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MODELING AND EXPERIMENTAL ANALYSIS OF VARIABLE SPEED THREE PHASE SQUIRREL-2

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MODELING AND EXPERIMENTAL  ANALYSIS OF VARIABLE SPEED THREE PHASE SQUIRREL-2 Powered By Docstoc
					INTERNATIONAL JOURNAL OF ELECTRICAL ENGINEERING
 International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 –
 6545(Print), ISSN 0976 – 6553(Online) Volume 4, Issue 3, May - June (2013), © IAEME
                          & TECHNOLOGY (IJEET)

ISSN 0976 – 6545(Print)
ISSN 0976 – 6553(Online)
                                                                                  IJEET
Volume 4, Issue 3, May - June (2013), pp. 128-140
© IAEME: www.iaeme.com/ijeet.asp
Journal Impact Factor (2013): 5.5028 (Calculated by GISI)
                                                                              ©IAEME
www.jifactor.com




     MODELING AND EXPERIMENTAL ANALYSIS OF VARIABLE
   SPEED THREE PHASE SQUIRREL CAGE INDUCTION GENERATOR

      1
          Lalit Kumar, 2Mrs. S. U. Kulkarni, 3Mohit.K.Shakya, 4Sachinkumar L.Sarwade
                           1
                              M.Tech Student, Electrical Engineering, BVUCOEP.
           2
               Asst.Prof. Electrical Engg. Bharti Vidyapeeth University College of engineering,
                                                   Pune, India.
                            3
                              Asst.Prof., Electrical Engineering, KJCOEMR, Pune.
                              4
                                M.E. Student, Electrical Engg., PVGCOET, Pune.


  ABSTRACT

          Induction machines have wide applications in renewable power system and
  particularly in wind turbine power systems. In case of standalone wind power system
  applications, for generating single phase electricity single phase or three phase induction
  machines can be used. Three phase induction generator can be used to generate single phase
  electricity at constant or above synchronous speed by using the two-series-connected-and-one
  isolated (TSCAOI) winding connection without an intermediate stage. In contrast with
  single-phase cage induction machines, three phase induction machines are significantly less
  expensive, more efficient, and smaller in frame size in comparison with their single-phase
  counterpart of similar power ratings.
          This paper introduces a novel cage induction generator and presents a mathematical
  model, through which its behavior can be accurately predicted. The proposed generator
  system employs a three-phase cage induction machine and generates single-phase, constant-
  frequency electricity at varying rotor speeds without an intermediate inverter stage. The
  technique uses any one of the three stator phases of the machine as the excitation winding and
  the remaining two phases, which are connected in series, as the power winding. The two-
  series-connected-and-one isolated (TSCAOI) phase winding configuration magnetically
  decouples the two sets of windings, enabling independent control. Electricity is generated
  through the power winding at both sub and super-synchronous speeds with appropriate
  excitation to the isolated single winding at any frequency of generation. An Experimental
  analysis and dynamic mathematical model, which accurately predicts the behavior of the
  proposed generator.

