Direct drive double fed wind generator

An electric machine topology characterized by single tooth winding in both stator and rotor is presented. The proposed machine is capable of operating as a direct drive double fed wind generator (DDDF, D3F), because it requires no gearbox. A wind turbine drive built around a D3F generator is cheaper to manufacture, requires less maintenance and has a higher energy yield than its conventional counterparts. The all tooth wound generator of a D3F turbine has a superb volume utilisation and lower stator I2R losses due to its extremely short end windings, efficient cooling and reduced active length. Both stator and rotor of a D3F generator can be manufactured in segments, which simplifies its assembly and transportation to the site, and makes its production cheaper.


Nomenclature
The International Electrotechnical Commission (IEC) defines in its publication 411 the nomenclature related to electric machines as: -Asynchronous machine (IEC 411-31-09): An alternating current machine in which the speed on load and the frequency of the system to which it is connected are not in a constant ratio; -Induction machine (IEC 411-31-10): An asynchronous machine of which only one winding is energized; -Synchronous machine (IEC 411-31-08): An alternating current machine in which the frequency of the generated voltages and the speed of the machine are in a constant ratio; -Double wound synchronous generator (IEC 411-32-03): A synchronous generator which has two similar armature windings mounted on the same magnetic structure and capable of supplying two separate circuits Existing literature in English language is abundant on misleading descriptions of a double fed machine, such as "Double fed induction machine", or "Double fed asynchronous machine".Both of these terms are inconsistent and in collision with binding nomenclature.Consequently, they ought not to be used in technical communications.Following the IEC Norm 411, here the term "Double fed machine", or, more particular "Double fed generator" for such configuration will be used.

Introduction
Ever since early beginnings of industrial development of wind turbines in the late eighties of the last century the main players in this promising new market were not the established electric machine manufacturers.Wind turbine manufacturing companies were founded and went bankruptcy at rates never seen before in the electric power generation industry, rarely surviving more than a decade on the market.One of the consequences of such almost chaotic situation is the wind energy price paradox: despite free primary energy, the price for a kWh from a wind power plant is a multiple of the price for a kWh from a conventional (gas, coal or nuclear) power plant.Apart from more or less volatile economical reasons for the price discrepancy, there is an objective cause on the engineering side of the problem: a mismatch between turbine characteristics and capabilities of a power train with conventional generator.By way of example, in most direct-drive wind turbines an expensive and energy lavish solution -a permanent magnet (PM) generator with a full size frequency converter -is used to operate a synchronous generator at a variable speed of the wind.
In this paper an alternative concept of wind generator with single tooth wound stator and rotor, D 3 F, is presented [1], which requires neither permanent magnets, nor gearbox in order to convert energy from wind into electricity.A D 3 F generator based wind turbine has the lowest investment costs and highest overall efficiency among all conventional turbine types.A D 3 F generator is simple to maintain and has a life expectancy inconceivable for wind turbines with gearbox.
In the following the performance of the two most widely spread solutions: gearbox and PM generator-based wind turbines are discussed.

Conventional double fed high speed generator with gearbox for wind turbines
Typical connection scheme of a double fed generator is shown in Fig. 1.
The high-speed wound rotor generator in Fig. 1 requires a gearbox in order to adapt its speed to low rotational speed of blades.Stator winding of the generator is connected directly or via a transformer to the power system, whereas the rotor winding requires a reduced size converter, with a rating typically up to 30% of the rated power.
Nowadays, a couple of decades after the advent of wind generators, the gearbox is still the Achilles heel of turbines operating with double fed generators.The gearbox is one of the components in wind turbines most prone to cause problems [2].Maintenance and gearbox replacement costs, along with the costs caused by energy production losses due to non-functioning gearboxes, build a large share of the expenses of operating wind power plants.Average material costs for major replacement of a wind turbine gearbox are the highest among all other components and can reach the amount of 230.000 € for a typical wind turbine; time needed for a gearbox replacement is almost 10 days [3].Reliability curve of a typical gearbox, shown in Fig. 2 [2], confirm the old rule of thumb that during the 20-year design life of a wind turbine its gearbox has to be replaced every 5-7 years.In other words, a wind turbine with gearbox consumes 3-4 gearboxes during 20 years of its operation.The poor reliability of a gearbox speaks volumes about this solution.
Premature failure of gearboxes increases cost of energy due to turbine downtime, unplanned maintenance, gearbox replacement and rebuilt, as well as increased warranty reserves.Both crucial components of a gearbox-gear teeth and bearings-are potential sources of gearbox failures.Besides roller and ring hardening cracks running from the surface of a roller or a ring toward its center of mass in a relatively straight line, as described in [10], the most frequent failure modes for bearings (Fig. 3) are: • Severe adhesion (scuffing), which relates to the transfer of material from one bearing surface to another due to welding and tearing.Damage due to adhesion typically occurs in areas of slip in narrow or broad bands along the direction of sliding.Scuffing areas appear to have a rough or matte texture.• Abrasion can be two-body (embedded particles on one bearing surface abrade the opposing bearing surface) and/or three-body (due to loose contaminants).Abrasion scratches on bearing surfaces are in the direction of sliding.Abrasion is usually caused by contamination of lubricant by sand, rust, machining chips, grinding duct, weld splatter, and wear debris.
The most frequent failure modes for gear teeth (Fig. 4), as described in [10] are fretting corrosion and high-cycle bending fatigue.It is interesting to know that in the final stage of highcycle bending fatigue usually sudden fracture occurs, which can be ductile, brittle or mixed-mode, depending upon material toughness and magnitude of applied stress.
It can be generally summarized that no wind turbine gearbox can survive without clean oil.Interestingly, some wind turbine gearboxes fail and others survive without a recognisable reason.

