11.vector control of wind driven self excited induction generator connected to grid using twenty four pulse ac

Control Theory and Informatics www.iiste.org
ISSN 2224-5774 (print) ISSN 2225-0492 (online)
Vol 2, No.1, 2012

Vector Control Of Wind Driven Self Excited Induction
Generator Connected To Grid Using Twenty Four Pulse AC-
DC Converter Employing Pulse Doubling Technique
Swati Devabhaktuni1* S.V.Jayaram Kumar2

1. Gokaraju rangaraju institute of engineering and technology, Hyderabad,Andhra Pradesh,India
2. Jawaharlal Nehru Technological Univeristy,Hyderabad,Andhra Pradesh,India.
* E-mail of the corresponding author: swatikjm@gmail.com

Abstract
This paper deals with multipulse AC-DC converters for improving the power quality in vector-controlled
wind driven self excited induction generator at the point of common coupling. These multipulse AC-DC
converters are realized using a reduced rating autotransformer. Moreover, DC ripple reinjection is used to
double the rectification pulses resulting in an effective harmonic mitigation. The proposed AC-DC
converter is able to eliminate up to 21st harmonics in the supply current. The effect of load variation on
Vector Controlled Self Excited Induction Generator(VCSEIG) is also studied to demonstrate the
effectiveness of the proposed AC-DC converter. A set of power quality indices on input AC mains and on
the DC bus for a VCSEIG fed from different AC-DC converters .The complete electromechanical system is
modeled and simulated in MATLAB using Simulink and simpower system block set. The simulated results
are presented and compared for regulating voltage and frequency of SEIG driven by wind turbine
Keywords: Autotransformer, Multipulse AC—DC converter, DC ripple reinjection, Pulse doubling,
VCSEIG.

1. Introduction
Since the 1930s, it has been known that a three-phase induction machine can work as a self-excited
induction generator (SEIG) [1, 2]. Due to their simplicity, robustness, and small size per kW generated,
induction generators are favoured for small hydroelectric and wind powered plants. One distinct advantage
of the SEIG is its much lower unit cost compared with the conventional synchronous generator [3]. Recent
widespread adaptation of power electronics and microcontrollers has enabled use of these generators with
increased efficiency, for power generation of up to 500 kW when connected to the power grid. In contrast,
the maximum power of a stand-alone SEIG does not go much beyond 15 kW [4].
In isolated applications, an induction generator operates with three AC capacitors connected to the stator
terminals, or with a power converter and a single dc capacitor [5]. Once an induction generator is excited,
the capacitor maintains the excitation. The minimum dc capacitance required for the initiation of voltage
buildup can be found in reference [6]. For a given capacitance,self-excitation can only be achieved and
maintained over certain load and speed ranges. Even when the capacitor is properly chosen, additional
mechanisms must be added to avoid demagnetization and to achieve better control of the voltage produced.
Some of these mechanisms include field-oriented techniques [7]. Contemporary research focuses on control
mechanisms using stator flux orientation as well as those using rotor flux orientation.
Harmonics can be reduced using different active or passive wave shaping techniques. However, they
require careful application and may produce unwanted side effects, particularly in the presence of power
factor (PF) correction capacitors. The most rugged, reliable and cost effective solution to mitigate these
harmonics is to use multipulse methods [4].
In multipulse converters, the autotransformer-based configurations provide the reduction in magnetic rating
as the transformer magnetic coupling transfers only a small portion of the total kVA of the induction motor
drive. Various 6-pulse-based rectification schemes have been reported and used in practice for the purpose

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Vol 2, No.1, 2012
of line current harmonic reduction [7]. With the use of a higher number of multiple converters, the power
quality indices show an improvement, but at the cost of large magnetics resulting in a higher cost of the
drive. To achieve similar performance in terms of harmonic current reduction, DC ripple reinjection has
been used.
This paper investigates a control system for an induction generator that uses the stator flux orientation.
Systematic analysis of this control system is carried out for wide ranges of both load and speed. The
induction generator supplies a variable dc load. This paper presents an autotransformer-based 24-pulse AC-
DC converter with reduced rating magnetics. A pulse multiplication technique is used to improve various
power quality indices to comply with the IEEE standard 519 [8]. This arrangement results in elimination up
to the 21*harmonic in the input line current.Moreover, the effect of load variation on the vector-controlled
induction motor drive (VCIMD) is also studied. The proposed AC—DC converter is able to achieve near
unity PF in a wide operating range of the drive.