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Keywords: Induction Generator in TSCAOI Configuration, Renewable Power System,
Mathematical Model.
I. INTRODUCTION
         The use of renewable energy as an alternative to low cost fossil energy, which was in
abundance, has never been considered as an economically viable option in the past. However,
the excessive, unnecessary, and inefficient use of fossil energy has now become a global
concern, owing to rapidly decreasing fossil resources, rising fuel prices, increasing demand
for energy, and, more importantly, the awareness of global warming and environmental
impact. Consequently, it has now become a common practice of governing bodies to place
more emphasis on energy saving, harnessing renewable energy, and particularly on energy
management through efficient generation, conversion, transmission, and distribution. This
initiative incited a new area of active research and development within both academia and
industry under the context of “green or clean or renewable” energy. Nuclear energy has
several advantages over coal in that no carbon dioxide or sulfur dioxide are produced, mining
operations are smaller scale, and it has no other major use besides supplying heat. The major
difficulty is the problem of waste disposal, which, because of the fears of many, will probably
never have a truly satisfying solution.
         Because of these problems, along with the rising energy demand in the 21st century
and the growing recognition of global warming and environmental pollution, energy supply
has become an integral and cross cutting element of every countries economy. In recent
years, more and more countries have polarized sustainable, renewable and clean energy
sources such as wind power and other forms of solar power are being strongly encouraged.
Wind power may become a major source of energy in spite of slightly higher costs than coal
or nuclear power because of the basically non-economic or political problems of coal and
nuclear power. This is not to say that wind power will always be more expensive than coal or
nuclear power, because considerable progress is being made in making wind power less
expensive. But even without a clear cost advantage, wind power may become truly important
in the world energy picture.
                  Wind power has now established itself as a mainstream electricity generation
source, and plays a central role in an increasing number of countries’ immediate and longer
term energy plans. After 15 years of average cumulative growth rates of about 28%, the
commercial wind power installations in about 80 countries at the end of last year totaled
about 240 GW, having increased by more than 40 times over that same period. Twenty two
countries have more than 1,000 MW installed. The following figure shows the 1999 WIND
FORCE 10 blueprint and actual development globally [1].




                       Fig 1: Total Wind Capacity Installed in India

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        There are various techniques for conversion of mechanical energy into electrical
energy. Typically, small renewable energy power plants rely mostly on induction machines,
because they are widely and commercially available and very inexpensive. Induction
generators are useful in applications such as mini hydro power plants, wind turbines, or in
reducing high-pressure gas streams to lower pressure, because they can recover energy with
relatively simple controls. It is also very easy to operate them in parallel with large power
systems, because the utility grid controls voltage and frequency while static and reactive
compensating capacitors can be used for correction of the power factor and harmonic
reduction. Although the induction generator is mostly suitable for hydro and wind power
plants, it can be efficiently used with prime movers driven by diesel, biogas, natural gas,
gasoline and alcohol motors. Induction generators have outstanding operation as either motor
or generator; they have very robust construction features, providing natural protection against
short-circuits, and have the lowest cost with respect to other generators. Abrupt speed
changes due to load or primary source changes, as usually expected in small power plants, are
easily absorbed by its solid rotor, and any current surge is damped by the magnetization path
of its iron core without fear of demagnetization, as opposed to permanent magnet based
generators. The induction generator has the very same construction as the induction motors
with some possible improvements in efficiency.




               Fig.2.Typical induction generator systems used in wind turbines

        Of the schemes illustrated in Fig. 2, fixed-speed cage three phase induction generators
are well known for their simplicity and low cost and operated at constant rotor speed to
generate electricity at constant frequency for both direct grid integration and standalone
operation. Usually, they are excited through a bank of capacitors and are incapable of
tracking maximum power that is available from the wind turbine when operated at constant
speed. Therefore, in order to extract maximum energy under varying wind speed conditions,
an intermediate power conversion stage, comprising an alternating-current (ac)/direct-current
(dc) and dc/ac back-to-back converter configuration, is employed between the generator and
the grid or the load [2]–[4]. The intermediate stage allows for the variable-speed operation of
the generator, but it essentially requires to be rated for the same or a fraction (in the case of
doubly fed induction generators) of the power level of the generator itself. Thus, such an
intermediate stage is often found to be economically unjustifiable for some applications,
particularly at micro power levels.
        Induction generators have been also employed to generate single-phase electricity,
particularly for standalone or residential use [5]. In [6] and [7], a self-excited and self
regulated single-phase induction generator has been reported for the generation of single-
phase electricity. In contrast, the analysis of the self-excitation of a dual-winding induction
generator has been presented in [8]. This paper, which uses a single-phase cage induction
machine with an auxiliary winding, has been extended by connecting an inverter to the
auxiliary winding to achieve more flexibility in power control. The experimental performance

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is investigated under varying operating conditions and it indicate that the machine can be
operated both at sub- and super-synchronousrotor speeds to generate electricity at constant
frequency.