Direct drive wind generators with permanent magnets
Serious problems with gearboxes used in early versions of wind turbines were probably the dominant factor that lead to the development of direct drive generators.Before their ongoing wide spread applications in direct drive wind turbines, high polarity synchronous generators were overwhelmingly used in hydro power plants, however at ratings exceeding those in wind turbines sometimes for a couple order of magnitudes.
Connection scheme of a direct drive generator with permanent magnets is shown in Fig. 5.The use of permanent magnets in electric machines has a long tradition, especially in the area of D.C. commutator motors and generators.However, before the advent of rare-earth based permanent Figure 4 Severe fretting corrosion of gear teeth [4] magnets, these attempts were severely limited by too a low coercive force of available AlNiCo magnets and/or by too a low residual flux density of ferrites.Rare-earth magnets with coercive forces above 1 MA/m made it possible to build rotating field machines resistant to demagnetization tendencies of armature reaction Ampèreturns.High coercive force opened doors for various applications of rare-earth permanent magnets in D.C. machines in which their intrinsic advantagegeneration of magnetic flux without electric losses -could be completely revealed.
The initial success based on the advantage of lossless field creation by permanent magnets in D.C. machines was wrongly thought to be applicable to rotating field (synchronous) machines.Whereas in a properly designed and constructed PM D.C. machine the air gap flux is practically independent of load and never exceeds its rated value, in a PM synchronous machine not only magnets, but also load Ampèreturns and phase shift of the armature current determine the amount of air gap flux.As a consequence, the replacement of the field winding with permanent magnets in a synchronous machine does not have by far such positive effects as in a D.C. machine.
In particular, a permanent magnet synchronous machine has following disadvantages vs. wound field synchronous machine: • Insufficient flux under load: Permanent magnets in a direct-drive wind generator are dimensioned in such a manner that no-load and rated voltage of the generator are almost identical.Since the magnet Ampèreturns are frozen, a loaded generator operates in underexcited mode Figure 7 The dependence of stator copper losses on the power factor of a PM machine (leading power factor, Fig. 6).Rated field Ampèreturns of a wound rotor synchronous generator, on the opposite, can reach up to 3 times higher values than its no-load Ampèreturns.Due to saturation, a triple field current can create no more than up to 50% more flux.Therefore, the rated rotor flux in a wound rotor machine is about 50% higher than its no-load value [5], whereas permanent magnets in a loaded PM machine cannot deliver more flux than at no load.Since the machine torque is directly proportional to its air gap flux, a wound rotor synchronous machine can generate 50% more torque per volume than a PM machine, considering the same level of no-load flux and stator current sheet.• Poor power factor as a source of increased stator copper losses: As opposed to a conventional field winding excited synchronous machine which can reach any power factor simply by varying its field current, the operating point of a PM generator is always in the II quadrant of the power chart [5], [12], i.e., underexcited.Rated values of PM generator power factor vary between 0.8 -0.9 leading for conventional, double-layer windings and 0.6 -0.8 leading for tooth-wound machines [6].
As a consequence of the poor power factor cos ϕ, stator copper losses P Cu in a 3-phase PM generator increase for a given output power P according to equation: with U denoting the terminal voltage and R Ph the phase resistance.Denoting by P Cu,min stator copper losses at unity power factor and by P Cu the same quantity at an arbitrary power factor, one obtains the ratio between the two as a function of the power factor as shown in Fig. 7 At cos ϕ = 0.7 stator copper losses of a PM generator are twice as large as at cos ϕ = 1!The price for elimination of field winding losses in PM generators is paid by a substantial increase of stator winding copper losses.
In addition, one should not forget that NdFeB magnets are metallic and that the pulsation of flux density inside magnets caused by stator MMF harmonics, teeth and slots generates eddy current losses in magnets.Eddy current losses in magnets not only deteriorate generator efficiency, but also increase magnet temperature.Since magnet flux decreases with increasing temperature of an NdFeB magnet, stator current has to be increased in order to compensate for loss of magnet flux.A higher stator current leads to even higher copper losses, which further deteriorate the generator performance.∑ Unpredictable development of magnet prices: Permanent magnets belong to the most expensive materials built into an electric machine.As a rule of thumb, about 50% of a PM generator price goes to the magnets.Paradoxically, permanent magnets used in "environment friendly" energy generation from wind are fabricated in an environmentally extremely hostile manner [7].The poor power factor of a PM generator means that the converter losses increase due to unnecessarily increased reactive component of the stator current, which further worsens the overall power train efficiency.

