Comparison of direct-drive and geared generator concepts for wind turbines
Summary (3 min read)
I. INTRODUCTION
- T HE objective of this paper is to compare five different generator systems for wind turbines, namely the doublyfed induction generator with three-stage gearbox (DFIG3G), the direct-drive synchronous generator with electrical excitation (DDSG), the direct-drive permanent-magnet generator , the permanet-magnet generator with single stage gearbox (PMG1G) and the doubly-fed induction generator with single-stage gearbox (DFIG1G).
- On the one hand, the resulting system combines some of the disadvantages of both the geared and direct-drive systems: the system has a gearbox and it has a special and therefore expensive generator and a fully rated converter.
- On the other hand, compared to direct-drive systems, a significant decrease in the generator cost and an increase in the generator efficiency can be obtained.
- The objective of this paper, therefore, is to compare five wind turbine generator systems, namely: 1) the DFIG3G as currently used; 2) the DDSG as currently used; 3) the DDPMG; 4) the PMG1G; 5) the DFIG1G.
- In [6] and [7] , more generator systems were compared and more criteria were taken into account.
A. Wind Turbine Modeling
- Table I gives the characteristics of the wind turbine that was used to compare the different generator systems.
- Fig. 3 illustrates the rotor speed, which is assumed to be proportional to the wind speed at maximum aerodynamic efficiency at low wind speeds and equal to the rated rotor speed at higher wind speeds (above 9 m/s).
- Integrating the area below the curve gives a value of 1.
- Table I also gives some approximate numbers for the cost of the rest of the wind turbine.
- Because the paper concentrates on the generator system, these numbers are not extensively validated and must be seen only as indicators.
B. Gearbox Modeling
- Some references suggest higher gear ratios [7] .
- At the moment, this is not seen as proven technology with a guaranteed lifetime.
- From the commercially available gearboxes, it appears to be cheaper to use gearboxes with more stages for higher gear ratios.
- This means that the losses are proportional to the speed EQUATION where P gearm is the loss in the gearbox at rated speed (3% of rated power for a three-stage gearbox [11] and 1.5% for a single-stage gearbox, see Table I ), n is the rotor speed (r/min), and n rated is the rated rotor speed (r/min).
D. Generator Modeling
- The different generators are modeled using equivalent circuit models.
- 2) The magnetic flux density crosses the air-gap perpendicularly.
- The effective air gap of the machine depends on the type of machine.
- The cross-section area of the conductor is the available slot area multiplied by the fill factor divided by the number of turns per slot: EQUATION ) where q is the number of slots per pole per phase, k sfil is the slot fill factor (60%), b sav is the average slot width, and h s is the slot height.
- To calculate the total iron losses, the specific iron losses in the different parts (teeth and yokes) are evaluated, multiplied by the weight of these parts, and added.
A. DFIG3G
- Because the stator is directly connected to the 50-Hz grid, the synchronous speed is 1000 r/min.
- The rotor side parameters are all referred parameters.
- The parameters of the second equivalent circuit can be calculated from the parameters of the first in the following way [15] : EQUATION ).
- The annual energy dissipation, determined from a combination of the losses with the Weibull distribution, is also depicted in Fig. 6 .
- The losses in the gearbox dominate the losses in this generator system: Roughly 70% of the annual energy dissipation in the generator system is in the gearbox.
B. DDSG
- From the electromagnetic point of view, larger air-gap diameters are better, but mechanical design, construction and transportation become more difficult.
- This 5-m air-gap diameter is a compromise between these criteria.
- The number of slots per pole per phase is two.
- Decreasing this number results in a significant increase in the excitation losses, mainly in part load.
- The annual energy dissipation, determined from a combination of the losses with the Weibull distribution is also depicted in Fig. 9 .
C. DDPMG
- Compared to the DDSG the number of poles is doubled to reduce the risk of demagnetizing the magnets and to reduce the dimensions of yokes and end-windings.
- Doubling the number of poles does not increase the excitation losses as in the DDSG because permanent magnets are used.
- The equivalent circuit of the permanent-magnet generator and the applied phasor diagram are depicted in Fig. 10 .
- Table II gives the annual energy yield, the annual dissipation, and the estimated costs.
