A study of generator system selection for large wind turbine generator system

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  • A Study of Generator System Selection for Large Wind Turbine Generator System

    MAMORU KIMURA,1 KAZUMASA IDE,1 KAZUO NISHIHAMA,1 MOTOO FUTAMI,1 MASAYA ICHINOSE,1 TETSUO FUJIGAKI,2 MOTONOBU IIZUKA,2 KAZUHIRO IMAIE,2

    YASUOMI YAGI,2 and JYUNJI TAMURA31Hitachi Research Laboratory, Hitachi Ltd., Japan

    2Hitachi Works, Power Systems, Hitachi Ltd., Japan3Kitami Institute of Technology, Japan

    SUMMARY

    This paper focuses on selection of wind turbine gen-eration systems that include generators, converters, andgears. We study three systems: a permanent magnet gener-ator (PMG) system, a doubly-fed generator (DFG) system,and a synchronous generator (SYG) system in terms of thesystem efficiencies and running costs. The system efficien-cies and running costs are calculated by considering therelationship between wind power and wind conditions.According to these results, the one-step gear PMG systemis the best choice for a large wind turbine system. 2007Wiley Periodicals, Inc. Electr Eng Jpn, 161(1): 5157,2007; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.20506

    Key words: wind turbine generator; system effi-ciency; running cost.

    1. Introduction

    As a result of the December 1997 COP3 (KyotoProtocol to prevent global warming) agreement, target val-ues for reducing greenhouse gases have been established,and the amount of carbon dioxide (CO2) exhaust has beenrestricted. Photovoltaic power generation, wind power gen-eration, and geothermal and hydraulic power generationnow represent power generation facilities that use naturalenergy without fossil fuels. Among these, wind powergeneration is gaining the most attention worldwide at pre-sent [1, 2]. In particular, if a turbine that uses lift in line withaerodynamics, a so-called modern wind turbine generator,is developed, usage levels would rise dramatically. Over the

    past 5 years, the growth rate has been about 32% annually,representing the fastest rate ever [35].

    Given this situation, construction has proceeded [6]in locations with a relatively low average wind speed ofroughly 7 m/s (Class 2, Class 3), because locations optimalfor wind power generation with good wind conditions, thatis, strong winds, year round (IEC61400 Class 1: averagewind speed per year of greater than 10 m/s) are few and farbetween. As a result, in wind power generation systems thatuse the constant-speed squirrel-cage-type inductive gener-ator that have been employed in the past, efficient operationin areas with lower wind speeds is problematic, and so avariable speed wind power generation system that is effi-cient even at lower wind speeds is needed [7].

    Development of a variable speed wind power genera-tion system is progressing at present in various ways atvarious sites using or not using gears, including a perma-nent magnet generator (PMG), a doubly-fed generator(DFG), and a synchronous generator (SYG). For instance,the National Renewable Energy Laboratory (USA) hasreleased an evaluation of the mechanical systems, includingthe cost and efficiency of gears and turbines [8]. However,there do not seem to be any reports calculating the runningcosts while considering the efficiency characteristics withrespect to the load of the generator system and the windappearance frequency.

    In this paper, the authors evaluate the various meth-ods given above in terms of functional features, runningcosts, and initial costs, and then report on the results of thisevaluation with gears taken into consideration in a PMG,DFG, and SYG for the high-capacity, next-generation 5-MW wind power generation system [9] under evaluation atpresent for use offshore in many countries.

    2007 Wiley Periodicals, Inc.

    Electrical Engineering in Japan, Vol. 161, No. 1, 2007Translated from Denki Gakkai Ronbunshi, Vol. 126-D, No. 3, March 2006, pp. 255260

    51

  • 2. Evaluation Method

    2.1 Wind power generation system

    Figure 1 shows the most common strategies for vari-able speed wind power generation systems at present. Theirfeatures are given below.

    (a) Doubly-fed generator (DFG)The rotation speed of the turbine is accelerated using

    a step-up gear. The rated number of rotations for the turbineis in general 1000 to 1500 (min1). Variable speed operationis possible, and lower costs can be achieved because thepower transformer need only have the capacity for theexcitation portion of the rotor. At present this is the mostcommon wind power generation system.

    (b) Synchronous generator (SYG)A generator with tens to hundreds of poles is directly

    connected to the turbine, which is then connected to thepower system via a power transformer with the same capac-ity as the generator. Because gears are not used, noise andmaintenance costs associated with gears can be reduced.

