manent magnet machines. The hybrid drive train can facilitate more nacelle arrangements and match the size of the generator and gearbox.

#### *(c) Brushed or Brushless Topology?*

In general, DC machines, wound rotor synchronous generators, wound rotor induction gen‐ erators all employ commutators, brushes or sliprings to access the rotating rotor circuits. Consequently, routine maintenance and replacement lead to some difficulties in wind pow‐ er applications, especially for offshore installations. Clearly it would be particularly desira‐ ble to rid of any components physically connected to the rotating parts of wind turbines. There are several ways of achieving this. Taking the DFIG for example, brushless doubly-fed generators (BDFGs) can be a solution. They use two windings on the stator (a power wind‐ ing and a control winding) with different pole numbers. The rotor can be of squirrel cage type and an indirect coupling of the two stator windings is established through the rotor. It is also possible to use a reluctance rotor in this topology where the machine has become a brushless reluctance generator [6, 14, 25]. By modifying the conventional machines, a higher reliability is achieved due to the absence of the brushes and slip rings. The penalty is the use of two machines in a machine case.

#### *(d) Two-Level, Multi-Level or Matrix Converter?*

Power electronics is recognized as being a key and enabling component in wind turbine sys‐ tems. Broadly, there are three types of converters widely used in the wind market. These are two-level, multi-level and matrix converters.

Two level power converters are commonly called "back-to-back PWM converters", as shown in Fig. 17(a). They include two voltage source inverters (with PWM control scheme) connected through a DC capacitor. This is a mature technology but suffers from high costs, high switching loss and large DC capacitors. Any power converters having three or more voltage levels are termed "multi-level converters". These are illustrated in Fig. 17(b). They are particularly favored in multi-MW wind turbines since they offer better voltage and pow‐ er capacity, lower switching loss and total harmonic distortion. However, the power elec‐ tronic circuits are more complex and costly.

**Figure 17.** Three types of power converters in wind applications. (a) [21], (b) [42], (c)[5].

On the contrary, matrix converters are different in the way of AC-AC conversion. They re‐ move the necessity of a DC stage and directly synthesize the incoming AC voltage wave‐ form to match the required AC output. As shown in Fig. 17(c), they generally have nine power electronic switches with three in a common leg. The elimination of DC capacitors im‐ proves the reliability, size, efficiency and cost of power converters. The downsides are the limited voltage (up to 86% of the input voltage), sensitivity to grid disturbances [26], and high conducting power loss.
