AbstractLarge Synchronous Generators are the

fundamental machines in nuclear, fossil and alternative energy generation. Large 4-pole turbo-generators reach 2191 MVA with torques of 12.6 MNm. This torque-level is even small in comparison with the Three Georges hydro generators, which have less speed and reach up to 96MNm. The market of both kinds of machines is a very limited one. The backbone of the energy market is given by gas-turbine- and wind-turbine driven generators. Beside smaller asynchronous and synchronous generators up to 6 MW, which are driven by the wind-turbine over a gearbox, direct drives are used with large synchronous generators either with permanent magnets or a field winding in the rotor. Generators with large numbers of machines per year as well as extreme large generators are based upon highly sophisticated design and calculation methods. Salient 4-pole synchronous generators for instance must be very reliable and optimized in design due to the number of machines, which are going into application each year. The development process is based for all large synchronous generators on analytical and numerical calculation methods. Numerical field calculation is a powerful tool for the design in these electrical machines. Developed methods and programs enable the skilled engineer to solve challenging field problems, which occur in reality. Analytical methods are nevertheless the backbone to calculate synchronous generators as a whole or to evaluate design modifications close to a verified design point.

Index TermsCalculation principles, energy conversion, large synchronous generators, research and development process, salient pole-generators, turbo-generators

I. INTRODUCTION VIDENCES of distinguished engineering efforts have to be accomplished in the development of turbo-

generators up to 2000 MW as well as in the development of wind-turbine generators and industrial machines, see fig. 1. It is inevitable to combine basic principles of

O. Drubel is with SIEMENS AG, Large Drives Dynamowerk

Nonnendammallee 72, 13296 Berlin, Germany (e-mail:oliver.drubel@siemens.com). The author thanks T. Hildinger from Voith Hydro Holding GmbH&Co. KG for the provision of the hydro-generator photographs. Additionally the author acknowledges Dr. Wetzel from Siemens AG in Nrnberg for the fluid dynamic calculation of the salient pole generator and Dr. Hartmann from Siemens AG in Berlin for the wind-turbine generator photograph..

electrical engineering with fluid dynamics, heat transfer and mechanical engineering. Synchronous machines have to endure centrifugal forces comparable to cars, which change their direction with a speed of 850 km/h, but with a turning circle of 625 mm. Large synchronous machines have the disadvantage of the need for insulation material in the rotor in comparison with other types of electrical machines like asynchronous machines or reluctance machines. The dc-field in the rotor does allow on the other hand the application of solid forged steel. Additional losses due to eddy currents in the rotor are only generated due to higher harmonics for instance slot harmonics of the fundamental, but not due to the fundamental sinusoidal flux component itself. Asynchronous machines, which use forged rotors to allow for high surface speeds have to endure disadvantages in losses and power factor. Nevertheless the synchronous machine has two further main advantages at these large ratings. Synchronous machines allow reaching efficiencies, which are about 0.5-2% higher than the efficiency from asynchronous machines and last but not least the apparent power of the generators can be controlled.

Fig. 1. Example of a 40 MVA salient pole-synchronous generator The synchronous generator has several historical roots. A

Challenges in Calculation and Design of Large Synchronous Generators

O.Drubel, Member, IEEE, VDE

E

978-1-4673-5658-9/13/$31.00 2013 IEEE 18

physical root of an a.c.-machine is given by Antoine Hippolyte Pixii in 1832; the first three phase salient pole synchronous generator has been built in 1887 by Friedrich August Haselwander, [1]. One year later Charles Bradley patented ideas for synchronous generators. This hundred years before started development is going on still with strong steps. Already 1970 turbo-generators with weight to power ratios of 800-1000 kg per MVA have been built, but gas cooled. Due to the application of numerical tools, which are used either in 2-dimensional versions or from time to time in 3-dimensional ones, it has been possible to keep the power density since then, but in air cooled machines, [2]. Numerical calculation methods are used to confirm an analytically determined design, [14], or to investigate ideas beyond existing design rules, [3]. Further applications are damage investigation, mechanics and hot spot investigation. They are still not powerful enough to substitute experience and analytical methods in damage prediction or design optimization. Nevertheless synchronous generator development is one of the highlights in developments of electrical engineering. A total overview for the necessary steps in the development up to a pilot generator layout is given within this paper.

