Analysis and Control Aspects of
Brushless
Induction
Machines
with Rotating Power Electronic
Converters
Elforsk rapport: 12:67
Naveed ur Rehman Malik
August 2012
Analysis and Control Aspects of
Brushless
Induction
Machines
with Rotating Power Electronic
Converters
Elforsk rapport: 12:67
Naveed ur Rehman Malik
August 2012
ELFORSK
Foreword
In order to investigate a new innovative concept of a brushless double fed
induction generator, the PhD-project V-308 was started within the research
program Vindorsk.
The work is carried by the PhD student Naveed ur Rehman Malik under the
supervision of Professor Chandur Sadarangani at the Royal Institute of
Technology, KTH.
This report describes the main results of the project as a summary in
complement to the Licentiate thesis.
Vindforsk – III is funded by ABB, Arise windpower, AQSystem, E.ON Elnät,
E.ON Vind Sverige, EnergiNorge, Falkenberg Energi, Fortum, Fred. Olsen
Renewables, Gothia wind, Göteborg Energi, Jämtkraft, Karlstads Energi, Luleå
Energi, Mälarenergi, O2 Vindkompaniet, Rabbalshede Kraft, Skellefteå Kraft,
Statkraft, Stena Renewable, Svenska Kraftnät, Tekniska Verken i Linköping,
Triventus, Wallenstam, Varberg Energi, Vattenfall Vindkraft, Vestas Northern
Europe, Öresundskraft and the Swedish Energy Agency.
Comments on the work have been given by a reference group with the
following members: Robert Chin, Luca Peretti and Jouko Niiranen at ABB and
Ramasamy Anbarasu at Vestas
Stockholm, November 2012
Anders Björck
Programme manager Vindforsk-III
Electricity and heat production, Elforsk AB
ELFORSK
ELFORSK
Sammanfattning
Denna rapport presenterar målet med projekt “V308” och redogör för
resultatet som har uppnåtts i projektet.
Målet med studien är att undersöka en innovativ topologi av en borstlös
dubbelmattad induktionsgenerator som minskar underhållskostnaderna och
förbättrar tillförlitligheten hos generatorn.
För att åstadkomma detta, har en variant av en borstlös dubbelmattad
induktionsgenerator undersökts där släpringarna och kolborstarna har ersatts
med en roterande kraftelektronisk strömriktare och en roterande matare.
Stationära och dynamiska modeller för den roterande kraftelektroniska
generatorn tillsammans med en återkopplad reglering har utvecklats och
tillämpats. Genom tillämpning av dessa modeller har det påvisats att
generatorn arbetar stabilt i både under – och översynkron tillstånddrift.
Vidare, är det påvisat att den roterande mataren tillsammans med den
kraftelektroniska strömriktaren är kapabel att mata reaktiv effekt till nätet
genom rotorlindningen och därmed kan förbättra effektfaktorn på generatorn.
Möjligheten att uppnå effektfaktor = 1 är också demonstrerad. Vidare, har det
påvisats att eftersläpningseffekten med konceptet kan återvinnas och
återmatas till nätet via generatorns stator.
Vidare, har generatorns beteende för både symmetriska och osymmetriska
felfall på nätet undersökts. Det har påvisats att genom att använda ”Passive
Resistive Network” - metoden har generatorn kapacitet att motstå stora
spänningsdippar. Vidare, visas att det genom att använda en så kallad
”Extended Vector Control”-reglering så erhålls en stabil drift även vid
osymmetriska felfall. Med denna reglering reduceras pendlingar i effekt,
ström, moment och DC-spänning kraftigt.
Utöver de ovan nämnda studierna, har en stationär modell utvecklats och
funktionaliteten studerats för en enkel-matad asynkronmaskin där den
roterande mataren är borttagen och rotorlindningarna kortslutna genom de
två
roterande
strömriktarna.
Genom
denna
konstruktion
kan
eftersläpningseffekten cirkulera i rotorlindningen och med hjälp av de två
roterande kraftelektroniska strömriktarna kan rotorströmmen användas för
att magnetisera asynkronmaskinen. Därmed kan en önskad effektfaktor
uppnås mot nätet. Beräkningen med den stationära modellen har verifierats
genom mätningar. En asynkronmotor som arbetar med effektfaktor = 1 är
önskvärt eftersom den reducerar kopparförlusterna i både transmissions och
distributions
linjer,
samt
förbättrar
systemets
verkningsgrad
och
spänningsstabiliteten på nätet.