II. INDUCTION GENERATOR IN TSCAOI CONFIGURATION

        Induction generators have been also employed to generate single-phase electricity,
particularly for standalone or residential use .A self-excited and self regulated single-phase
induction generator has been reported for the generation ofsingle-phase electricity. In [9], a
novel method for self-regulated single phase induction generator by using an ac adjustable
capacitor is introduced. All these reported schemes employed a single-phase induction
generator and an auxiliary winding in some cases or a three phase induction generator to
generate single-phase electricity at constant or above synchronous speed. In contrast with
single-phase cage induction machines, three phase cage induction machines are less
expensive and small in size for a similar power rating. As, explained in [10] three-phase cage
induction machine can be used as a single-phase generator under both sub- and super-
synchronous variable-speed conditions without an intermediate inverter stage. The technique
uses one of the three windings in isolation for excitation and the remaining two, which are
connected in series, as the power winding for the single-phase electricity generation. The
three-phase cage induction machine is mathematically modeled in the proposed two-series
connected- and-one-isolated (TSCAOI) phase winding configuration. The proposed
technique allows for both energy storage and retrieval through the excitation winding and is
expected to gain popularity, particularly in small-scale applications, being relatively simple
and low in cost. The following figure shows the proposed induction generator in the TSCAOI
configuration.




              Fig 3.Proposed Induction Generator in the TSCAOI Configuration

         Cage induction machines are undoubtedly the workhorse of the industry and can be
still regarded as the main competitor to permanent-magnet machines. This is because they are
self starting, rugged, reliable, and efficient and offer a long trouble free working life. Of these
cage induction machines, three phase machines are significantly less expensive, more
efficient, and smaller in frame size in comparison with their single-phase counterpart of
similar power ratings. Consequently, three-phase cage induction motors are economically


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more appealing and have thus become the preferred choice for numerous applications, even at
derated power levels.
        The proposed novel technique uses a three phase cage induction machine, exploiting
its economical advantage, to generate single-phase electricity at variable rotor speeds without
an intermediate inverter stage. The technique configures the three stator windings of the
three-phase cage induction machine in a novel way to create separate or rather decoupled
excitation and power windings. In this configuration, any one of the three phase windings is
solely used in isolation for excitation, whereas the remaining two are connected in series to
generate power at a desired frequency while the rotor is driven at any given speed.
Alternatively, the machine can be also configured in such a way that the two series-connected
windings provide the excitation while the single winding generates.
        As mathematically shown, the TSCAOI winding configuration magnetically
decouples both excitation and power windings from each other and thus allows for
independent control as in the case of a single-phase induction motor with an auxiliary
winding. In the proposed technique, excitation for the generator is provided through the
single winding to study the voltage build up process at no load and at load.

III. MATHEMATICAL MODEL

While doing mathematical modeling, certain assumptions are made, they are as follows:
1. Uniform air gap.
2. Balanced rotor and stator windings with sinusoidally distributed mmf.
3. Inductance vs. rotor position is sinusoidal &
4. Saturation and parameter changes are neglected.
              Fig. 4 shows the stator and rotor with respect to αβ frame. The first step in the
mathematical modeling of an induction machine is by describing it as coupled stator and rotor
three-phase circuits using phase variables, namely stator currents ias, ibs, ics and rotor
currents iar, ibr, icr ; in addition to the rotor speed ωr and the angular displacement φr
between stator and rotor windings.




                        Fig 4.Stator and Rotor with respect to αβ frame

       The electrical parameters of machine are expressed in terms of a resistance matrix R
[3x3] and an inductance matrix L [3x3] in which the magnetic mutual coupling elements are
functions of position r. So that, for instance, the current vector is I = [ias ibs ics iar ibr icr]t,
representing stator and rotor currents expressed in their respective stator and rotor frames.


