Modelling and simulation analysis
of static drive for large
synchronous machine
Arun Kumar Datta*
Central Power Research Institute, India
Abstract
These devices named ASD, VFD etc. are boon to AC
motors. Among them the thyristor converter has became
firmly established as the static electrical power conversion
equipments of many types ranging from a few hundred
watts to tens megawatts. Though the concept of frequency
converter is very old but for the large electrical machines
this soft starting technology is thought of at a later stage.
Under the technology Static Frequency Converter (SFC) is
envisaged, designed and applied on many large machines
[2] - [4]. Uses of SFC has increased in the recent years
in the field of aviation industry, computer installations,
communications, military installations, motor speed
control, ships and power transmission. These systems use
synchronous motor as well as induction motor which are
smoothly controlled by variable speed drives. Synchronous
motor functions as commutator-less DC motor when the
phase of the stator current is controlled by a rotor position
sensor. The advent of reliable high power thyristors now
makes the synchronous motor a viable competitor to the
commutator machines and induction motors in many
applications [5] – [7]. SFC gives wide range of speed with
long-term stability and good transient performance of
synchronous motors.
Synchronous machines have many excellent features.
Starting problem has made its use limited in nature.
But by virtue of modern static drives it is being used
in a widespread manner. One large rating synchronous
machine and its drive system are taken as study object
in this paper. This machine is installed at a laboratory
of Central Power Research Institute, India. It can be
operated in dual mode i.e. as a motor or as a generator.
In motor mode it attains its rated speed. In generator
mode its output is utilised to perform short circuit tests
on electrical power equipment. Starting, running and
braking of this machine are done with a static drive called
Static Frequency Converter (SFC). Detailed analysis of
this static drive is done in this paper by creating a virtual
model of the system. Model parameters are taken from
the real system. Validation of the model is also carried
out by comparing simulated waveforms with the actual
recorded from the SFC system. Though the study is
carried out on a specific system but the findings are
applicable to all such similar machines.
1. Introduction
2. Starting a synchronous
machine
AC machines are comparatively cheaper than DC machines
for not having any commutator but its use was very limited
being a fixed speed machine. Speed of AC machine
depends on supply frequency which was normally fixed
prior to the application of static devices [1]. Of course,
various means for controlling the speed of AC machines,
connected to a fixed frequency supply had been devised.
These are obsolete now with a static variable frequency
supply. After the invention of semiconductor devices
various static starting devices for motors are developed.
Starting a synchronous motor has always been a problem
as it is not a self starting machine. Variety of systems has
been developed over the years as a starting device. The
choice of the methods employed depends very much on
the particular requirements and conditions [8]. The key
techniques to start the synchronous motors are depicted
in a simplified form in Figure 1. These are briefly
summarized hereafter.
*arun@cpri.in
KEYWORDS
Alternators, AC-DC power conversion, capacitive coupling, electric discharge machining, frequency converter, Fourier
transforms, inductive coupling, simulation, Shaft voltage, synchronous machines, static excitation, thyristor.
Cigre Science & Engineering • N°4 February 2016
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Figure 1. Starting methods of synchronous machines.
2.1 Starting with starter motor
high reactive current (6 to 7 times the rated current)
and therefore drops the voltage in the supply network.
The additional transformer reduces the voltage drop but
increases the start-up time. Braking is possible in this
system with additional equipment.
Synchronous motor is fitted with a flange mounted starter
motor which accelerates the rotor of the synchronous
machine to the required speed. The starter motor is either
an induction motor with starting resistors or a DC motor.
But DC motor is not preferred for high speed machine.
The starting motor has to be mounted on the free shaft
end of the synchronous motor and thus making the group
longer. Braking is possible with additional equipment.
This method is mainly used, when opposing torque and
therefore the rating of the starting system is not more than
about 5% of the synchronous machine rating.
2.3 Frequency starting with asynchronous generator
The synchronous machine is started from the supply
system by using an auxiliary rotating group, consisting of
an induction motor and an asynchronous generator. The
accelerating power is governed by the starting resistor in
the rotor circuit of the asynchronous generator.