ELFORSK
Summary
This report presents the aims of the project “V308” and describes the results
which have been obtained.
The purpose of the study is to investigate an innovative topology of brushless
doubly-fed induction generator in order to reduce maintenance costs and
improve the reliability of the generator.
In order to do so, a Brushless Doubly-Fed Induction Generator is considered
in which the slip rings and carbon brushes are replaced by power electronic
converters and a rotating exciter. The steady-state and dynamic model of the
Rotating Power Electronic Generator together with its closed-loop control is
developed and implemented. Through these models, it is demonstrated that
the generator shows stable behavior in both the sub-synchronous and supersynchronous modes of operation. Moreover it is shown that the rotating
exciter together with the power electronic converters is capable of injecting
reactive power from the rotor thereby improving the power factor of the
generator. The capability of obtaining unity power factor operation is also
illustrated. Additionally it is demonstrated that the slip power recovery
scheme through mechanical means is successful and hence requirement of
slip rings and carbon brushes is not necessary. The slip power is recovered via
the stator of the RPE-BDFIG hence increasing the efficiency of the generator.
Additionally impact of the generator is investigated under symmetrical and
unsymmetrical grid faults. It is shown that through the use of a Passive
Resistive Network, the generator has the capacity of withstanding deep
voltage dips. Moreover it is shown that through the use of Extended Vector
Control the generator shows stable behavior and oscillations in the mechanical
and electrical components of the generator such as power, current, torque
and DC-link capacitor are reduced to a considerable extent as compared to
when Extended Vector Control is not used during unsymmetrical conditions.
In addition to the above studies a steady-state model of a single-fed induction
machine is also developed and investigated where the rotating exciter is
removed and the rotor windings are short-circuited through the two rotating
power electronic converters. In this way slip power circulates in the rotor and
with the help of the two rotating electronic converters, rotor current is used to
magnetize the induction machine thereby improving the power factor. The
steady state model is verified through experimental results. Operation of an
induction machine at unity power factor is a desirable property as it reduces
copper losses in the transmission and distribution network, improves the
system efficiency and strengthens the voltage stability of the grid.
ELFORSK
Innehåll
Abbreviations
1 1 Introduction
2 2 Modeling and Results
1.1 1.2 2.1 2.2 2.3 2.4 Types of Generator Topologies .......................................................... 2 Aims of the Project .......................................................................... 5 6 Configuration of the Rotating Power Electronic Doubly-Fed Induction
Generator ...................................................................................... 6 Results .......................................................................................... 8 Low Voltage Ride Through (LVRT) ...................................................... 9 Rotating Power Electronic Induction Drive using Lindmark Concept ....... 14 3 Conclusions
15 4 Publications
16 ELFORSK
Abbreviations
BDFIG
Brushless Doubly-Fed Induction Generator
DFIG
Doubly-Fed Induction Generator
EVC
Extended Vector Control
LVRT
Low Voltage Ride Through
PRN
Passive Resistive Network
PLL
Phase Locked Loop
RPE-BDFIG Rotating Power Electronic Brushless Doubly-Fed Induction
Generator
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1
Introduction
In the wind industry different types of generators are in use for producing
electrical power. Traditionally generators were directly connected to the grid
and hence operated at constant speed. For fixed speed generators efficiency
of power conversion from the wind is low as speed of the generator should
vary with the speed of the wind for maximum power extraction. With
continuous progress in power electronic converters for high power range,
variable speed drives were employed in the wind industry. As a result trend
shifted towards using variable speed drives and use of such drives increased
rapidly in the last decade.
1.1
Types of Generator Topologies
In the arena of variable wind generators, there are different types of variable
speed wind generators which have been in use. One of the configurations of
the generator uses full scale converters in which the converter is connected
between the stator of the generator and the grid. In this type of configuration,
full power of the generator flows through the converter and hence the
converter is rated for full power rating of the generator as shown in Figure
1-1. However since the speed change required for extracting power from the
wind is limited therefore use of full scale converter topology is not cost
effective.
Figure 1-1: Generator with full scale converter.
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Figure 1-2: Vestas wind turbine V80 uses Doubly-Fed Induction Generator (DFIG).
Figure 1-3: Doubly-Fed Induction Generator (DFIG).
In order to reduce the size of the power electronic converter, an alternative
topology known as Doubly-Fed Induction Generator (DFIG) is widely used and
is one of the most famous topologies in the wind industry. Figure 1-2 shows
Vestas V80 wind turbine which employs DFIG.