II. DEVELOPMENT TARGETS Development targets for large synchronous generators

depend strongly on the individual planned application. In principle two families of machine design can be identified, see fig. 2. One family of generators consists of machines, which are first developed for a unique plant. These generators are optimal adjusted to the individual site conditions of for instance a hydro plant with the individual water-turbine. Another case could be a generator, which fulfills the requirements of the turbine train operating at the technical boundary line in a large steam power plant.

Fig. 2. Development targets for large synchronous machines Once the machines have been built, their design is

reused on power plants with the same performance conditions. Main target of these machines is that they

fulfill, what has been guaranteed without having the chance to do any corrections on the design in a second go. The guaranteed efficiency, the apparent- and active power are predefined during the sales process. Constrains like shaft vibration levels, noise levels or insulation test voltages have to be fulfilled due to international standards or local requirements, which are often merged in plant requirement specifications. Last but not least the individual grid connection underlies the regional grid codes.

The second family of generators represents a series of machines, which are affiliated by a strong basis of identical components. These components are characterized by same drawings and parts lists. This second class or family of generators has to be developed up to an optimum regarding market sector requirements, reliability and production methods. Less adjustment to the individual site condition has to be compensated with more efforts to streamline the material utilization for the rated operation point of the machine.

III. DESIGN HIGHLIGHTS Royal league in torque and size of electrical machines

are generators in large hydro power plants with 96 MNm, see fig. 3,

Fig. 3. Large 840 MVA hydro generator with 75 rpm

The design of these generators depends strongly on the site conditions of the river or mountains, where the plant shall be erected. The hydro plant Itaipu, see fig. 4, in Brazil and Paraguay has 20 turbines, which drive generators with 823 MVA each at 90.9 rpm.

This single hydro plant holds the record of 95000 GWh electrical power production in one year. This amount of energy would have been sufficient to replace all German nuclear plants, when they where in operation. The stator current of one generator has a rated value of 26400 A. Even though this current level is relatively high, it can be handled in the active part in a reasonable way due to the high no. of pole pairs. Design challenges are given by the rotor dimensions both in the bearing design as well as the

Development Targets for Large Synchronous

Generators

One of a kind x Optimal adjusted to

site requirements x Highly reliable and

efficient by individual design

Machine Series x Optimal adjusted to

individual energy segments

x Reliable and efficient by standardized components

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manufacturing accuracy because of the weight.

Fig. 4. Hydro generator with 823 MVA and 90.9 rpm

The series product of low speed large synchronous generators is given in the wind-energy sector, see fig. 5.

Fig. 5. Direct drive 6 MW synchronous generator

These permanent magnet wind-power generators are designed for low voltage converter operation with 6 MW up to 11 rpm. Due to the minimization of components within the tower head this kind of series design has strong advantages in reliability and maintenance for offshore applications.

Different highlights are dominant within the design of a 2- or 4-pole turbo-generator. An example of a turbo-generator is shown in fig. 6. Two pole machines have only two parallel current circuits, which are complete symmetrical. Therefore half of the rated machine current has two be handled within one slot. The voltage level must often be adjusted accordingly. The generator in fig. 6 is operating in the plant Schwarze Pumpe. The generator is hydrogen cooled with a stator winding voltage of 27 kV. Several generators exist with this voltage level. Nevertheless the control of the electrical field requires

strong experience.

Fig. 6. Hydrogen-cooled 1000 MVA, 2-pole turbo-generator

Directly water cooled 2- or 4-pole turbo-generators are most often used in steam power plants. Steam power plants are adjusted to the available fuel for heat generation. Gas-turbine plants are more standardized with a shaft power, which allows often the application of air cooled generators. Air cooled generators have several advantages against hydrogen cooled design. They do not need the auxiliary systems to generate hydrogen for gas cooling or even adequate water quality. Additionally the housing does not need to cope with the hydrogen gas pressure. On the other side they are limited in performance and efficiency. Fig. 7 shows a published photograph of the largest air cooled turbo-generator, [4].

