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Abstract--This paper presents a simple control structure for

the stand alone operation of Self Excited Induction Generator

used to operate under variable loads. The proposed system uses

PWM Voltage Source Inverter, DC-DC Boost Converter and

diode bridge Rectifier with SEIG. The required reactive power

for the Self Excited Induction Generator is supplied by means of

a capacitor bank and the PWM converter to build up the voltage

of the SEIG. The DC-DC Boost Converter maintains constant dc

link voltage at the input terminals of PWM Voltage Source

Inverter. The consumer load is connected across VSI through LC

filter. The proposed control strategy has good ac voltage

regulation at all loads and no harmonic problems. The

performance of proposed controller is simulated using MATLAB

SIMULINK to demonstrate its capability as a voltage regulator

and a harmonic eliminator. This scheme can be efficiently used

for decentralized power generation in rural and remote areas.

Index Terms-- DC-DC converter, PWM VSI, SEIG, Self

Excited Induction Generator, Solid State Controller, Voltage

Regulator

I. INTRODUCTION

N Induction motor operated as a induction generator with

terminal capacitors is known as Self excited induction

generator (SEIG) which has been a subject of

considerable research over last few decades [1-3] because of

its perception as the simplest energy conversion device to

produce electricity in off-grid, stand alone mode using

different types of prime movers and employing different

conventional and renewable energy resources such as oil, bio-

fuel, wind and small hydro. For its simplicity, ruggedness,

robustness and small size per generated kilowatt the SEIG is

favored for small wind and hydro power plants. However, in

reality SEIG has not yet been able to penetrate the consumer

market to replace other traditional systems in a substantive

way and to exploit its advantages. This dictates efforts to be

made to bridge the gap and to take the concept to a stage of

S S Murthy is with the Department of Electrical Engineering, Indian

Institute of Technology, Delhi, New Delhi -110016 , India (e-mail:

ssmurthy@iitd.ac.in).

Rajesh Kr. Ahuja, Research Scholar is with the Department of Electrical

Engineering, Indian Institute of Technology, Delhi, New Delhi -110016 ,

India (e-mail: rajeshkrahuja@gmail.com).

successful deployment in the field. There are many practical

factors to be addressed in this direction.

The use of modern control techniques discussed in [9-10]

involves power electronic converters to improve the

performance of the system but overall control structure is

complex in nature. Based on the instantaneous reactive power

theory, using a capacitor bank and an inverter simultaneously

without any mechanical sensor in the rotor has been proposed

[4]. However, there is a poor ac voltage regulation problem

especially at low speeds. An Induction machine with a diode

bridge rectifier and a PWM converter for a stand alone power

generation based on the rotor field orientation has been

proposed to control the output voltage of diode bridge rectifier

[5]. This system is having a serious voltage harmonic problem.

This paper attempts to provide a new and simple excitation

control strategy for Self excited induction generator to mitigate

the above problems. The Proposed scheme uses a capacitor

bank, PWM Voltage Source Inverter, DC-DC Boost Converter

and diode bridge Rectifier simultaneously. The load voltage is

regulated irrespective of rotor speed and changing loads. The

control scheme does not require any information of rotor

position and speed which eliminates need for mechanical

sensors and reducing overall cost.

II. PRINCIPAL OF OPERATION OF CIRCUIT

The Proposed overall system block diagram is shown in

Fig.1.The system uses PWM Voltage Source Inverter, DC-DC

Boost Converter and Rectifier with SEIG .The consumer load

is connected across VSI through LC filter. In First stage the

AC power developed by SEIG is converted into DC power

through diode bridge rectifier, at second stage this DC power

is fed to Boost DC-DC converter and finally the output of DC-

DC converter is converted into AC power by three phase VSI

which is supplied to three phase load. So this is AC-DC-DC-

AC power conversion scheme. The Reactive Power required

by Induction Generator and Load is supplied by VSI and three

phase delta connected excitation capacitors connected across

SEIG Terminals. At starting the value of excitation capacitors

is so chosen to get slightly more than rated voltage across

terminals of SEIG at no load.