In this topology one of the power electronic converters is connected to the
rotor of the induction generator and second to the grid while stator is directly
connected to the grid as shown in Figure 1-3. It is a known fact that in an
induction generator, active power in the rotor varies with the deviation in
rotor speed with respect to stator flux speed. As a result, since the deviation
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of rotor speed from its synchronous speed is limited (commonly denoted by
normalized quantity “slip (s)”), therefore the active power in rotor is also
limited and varies in proportion with “slip (s)”. This implies reduction in power
rating of the converter which needs to be around 30% of the power rating of
the machine for speed ranges between ±25%. This is a huge advantage in
terms of cost savings and hence has lead to commercial success of the
Doubly-Fed Induction Generator (DFIG). However, DFIG’s also suffer from
many disadvantages. One of the main disadvantages is that these generators
employ slip-rings and carbon brushes in order to extract active power from
the rotating part of the machine and deliver it to the stationary grid. It is a
well-known fact that carbon brushes wear out with passage of time and also
produce carbon dust during their operation which affects the insulation of the
windings and hence reduces the lifetime of the generator. Moreover carbon
brushes are sensitive to the air temperature, humidity and thermal heating
due to handling of large magnitudes of rotor currents. Due to aforementioned
reasons, carbon brushes need to be replaced after every few months. This
implies that the maintenance costs and down time of the generator increases.
In addition maintenance of offshore wind turbines is tedious and difficult since
inspection and maintenance is dependent upon weather conditions as turbineunits are not accessible during certain time periods of the year. Therefore it is
of vital importance that the doubly-fed generators are designed in a manner
so as to recover slip power utilizing more reliable means than carbon brushes
and slip rings. Therefore, a DFIG without carbon brushes and slip rings will be
a huge advantage in terms of cost savings and better reliability and
robustness.
Figure 1-4: Conventional cascade Brushless Doubly-Fed Induction Generator (BDFIG).
In the past, several authors have studied and developed doubly-fed machines
without slip rings and carbon brushes. One of the well-known topology is the
Brushless Doubly-Fed Induction Generator (BDFIG). These types of
generators are very useful in a sense that they have provision of recovering
slip power without using slip rings and carbon brushes. These machines
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employ an extra machine called as auxiliary machine. The rotor of the
auxiliary machine is coupled electrically and mechanically to the rotor of the
main machine (DFIG) as shown in the Figure 1-4. The stator of the auxiliary
machine is connected to the grid via the two power electronic converters. The
slip power is recovered from the rotor of the DFIG via the auxiliary machine
using the induction principle. Despite the fact that this type of generator is
capable of recovering slip power in a brushless manner, it has not been able
to achieve commercial success as was expected due to several reasons:
• One of the primary reasons has been that it employs two induction
machines and therefore suffers from low power density.
• Losses of the generator are high since it utilizes two induction
machines carrying three phase AC currents in its four windings.
• Moreover due to induction nature of the generator, it suffers from low
power factor.
1.2
Aims of the Project
Due to aforementioned reasons an alternate topology of the brushless
configuration is being studied and analyzed in this project where the second
induction machine (auxiliary machine) in the Brushless Doubly-Fed Induction
Generator (BDFIG) is replaced by a synchronous machine and power
electronic converters are moved from the stator of the auxiliary machine to in
between the rotors of the two machines as shown in Figure 2-1 in the
following chapter. It is expected that by doing so better power factor control
over a wide range with lower power rating of the power electronic converter
and smaller generator size can be achieved. Moreover it is expected that the
newer configuration will result in higher power density than the conventional
Brushless Doubly-Fed Induction Generator (BDFIG). The details and analysis
on different aspects of the new configuration is given in the subsequent
chapter.
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2
Modeling and Results
2.1
Configuration of the Rotating Power Electronic DoublyFed Induction Generator
Basic working principle of operation of the Rotating Power Electronic Brushless
Doubly-Fed Induction Generator (RPE-BDFIG) is based on the same principle
as that of the conventional slip-ring Doubly-Fed Induction Generator (DFIG)
but without the use of carbon brushes and slip rings i.e. means of recovering
slip power is brushless in nature. The Rotating Power Electronic Brushless
Doubly-Fed Induction Generator (RPE-BDFIG) consists of two machines as
shown in Figure 2-1; one is the main machine which is a wound rotor
induction machine (also referred to as Doubly-Fed Induction Generator
(DFIG)) and the other machine is the exciter which is modeled as a
synchronous machine. The two machines are coupled mechanically and
therefore rotate with the same speed. The rotor windings of the two machines
are connected with each other via two power electronic converters. Therefore
the frequency and magnitude of the currents and the voltages are different in
the two rotors. In addition mechanical means are employed to recover slip
power which is then added to the main shaft. The mechanical power which is
added/subtracted to/from the shaft contributes again for generating electrical
power and the process continues in a cyclic manner as shown in Figure 2-2
and Figure 2-3. This implies that the input turbine mechanical power produces
electrical power which is delivered to the grid and the slip power which is
compensated by the exciter in a cyclic fashion either by generating
mechanical or electrical power which ultimately is delivered to the grid in the
form of electrical power. The fact that the mechanical power supplied by the
turbine is equal to the generated power delivered to the grid (for a lossless
generator) implies that slip power recovery scheme is successful.
