Conventional Wind Generators

Conventional wind generators are wind generators that are designed and manufactured from tradi- tional electrical machine concepts. These set of wind generators have been in existence from the on- set of modern wind turbines. In Cao, Xie and Tan (2012) [4], the three main types of traditional wind generators are presented as: direct current (DC), alternating current (AC) synchronous and AC asyn- chronous generators. Asynchronous AC generators are usually squirrel cage rotor induction genera- tors (SCIGs), wound rotor IGs (WRIGs) and doubly–fed IGs, so–called DFIGs, whereas synchronous generators include PMSGs and WRSGs, sometimes ascribed as electrical excited SGs (EESGs).

Information on DC generators, its theory, design and operation are well established, even by a simple online Google search. In undertaking such online research, some conceptual terms like fields, armature, stator, rotor, brushes, commutators, etc., which are general terminologies in the design of electrical machines, are displayed. In reality, DC machines acting as wind generators are not com- mon. In fact, Cao, Xie and Tan (2012) [4] purportedly reported that when they do exist as wind gen- erators, such wind turbines are usually in low power installations. One reason for their unpopular wind generator potentials might be attributed to its high maintenance costs due to inherent presence of commutators and brushes.

In Polinder et al’s (2006) [11] study, where some current and future wind turbine generator sys- tems were discussed, DC generator topologies, actually, did not feature. A similar outcome is ob- served in Li, Chen and Polinder (2006) [15]. Needless to say that prospects of DC generators for WPG is bleak, and with almost no account of its existence among wind generator manufacturers as appraised in studies by Matveev (2011) [21] and Ragheb (2014) [25]. However, some studies do abound where DC generator have been investigated for WPG, but they are usually operated in LS drivetrains, which unfortunately may further complicate both the manufacturing and maintenance challenges of the generator, Cao, Xie and Tan: 2012 [4] and Madani: 2011 [26].

On the other hand, induction generators are so popular for wind generator designs such that they are currently ranked as the highest used machines in the industry, Cao, Xie and Tan: 2012 [4], de Vries (2012) [13] and Zou: 2015: [27]. In particular, Zou (2015) [27] discussed the nitty gritty of using IGs for wind power systems, which, for the sake of avoiding redundant duplications, would not be comprehensively rehashed here. To this end, the common drivetrain topologies of IGs as used for WPG have been reproduced as shown in Fig. 1.5.

The SCIG system is dubbed as “cheap” in Polinder, et al (2013) [7], but it is usually operated at fixed speed, thereby limiting the power capture, among other issues. In the same vein, Cao, Xie and Tan (2012) [4] wrote about SCIGs that they are “simple, reliable, inexpensive and well developed… but they draw reactive power from the grid and thus some form of reactive power compensation is needed”. Furthermore, because SCIGs are grid–tied, it means that they do not permit adjustment of their output voltage, and they also experience limitations such as audible noise, low efficiency and high maintenance costs mainly due to its operation in multi–stage geared wind generator drivetrains, de Vries: 2012 [13], Ragheb: 2014 [25] and Zou: 2015 [27]. But to give credit to the earliest develop- ers of modern wind turbines, SCIGs are distinguished in wind generators’ hall of fame as the wind generators used in the popular Danish concept which ranged up to 1.5 MW between 1980 and 1990.

On the other hand, WRIGs are slightly better than SCIGS in that they can be operated at varia- ble speeds via a rotor resistance slip control, but in a very limited range. Today, DFIGs are the reign- ing superpower, not only among IGs, but among wind turbine generators generally. They have been designed to reach a capacity of 5 MW, according to Zhu and Hu (2013) [10]. The advantage of DFIGs is that they require only some percentage (20–34 %) of the generator nominal power to devise the ratings of the SSCs, which assist them to provide a wider range of speed variation compared to WRIGs, Zhu and Hu: 2013 [10] and de Vries: 2012 [13]. The known disadvantages of DFIGs are: presence of slip–rings and the use of a three–stage gearbox, as well as grid interconnection challenges as detailed in Polinder et al (2006) [7] and Zhu and Hu (2013) [10].

Now, enter in the synchronous generators (SGs). SGs are growing in popularity today because, unlike DFIGs, they are fully decoupled from the grid with a fully–rated power converter (FPC), thus facilitating a wider variable speed range with superior grid compliance, de Vries: 2012 [13]. In some cases, they are operated as direct–drive systems, thus improving their drivetrain reliability, Dubois: 2004 [28]. However, Bang et al (2008) [29] and Semken et al (2012) [30], both agree that the main challenge with SGs, if designed for gearless drives, is a resulting large volume, which increases the generator costs, especially when rare–earth PMs are used.

As a matter of fact, PMSGs coupled with very high PM volumes pose added risk of PM de- magnetisation due to poor thermal dissipation, in addition with associated motive forces on their ro- tor–housed PMs, Madani: 2011 [26], Chen et al: 2015 [31] and Sjökvist: 2014 [32]. Also, their fields, based on PMs, are not controllable. However, because PMSGs are self–excited, the problems of brushes and slip rings qualify them as very robust electrical machine candidates.

Next are the WRSGs or EESGs, with wound–fields (WFs) replacing PMs, which makes them advantageous in terms their ability to produce reactive power and regulate output voltage by the regu- lation of the field current, Madani: 2011 [26] and Zhu and Hu: 2013 [10]. Nevertheless, they experi- ence maintenance and efficiency issues. The different drivetrain topologies so far discussed for SGs are summarised as shown in Fig. 1.6.

Table 1.2. Quantitative comparison of three major wind generators7 [4]

Parameter

DFIG Synchronous generators

geared direct drive

– – PM EE PM

1–stage 3–stage 1–stage – –

Airgap diameter (m) 3.6 0.84 3.6 5 5

Stack length (m) 0.6 0.75 0.4 1.2 1.2

Iron weight (ton) 8.65 4.03 4.37 32.5 18.1

Copper weight (ton) 2.72 1.21 1.33 12.6 4.3

PM weight (ton) 0.41 1.7

Generator active material cost (k€) 67 30 43 287 162

Gearbox cost (k€) 120 220 120

Converter cost (k€) 40 40 120 120 120

Generator construction cost (k€) 60 30 50 160 150

Total generator system cost (k€) 287 320 333 567 432

Annual electricity yield (MWh) 7760 7690 7700 7740 7890

Yield/total cost (kWh/k€) 4.22 4.11 4.09 3.67 3.98

A detailed quantitative performance comparison of the traditional wind generators as reported in Cao, Xie and Tan (2012) [4] is reproduced in Table 1.2. Based on conclusions drawn in section 1.2, it is not surprising to note that, among other things, the 1–stage geared (MS) DFIG present an obvious advantage in terms of energy yield per total cost compared to the other wind generator drivetrains.

In summary, most conventional wind generators employ a multi–stage gearbox together with commutator brushes, expensive rare–earth PMs, and slip–rings in the rotors of e.g., DC wound–field configurations. Meanwhile, the use of PMSGs poses two major problems, high cost of PMs and de- magnetisation risks. Hence, for multi–MW systems, these problems are necessarily scaled up. How- ever, there are emerging brushless wind generators today, with some having stator–active and robust rotor qualities. These brushless machines are very attractive because they lack the use of brushes in all ramifications, and their hotspots are easily exposed to cooling schemes. Thus, the next section will be used to discuss the generality on the state–of–the–art of non–conventional, viz., brushless wind generators and their potentials for the proposed geared MS wind generator drives.