2.4 Frequency starting with synchronous auxiliary
machine
2.2 Starting with transformer or direct connection
The synchronous machine is either connected directly
or through a transformer to the power supply. The
machine is started in an asynchronous manner. This
method results in short start-up times but will draw a
The synchronous motor can be started with an existing
turbine-generator set. The two machines are electrically
coupled together at standstill or at low speed and then
Cigre Science & Engineering • N°4 February 2016
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Figure 2. SFC connected with short circuit alternator.
reverse the polarity of the output dc voltage and hence
feed power back to the ac supply from the dc side. Under
such condition the converter is said to be operating in the
inverting mode. The thyristors in the converter circuit
are commutated with the help of the supply voltage in
the rectifying mode of operation and are known as line
commutated converter. The same circuit while operating
in the inverter mode requires load side counter e.m.f. for
commutation and is referred to as the load commutated
inverter. Static frequency converter (SFC) converts supply
frequency to load requirement frequency with static
technology. High rating thyristor based SFC is in use
worldwide for starting and speed control of AC motors
by providing a power supply of variable frequency and
voltage simultaneously. It has a feature of four quadrant
operations. With the introduction of high power IGBTs
the soft starters are started utilizing this technology for
the medium to moderately high power range. However
in very high power application still thyristor has an edge
over IGBTs.
run up at variable frequency. Since this method is used
primarily for pump storage, the last generator cannot be
used as a pumping motor, unless in case of ternary unit.
2.5 Starting with static frequency converter (SFC)
Most of the above discussed methods are very poor
in terms of energy efficiency. Besides that they also
need huge investments of money and time. In this last
method synchronous machine is started from standstill
by applying a phase synchronized, variable frequency
generated by solid state power conversion equipment.
The starting system can be located remotely from the
motor with one start-up system applied for several
motors. The system can also be used for driving, braking
or reversing large synchronous machines. It is based on
current source inverter (CSI). Current source inverter fed
synchronous motors, hereafter also called commutatorless motors, have come into wide use in industry as a
kind of variable speed motors, which can be controlled
with performances near to DC motors. Furthermore,
this system has a rigid structure and is easy to maintain
like AC motors [9].
3. SFC for specially designed
synchronous machine
The three phase fully controlled bridge converter used
in SFC has been probably the most widely used power
electronic converter in the medium to high power
applications [10]. Multiphase circuits are generally
preferred when large power is involved in order to
reduce harmonics. The controlled rectifier can provide
controllable output dc voltage in a single unit instead
of a three phase autotransformer and a diode bridge
rectifier. The controlled rectifier is obtained by replacing
the diodes of the uncontrolled rectifier with thyristors.
Control over the output dc voltage is obtained by
controlling the conduction interval of each thyristor. This
method is known as phase control and converters are
also called phase controlled converters. Since thyristors
can block voltage in both directions it is possible to
A specially designed synchronous machine called short
circuit alternator is used as a source of energy for short
circuit tests on electrical power equipment. It is ought to
know that this alternator runs without a prime mover. It
is started as synchronous motor and can be converted
to generator whenever requires to feed energy for short
circuit tests.
SFC works on the principle of LCI (Load Commutated
Inverter) and is very much popular in the field of gas
turbine base power plant, pump storage power plant and
railways [11] - [15]. LCI operation is simple and reliable. It
uses load-commutated, phase-controlled power thyristor
technology to supply power to the stator windings of a
Cigre Science & Engineering • N°4 February 2016
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Figure 3. SFC configuration.
power with a six pulse phase-controlled rectifier known
as network bridge (NB). It is controlled in such a way
as to produce a unidirectional current of controllable
magnitude in smoothing inductor. Smoothing reactor
filters out the ripples from the DC current in the second
stage. In the third stage this current is then switched in a
thyristor inverter called machine bridge (MB), to provide
variable frequency three phase current to a synchronous
motor. MB produces three-phase alternating current,
the frequency of which is varied from a very low value
up to the nominal value [23]. The current waveform of
the inverter is of the six-step type. Voltage waveform
is dependent on the properties and loading of the
synchronous motor. Precise speed control can be
achieved through inverter frequency control. Reversal
of rotation can also be achieved by reversal of the phase
sequence of the thyristor switching as the frequency goes
through zero value. Reversal of power flow is achieved by
reversing the polarity of the voltage in the direct-current
link between the rectifier and the inverter. Both these
bridges can also be made to operate in vice versa mode
depending upon the machine requirements e.g. braking
etc.Thus, full four-quadrant operation of the drive is
possible. With these features of SFC, the machine can be
operated in dual mode i.e. motor and generator. Thyristor
firing angle in the NB and MB are set by SFC controller
with various feedback loops (Figure 3). This controller is
popularly known as power electronic controller (PEC)
which acts very fast in µsec range during short circuit
test sequence [24], [25].