    (c) Permanent magnet generator (PMG)As with the SYG, a generator with multiple poles is

    connected to the turbine, which is then connected to thepower system via a power transformer with the same capac-ity as the generator. Along with having the advantages of aSYG, in a PMG, maintenance costs can be further reducedbecause there are no slip rings. With no loss on the rotorside, high efficiency can be achieved. Moreover, there are

    examples of generator speeds being accelerated through theuse of gears.

    The authors will evaluate the three systems describedabove.

    2.2 Design policy

    The authors performed their evaluation using a gen-eration system with an output of 5 MW for their targetsystem in order to address the increased size of future windturbine generation systems. In a gearless SYG or PMG, anultralow-speed generator with 6 to 10 min1 rotations isused, and in a DFG, a synchronous speed of 1000 min1 isused, with a slip range of 20%. Further, the gears areassumed to provide a fivefold increase per each, and therated speed for a three-step gear increases to 1200 min1.The output obtained is proportional to the cube of the windspeed, and the authors performed their evaluation under theassumption of 5 MW at 10 rpm, and 3.65 MW at 9 rpm.

    Figure 2 shows the flowchart for the evaluation of thepower generation systems. In each method for designing arotating machine, the parameters are calculated, then themechanical loss, iron loss, copper loss, and stray loss arecalculated using the same method for the three machinetypes, and finally, the generator efficiency is calculated.Moreover, the inverter efficiency is calculated for eachsystem, and in the PMG and DFG, the system efficiency iscalculated by multiplying it by the gear efficiency. Effi-ciency during a partial load is found in the same way.

    The types of loss taken into consideration here areiron loss, mechanical loss (bearing loss, wind loss), copperloss, and stray loss. The equations for each are given below.

    Iron loss (Wi)

    Fig. 1. Wind turbine system. Fig. 2. Flowchart for estimation of system efficiency.

    (1)

    52

  • Iron loss is calculated based on Eq. (1). The iron losscoefficients Ke and Kh are calculated using Eq. (2) andbased on catalog data [10] at 50 and 60 Hz. The iron loss iscalculated for the teeth region and the core back area, andthe magnetic flux density B used applies to the teeth andcore back magnetic flux when calculated for a loadlessinduced voltage.

    Mechanical loss (Wm)The mechanical loss taken into consideration here

    includes bearing loss, and the wind loss resulting from thegap between the stator and the rotor. Each is explainedbelow.

    Bearing loss (Wb)Ball bearing loss is calculated using the equation

    given in Ref. 11 and based on the coefficient of friction forthe bearings used. Also, the type of bearing loss is taken tobe for a ball bearing, and the bearing dimensions are se-lected with the load resistance in mind.

    Wind loss (Ww)The wind loss is calculated based on the relationship

    between the equation for wind loss given in the references,the Reynolds number, and the coefficient of friction, andthe coefficient of viscosity for air.

    Copper loss (Wc)Copper loss is calculated in accordance with Eq. (3)

    per phase. The rotor copper loss (Wr) and the stator copperloss (Ws) are each calculated for the DFG and SYG, andonly the stator copper loss is calculated for the PMG. Here,R is the winding resistance () per phase, and I is the currentvalue (A):

    Stray load loss (Wstr)Here, the stray load loss is calculated as follows

    regardless of the type of generator and using the IEC 60034rating for an induction drive:

    Here, represents the stray load loss evaluation coefficient;P0, the rated output (W); It, the current value (A); IRt, therated current value. Based on IEC 60034 and JEC 2137, is set to 0.009 because here 5 MW is used.

    2.3 Generator characteristics

    The characteristics of the PMG, SYG, and DFG aregiven as examples in the evaluation in this paper.

    (a) PMGFigure 3 shows the output versus efficiency curve for

    a gearless, one-step, two-step, and three-step system. Basedon the figure, the efficiency of the gearless PMG is highest,and it is clear that flat efficiency characteristics appear atabove 95% for 20 to 30% of the rated output.

    Figure 4 shows the results of calculating the loss in agearless PMG. Based on the figure, the iron loss is highest,causing a drop in efficiency at low output levels, and thecopper loss then exceeds the iron loss as the output rises.

    (b) SYGFigure 5 shows the results of calculating the loss in a

    gearless SYG. Based on the figure, the iron loss is highest,and the copper loss is highest overall because of secondarycopper loss due to the magnetic field created, causing a dropin efficiency at low output levels. It is clear that the copperloss is greater than in the PMG.

    (2)

    (3)

    (4)

    Fig. 3. Characteristics of output power versus efficiency(PMG).

    Fig. 4. Estimation results of loss (PMG).