Electric machine with single tooth winding on one side of air gap
Single tooth winding is a special type of fractional slot winding with a coil pitch equal to the slot pitch.Being patented already in 1895 [8], Fig. 8, this machine configuration is widely used in stepping motors [9], as well as in some niche applications, such as ski lift drives (Fig. 9), [6].The ski lift drive motor (Fig. 9) manufacturer offers also wind turbines based on PM generators with single tooth stator winding.
Coils of a single tooth wound machine have obviously the shortest end windings among all winding types and, therefore, comparably lower I 2 R losses and leakage inductance.

Electric machine with single tooth winding on both sides of air gap
Figure 8 Cross-sectional view of the first ever tooth wound machine [8] Figure 9 Stator detail of a tooth wound ski lift motor [6] Figure 10 The prototype of an electric machine with single tooth winding on both sides of air gap [11] In [1] an electric machine with single tooth winding on each side of air gap is described.Based on this disclosure a machine prototype was built (Fig. 10) and tested [11].The machine shown in Fig. 10 not only has extremely short end windings, and consequently low end winding resistance and leakage reactance, but it also can be cooled more efficiently than conventional machines.Fresh air is blown directly to end windings and coil portions in slots, which is an additional reason why this type of machine has a better torque to volume ratio than a conventionally wound machine.
Measured torque-speed curve of the machine prototype in Fig. 10 during starting transient is shown in Fig. 11.Rotor coils during the measurement were short-circuited, and the measured curve corresponds to that of an induction motor starting characteristic.One recognises in Fig. 11smooth M(n) curve, without excessive torque pulsations.
Rotating air gap flux density created by a single tooth winding can have substantially large higher harmonics.If the orders of higher harmonic components in both stator and rotor created rotating air gap flux density components match each other, a pulsating torque will be generated due to such harmonics.In addition, currents creating higher harmonic torques produce their own I 2 R losses, which increase stator and rotor winding temperatures.Therefore, special attention has to be paid to the proper design of stator and rotor windings [1].
Operating chart of a double fed electric machine with single tooth winding on both sides of the air gap is shown in Fig. 12 [12].In this Figure a curve representing a set of operating points of a permanent magnet generator having the same synchronous reactance of 167 % and the same stator as the single tooth wound machine is shown for the purpose of comparison.
By comparing performances of the two machines in Fig. 12 one concludes that a machine with single tooth winding on both sides of the air gap is superb to a permanent magnet excited machine.This is a consequence of the fact that the field current of the former can be varied in wide range, thus Operating range of a machine with single tooth winding on both sides of air gap (blue shaded area) [12] and operating curve of a permanent magnet machine (red)

