**5. Performance Comparisons**

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

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

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

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 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‐

may not be greater than 10 MW.

196 Advances in Wind Power

*(c) Brushed or Brushless Topology?*

of two machines in a machine case.

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

two-level, multi-level and matrix converters.

tronic circuits are more complex and costly.

and match the size of the generator and gearbox.

A quantitative comparison of DFIGs, synchronous and PM generators is listed in Table 1. It can be seen that direct drive wind turbine generators are larger in size but shorter in length compared to geared counterparts. From this limited range of data, three-stage geared DFIGs appear to be lightest; conventional synchronous generators are the heaviest and the mostly costly machines.

In addition, a performance comparison of different wind turbine generators is summarized in Table 2.


**Table 1.** Quantitative comparison of three major wind turbine generators [38; 30].



**Table 2.** Overall performance comparison of different wind turbine generators (partially, 3; 20).