high efficiency synchronous motor. The power circuit has
a source rectifier connected to the power supply, a DC
link reactor and an inverter connected to the synchronous
motor (Figure 2). The source rectifier connected with the
reactor acts as a DC current source. Its output current is
impressed at the DC input of the machine side inverter
[16]. The LCI controls the motor torque to regulate
motor speed. Motor torque is controlled through the
DC link current. The only problem faced in the LCI is
for the load side SCR commutation at low speed where
natural commutation doesn’t take place hence forced
commutation is used [17]. This is happened due to low
induced emf generation. Normally this problem persists
upto 10% of the rated speed.
Though SFC is used to run a synchronous motor,
however implementation of this technology in a short
circuit testing plant is actually eliminated a high power
motor used as a prime mover for a large synchronous
generator [18] – [20].
4. sfc operation
Frequency converters are of many types depending
on the technology and application. The general
configuration (Figure 3) of SFC here is a combination
of two 6-pulse thyristor bridges with an intermediate dc
link reactor [21], [22]. Hence it is a three stage process.
In the first stage, AC power frequency is converted to DC
Cigre Science & Engineering • N°4 February 2016
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Figure 4. Simulink model of SFC used in short circuit alternator.
5. sfc Simulink model
Parameter
Value
Input source
33 kV, three phase
Transformer nominal power
3.5 MVA
Transformer primary input, Delta winding
33 kV, 50Hz
Transformer secondary output, Star winding
1.72 kV
Thyristor bridge
3 arms
Snubber resistance
2000 Ω
Snubber capacitance
0.1 µF
Link reactor
4.8 mH
Base voltage
12 kV
X/R ratio
20
Motor voltage
1.72 kV
3-ph S.C. level at base voltage
1500 MVA
To understand the intricacies of SFC system, simulation
technique is always a better solution. In line with this a
MATLAB/Simulink software is applied to model the
whole system. Model parameters are the real data (Table
I) from the working system. The main power source is the
grid supply at 33 kV level. It has to be step down to 1.72 kV
level as SFC operates at this voltage. To create SFC model
one three phase source, one step down transformer, two
thyristor bridges are taken. A reactor is placed in between
these bridges. 33 kV/1.72 kV step-down transformer is
inside the rectifier block. Rectifier and inverter both are
in six pulse configuration. The other requirements are
control circuits and scope window at the requisite place.
Simulink model of the system is shown in Figure 4.
It is necessary to validate the created Simulink model
before analysing the system through it. For this the model
is run and waveforms for simulated motor voltage (Figure
5) and simulated motor current (Figure 7) are traced.
These waveforms are compared with the waveforms
drawn from the real system (Figure 5 & 6 and Figure 7
& 8). Real and simulated traces are found to be similar in
nature, which authenticates the validation process.
Table I. SFC model parameters.
Figure 5. Simulated motor voltage.
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Figure 6. Motor voltage from the real system.
6. Simulation analysis
at motor terminals the system also feeds harmonics at
the source side. Source voltage doesn’t deform much
(THD=2.46%), but the source current sees a very high
level of THD at 18.12%. All these THD values are given
in Table II. Field winding of synchronous machine also
gets a pulsating DC from a controlled rectifier containing
numbers of high level of harmonics. These harmonics
causes inductive and capacitive couplings in the whole
system of synchronous machine including the static drive
[26]. This is one of the main reasons of creating shaft
voltages in the machine. Other reason of shaft voltage
is the presence of common mode voltage (CMV) which
is also investigated later. Some other negative impacts of
high THD are deterioration of winding insulation apart
from distorting the neighbouring electronic circuits.
After the model validation, simulated source voltage and
simulated source current are recorded and depicted in
Figure 9 and Figure 10. Motor voltage and motor current
from the real and simulated systems are already shown in
Figure 5 to Figure 8. Simply looking, current and voltage
waveforms of the SFC system are not sinusoidal rather
distorted in nature. These simulated waveforms are
processed through Fast Fourier Transformation (FFT).