Fig. 7. Design of the world largest air cooled 2-pole turbo-generator, [4]

This generator represents the limits of mechanical dimensions in rotor diameter at 3000 rpm. The possible rated power of such a kind of generator is given between 400-500 MVA. Even in this high end machines of air cooled design the weight to power ratio is nearly by a factor of two better in a directly water cooled synchronous machine. This is directly evident by a comparison with the physical parameters of air with pressurized hydrogen gas. The heat capacity of air is more than a factor of 14 lower in comparison with hydrogen gas. The friction of gas is a fraction of the friction of air allowing much higher

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hydrogen flow than air flow through the directly cooled rotor winding. Additionally due to the reduced friction the cooling holes in the rotor winding have a reduced cross section, allowing for more rotor copper in the same rotor slot dimensions.

IV. CHALLENGES WITHIN THE DESIGN PROCESS The development of large synchronous generators

requires the application of a sophisticated design process, see fig. 8.

Fig. 8. Development process for large synchronous generators towards the pilot design A first mile stone is given already by a clear definition of the main product requirements and boundary conditions.

An estimated 70%-80% of product developments need some laps of honour due to non adequate project or segment knowledge, which shall be specified within the project management. Only few experts are really capable to combine the technical feasible with the needs of the application. Within the next step electrical, fluid dynamic, thermal and mechanical constraints have to be challenged to allow a first machine layout. Special attention is given during this development part towards rotor-dynamic and transient-torsional calculation, [5]. Based on the first layout more detailed investigations shall be done especially on the rotor with its centrifugal forces. This will be based on design drafts, which can be used as basis for mechanical stress and or stiffness calculations. Tentative mechanical limits may require electrical design adjustments. Afterwards the detail design of the machine will be elaborated. Again special attention must be paid towards typical areas like the end-winding of two pole generators, [6, 7], see fig. 9.

Fig. 9. End winding design of a 2-pole turbo-generator, [6]

V. CALCULATION METHODS FOR LARGE MACHINE DESIGN The design of electrical machines requires the

combination of all main physical fields. It is neither possible nor effective to solve the electrical, thermal, mechanic and fluid dynamic field equations as a whole in one go. Engineering ingenuity is especially necessary to ask the right questions, which will be investigated in detail then.

The experienced engineer compensates in a first step the accurate fluid dynamic calculation by well known possible figures for stator and rotor current density. Special areas, which require further cooling improvement beyond the existing experience, may be investigated in detail.

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The air- or gas-flow is simulated by the Euler equation (1).

pmFvv

tv

tv grad1

dd

U xww

&&&&&

(1)

This equation-system can be solved numerically. An

example of a solution for a salient turbo-generator is shown in fig. 10.

Fig. 10. Flow calculation at the rotor of a salient pole synchronous generator Based on the flow distribution it is possible to determine

local heat transfer coefficients. Especially the stator current is directly given by the

rated operation point within a synchronous generator. More complex is the determination of additional losses in damper cages, exciter losses or massive iron material, [3, 8, 9, 13] or during converter operation at start up, [8, 10, 12]. A real challenge is the evaluation of excessive current densities, which occur in these large machines during transients like three phase short circuits or faulty synchronization. This is done by the solution of the transient eddy current equation (2):

MPJPJ gradww '

tAA&&

(2)

The solution of this equation is given in a 60 Hz 665 MVA turbo-generator during a three phase short circuit in fig. 11.

A very similar equation structure than the eddy current equation is given by the heat Laplace equation (3):

tcp

ww ' -U-O el (3)

The solution behavior of the heat equation is of good

nature. Additionally temperature is a scalar and the

transient equation part is only in some few cases for instance during ceiling important.

Fig. 11. Eddy current calculation in the massive rotor of a 2-pole turbo-generator during transients, [11]

Last but not least the equation system is given for the

mechanic description of the synchronous machine by the Bertraminische equation system (4).

01

1

01

1

01

1

xz2

yz

zx2

zx

yx2

xy

ww

ww

www

'

ww

ww

www

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ww

ww

www

'

zV

F

yV

F

yz

xV

F

zV

F

xz

xV

F

yV

F

yx

VQW

VQW

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0div1

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1

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1

z

2

2

z

y

2

2

y

x

2

2

x

ww

ww

'

ww

ww

'

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ww

'

VF

zV

F

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VF

yV

F

y

VF

xV

F

x

&

&

&

QQV

QV

QQV

QV

QQV

QV

(4)

481. A/mm2

350. A/mm2

a)

22

This system is solved to determine local stresses in machine parts. The support component of a rotor screw has been optimized by its application in fig. 12

Fig. 12: Mechanic stress calculation within a support component for a screw in the rotor of a turbo-generator

VI. CONCLUSION Design and development of large synchronous machines

require the application of the complete toolbox of modern field calculation in electric, mechanic, thermal and fluid dynamic calculation. The combination of years of experience with available numerical tools allowed strong development steps in the direction of machine utilization, reliability and machine efficiency. These improvements show more and more success in the development of large series machines, which gives the basis to a chance in the energy market towards alternative wind-turbine driven generators or gas-turbine driven plants with all the flexibility of load scheduling in energy systems.