The output of the SEIG is directly connected to the three

phase full bridge diode rectifier and the rectified bridge output

is connected to the Boost DC-DC converter. The developed

AC Power at SEIG terminal is fed to three phase diode

A Novel Solid State Voltage Controller of Three

Phase Self Excited Induction Generator for Decentralized Power Generation

S S Murthy, Senior Member, IEEE, and Rajesh Kr. Ahuja, Student Member, IEEE

A

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rectifier. As the terminal voltage of SEIG drops from no load

to full load so the dc voltage at the output of uncontrolled

rectifier varies accordingly .This DC voltage is the input of

Boost DC-DC converter. At all loads Boost DC-DC converter

maintains almost constant voltage at the input terminals of

VSI.

Stator

Induction

Generator

Vdc

PWM Current

Controller +_

PI

Controller

Vref

Gate

Pulses

IL

Sine Triangle

PWM Controller

Balanced

Three

Phase

Load

S1

S4 S6 S2

S3 S5

VSI

S1-S6

RECTIFIER DC-DC

CONVERTER

Prime

mover

C1 C2

BOOST

DC-DCCONVERTER

abc to dqo

trancformation +

Vd,Vq

ref

_ PI

Controller

hypot

modulation

index

Vabc

load

Voltagr Regulator

Three

Phase

Delta Connected

Capacitor

Fig. 1. Schematic diagram of proposed system

The VSI is designed to operate by Sine Triangle PWM gate

pulses for 415 V at 50 Hz. As the input voltage to VSI is

almost constant from no load to full load which gives constant

voltage and frequency at all loads. For more reliable operation

modulation index control is also used. With the above solid

state controller voltage and frequency at load terminals

remains nearly constant from no load to full load as desired for

stand alone operation.

III. MODELING OF SELF EXCITED INDUCTION GENERATOR

Given that the SEIG’s main drawbacks are its widely

varying voltage and frequency, studying the machines transient

behavior using its dynamic model is essential for designing

suitable control system. Fig. 2 shows the d-q model of

induction machine with excitation capacitors feeding R-L load,

with usual notations.

Considering the inductive load current and excitation

capacitor voltages as state variable it is seen that

s c l

q q qi i i= + (1)

s c l

d d di i i= + (2)

c s

q qi Cpv= (3)

c s

d di Cpv= (4)

( )s l l l

q qv i R L p= + (5)

( )s l l l

d dv i R L p= + (6)

Fig. 2 d-q model of induction machine with excitation capacitors and load

Combining the above equations in the standard d-q model

equation[8] of the induction machine we get the dynamic

model of a SEIG complete with excitation capacitors and R-L

load as:

( ) 0 0 0 0 0 0

0 ( ) 0 0 0 0 0

0 ( ) 0 0 0 0

0 ( ) 0 0 0 0

1 0 0 0 0 0 0

0 1 0 0 0 0 0

0 0 0 0 0 1 0 ( ) 0

0 0 0 0 0 0 1 0 (

s s s srq

s s s srd

sr r sr r r r r

r sr sr r r r r

l

q

l

d

l l

l l

v R Lp M p

v R LP M p

M p k M R Lp k L

k M M p k L R Lp

i Cp

i Cp

R Lp

R Lp

ω ωω ω

+

+ +

− − + = − − − +

− +

s

q

s

d

r

q

r

d

c

q

c

d

l

q

l

d

i

i

i

i

v

v

i

i

(7)

IV. MODELING OF VSI

DC link Capacitor Voltage is governed by the equation

dc dcdV I

dt C= −

(8)

Where Vdc is the capacitor voltage and Idc is the current

flowing through it as shown in figure

Using the Sine Triangle PWM switching function Vao , Vbo and Vco is

ids id

r

vds vd

r

vqs

vqr

iqs

iqr

d

iqc

iql

idc

idl

Ll

Ll

Rl

Rl

ωr

Tr

3

ao 0.5 sin( ) ( )dc cV mV wt highfrequency M N termsω ω= + ± (9)