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Variables:

       V= [vas vbs vcs 0 0 0]t…………………………...(1)

      I= [ias ibs ics iar ibr icr]t…………………………..(2)
Whereas,
      vas= Vs*sin(ωt),

       vb s= Vs*sin(ωt+   ),

         vcs=Vs*sin(ωt- )
Vs= peak voltage
ω= 2πf, is the phase angle in radians
t is the time in seconds
f = frequency in cycles per second (Hz) &
t=Transpose of the matrix
Matrix analysis of Induction Machine




       The transformation matrix for conversion of abc frame stator and rotor quantities into
‘eo’ and ‘αβ’ frames respectively are given as follows,



                     ………………………………….. (3)



                                                         ……………………... (4)

The voltage equation for 3-phase induction machine in abc frame, is represented as
following,

[vs,abc] =[Rs][is,abc] + p {[Ls][is,abc]} + p {[Lsr][ir,abc]}……………………………..(5)

[vr,abc] =[Rr][ir,abc] + p [Lsr]T [is,abc] + p {[Lr][ir,abc]}……………………………..(6)

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        For, squirrel cage induction generator the rotor voltage is zero as the rotor is short
circuited. As per the requirement of the TSCAOI configuration, the voltage and currents of
the stator and rotor have to be transformed into the ‘eo’ frame and ‘αβ’ frame respectively.
This can be done using the transformation matrices Q and Kr for stator and rotor respectively.

[vseo]T= [Q][vsabc] …………………………………………..(7)

[vr αβ]=[Kr][vrabc] …………………..…………………….....(8)

So, after substituting the values of (3), (4), (5)and also (6), in equations (7) and (8), we will
get the following equations,

[vs,eo] =[Q][Rs][Q]−1[is,eo] + [Q]p{[Ls][Q]−1[is,eo]} + [Q]p{[Lsr][Kr]−1[ir,αβ]} ………………(9)

[vr,αβ] =[Kr][Rr][Kr]−1[ir,αβ] + [Kr]p {[Lsr]T [Q]−1[is,eo]} + [Kr]p {[Lr][Kr]−1[ir,αβ]……….(10)

After lengthy manipulations and substitutions we will get the following equations,



                                                                                         ... (11)




                                                                             ……………. (12)

        Where, ωris the rotor speed in electrical radians per second. By observing above
equation, we can say that the power winding and the excitation winding of the stator are
totally decoupled.
        To complete the machine model, it is necessary to select state variables and derive the
appropriate equations for integration. In this case, the elements of the machine current vector
are chosen as the state variables. Equation (13) shows the state space model using the
winding currents as the phase vector, as derived from (11) and (12), i.e.,

p[i]=[A][i]+[B][v] …………………………………… (13)

Where,

          [i]= [iseisoirαirβ]t
         [v]= [vseo]=[vsevso]t




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LM= Lms
Lss=Lls+ LM
Lrr= Llr+ LM
D=(LssLrr- LM2)
D1=LlsLrr+ LlrLM

The electromagnetic torque of the machine can be derived from


                                ..... (14)

        Where, P denotes the number of poles. Equation (14) in “abc” quantities is
transformed into the “eo” and “αβ” frames and can be given by



                                        …. (15)

       Equation (15) represents the torque components due to both load and excitation
currents. At the steady state, the torque given in (15) is equal to the turbine torque. The
equation of the motion of the generator is given by



                             ……… (16)

Where, J (in kg · m2) is the inertia and Tp(in Nm) is the torque of the prime mover.

        The above mathematical model shows the conversion of three phase induction
machine which can be used for the single phase power generation when the prime mover
torque is given to rotate the rotor.



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IV. EXPERIMENTAL ANALYSIS




                                                       Fig. 5. Experimental setup

        In order to demonstrate the practical viability of the proposed concept, a four-pole
3.7KW ,415V, 7.3A cage induction machine in TSCAOI configuration was used. The
experimental setup is shown in Fig. 5.1(a). The rotor of proposed 3.7KW induction generator
was driven by another DC motor to emulate the variable wind conditions, to supply power
to a standalone and electronic load at constant voltage at 50 Hz under varying rotor speeds. A
experimental observation have taken on different capacitor (25-µF, 50-µF, 75-µF, 105-µF),
shown as C0 in Fig. 5, was employed to reduce the reactive-power requirement of the
excitation source and to keep the magnitude of the excitation current below the rated value of
the machine.

