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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:[email protected]). 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
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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
'
ww
ww
www
'
zV
F
yV
F
yz
xV
F
zV
F
xz
xV
F
yV
F
yx
VQW
VQW
VQW
0div1
21
1
0div1
21
1
0div1
21
1
z
2
2
z
y
2
2
y
x
2
2
x
ww
ww
'
ww
ww
'
ww
ww
'
VF
zV
F
z
VF
yV
F
y
VF
xV
F
x
&
&
&
QQV
QV
QQV
QV
QQV
QV
(4)
481. A/mm2
350. A/mm2
a)
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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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