8/13/2019 04207467

1/3

546 IEEE TRANSACTIONS ON ENERGY CONVERSION, VOL. 22, NO. 2, JUNE 2007

An Electronic Load Controller for the Self-Excited Induction Generator

Juan M. Ramirez, Member, IEEE, and Emmanuel Torres M, Student Member, IEEE

AbstractThis letter is aimed at designing an electronic loadcontroller (ELC) for a self-excited induction generator (SEIG) ona stand alone application. With constant input power and fixedvalue of capacitance, the induced voltage varies with the appliedload. This paper proposesan ELCscheme whose control strategy issimple and reliable. Antiparallel insulated-gate bipolar transistor(IGBT) switches are used to control the dump load connection anddisconnection. The proposed ELC has been tested under severalcritical situations, providing an excellent voltage and frequencyregulation.

IndexTermsAC motors, control equipment, electric machines,induction generators, insulated gate bipolar transistors (IGBTs).

I. INTRODUCTION

THE INCREASING rate of the depletion of conventional

energy sources has given rise to an increased emphasis on

renewable energy sources such as wind, mini/microhydro, etc.

In renewable energy applications of low and medium power (up

to 100 kW), the induction generator offers several advantages.

In low power rating applications, uncontrolled turbines are

preferred, as they maintain the input hydropower constant; thus,

requiring the generator output power to be held constant while

varying consumer loads. A dump load in shunt with the con-

sumer load is necessary to keep the electrical load constant at the

generator terminals; thus, the self-excited induction generator

(SEIG) can operate with constant input power.

In this letter, a novel electronic load controller for SEIG is

proposed.

II. SELF-EXCITEDINDUCTIONGENERATORMODELING

The used SEIG in d-qcoordinates is represented in Fig. 1 [1].

The induction machine employed as SEIG in this letter, is a

three-phase squirrel cage rotor with the specifications2 kW,

120/208 V, 15.2/8.8 A, 60 Hz, and 4 polesand the main

parametersrs= 0.6 , rr = 1.06 , Lls= Llr = 6.4 mH,LM = 51.3mH.

When the induction machine is used as SEIG the magnetizing

saturation is the main factor in the voltage build up and stabiliza-tion dynamics. Here, the saturation is estimated by driving the

induction machine at synchronous speed, and taking measure-

ments when the applied voltage is varied from 0% to 120% of

the rated voltage at rated frequency. The corresponding adjusted

polynomial is included into the model of the SEIG to take into

account the saturation.

Manuscript received November 7, 2006; revised October 30, 2006. This letterwas supported by Consejo National De Ciencia Y Technologia (CONACYT)under Grant 43478. Paper no. PESL-00089-2006.

The authors are with Cinvestav, Guadalajara, Mexico (e-mail: jramirez@gdl.cinvestav.mx; etorres@gdl.cinvestav.mx).

Digital Object Identifier 10.1109/TEC.2006.895392

Fig. 1. Model of the SEIG:d-axis. There is a similar model for the q-axis.

The capacitor value is selected from the no-load condition

(switchs is open in Fig. 1), and the self-excitation currents are

obtained as in [1], from which the stator current becomes

iqs = U(s)

As6 + Bs5 + Ds4 + Es3 + F s2 + Gs + H (1)

where s is the Laplace operator, U(s) denotes the numeratorthat is independent on the capacitors initial condition and on

the machine parameters. When the denominator of (1) is set to

zero

As6 + Bs5 + Ds4 + Es3 + F s2 + Gs + H= 0. (2)

If any of the roots has a positive real part then there will be self-

excitation [2]. This way, it is possible to compute the minimum

speed and capacitance for self-excitation for the no-load case.

When the SEIG is loaded (switch s is closed), a similar analysis

to compute the roots with positive real part is accomplished.

Once the suitable capacitance and speed values for self-

excitation are selected, the roots with positive real part can

be used to calculate theLMvalue which gives a root having a

zero real part (steady-state condition). The computed LMvalue

and the relation between the magnetizing inductance LM and

phase voltage Vph are used to compute the terminal voltage,and the imaginary part of the root gives the frequency of self-

excitation [2].

III. SYSTEMDESCRIPTION

The block diagram of the proposed SEIG-electronic load con-

troller (ELC) for load voltage regulation is illustrated in Fig. 2.

The SEIG-ELC is constituted by an induction machine driven by

a prime motor, a three-phase capacitor bank, and an electronic

load controller [1][3].

The proposed ELC is a chopper circuit per-phase (Sa, Sb,

andS cswitches) in series with the dump load, and it becomes

simpler than that in [3], Fig. 3.Rdrepresents the dump load.

0885-8969/$25.00 2007 IEEE

8/13/2019 04207467

2/3

R AMIREZ AND TOR RES M: AN ELECTRONIC LOAD CON TROLLER FOR THE SELF-EXCITED INDU CTION GENERATOR 5 47

Fig. 2. Schematic block diagram of the proposed SEIG-ELC.

Fig. 3. Switch configuration per phase.

The induced voltage vmt and theload current imL are measured.

vmt is compared with the reference voltagevref. Ifvmt =vref,

the control does not take any action. Ifvmt =vref, the controlsystem performs the following actions. Using vmt and i

mL, the

instantaneous load resistance is estimated as

ReL= Vmt

imL(3)

where the superscripts m and e denote the measured and esti-

mated variables, respectively. The apparent resistance value that

ELC must provide to the system is calculated by

Red= ReLReL

RL 1

. (4)

The current through the ELC is computed using (5)

ied= Vref

Red. (5)

This current iedis fed into a proportional controller whose output

is the modulating signal i0, which is compared with a triangular

carrier signal ip to obtain the insulated-gate bipolar transis-

tors (IGBTs) gate signals. The frequency ofip is 500 Hz. The

Fig. 4. Dynamic response for unbalanced conditions, phasec.

switching logic is defined as follows:

Ifi0 > ip, the St= 1 else when i0 < ip, the St= 0 (6)

where St represents the state of Sa, Sb, or Sc. When St= 0,the IGBT is open and when St = 1, the IGBT is closed. Suchstrategy takes the induced voltage to the reference value to keep

it constant under any load condition.

IV. EXAMPLE

As the control strategy is per phase, the proposed ELC can

operate under unbalanced conditions without caring for the un-

balanced degree. Voltage and frequency remain constant and the

system operates satisfactorily. Fig. 4 shows the operation of the

8/13/2019 04207467

3/3