Figure 2-1: Rotating Power Electronic Brushless Doubly-Fed Induction Generator (RPEBDFIG).
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As far as configuration of the RPE-BDFIG is concerned, the stator of the IM is
directly connected to the grid while its rotor is connected to one of the power
electronic converters, see Figure 2-1. The second power electronic converter
is connected to the three phase rotor windings of the exciter. The exciter
operates in an inverted configuration with the three phase winding on the
rotor and the DC winding on the stator. Slip power from the rotor of the
induction machine is fed to the rotor of the exciter through the two power
electronic converters.
Therefore for a ±25% speed range around
synchronous speed, rotor of the exciter carries high frequency currents while
rotor of the IM operates at a lower frequency. Conversion in frequency and
magnitude of the currents and the voltages between the two rotors is
performed by the two power electronic converters. Wireless means are to be
used for switching of the power electronic converters.
The RPE-BDFIG also operates in two modes of operation similar to
conventional BDFIG and slip-ring DFIG. The two modes of operation are
explained below.
Super-synchronous Mode
Super-synchronous mode is the normal mode of operation for a generator
where the rotational shaft speed is greater than the synchronous speed. The
synchronous speed of the RPE-BDFIG is decided by the grid frequency. In this
mode of operation power leaves the generator from both the stator as well
the rotor terminals as shown in Figure 2-2. Slip power leaves the rotor of the
main machine and enters the rotating exciter through the two power
electronic converters. In this mode, exciter operates as a motor and adds
torque to the common shaft of the generator. As a result electromechanical
torque generated by the main machine is the sum of the torques supplied by
the wind turbine and the exciter.
Figure 2-2: Power flow in super-synchronous mode.
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Figure 2-3: Power flow in sub-synchronous mode.
Sub-synchronous Mode
In sub-synchronous mode, power enters the rotor of the induction generator
from the exciter via the rotor power electronic converters. Therefore the
exciter operates as a generator extracting the torque from the shaft and
producing electrical power which is equivalent to slip power as shown in
Figure 2-3. The net electromagnetic torque generated is the difference
between the turbine and the exciter torque.
A complete analysis of the steady state model of the generator is given in
Publication I while the dynamic model, equivalent circuit of the RPE-BDFIG
and its control aspects are dealt within Publication II. The feed-back control
for the RPE-BDFIG is implemented using vector control. The dq reference
frame is aligned using grid flux orientation. The stator of the RPE-BDFIG is
synchronized with the grid using Phase Locked Loop (PLL) estimator. A total
of four controllers are used; two current controllers, one speed controller and
one voltage controller as explained in detail in Publication II.
2.2
Results
Some of the results are presented here for completeness of the report. Figure
2-4 shows that generated output electrical power is equal to the input
mechanical power minus the losses. This proves that the slip power in the
rotor is successfully recovered via the stator of the RPE-BDFIG. Moreover
Figure 2-5 (left) shows that the generator is operating at unity power factor.
This shows that the exciter is capable of not only handling active power of the
generator but also reactive power in such a way that the rotor takes over the
reactive power requirements of the generator. This is a noteworthy
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observation as unity and capacitive power factor operation implies better
voltage stability of the grid and lower transmission losses. Besides Figure 2-5
(right) shows that the voltage of the rotating capacitor reaches steady state
after being subjected to transients due to changes in turbine power.
The analysis in Figure 2-4 and Figure 2-5 shows that the RPE-BDFIG reestablishes steady-state point of operation after being subjected to
mechanical transients such as changes in torque and mechanical speed.
Furthermore, it is shown that this generator has stable modes of operation at
different speeds such as above and below the synchronous speed.
Figure 2-4: Input power supplied by the turbine (left) and the generated electrical power
delivered to the grid (right).