2. System Configuration

The proposed system is as shown in Fig.1

Fig.1.Overall system description

2.1. System Description

Fig. 1 shows the wind driven self excited induction generator with excitation capacitor, consumer loads,
and conventional six-pulse diode rectifier. The diode bridge is used to convert ac terminal voltage of SEIG
to dc voltage. The output dc voltage has the ripples, which should be filtered, and therefore, a filtering
capacitor is used to smoothen the dc voltage. An inverter is used to provide AC voltage across the grid
which is the load for SEIG. The sensed terminal voltage is compared with reference voltage and error signal
is processed through PI controller. According to the principle of operation of the system, the suitable value
of capacitors is connected to generate rated voltage at desired power. The input power of the SEIG is held
constant at varying consumer loads.

2.2. Design Of The Model

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Vol 2, No.1, 2012

Fig.2. Twenty Four pulse Diode Bridge

In the above fig.2 the constant prime mover shaft was connected to Induction generator shaft. In this paper
uncontrolled wind turbine is used as a constant prime mover. A.3.5KW, 440V, 50HZ Induction machine is
used as SEIG and modeled using available power electronics block set like diode bridge rectifier and
connected to a 440 V grid supplying to the college laboratories. Simulation is carried out in MATLAB
version of 10 above at discrete step of 50E-6.Detailed simulation and analysis are given in the fallowing
section. The parameters of the induction machine are Rs = 0.69 Ω, Rr = 0.74Ω, Lls = Llr = 1.1 mH, J =
0.23kg/m2, Lss = Lls + Lm and Lrr = Llr + Lm. Excitation capacitor C = 15 µF/ phase and Capacitor at rectifier
C=3200 µF. The magnetic rating are 12-pulse-based converter: Autotransformer rating 12Kva,Interphase
transformers 2.7kVA, passive filter 3kVA.References.

3. Control scheme

Stator flux oriented control is used in this paper. Its accuracy is dependent only on the stator resistance
variation. In addition, it is insensitive to the variation in the leakage inductance of the machine. In
induction motor the application of stator flux oriented control the parameter variation of resistance Rs
tends to reduce the accuracy of the estimated signal at low voltage [1]. However, at higher voltage the
effect of parameter variation in Rs can be neglected. Flux estimation accuracy in rotor flux oriented
control is affected by rotor parameters. The rotor resistance variation becomes dominant by
temperature and skin effect in squirrel cage induction machines [3].Compensation of this parameter is
difficult because of inaccessibility, but it easier to compensate Rs [4].

3.1. Determination of control parameters

To find the decoupling signal given in Fig. 1 some of the equations of an induction machine in the
excitation reference frame are considered[5]. Since in the stator oriented reference frame all variables
are expressed in a reference frame oriented to the stator flux linkage space vector, a mathematical
model needs to be developed to find the relationship between the stator flux linkage and the stator
currents

(1)

(2)

(3)

(4)

(5)

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Vol 2, No.1, 2012

(6)

From equations (1) and (2)

(7)
And from equation (3) and (4)

(8)
Where
(9)
From the equations (1) and (2) the rotor currents can be expressed as
(10)
(11)
With stator flux oriented vector control, illustrated in Fig.4, the total stator flux is aligned along the d-
axis of the synchronously rotating reference frame so that

(12)
And

(13)
Then
(14)
Equations (7),(8),(9) and (10) are substituted in the voltage equations given in (5) and (6) to eliminate
the rotor flux linkages and rotor currents from the induction machine equations and to express the
machine equations in the reference frame fixed to the stator flux linkage space vector. Simplifying the
equations using (12), (13) and (14) the new expressions are given by

(15)

(16)
where p is the differential operator, i.e. and is the slip frequency expressed as

As given in (15), with the stator flux oriented vector control, which is based on an impressed stator
current controller, the total stator flux linkage, , and the q-axis stator current, ,are coupled. This
means that any change in without changing will cause unwanted transients to occur in the
stator flux linkage.
The undesirable coupling can be eliminated by utilizing a decoupling circuit in the flux linkage control
loop. The decoupling circuit is implemented at the output of the stator flux linkage controller. The
stator flux linkage controller is a PI controller and its output is designed to be the d-axis current
required to produce the reference stator flux linkage say given by

(17)
where G is the transfer function of a PI controller.
However, due to the coupling problem does not have total control over the stator flux linkage. The
reference d-axis current, with full control of the stator flux linkage is expressed as

(18)

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Vol 2, No.1, 2012
Substituting (17) in (18) gives

(19)
Substituting eq(19) in eq(15) gives,

(20)
For decoupled control with the help of the two last terms of Equation (20) can be cancelled i.e,

(21)
So that,

(22)
The angular slip frequency is available from (16) and is given by

(23)
The reference flux linkage required at any speed is calculated based on this maximum flux linkage,
which corresponds to the minimum rotor speed, . Hence at any rotor speed, , the
reference stator flux linkage is given by,

