Induction-Generator-Based System Providing Regulated Voltage With Constant Frequency.pdf

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IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 47, NO. 4, AUGUST 2000

Induction-Generator-Based System Providing

Regulated Voltage with Constant Frequency

Enes Gonçalves Marra , Associate Member, IEEE, and José Antenor Pomilio , Member, IEEE

Abstract— The electrical characteristics of an isolated induc-

tion-generator-based system are improved through the association

with a voltage-source pulsewidth modulation (PWM) inverter. The

electronic converter allows the achievement of a better system be-

havior in many aspects: voltage regulation, frequency stabilization,

and reactive power compensation. The system operation strategy

consists of maintaining constant synchronous frequency at the in-

duction generator via an association with a PWM inverter. The

system power balance and the generator voltage regulation may be

accomplished by two different means: through the rotor speed reg-

ulation, or by sending part of the energy stored in the inverter dc

side to the grid through a single-phase line, in case the rotor speed

is not regulated and a single-phase grid connection is available. The

obtained results demonstrated the system is stable, robust, and an

effective source of regulated three-phase voltages.

Index Terms— Energy conversion, energy resources, induction

generator, pulsewidth modulation inverter.

Fig. 1.

Capacitor-excited IG system, isolated from the utility grid.

eration systems, such as low-head microhydroelectric plants and

fuel engine driven generation systems. Two distinct structures

are presented. In one of these structures, the generator’s shaft

speed is regulated. The other structure does not comprise speed

governor, and the system acts as cogenerator, sending energy

to a single-phase grid, as a strategy to control the IG terminal

voltage.

The cogenerator structure is appropriate to be employed in

areas such as light manufacturing or agricultural areas where

electric power available is only single phase. Customers in these

areas may request three-phase power from the utility and find it

is uneconomical for the utility to meet a relatively small three-

phase need [3], [4].

Both proposed systems are intended to be sources of regu-

lated voltage with constant frequency, whose energy quality

is good enough to feed sensitive loads, such as micropro-

cessor-controlled ones.

I. I NTRODUCTION

I T IS FREQUENTLY stated that cage rotor induction ma-

chines (IMs) are robust, inexpensive compared with dc and

wound-rotor synchronous machines, require little maintenance,

and have high power-weight ratio (W/kg). Despite these favor-

able features, IM’s are hardly employed as generators due to

their unsatisfactory voltage regulation and frequency variation,

even when driven under constant speed and feeding loads which

consume active power [1], [2].

Wound-rotor synchronous generators are reliable suppliers

of regulated three-phase constant frequency voltage, provided

the dynamic response of the speed governor is able to main-

tain constant rotor velocity during the occurrence of load power

variations. Nevertheless, they are expensive machines due to

the maintenance required by the excitation system, which con-

tains slip rings, brushes, or rotating rectifiers, in addition to field

current control circuits. Therefore, a cost-effective and techni-

cally reliable alternative to wound-rotor synchronous generators

would be welcome.

The aim of this investigation is to propose an induction gen-

erator (IG) application as an alternative to wound-rotor syn-

chronous generators to be employed in low-power isolated gen-

II. I SOLATED C APACITOR -E XCITED IG S YSTEM

Fig. 1 presents a system in which a capacitor-excited IG op-

erates isolated from the utility grid. In this circumstance, the ac-

tive power of the ac load affects considerably the amplitude and

the frequency of the voltage at the IG terminals. In this case, the

synchronous frequency is not constant, even if the rotor speed

is kept constant by the action of a speed governor.

Assuming that the mechanical, electrical, and magnetic losses

are negligible, the electric power converted by the generator is

given by the product between the rotor speed and the generator

torque.

Supposing the rotor speed is invariable, the increase of the

active power required by the ac load yields a drop in the stator

frequency, as it is the only possible way the IG can raise its rotor

slip frequency and consequently elevate the torque, so that it is

able to suit the load power demand.

Fig. 2 illustrates qualitatively a situation in which the induc-

tion generator was feeding a unity power-factor load so that the

steady-state operation point is “A.” The synchronous frequency

Manuscript received February 12, 1999; revised September 20, 1999. Ab-

stract published on the Internet April 21, 2000. This work was supported by

Coordenação para o Aperfeiçoamento de Pessoal de Ensino Superior (CAPES)

and Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP).

E. Gonçalves Marra is with the School of Electrical Engineering, Federal

University of Goiás, 74605-220 Goiânia, Brazil (e-mail: enes@ieee.org).

J. Antenor Pomilio is with the School of Electrical and Computer Engi-

neering, State University of Campinas, 13081-970 Campinas, Brazil (e-mail:

antenor@dsce.fee.unicamp.br ).