Figure 2-5: Stator Power Factor (left) and the rotating capacitor voltage (right).
2.3
Low Voltage Ride Through (LVRT)
Impact of symmetrical voltage dips and unsymmetrical voltage dips on the
RPE-BDFIG is investigated in Publication III and Publication IV
respectively. It is a known fact that Doubly-Fed Induction Generators are
quite sensitive to voltage dips in the grid. Moreover protective measures are
required to save the converter from damage due to over currents during
these voltage dips. In addition, in the case of unsymmetrical faults, stator of
the generator experiences voltages with different magnitudes and phase angle
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jumps in different phases. The unbalance in the generator stator voltages
results in an extra heating of the generator windings due to negative
sequence currents and an increase in the iron losses due to higher frequencies
of these currents. Moreover oscillations in power, currents, torque and
capacitor voltage results in a reduction in the lifetime of the electrical and
mechanical components connected to the generator such as gearbox,
generator shaft, and the DC-link capacitor. Therefore it is of immense
importance that the impact of the voltage dips is handled in a suitable
manner. Passive Resistive Network (PRN) technique was employed for
successful ride through of the generator during symmetrical voltage dips while
Extended Vector Control (EVC) method was used for ride through of the
generator during unsymmetrical voltage dips.
The analysis has been performed for various voltage dips occurring due to the
symmetrical and unsymmetrical faults in the network. German grid code
standards (E.ON) as shown in Figure 2-6 were used for specifying the voltage
dips as they are one of the most stringent grid codes. For the case of
symmetrical voltage dips, the analysis was done up till 15% of the remaining
nominal voltage (i.e. voltage dip of 85%) which was applied for 325 msec as
specified by the E.ON grid code standards. Similarly for unsymmetrical
voltage dips, the analysis has been performed for various magnitudes of the
voltage dips including 85% voltage dips. In the case of unsymmetrical voltage
dips, the analysis of the generator was done for a single-phase-to ground
fault and phase-phase fault and it is seen that the generator is capable of
successfully riding through these faults. In both cases the dip was modeled as
a rectangular waveform which represents a worst case scenario.
The limit curve for the magnitude and the corresponding duration of the
voltage dip as illustrated in the E.ON grid code standard is depicted in Figure
2-6. In region I wind turbines are not allowed to be disconnected and should
provide reactive current (at a rate of 2% of the rated current for each percent
of the voltage dip) if the voltage drops below 90% of the nominal value at the
generator terminals. In region II wind turbine units are allowed temporary
disconnection from the grid and should re-synchronize again within 2 seconds
after the fault clearance. In region III, wind turbine units are allowed to be
disconnected from the grid.
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Figure 2-6: E.ON grid code standard.
Results for symmetrical voltage dips
Figure 2-7 (left) to Figure 2-10 (left) shows the behaviour of the generator
when it is subjected to 60% voltage dip without LVRT protection while Figure
2-7 (right) to Figure 2-10 (right) shows the behaviour when PRN is used as a
LVRT measure and it is seen that through PRN, the generator is successfully
able to ride through the fault without any damage to the converter and the
exciter as the rotor powers are within limits. Moreover rotor speed is also
under control.
Figure 2-7: Exciter rotor power without the LVRT technique (left) and exciter rotor power
with the LVRT technique (right) during 60% voltage dip.
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Figure 2-8: DFIG rotor power without the LVRT technique (left) and DFIG rotor power
with the LVRT technique (right) during 60% voltage dip.
Figure 2-9: Capacitor rotor voltage without the LVRT technique (left) and with the LVRT
technique (right) during 60% voltage dip.
Figure 2-10: Rotor speed without the LVRT technique (left) and with the LVRT technique
(right) during 60% voltage dip.
Results for unsymmetrical voltage dips
Figure 2-11 (left) to Figure 2-13 (left) shows the behaviour of the generator
when it is subjected to 50% voltage dip during single-phase unsymmetrical
voltage dips without EVC strategy while Figure 2-11 (right) to Figure 2-13
(right) shows the behaviour when EVC strategy is used. It is seen that
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through the use of EVC control, the generator shows stable behaviour and the
oscillations in power, torque and DC-link capacitor are reduced to a
considerable extent.
Figure 2-11: Torque of the generator during single-phase-to-ground fault without EVC (left)
and with EVC (right) during 50% voltage dip.
Figure 2-12: Exciter rotor power during single-phase-to-ground fault without EVC (left) and
with EVC (right) during 50% voltage dip.