0.5 sin( 120 ) ( )bo dc cV mV wt highfrequency M N termsω ω= − + ±o

(10)

0.5 sin( 120 ) ( )co dc cV mV wt highfrequency M N termsω ω= + + ±o

(11)

Where m = modulation index, ω = fundamental frequency in

r/s ( same as modulating frequency) and phase shift of output

depends upon the position of modulating wave

M+N is the odd integers

The Line to Line voltages generated by the VSI can be derived

as:

Vab = Vao - Vbo (12)

Vbc = Vbo - Vco (13)

Vca = Vco - Vao (14)

V. MATHEMATICAL MODEL OF THE R- L LOAD

Equations of R-L load in d-q frame are as follows:

dsds L ds L e L ds

diV R i L L i

dtω= + −

(15)

qsqs L qs L e L qs

diV R i L L i

dtω= + +

(16)

where Vds and Vqs , Ids and Iqs are respective axis voltages

and currents

VI. DESIGN OF PI CONTROLLER AND CONTROL OF BOOST DC-

DC CONVERTER

The equation in Laplace transformation of the output signal

of the PI controller is indicated by (17).

( ) Ip ( ) ( )K

I s = K +s

( )( )ref s dc sV V− (17)

where Kp , KI are the proportional and integral gain of the PI

controller. Vref (s) is Laplace transformation of reference

voltage and Vdc(s) is the Laplace transformation of dc link

voltage

The discrete form of the above equation is as follows:

( ) ( ) ( ) ( ) ( )[ ]( ) ( )p s I

p

I k I k-1 K + T K

K 1 1

ref dc

ref dc

V k V k

V k V k

−= +

− − − − (18)

where [ Vref (k) - Vdc (k) ] is the voltage error at sampling time k

, [ Vref (k-1) - Vdc (k-1) ] is the voltage error at sampling time (k-1)

and Ts is the sampling time period.

This output I(k) is compared with fixed frequency sawtooth

wave to generate the switching signal for the gate of IGBT of

Boost DC Chopper. These switching pulses maintain the

constant DC bus voltage at the input of PWM-VSI.

A. Simulation Results

The Self Excited Induction Generator with proposed AC-

DC-DC-AC power conversion scheme is extensively simulated

on computer using MATLAB SIMULINK. The proposed

system has been simulated under various load conditions to

validate the operation of solid state controller. The simulation

set up used in MATLAB SIMULINK is shown in Fig.2 and

Fig. 3 .

Fig 2. Simulink model of proposed system

Fig 3. Simulink model of VSI subsystem

Here the simulation is carried out on a 7.5 kW , 415 / 230

V, 50 Hz star / delta connected machine. It requires 3.5 KVAR

at no load and 8.8 KVAR on full load for rated voltage. In this

4

system capacitance connected across the SEIG terminals is of

3.5 KVAR as remaining KVAR is supplied by VSI.

Due to drop of SEIG terminal voltage from no load to full

load i,e 420 V to 353 V , dc voltage at the output of the

uncontrolled rectifier varies from 580 V to 450 V, which is the

input of DC-DC converter. At all loads Boost DC-DC

converter maintains almost constant voltage at the input

terminals of VSI i.e. 750 V and the VSI is designed to operate

by Sine Triangle PWM gate pulses for 415 v at 50 Hz. As the

input voltage to VSI remains almost constant from no load to

full load which gives constant voltage and frequency i,e

415V,50 Hz at all loads as shown in Fig. 4 .

B. Performance of the system feeding Linear Loads

Figs. 4 - 6 shows the performance of the proposed system

with balanced 0.8 PF load. It is observed that the voltage

building up process takes around 1.1s to attain the steady state

voltage of the SEIG.