- Iron losses are not negligible; at wind speeds up to 8 m/s, they are larger than the copper losses and over 15% of the annual dissipation in the generator system is in the iron.
D. PMG1G
- The rated speed of 90 r/min is still low.
- Therefore, this generator is also built as a ring machine with a large radius.
- The air-gap diameter is chosen as 3.6 m to eliminate the most important transportation problems.
- Fig. 12 depicts some results from the model as a function of the wind speed: voltage, current, power, generator efficiency, generator system efficiency (including losses in the converter and the gearbox), and losses.
- The annual energy dissipation, determined from a combination of the losses with the Weibull distribution, is also depicted in Fig. 12 .
E. DFIG1G
- For the same reasons as for the PMG1G, the air-gap diameter of the DFIG1G is 3.6 m.
- The synchronous speed of the induction generator is chosen at 75 r/min, so that at the rated speed, there is still some margin both in speed and power for control purposes.
- The magnetizing current of this induction machine is rather large due to the considerable air gap and the high number of pole pairs.
- The DFIG1G is controlled in the same way as the DFIG3G.
- The annual energy dissipation, determined from a combination of the losses with the Weibull distribution, is also depicted in Fig. 13 .
IV. COMPARISON AND DISCUSSION
- The DFIG3G is the lightest, low cost solution with standard components, explaining why it is most widely-used commercially.
- Compared to the generator systems with gearbox, it is more expensive.
- Surprisingly, the DFIG1G seems the most interesting choice in terms of energy yield divided by cost.
- The DDSG with electrical excitation used by Enercon claims improved reliability including immunity to problems from voltage disturbances due to grid faults as a result of the use of a fully rated converter.
- An integral design of the turbine and the generator system including manufacturing, transportation, and installation may considerably affect the price of a wind turbine.
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Citations
2,465 citations
1,023 citations
Cites background from "Comparison of direct-drive and gear..."
...1 Performance comparison of different wind generator systems Some comparisons of different wind generator system have been conducted by some researchers [8–13, 17, 19, 24, 26, 39–41]....
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...Additionally, a variety of innovative concepts of wind turbines appear, for example, an interesting alternative may be a mixed solution with a gearbox and a smaller low speed permanent magnet synchronous generator (PMSG) [7–9], because direct-drive wind generators are becoming larger and even more expensive for increasing power levels and decreasing rotor speeds....
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...3 Discussions of comparison criteria Various criteria may be used for comparing different wind generator systems, including the torque density, the cost per torque, the efficiency, the active material weight, the outer diameter, the total length, the total volume, the total generator cost, the annual energy yield, the energy yield per cost, the cost of energy and so on [8, 9, 12, 13, 16, 17, 21, 25]....
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...[9] have also presented a detailed comparison of five 3 MW different generator systems for variable speed wind turbine concepts, which are a DFIG system with three-stage gearbox (DFIG 3G), a direct-...
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...† DFIG 3G is the lightest and low-cost solution with standard components according to [9]....
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Cites background from "Comparison of direct-drive and gear..."
...3b compares the direct driven (DD) synchronous generator (SG) with the 3 stage gearbox (3G) Doubly-Fed Induction Generator (DFIG), using the latter as the base value for both weight and losses [8]-[9]....
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694 citations
Cites background from "Comparison of direct-drive and gear..."
...tear, reduced life span, reduced efficiency and need for regular maintenance [30], [50]....
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...5 times heavier compared to the three-stage gearbox based induction generators [50], [54]....
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References
277 citations
242 citations
"Comparison of direct-drive and gear..." refers background or methods in this paper
...where ls is the stack length in axial direction, rs is the stator radius, Ns is the number of turns of the phase winding, kw is the winding factor [14], [15], p is the number of pole pairs, and geff is the effective air gap....
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...The no-load (motional) voltage induced by this flux density in a stator winding can be calculated as [15], [16]...
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...The magnetizing inductance of an AC machine is given by [14], [15]...
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...Using Fourier analysis, the fundamental space harmonic of this flux density can be calculated as [15], [16]...
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...The parameters of the second equivalent circuit can be calculated from the parameters of the first in the following way [15]:...