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  • (c) DFGFigure 6 shows the output versus efficiency curve for

    the DFG. It is clear that efficiency when the DFG charac-teristics have a slip of 0 is highest, and that when slip occurs,efficiency drops due to the resulting rotor loss, and as aresult, the DFG characteristics include a drop in efficiencyat rated points when the variable speed range is broad.

    3. Evaluation Results

    3.1 Comparison of systems

    Figure 7 shows the efficiency curves for all of thegenerators. Based on this figure, it is clear that the gearlessPMG has the highest-efficiency in all regions. In the PMG,high-efficiency results are seen because the rotor copperloss is low. Moreover, the efficiency of a one-step PMG and

    a three-step DFG is about the same. The reason for this isthat whereas the PMG requires a converter with the samecapacity as the generator, in the DFG, there is a differencein efficiency lost in the converter because only a converterwith a capacity for the generator slip is needed. However,the DFG also has the disadvantage of not being able tohandle a significant variable speed range.

    Table 1 compares the efficiency by size for eachsystem. The results clearly show that due to differences inrotor speed, the DFG has the smallest size and weight. Anexamination of recent 5-MW prototypes shows that thereare many three-step DFGs, and a goal is to reduce theweight of the nacelles [12, 13]. This is because it is difficultto set up a heavy object, here a nacelle with a generator andgearing in a 5-MW-class turbine with a diameter over 100m, at high elevations. When the cost of constructing a windturbine is considered, this is not practical given that thegenerator alone weighs several hundred tons. Thus, theone-step PMG system and the three-step DFG system canbe considered as able to reduce the cost during turbineconstruction and operate at high efficiencies.

    Fig. 5. Estimation results of loss (SYG). Fig. 7. Characteristics of system output power versussystem efficiency.

    Table 1. Comparison of 5-MW-class wind turbinegenerator system

    Fig. 6. Estimation results of loss (DFG).

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  • 3.2 Comparison of running costs

    Under wind power generation, it is difficult to obtainsteady power levels at all times because the source is naturalenergy. At the present time, commercial success is evalu-ated for an average wind speed of above 7 m/s. However,the income of the wind power generation system is deter-mined by whether or not it can generate power at lowerwind speeds.

    The authors evaluated the annual power generationcapacity resulting from differences in generator efficiency.Using a wind speed appearance distribution model [14],they simulated the annual output capacity of a turbine andcalculated the capacity while taking into consideration theoperating range of the turbine.

    Figure 8 shows the operation speed and turbine effi-ciency for a three-step DFG and a one-step PMG, and thedifference in annual income when a sale price of 8/kWhis assumed for the average wind speed. The PMG canoperate a turbine at high efficiency because it can handle asubstantial variable speed range. However, the DFG oper-ates at a constant number of rotations because its variablespeed range is exceeded when the average wind speed isbelow 7 m/s due to the variable speed range being 20%. Asa result, turbine efficiency drops significantly at lower wind

    speeds. Moreover, in terms of efficiency characteristics forthe generator, the PMG tends to have a higher powergeneration capacity when the average wind speed is lowerbecause it can operate at a higher efficiency than the DFGcan at low speeds. Consequently, the income difference ishighest when building at a site where the average windspeed is about 5 m/s, specifically about 2.5 million peryear.

    Wind turbine generation systems have the advantageof no fuel costs as a part of the running costs, and as a result,efficiency differences in the generators reflect directly dif-ferences in income. Income differences can be allocated todepreciate the initial costs as is, and so under the evaluationconditions in this paper, a one-step PMG system has thegreatest cost/benefits under a variety of wind conditions fora wind power generator operator.

    4. Conclusion

    The authors considered the selection of a 5 MW-classnext-generation large-capacity wind power generation sys-tem, and then by calculating the generator efficiency, con-verter efficiency, and gear efficiency, evaluated the optimalcombination for a wind power generation system with theweight of the system also considered. The conclusions ofthe evaluation are summarized below.

    (1) The authors showed that when attention is givento increasing generation capacity in a region with lowerwind speeds, the PMG system yielded increased generationcapacity in such regions because it had minimal rotorcopper loss.

    (2) The authors showed that because a DFG systemcan use a converter with a smaller capacity, its efficiency ishigher compared to a PMG system or a SYG system, bothof which require a converter with a capacity equivalent tothe generator capacity.

    (3) The authors showed that the income differencebetween a three-step DFG system and a one-step PMGsystem is greatest when constructing the system at a sitewith an average wind speed of around 5 m/s, specifically2.5 million per year.

    (4) The...

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