VII. REFERENCES [1] G. Neidhfer, Michael von Dolivo-Dobrowolsky und der Drehstrom

(Michael von Dolivo-Dobrowolsky and the three-phase current), Geschichte der Elektrotechnik Band 19, VDE Verlag Berlin, 2004

[2] R. Joho, J. Baumgartner, T. Hinkel, C.E. Stephan, M. Jung, Type tested air-cooled turbo-generator in the 500 MVA range, CIGRE Session 2000, Paper 11-101

[3] G. Traxler-Samek, R. Zickermann, A. Schwery, Cooling Airflow, Losses, and Temperatures in Large Air-Cooled Synchronous Machines, Industrial Electronics, IEEE Transactions on Volume: 57 , Issue 1

[4] O. Drubel, R. Joho, Y. Sabater, Worlds Largest Air Cooled Turbo-Generator in Commercial Operation, PowerGen, 13-15 Sept. 2004, Manama, Bahrain

[5] M. Freese, J. Rosendahl, S. Kulig, Torsional behaviour of large synchronous machines during asynchronous start-up and system disturbances Electrical Machines (ICEM), 2012 XXth International

[6] G. Grning, Elektromechanisches Verhalten von Stnderwickelkpfen groer Turbogeneratoren bei stationrem Betrieb und elektrischen Strungen (Electro-mechanical behavior of end windings from large turbo-generators during steady state operation and transients), Dissertation, TU Dortmund, Shaker Verlag Aachen, 2007

[7] S. Exnowski, Exitability of different modes of vibration of stator end windings, Proceedings of IEEE IECON 2012, pp. 1781-1785

[8] O. Drubel, Converter Applications and their Influence on Electrical Machines, Lecture Notes in Electrical Engineering vol.232, Febr. 2013, Springer Verlag 2013

[9] O. Drubel, Converter Dependent Design of Induction Machines in the Power Range below 10MW, Proceedings of IEEE IEMDC 2007, SS 5.2, Antalya

[10] O. Drubel, Current distribution within multi strand windings for electrical machines with frequency converter supply, Compel, 2003

[11] O. Drubel, Die Berechnung der elektro-magnetischen und thermischen Beanspruchung von Turbogeneratoren whrend elektrischer Strflle

mittels Finiter-Differenzen-Zeitschritt-Methode (Calculation of electro-magnetic and thermal stress of turbo-generators during transients by application of finite-differences), AfE, Vol. 82, No. 6, Nov. 2000, S. 325-336

[12] O. Drubel, M. Hobelsberger, Medium frequency shaft voltages in large frequency converter driven electrical machines Electrical Engineering (AfE), Volume 89 no. 1 Oct. 2006, pp. 29-40E.

[13] O. Drubel O., R. Gantenbein, A. Izquierdo, M. Klocke, Current flow and losses in brushless exciters with polygon-connected windings and dc rectifiers], Electrical Engineering (AfE) 2007

[14] M. Canay, Ersatzschemata der Synchronmaschine sowie Vorausberechnung der Kenngrssen mit Beispielen (Equivalent circuit diagram of the synchronous machine and its parameter calculation with examples machines), These, Polytechnique de lUniversite de Lausanne, Juris-Verlag Zrich, 1968

VIII. BIOGRAPHIES

Oliver Drubel (M1993) was born in Plettenberg in Germany, on July 4th, 1972. He graduated and received his PhD from the TU Dortmund, and studied additionally at the University of Southampton. He got the permission to teach the subject of large electrical machines based on the habilitation procedure about Converter Applications and their Influence on Large Electrical Machines at the TU Dresden.

His employment experience includes ALSTOM in Switzerland and Siemens in Nrnberg in Germany. Actually he is engaged with Siemens in Berlin. His special fields of interest are electrical machines and drives above 500 kVA.

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