Figure 11 to Figure 14 show frequency spectrums of
all the simulated parameters e.g. source voltage, source
current, motor voltage and motor current. FFTs reveal
that the level of harmonics in motor voltage is 20.74%
whereas; motor current contains 30.11% THD. THD
level below 5% is considered as safe. Besides high THDs
Figure 7. Simulated motor current.
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Figure 8. Motor current from the real system.
Figure 9. Simulated source voltage.
Figure 10. Simulated source current.
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Figure 11. Frequency spectrum of simulated source voltage.
Figure 12. Frequency spectrum of simulated source current.
Figure 13. Frequency spectrum of simulated motor voltage.
Cigre Science & Engineering • N°4 February 2016
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Figure 14. Frequency spectrum of simulated motor current.
%THD
source
voltage
%THD
source
current
%THD
motor
voltage
%THD
motor
current
2.46
18.12
20.74
30.11
8. References
[1] Yuko Shinryo, Isamu Hosono and Keijiro Syoji, “Commutatorless DC
Drive for Steel Rolling Mill”, IEEE/IAS Annual Meeting, pp.263-271,
1977.
[2] Oliver Drubel Max Hobelsberger, “Static frequency converters with
reduced parasitic effects”, 35th Annual Power Electronics Specialists
Conference, PESC 04, 2004, IEEE.
Table II. Harmonic distortion due to SFC.
[3] Oliver Drubel, “Converter Dependent Design of Induction Machines
in the Power range below 10MW”, 1-4244-0743-5/07 2007 IEEE.
7. Conclusion
[4] P. Langhorst, C. Hancock, “Simple Truth about Motor Drive
Compatibility”, MagneTek Publication, pp. 24-28, July 1996.
SFC is best among the known techniques of starting a
synchronous machine. With SFC the machine can be
started softly without much loading the supply source.
Other than starting and speed control, SFC can also
be used for braking and reversal operation of large
machines. In other way all the four quadrant operations
can be achieved with SFC. To know the performance of
SFC on this large generator, it is not advisable to perform
any kind of experiment on a live system. The suitable
solution envisaged for this study is the simulation
technique. By virtue of MATLAB/Simulink software,
SFC is modeled from the actual parameters. Simulation
results are validated with the waveforms drawn from
the real bridges in SFC. FFT analyses on the simulated
waveforms are carried out. The significant findings from
the frequency spectrums are the presence of high level of
harmonics in the motor terminals as well as source side.
[5] Frank J. Bourbeau, “Synchronous Motor Railcar propulsion”, IEEE/
IAS 1974-Part I, pp. 533-540, 1974.
[6] John A. Allan, W. A. Wyeth, Gordon W. Herzog and John A. I.
Young, “Electrical Aspects of the 8750 hp Gearless Ball-Mill Drive at
St. Lawrence Cement Company”, IEEE Transactions on Industrial
Applications, Vol. IA-11, pp. 681-687, Nov/Dec.1975.
[7] H. Stemmler, “Drive System and Electronic Control Equipment of the
Gearless Tube Mill, Brown Boveri Review, pp. 120-128, March 1970.
[8] Beat Mueller, Thomas Spinanger and Dieter Wallstein, “Static
Variable Frequency Starting and Drive System for Large Synchronous
motors”, Conf. Rec. IEEE/IAS 1979 Annual Meeting, pp.429-438,
1979.
[9]Fumio Harashima, Haruo Naitoh and Tohsimasa Haneyoshi,
“Dynamic Performance of Self-Controlled Synchronous Motors
Fed by Current-Source Inverters”, IEEE Transaction on Industrial
Applications, Vol.IA-15, pp. 36-46, Jan/Feb. 1979.
[10]Bimal K. Bose, “Power Electronics and AC Drives”, Prentice – Hall,
New Jersy, USA, 1986.
Shaft voltage is the outcome of high THD which must be
arrested by some suitable measure for safe operation of
any large machine. Modification of the source and use of
filters are among the potential solutions of shaft voltage
but source modification with multilevel PWM inverter
is the best bet.
[11]Hisanori Taguchi, Shinzo Tamai, Yasuhiko Hosokawa and Akinobu
Ando, “APS Control Method for Gas Turbine Startup by SFC”,
International Power Electronics Conference, pp. 264-269, IEEE, 2010.
[12]Shin-Hyun Park, Seon-Hwan Hwang, Jang-Mok Kim, Ho-Seon Ryu,
Joo-Hyun Lee, “A Starting-up control algorithm of large synchronous
generation motor for Gas Turbosets”, IEEE International Symposium
on Industrial Electronics ISIE, pp. 502-508, 2008.