(24)

3.2. Design of the proposed 24-pulse AC-DC converter

This section presents the design technique for achieving 24-pulse rectification in the proposed AC-DC
converter. To achieve the 24 pulse rectification, the necessary requirement is the generation of two sets of
line voltages of equal magnitude that are 300 out of phase with respect to each other.The number of turns
required for the 00 and 300phase shift is calculated as follows:
Consider phase A:

(25)
Assume the following set of voltages:

(26)
Similarly,

(27)
Where V is the RMS value of the phase voltage.Using the above equations, can be
calculated.These equations result in for the desired phase shift in the auto
transformer.The phase-shifted voltages for phase A are:

(28)
The kVA rating of the interphase transformer and the Zero Sequence Blocking Transformer(ZSBT) is also
calculated.
The AC-DC converter output voltage and the voltage across the interphase reactor is given by:

(29)
is an Ac voltage ripple of six times the source frequency.

3.3. Design of interphase reactor and ZSBT

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Vol 2, No.1, 2012

Pulse multiplication has been obtained for controlled converters with a tapped interephase reactor and two
additional diodes. The turns ratio of the interphase reactor is given by =0.2457.The waveforms of the
diode currents are as shown Fig.3(a) and (b) respectively.
The ZSBT helps in achieving an independent operation of the rectifier bridges,thus eliminating the
unwanted conducting sequence of the rectifier diodes..The voltage waveforms across ZSBT and IPT are as
shown in Fig.4(a) and (b) respectively.

(a) (b)
Fig.3.(a).Tapped interphase reactor along witht the diodes (b).Diodes D1 and D2 voltage waveforms

(a) (b)
Fig.4.(a).Voltage waveforms across IPT (b) Voltage waveform across ZSBT

4. MATLAB based Simulation and Results

The simulation stator oriented vector control is implemented using MATLAB/SIMULINK. The features in
the Power Systems Blockset are used to model an inverter, rectifier and all circuit components. The
induction machine model in the Power Systems Blockset is modified to include speed as an input and to
update the variation of magnetizing inductance as the voltage builds up during self-excitation. To get the
right control parameters and performance it is simply a matter of tuning the PI controllers in the DC voltage
controller and flux linkage controller given in Fig. 1.
The dynamics of the DC voltage at the start of the voltage build up process, for a rotor speed of 1.48p.u
with capacitance value of 3200µF is as shown in Fig.10(a). When the capacitance is large it takes longer to
reach its steady state value. If the capacitance is too small there will not be enough exciting current and as a
result there will not be voltage build up. Fig.10(b) shows the no load build up of generated line to line
voltage at the terminals of the induction generator during the start of self excitation. The voltage build up
process is under the no load condition. If there is load, with magnitude above a given minimum value, the
voltage build up process will fail.

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Vol 2, No.1, 2012
The frequency of the generated voltage is estimated as :

(30)
Here, transient waveforms of the generator voltage (Vabc), generator current (Igabc), Speed of the
generator ,Electromagnetic torque, rectifier current , voltage at capacitor,inverter voltage, grid voltage, grid
current are given under the sudden application and short circuit at grid for conventional six pulse rectifier is
as shown in Fig.5 respectively.The Simulink model for the six pulse diode rectifier is as shown in Fig.6.

(a) (b)

(c) (d)

(e)
(f)

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Vol 2, No.1, 2012

(g)
(h)
Fig.5. The experimental transient waveforms (a).Generated voltage (b). Line current.
(c).Rotor speed (d).Electromagnetic torque (e).Rectifier current
(f).Voltage at capacitor (g).Inverter voltage (h).Grid voltage
and grid current.

The proposed AC-DC converter along with the VCSEIG is simulated in the MATLAB environment
along with the Simulink The load is suddenly applied at the grid at 0.8 to 1Seconds and the
shortcircuit occurs at grid at 1 to 1.2 seconds.The variation is due to these load variations are very
slightly observed due to the application of the vector control and 24pulse..Fig.5.Shows the dynamic
performance of the proposed AC-DC converter at start and load perturbation on VCSEIG.It consists
of Generated voltage,supply current,rotor speed,electromagnetic torque,inverter voltage and DC link
voltage.

Fig.6. Simulink/MATLAB model for Twenty Four pulse controller

5. Conclusions

The voltage build up process of an induction generator with a single capacitor on the DC side of the
inverter using stator flux oriented vector control is discussed. Since the induction generator operates at
a frequency away from the DC frequency any integration offset error is removed by a low pass filter at
a reasonable small time constant.
The total flux is aligned to the d-axis of the stator flux in the excitation reference frame. A decoupling
signal is also generated the effect of q-axis current on the d-axis flux. The main advantage of stator
flux oriented vector control is the magnitude of the estimated flux depends only on the stator
resistance. Unlike the rotor resistance the variation of stator resistance depends manly on temperature.