                                                 8.0

                                                 7.0
                                                                              C=50µF,Vse=150V
                                                 6.0
                        Excitation Current (A)




                                                 5.0
                                                                              C=50µF,Vse=100V
                                                 4.0

                                                 3.0
                                                                              C=75µF,Vse=100V
                                                 2.0

                                                 1.0

                                                 0.0
                                                   1300 1400 1500 1600 1700   C=75µF,Vse=150V

                                                       Rotor Speed (rpm)


                                                                   (a)


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                                        250.0

                                                                    C=50µF,Vse=1
                                        200.0                       50V
                                                                    C=50µF,Vse=1

                       Output Voltage
                                        150.0                       00V


                                                                    C=75µF,Vse=1
                                        100.0                       00V


                                         50.0
                                                                    C=75µF,Vse=1
                                                                    50V
                                          0.0                       C=105µF,Vse=
                                            1300     1500    1700   100V
                                                Rotor Speed(rpm)

                                                            (b)

                                        300.0

                                        250.0                       C=50µF,Vse=1
                                                                    50V
                       Output Power




                                        200.0
                                                                    C=50µF,Vse=1
                                        150.0                       00V

                                        100.0
                                                                    C=75µF,Vse=1
                                         50.0                       00V

                                          0.0
                                            1300     1500    1700
                                                                    C=75µF,Vse=1
                                                Rotor Speed(rpm)    50V


                                              (c)
   Fig.6. Rotor speed verses (a) Excitation current (b) Output voltage (c) Output power for
                        different rotor speeds and different capacitor.

        Fig. 6 The graph between roter speed verses diffirent quantity is plotted as shown in
Fig.6. It is observed from Fig.6 (a) that the excitation current is minimum for 50 µF capacitor
connected across power winding. The variation of the output voltage versus the rotor speed is
shown in Fig.6 (b). The impedance seen by the excitation source is complex in nature, being
dependent on both the excitation frequency and the slip frequency. It appears from results
that the variation of both the output voltage and current is largely governed by the rotor-
speed. It is observed from above Fig. 6(a) and (c) that with increse in excitation voltage and
value of the capacitance the output power and output voltage increses.But it is also observed

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that before or after the perticular range of rotor speed the output power and output voltage
decreases.At this speed the excitation current is minimum in each case of diffirent capacitor
value.It is seen that at this speed with increse in capacitor value the excitation current
increses but it is in safe limit of excitation current upto Co =105 µF.

                                            600.0
                                            500.0
                                                                         C=50µF,Vse=150V
                                            400.0
                                            300.0
                     Excitation Power (W)




                                            200.0                        C=50µF,Vse=100V
                                            100.0
                                               0.0
                                                                         C=75µF,Vse=100V
                                            -100.013001400150016001700
                                            -200.0
                                            -300.0                       C=75µF,Vse=150V
                                            -400.0
                                            -500.0
                                                     Rotor Speed (rpm)


Fig. 7. Rotor speed verses Excitation power for different rotor speeds and different capacitor

        Fig.7 demonstrates the variation of the power of the excitation source for different
rotor speeds for different value of capacitor. The positive power and the negative power
indicate the power supplied and absorbed by the source, respectively. The rotor speed was
simply increased or decreased by increasing or decreasing the torque setting of the prime
mover of the proposed generator that is DC motor. It is seen that after adding the more value
of capacitor the excitation power increases due to increase in the value of excitation current
as shown in Fig.7.
         Fig.8 shows the relation between efficiency of the proposed generator verses output
power. It is observed that with increase in output power efficiency is more around
1500rpm.The efficiency is less with increases in speed beyond 1600rpm.With increase in
capacitance value output power is increases at same excitation voltage. It should be noted that
the excitation voltage can never be as high so that the rated current should not be exceeded
and the flux required to generate output voltage across the power winding is less. The
performance of the proposed generator was evaluated by measuring the efficiency for a range
of output and excitation power levels, as shown in Fig.8. A maximum efficiency of
approximately 60% is observed.
        The above observations conclusively prove that the newly developed Induction
generator has certain distinct qualities and advantages which promise to make it a success for
consumer applications for remote application. The experimental investigation reported in this
part of the project proves the technical viability of the use of induction generator in TSCAOI
configuration to generate single phase power.