PM
counteracting the armature reaction created by the load, whereas the permanent magnet machine has a fixed excitation not capable to react to the Ampèreturns of the load current.
7. A 1.6 MW, 19 rpm direct drive wind turbine: Performance comparison of a PM and a D 3 F generator Flawless operation of the machine model presented in the previous section was one of the reasons to implement the principle of single tooth winding on large scale, e.g. in design of a wind turbine in the power range of couple of MW. Circuit diagram of this application is shown in Fig. 13.Compared with circuit diagrams of conventional solutions, shown in Figs. 1 and 5, the wind turbine represented schematically in Fig. 13 is simpler, cheaper to manufacture, easier to maintain and has a higher overall efficiency.A higher overall efficiency is a consequence of the fact that there is one power conversion and loss generation component less (gearbox), and another component (converter) has a significantly lower rated power, and, consequently, notably lower losses.
Crucial design data of a 1,6 MW, 19 rpm D 3 F wind generator are presented in Table 1.For purpose of comparison, data of a conventional PM generator of the same rating are also given in this Table.
Both generators in Table 1 have the same air gap diameter.The PM generator has longer end windings and the total stator length of 1081 mm, as a consequence of the stator coil pitch of 1 -7.The D 3 F generator is better cooled and has a coil pitch of 1 -2, which results in the stator overall length of only 625 mm.Therefore, the total stator length of the PM generator is 1081 mm / 625 mm, or 73% larger than for a D 3 F generator, a clear disadvantage relative to the latter.The proposed D 3 F generator is more compact, has shorter active part and, therefore, lower stator losses and end winding leakage inductance.Besides, it requires less copper for the stator winding.In addition, the D 3 F generator does not need permanent magnets in order to operate, which in the case of the PM generator weigh almost 1 metric ton (938 kg).
Cooling of a D 3 F generator is significantly more efficient than in case of the PM generator with conventional winding.Whereas the active length portion of coils of the PM generator is cooled indirectly by air, coils of the D3F generator are cooled directly along their complete axial length, which helps increase its rated power.
In Fig. 14 a segment with coils of a fundamental pole of a D3F generator is shown.The segment structure enables simple assembly of the generator on site, as well as replacement in case of winding failures.Direct air cooling of coils allows significantly higher current density than in a machine with conventional distributed windings (6 A/mm 2 in both stator and rotor windings of a D3F generator vs. 3.89 A/mm 2 in stator windings of a PM generator).Blue arrows in this Figure denote cooling air, which gets heated on its way axial through the coils.
Performance data of the two generators at the rated operating point are compared in Table 2.In order to generate the same torque as its pendant the PM generator, current density in the D 3 F generator is significantly lower (3.27A/mm 2 as compared to 3.89A/mm 2 ), and the operating temperature of the D 3 F generator is far below the one given by wire insulation class.
Considering current density of 6 A/mm 2 , which is a typical value for directly air-cooled conductors, one obtains that a D 3 F generator can generate 2993 kW, or 84% more power than a PM generator from the same volume!

Conclusions
In the paper a novel type of wind generator, the Direct Drive Double Fed (D 3 F) was introduced and its characteristics were compared with those of conventional solutions.A D 3 F generator does not need a gearbox, as opposed to a conventional wind turbine with a double fed wind generator.It does not require permanent magnets and a full-size converter, as is the case with a conventional direct drive PM wind turbine.
The advantages of a wind turbine with a D 3 F generator over conventional turbine drives are numerous, in particular: -Lower manufacturing costs, since no permanent magnets, no gearbox and no full size converter are needed for operation of a wind turbine with a D 3 F generator.In addition, both stator and rotor can be manufactured in segments, which simplifies transportation, mounting on site and replacement of damaged coils in later operation; -Improved energy harvest due to higher overall efficiency, as a consequence of elimination of superfluous components (gearbox, full size inverter) and losses in them; -Better cooling, since the complete coils of a D 3 F generator are directly cooled by air; -Extended life expectancy and cheaper maintenance, since the wind turbine with a D 3 F generator operates without the most unreliable component-the gearbox; -Low servicing costs, short down times, not only due to absence of a gearbox, but also because of exchange-friendly coils; -Segmental stator and rotor construction can be applied, which simplifies assembly of the generator on site and shortens outages.

Figure 1 .Figure 2 .
Figure 1.Connection scheme of a conventional double fed generator

Figure 5 Figure 6
Figure 5 Connection scheme of a wind turbine with a permanent magnet generator

Figure 11
Figure 11 Measured torque-speed curve of the machine in Fig. 10 with short-circuited rotor coils during starting transient

Figure 13
Figure 13 Connection scheme of a wind turbine with an all tooth wound generator Fig.15, and current density along with flux lines distribution at rated load of the studied generator in Fig.16.As expected, a properly designed D3F generator develops a smooth torque at rated operating point, Fig.15.

Figure 14
Figure 14 Direct air cooled coils of a fundamental pole segment of a D 3 F generator

Table 1 .
Design data comparison between a 1.6 MW, 19rpm PM and D 3 F generator

Table 2 .
Performance data comparison of a 1.6 MW, 19rpm PM and D 3 F generator Torque of the analyzed 1.6 MW generator as a function of time is shown in