40

Vol 2, No.1, 2012
If the variation of stator resistance is causing a significant error then a compensation block can be
added in the model.
The power quality has been improved using 24pulse uncontrolled diode rectifier and an inverter and
harmonics are reduced compared to 6 pulse. The proposed method is able to achieve unity PF with a good
DC link voltage regulation in the wide operating range of the drive, and has resulted in a reduction in the
rating of the magnetic, leading to the saving in the overall cost of the drive. It can easily replace the
existing six-pulse converters without much alteration in the existing system layout and equipments.

Appendix
1. Machine Parameters
The parameters of the 3.5 kW,440V, 7.5A, 50 Hz,4-pole induction machine are given below.
Rs = 0.69 Ω, Rr = 0.74Ω, Lls = Llr = 1.1 mH, J = 0.23kg/m2,
Lss = Lls + Lm and Lrr = Llr + Lm.
2. Excitation capacitor C = 15 µF/ phase and Capacitor at rectifier C=3200 µF
3. Air gap voltage:
The piecewise linearization of magnetization characteristic of machine is given by:
E1=0 Xm≥260
E1=1632.58-6.2Xm 233.2≤Xm ≤260
E1=1314.98-4.8Xm 214.6≤Xm ≤233.2
E1=1183.11-4.22Xm 206≤Xm ≤214.6
E1=1120.4-3.9.2Xm 203.5≤Xm ≤206
E1=557.65-1.144Xm 197.3≤Xm ≤203.5
E1=320.56-0.578Xm Xm ≤197.3

4. Magnetics Rating:
12-pulse-based converter: Autotransformer rating 12Kva,
Interphase transformers 2.7kVA, passive filter 3kVA.References

References

[1] O. Ojo, O. Omozusi, and A. A. Jimoh, “The operation of an inverter assisted single phase induction
generator,” IEEE Trans. Ind. Electron., vol. 47, no. 3, pp. 632–640, Jun.2000.
[2] D. Henderson, “An advanced electronic load governor for control of micro hydroelectric generation,”
IEEE Trans. EnergyConverse.,vol. 13, no. 3, pp. 300–304,Sep. 1998.
[3] IEEE Guide for Harmonic Control and Reactive Compensation of Static Power Converters, IEEE
Standard 519-1992.
[4] R. Bonert and S. Rajakaruna, “Self-excited induction generator with excellentvoltage and frequency
control,” Proc. Inst. Electr. Eng. Gener.Transm. Distrib., vol. 145, no. 1,
[5] Malik, N.H., and Al-Bahrani, A.H., “Influence of the Terminal Capacitor on the Performance
Characteristics of a Self-Excited Induction Generator ”, IEE Proc C., Vol. 137, No. 2, March 1990, pp.
168-173.
[6] Nandakumar, V.N, Yadukumar, K., Sureshkumar, T., Ragupathi, S., and Hegde, R.K., “A
WindDriven Self-Excited Induction generator With Terminal Voltage Controller and Protection
Circuits”, IEEE Power Conversion Conference,1993, pp. 484-489.
[7] Bonert, R., and Rajakaruna, R., “Self-Excited Induction Generator with Excellent Voltage and
Frequency Control”, IEE Proc.-Gener. Transm. Distrib., Vol.145, No.1, January 1998, pp.33-39.
[8] Mitronikas, E.D., Safacas, A.N. and Tatakis, E.C.,“A new stator resistance tuning method for
statorflux- oriented vector-controlled induction motor drive”, IEEE Transactions on Industrial
Electronics, Vol. 48 Issue: 6, Dec. 2001, pp. 1148 –1157.
Swati Devabhaktuni received the B.Tech degree in electrical and electronics engineering
from V.R.Siddhartha Engineering College,Andhra University, India in 2001, and the
M.Tech degree in control systems from J.N.T.U University, in 2004. Currently, she is a
Associate professor in Gokaraju rangaraju Institute of engineering and

41

Vol 2, No.1, 2012
technology,Hyderabad,She is also a Research Scholer in J.N.T.U University, Hyderabad. Her research
interests are the power electronics, AC motor drives , and control systems.

Dr. S. V. Jayaram Kumar received the M.E. degree in electrical engineering from the
Andhra University, Vishakapatnam, India, in 1979. He received the Ph.D. degree in
electrical engineering from the Indian Institute of Technology, Kanpur, in 2000.
Currently, he is a professor at Jawaharlal Nehru Technological University, Hyderabad. His
research interests include FACTS and Power System Dynamics,A.C drives.

42

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