Cigre Science & Engineering • N°4 February 2016
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[13] Zhang Yu-Zhi, “Study of Process of Starting Pumped Storage Machines
by Static Frequency Converter with Field Current Controlled”, IEEE
International Conference on Signal Processing Systems, pp. V1-224 –
V1-227, IEEE2010.
[23]Tsorng-Juu Liang, Jiann-Fuh Chen, Ching-Lung Chu, Kuen-Jyh
Chen, “Analysis of 12 Pulse Phase Control AC/DC Converter”, IEEE
International Conference on Power Electronics and Drive Systems,
PEDS’99, Hong Kong, pp. 779-783, July 1999.
[14] Robert B., Fisher P.E., “Introduction of Static Frequency Converters on
SEPTA’s 25Hz Commuter Rail System”, pp.149-155, IEEE.
[24]Ho-Seon Ryu, Bong-Suck Kim, Joo-Hyun Lee and Ik-Hun Lim, “A
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EPE-PEMC, pp.1496-1499, 2006, IEEE.
[15]Hatua, K., Ranganathan, V. T., “A novel VSI- and CSI-fed active–
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[25]Arun Kumar Datta, G. Venkateswarlu, M. A. Ansari, N. R. Mondal,
“Excitation Control during Short Circuit Test Sequence of 1500 MVA
Short Circuit Generator”, International Conference on Advances in
Computer, Electronics & Electrical Engineering, pp. 207-211, 25-27
March 2012, Mumbai.
[16] John Rosa, “Utilization and Rating of Machine Commutated InverterSynchronous Motor Drives”, IEEE Transactions on Industrial
Applications, Vol. IA-15, pp.155-164, Mar./Apr. 1979.
[26]Datta Arun Kumar, Dubey Manisha, Jain Shailendra, “Effect of
Static Power Supply in Alternator Used for Short-Circuit Testing
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9. Biography
[18] Datta Arun Kumar, Manisha Dubey, N. R. Mondal, B. V. Raghavaiah,
“Motor-less operation of Short Circuit generator – A CPRI Perspective”,
International Conference on Electrical Power and Energy Systems
(ICEPES-2010), pp.439-445, 26-28 August 2010 MANIT, Bhopal.
Arun Kumar Datta, Ph.D. in Electrical Engineering
from MA National Institute of Technology (MANIT),
Bhopal, India. Earlier he did his Post Graduation
(M.Tech.) from MANIT, Bhopal and Graduation from
Govt. Engineering College, Bilaspur. He is employed
in Central Power Research Institute (CPRI), Bhopal,
India since 1993 and looking after the Operation &
Maintenance of two 1500MVA short circuit generator
plants and a medium voltage substation. He had
undergone training at many places including the works
of M/s. Alsthom, France. He is also looking after the
Quality Assurance activities of CPRI. He is a Certified
Energy Auditor from Bureau of Energy Efficiency
(BEE), India and also a member of Institution of
Engineers (India). He has attended many International
& National Conferences/Seminars and has many
technical papers on his credentials.
[19]Datta Arun Kumar, Dubey Manisha, Jain Shailendra, “Investigation
of bearing currents in dual mode operation of synchronous machine
with static excitation system”, pp. 45-53, Electrical and Electronics
Engineering: An International Journal (ELELIJ) Vol. 2, No 4,
November 2013.
[20]Datta Arun Kumar, Ansari M. A., Mondal N. R., Raghavaiah B. V.
“A Novel Use of Power Electronics: Prime Mover-less Alternator with
Static Drive & Excitation System”, International Journal of Electronics
& Communication Technology (IJECT), Vol. 3, Issue 1, pp. 472-475,
January - March 2012.
[21] Tore Peterson, Kjell Frank, “Starting of large synchronous motor using
static frequency converter”, paper 71 TP 519-PWR for IEEE Summer
Meeting and International Symposium on High Power Testing,
Portland, Ore., pp. 172-179, July 18-23, 1971.
[22]Gordon R. Slemon, Sashi B. Dewan and James W. A. Wilson,
“Synchronous Motor Drive with Current-Source Inverter”, IEEE
Transactions on Industrial Applications, Vol. IA-10, pp. 412-416, pp.
May/June 1974.
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