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                                                 90.0
                                                 80.0
                                                                                    C=50µF,Vse=150V
                                                 70.0                               ,N=1500rpm
                                                 60.0
                                  %η generator   50.0                               C=50µF,Vse=150V
                                                 40.0                               ,N=1400rpm

                                                 30.0
                                                 20.0                               C=50µF,Vse=150V
                                                                                    ,N=1600rpm
                                                 10.0
                                                  0.0                               C=50µF, Vse=150
                                                        0.0   100.0 200.0 300.0     V,N=1650rpm

                                                              Outpu Power


                                                                        (a)

                                  100
                                   90
                                   80
                   %η generator




                                   70
                                   60
                                   50
                                   40
                                   30                                                    C=75µF, Vs
                                   20                                                    e=150V,…
                                   10
                                    0
                                                  0            100      200       300
                                                               Output Power

                                                                        (b)

  Fig.8 Output power verses efficiency of for different rotor speeds and different capacitor


        The above mentioned experimental results can be verified through approximate
theoretical analysis.
         In the proposed TSCAOI configuration of the machine, the power can be generated
through both the excitation and power windings, whereas the var requirement of the machine
is met by the excitation winding, supplemented by a capacitor connected to the power
winding. This is because the total var requirement of the three-phase machine can never be
met by the single excitation winding without exceeding its rated current, even at zero
excitation real power.

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V. CONCLUSION
       The mathematical model of a three-phase induction generator in TSCAOI
configuration are presented in a step-by-step manner. The above experimental verification is
analyzed for different value of capacitor at different rotor speed. The above experimental
analysis indicates that the proposed machine can be used to generate the power at sub or
super synchronous speed at constant frequency at any excitation voltage.
       As, we have seen the experimental results, the novel cage three phase induction
generator can be used for the single phase power generation. The proposed generator is easy
to implement and low in cost and it is an ideal machine for small-scale renewable energy
applications. The project conclusively proves that the new proposed machine described here
is technically viable and commercially reliable due to its simplicity, ruggedness and
maintenance free operation. The experiments conducted are detailed along with the
corresponding results. Special tests to identify the parameters for analysis have been
explained with typical results. The voltage drop from 264V to 220V at 420W output power
at 1540 rpm for Co of 105µF show good performance of the proposed machine.

REFERENCES
1.    GLOBAL Wind Energy Outlook 2012, Global wind energy council- November 2012
2.    R. C. Bansal, “Three phase self excited induction generators: Overview,” IEEE Trans. Energy
      Convers., vol. 20, no. 2, pp. 292– 299,Jun. 2005.
3.    J.-C. Wu, “Novel circuit configuration for the compensation for the reactive power of induction
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4.    B. Singh, S. S. Murthy, and S. Gupta, “STATCOM based voltage regulatorfor self-excited
      induction generator feeding non-liner loads,” IEEE Trans
5.    T. F. Chan and L. L. Lai, “Single phase operation of a three-phase induction generator using a
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      Sep. 2007.
6.    S. S. Murthy, “A novel self-excited self-regulated single phase induction generator—Part 1,”
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7.     L. Shridar, B. Singh, and S. S. Murthy, “Selection of capacitors for self regulated short shunt
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8.    O. Ojo and I. Bhat, “An analysis of single phase self excited inductiongenerators,” IEEE Trans.
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9.    Self-Regulated Single-Phase Induction Generator, M. M. Ahmed and M.Y. Abdelfatah
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10.   U. K. Madawala, T. Geyer, J. Bradshaw, and D. M. Vilathgamuwa,“A model for a novel
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11.   Analysis Of An Isolated Self-Excited Induction Generator Driven By AVariable Speed Prime
      Mover, D. Seyoum, C. Grantham and F. Rahman School of Electrical Engineering and
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12.   N. S. Wani and W. Z. Gandhare, “Voltage Recovery of Induction Generator using Indirect
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13.   Youssef A. Mobarak, “Svc, Statcom, and Transmission Line Rating Enhancments on Induction
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