At 1.2s ,1.5s and 2.0s a balanced delta connected load of 1

kW 0.8PF, 4.5 kW 0.8PF and 6 kW 0.8 PF respectively is

applied. At 2.5s a balanced delta connected full load of 7.5

kW, 0.8 PF is applied. Fig.4 and Fig.5 shows the load current

and load voltage transient responses for the step changes of the

load from 0 to 7.5 kW.

Fig. 4. Transient waveform of current and voltage in one phase at load

terminals

The controller is demonstrated along with load voltage, load

current and THD which shows the controller maintains almost

the constant voltage and frequency at different loads.

Figs. 7 - 9 demonstrate the performance of controller

regarding THD of load voltage and load current.

Fig. 5. Transient waveform of rotor speed, current and voltage at SEIG

terminals

Fig. 6. Transient waveform of current through boost inductor

The total harmonic distortion (THD) of load voltage and

load current observed at different loads are less than 5% as

shown in Figs. 7 - 9.

Fig. 7. Waveform and harmonic spectra of consumer load current under

the condition of balanced 0.8 pf load

5

(a)

(b)

Fig. 8. Waveform and harmonic spectra of consumer load current at different load

instants under the condition of balanced 0.8 pf load

(a)

(b) Fig. 9. Waveform and harmonic spectra of load voltage at different load

instants under the condition of balanced 0.8 pf load

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VII. CONCLUSION

A simple solid state constant voltage regulator of Three

phase Self Excited Induction Generator is proposed. Detailed

MATLAB/ SIMULINK based simulation studies have been

carried out to demonstrate the effectiveness of the control

scheme. The controller is able to maintain the constant voltage

at the consumer load terminals during step change in load. The

proposed controller structure is simple in nature and does not

require any mechanical sensors reducing the overall cost of

hardware. The voltage and current THD of load is found to be

satisfactory at all loads.

VIII. REFERENCES

[1] S. S. Murthy, O.P. Malik, and A.K. Tandon, “Analysis of self excited

induction generators,” Proc.1982 IEE, vol. 129, Pt. C, no. 6, pp. 260–

265,Nov.1982

[2] A. K. Tandon, S. S. Murthy, and G. J. Berg, “Steady state analysis of

capacitor self excited induction generator,” IEEE Trans Power

Apparatus and Systems, vol. PAS-103, no. 3, pp. 612-617, March 1984

[3] S. S. Murthy, O. P. Malik and P.Walsh, "Capacitive VAR requirements

of Self-excited Induction Generators to Achieve Desired Voltage

Regulation", IEEE Conference Record of Industrial and Commercial

Power Systems Technical Conference. Milwaukee (USA), pp124-128,

Jun. 1983.

[4] R.Leidhold , G. Garcia. M.I Valla,” Induction generator controller based

on the instantaneous reactive power theory,” IEEE Trans. Energy

Convers., vol. 17 , no. 3, pp. 368-373,sep. 2002

[5] M. Naidu and J. Walters,” A 4-kW 42-V induction machine based

automotive power generation system with a diode bridge rectifier and a

PWM inverter,” IEEE Trans. Indl. Appl., vol. 39, no. 5,pp. 1287-1293,

Sept./Oct. 2003.

[6] Bimal.K.Bose,” Modern Power Electronics and AC Drives”, Pearson

Prentice Hall , Fifth Impression, 2008

[7] Ned Mohan, Tore M. Undeland, William P. Robbins , “ Power

Electronics “, Wiley India (p) Ltd. Third Edition , 2007

[8] Howard E. Jordan, “Digital Computer Analysis of Induction Machines

in Dynamic systems” IEEE Trans. on Power Apparatus and systems,

Vol. 86, No 6, pp. 722-728. June 1967

[9] B. Singh, S.S. Murthy, S. Gupta, “Analysis and design of a STATCOM-

based voltage Regulator for self-excited induction generators” IEEE

transactions on Energy Conversion, Vol 19, N0. 4, December 2004

[10] W Luenchen, Y.H Lin, H S Gau , “ Statcom Controls for a self excited

induction generator feeding random loads.,” IEEE transactions on

Power Delivery”, Vol. 23, no.4, Oct. 2008