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185 citations
"Comparison of direct-drive and gear..." refers methods in this paper
...The model used here divides them into three parts [13]: 1) a small part that is constant and consists of power dissipated in power supplies, gate drivers, control, cooling systems and so on [9]; 2) a large part that is proportional to the current and consists of switching losses and conduction losses; 3) a part that is proportional to the current squared and con-...
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...The machine parameters are calculated in conventional ways [9]....
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154 citations
"Comparison of direct-drive and gear..." refers background in this paper
...However, [3]–[7] claim benefits for permanent magnet excitation, which elim-...
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...the cost of the permanent magnets and the power electronics is decreasing and because further optimization and integration of the generator system is possible [3], [18], [19]....
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Frequently Asked Questions (18)
Q2. What are the future works mentioned in the paper "Comparison of direct-drive and geared generator concepts for wind turbines" ?
Very important design aspects for which further work is needed are reliability and availability [ 20 ].
Q3. What is the phasecurrent in the PMG1G generator?
The phasecurrent is in the middle between the terminal voltage and the voltage induced by the magnets in order to reduce the saturation and to get a compromise between the converter rating and the generator rating.
Q4. What is the phase current in the DDSG?
The phase current leads the phase voltage a little in order to reduce saturation and excitation losses while a larger rating of the converter is not necessary.
Q5. What are the advantages of the DFIG3G?
Manufacturers supplying the DFIG3G use generator and converter components which are close to industrial standards yielding benefits in standardization, cost, and reliability.
Q6. How much is the rated speed of the generator?
With a gear ratio of 80, the rated speed of the generator is 1200 r/min, so that at rated speed, there is still some margin for control purposes.
Q7. What is the simplest way to calculate the parameters of the second equivalent circuit?
The parameters of the second equivalent circuit can be calculated from the parameters of the first in the following way [15]:Ls = Lsσ + Lsm;RR = RrL2 sL2smLL = LsσLs Lsm + LrσL2 sL2sm . (15)To simplify the calculations, the second equivalent circuit has been used.
Q8. What is the wind turbine's pitch angle?
Using these characteristics, the available shaft power P can be calculated as a function of the wind speed as [2], [10]P = 1 2 ρairCp(λ, θ)πr2v3w (1)where ρair is the mass density of air, r is the wind turbine rotor radius, vw is the wind speed, and Cp(λ, θ) is the power coefficient or the aerodynamic efficiency, which is a function of the tip speed ratio λ (tip speed divided by wind speed) and the pitch angle θ.
Q9. What is the popular type of wind turbine?
The only commercially successful large direct-drive wind turbine manufacturer, Enercon, uses this system but they claim other benefits from the system.
Q10. What is the average annual energy dissipation in the generator system?
The losses in the gearbox dominate the losses in this generator system: Roughly 70% of the annual energy dissipation in the generator system is in the gearbox.
Q11. What is the reluctance of the iron of the magnetic circuit?
The factor representing the reluctance of the iron of the magnetic circuit is calculated as [17]ksat = 1 + 1Hggeff ∫ lFe 0 HFedlFe (7)where HFe is the magnetic field intensity in the iron, estimated from the BH curve.
Q12. Why is the DFIG1G the expensive generator?
Because it is mainly built from standard components consisting of copper and iron, major improvements in performance or cost reductions cannot be expected.
Q13. What is the current of the DFIG1G?
The magnetizing current of this induction machine is rather large due to the considerable air gap and the high number of pole pairs.
Q14. Why are the numbers in Table The authornot extensively validated?
Because the paper concentrates on the generator system, these numbers are not extensively validated and must be seen only as indicators.
Q15. What is the BH of a permanent magnet?
In permanent-magnet machines, this factor representing saturation is much smaller than in the other machines because the effective air gap is much larger due to the low permeability of the magnets.
Q16. What is the annual energy yield of the DDSG?
The annual energydissipation, determined from a combination of the losses with the Weibull distribution, is also depicted in Fig.
Q17. What is the energy dissipation of the DFIG1G?
The annual energy dissipation, determined from a combination of the losses with the Weibull distribution, is also depicted in Fig. 13.
Q18. What is the smallest part of the losses in the generator?
Iron losses are not negligible; at wind speeds up to 8 m/s, they are larger than the copper losses and over 15% of the annual dissipation in the generator system is in the iron.