Figure 2-13: Rotating capacitor voltage during single-phase-to-ground fault without EVC
(left) and with EVC (right) during 50% voltage dip.
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2.4
Rotating Power Electronic Induction Drive using
Lindmark Concept
This section presents the operation of a single-fed induction machine in which
the power electronics in the rotor are used to magnetize the machine. This
type of drive uses controllable capacitors through the two power electronic
converters mounted on the rotor of the induction machine in order to inject
reactive current from the rotor to the stator as shown in Figure 2-14. The
stator of the machine is directly connected to the grid. Lindmark concept is
used to attain unity power factor. In Lindmark concept, the voltage vector
produced from the two rotating power electronic converters is injected to the
rotor terminals in a manner so as to move the rotor current. Movement of the
rotor current affects the stator current. Phase of the stator current with
respect to the stator voltage is changed in a manner so as to improve the
power factor of the machine. Using this technique a single-fed induction
machine where the rotor takes over the magnetization of the machine and
delivers the reactive power is demonstrated. Steady state model of the
machine is also developed and is verified through experimental tests in
Publication V. Interested reader is encouraged to refer to the two
publications for further details and results.
Figure 2-14: Configuration of the induction machine using Lindmark concept.
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3
Conclusions
In this project a novel topology of brushless doubly-fed induction machine
was introduced in which the slip rings and carbon brushes were replaced by
power electronic converter and the exciter rotating with the rotor.
During the project three main challenges were solved. One of them was that
for the first time, a dynamic model for this generator was developed together
with a feedback control. It was shown that the generator was successfully
able to operate at stable points. Moreover it was shown that the rotating
exciter magnetized the generator from the rotor thereby improving the
stability of the grid. Secondly, a suitable low voltage ride through technique
called Passive Resistive Network, was implemented for this type of generator
and it was shown that this type of generator is successfully able to handle
symmetrical voltage dips in the grid. Thirdly it was shown that Extended
Vector Control, strategy works well for this type of generator and it was
shown that the oscillations in different components of the generator was
reduced to considerable extent during the unsymmetrical grid faults.
Moreover the generator is capable of withstanding uptill 85% of symmetrical
and unsymmetrical voltage dips thus fulfilling the requirements of the E.ON
grid code standards.
Due to aforementioned analysis, this new type of generator can become a
competitive and cost effective solution for offshore wind farms with lower
maintenance costs and enhanced reliability. It can be a step forward towards
new generator design technologies.
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4
Publications
The detailed analysis, working principle, modeling, control aspects and results
of the project are given in the following publications.
[Publication I]: Malik, Naveed ur Rehman; Sadarangani, C.; , “Brushless
doubly-fed induction machine with rotating power electronic converter for
wind power applications,” Electrical Machines and Systems (ICEMS), 2011
International Conference on , vol., no., pp.1-6, 20-23 Aug. 2011.
[Publication II]: Malik, Naveed ur Rehman; Sadarangani, C.; , “Dynamic
modeling and control of a brushless doubly-fed induction generator with a
rotating power electronic converter,” accepted and to be published in ICEM
2012 being held in Marseille, France, September 2-5, 2012.
[Publication III]: Malik, Naveed ur Rehman; Sadarangani, C.; , “Behavior of
a brushless doubly-fed induction generator with a rotating power electronic
converter during symmetrical voltage sags,” accepted and to be published in
ICEM 2012 being held in Marseille, France, September 2-5, 2012.
[Publication IV]: Malik, Naveed ur Rehman; Sadarangani, C.; , “Extended
vector control of a rotating power electronic brushless doubly-fed induction
generator under unsymmetrical voltage sags ,” accepted and to be published
in IECON 2012, Annual Conference on IEEE Industrial Electronics Society
being held in Montreal, Canada , October 25-28, 2012.
[Publication V]: Malik, Naveed ur Rehman; Sadarangani, C.; Lindmark, M.; ,
“Theoretical and experimental investigation of the self-excited rotating power
electronic induction machine,” IECON 2011 - 37th Annual Conference on IEEE
Industrial Electronics Society , vol., no., pp.2048-2053, 7-10 Nov. 2011.
[Publication VI]: Malik, Naveed ur Rehman; Sadarangani, C.; Cosic, A.;
Lindmark, M., “Induction machine at unity power factor with rotating power
electronic converter,” published in International Symposium on Power
Electronics, Electrical Drives, Automation and Motion (SPEEDAM 2012), 20-22
June 2012.
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