Publisher Item Identifier S 0278-0046(00)06814-3.

MARRA AND POMILIO: IG-BASED SYSTEM

909

in the magnetization characteristic and in the excitation bank ca-

pacitive reactance.

The capacitance could be increased even more, in order to

recover the capacitive reactance . In this case, the slope of

the capacitor-bank voltage characteristic will return to its pre-

vious value, however, the steady-state operation point in the

magnetization characteristic will now be “ ” instead of “A,”

as the frequency remains . The new operation point at the

torque characteristic (Fig. 2) would depend on the behavior of

the ac load under voltage variations.

It should be highlighted that the voltage drops at the stator and

rotor resistance and leakage reactances are not the main cause

of the poor voltage and frequency regulation in the isolated IG.

The fundamental factor that affects the IG voltage regulation is

the influence of the frequency on the generator magnetization

characteristic.

Note that the voltage and frequency variations presented pre-

viously were caused by increments made exclusively in the ac

load active power. In case the ac load inductive reactive power

increases, the voltage reduction would be even higher, due to the

demand of capacitive reactive power from the excitation bank to

compensate for that.

Reductions at the rotor speed as a result of torque elevations,

due to a nonregulated shaft speed, would degenerate voltage and

frequency even more.

Substantial efforts have been made to overcome the poor

voltage regulation of the isolated induction generator under load

active and reactive power variations [5]. These efforts have been

concentrated on different types of voltage regulators acting

as volt–ampere-reactive controllers, based on series-shunt

capacitor compounds [1], [5]–[8], switched discrete capacitor

banks [9]–[11], thyristor-switched inductors [12], or saturated

reactors [13], [14]. Such approaches rely on contactors, relays,

or semiconductor switches.

Although the methodologies mentioned attain valuable im-

provement in voltage regulation, they have solved the problem

only partially, as the frequency is yet variable. Besides that, the

generator still experiences variation in its magnetization char-

acteristic with the frequency, which leads to the requirement of

a wide range of capacitance values at the excitation bank. How-

ever, an excessive increase in the capacitance would deeply

saturate the generator, leading to voltage waveform distortions.

This analysis leads to the conception of a strategy which

maintains constant frequency at the IG stator terminals and,

simultaneously, guarantees reactive power both to magnetize

the generator and to compensate for the ac load demand.

The constant-frequency approach ensures that the

steady-state operation of the IG will take place following

only one torque and magnetizing characteristic curves, both

regarding the constant stator synchronous frequency.

A generation system based on this modus operandi has to

comprise three indispensable parts, namely, the induction gen-

erator itself, a voltage regulator, and a device which fixes the

frequency, magnetizes the generator, and compensates for the

ac load reactive power requisites.

It is important to mention that a constant-frequency system

like this is suitable to work driven by energy sources which

cause relatively narrow ranges of speed variations, such as

Fig. 2. Torque-speed characteristics of the induction generator, for different

synchronous frequencies ( f >f ).

Fig. 3. Magnetization characteristics of the induction generator, for different

synchronous frequencies ( f >f ).

( ) of the stator magnetomotive force (MMF) is equal to in

point “A.” The point “A” of the IG torque characteristic (Fig. 2)

corresponds to an equivalent steady-state point “A” in the gen-

erator magnetization characteristic, as shown in Fig. 3.

When the active power required by the ac load increases, the

synchronous frequency decreases from to , producing a

torque increment to match the higher power demand. Thus, the

new stable steady-state operation point is steered to point “B.”

Notice that the speed governor is supposed to maintain the rotor

speed constant.

The frequency drop to reduces the magnetization-charac-

teristic voltage ( ) in the same proportion, assuming that the

air-gap flux is kept constant, i.e., is constant.

In addition to the change in the magnetization characteristic,

the frequency reduction affects the capacitive reactance of the

excitation bank (

), according to (1).

and

are the

capacitive reactance correspondent to the frequencies

and

, respectively,

(1)

Altogether, the resulting effect of increasing the ac load ac-

tive power is the IG terminal-voltage reduction, due to changes

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IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 47, NO. 4, AUGUST 2000

After startup, the IG provides the energy required to charge

and to supply the losses. The PWM inverter control circuit

is also fed by , by means of a forward dc–dc converter.

The fundamental frequency of the PWM inverter output

voltage is maintained constant at 60 Hz, yielding a constant-fre-

quency busbar at the IG leads.

The IG terminal voltage waveform is sinusoidal due to the ac-

tion of the

filter, which attenuates the high-frequency

voltage components.

The capacitance is rated to match the IG self-excitation

requisites during the startup. After the definition of , is

rated to set the filter cutoff frequency ( ) (for example, one

decade below the switching frequency of the PWM inverter) as

Fig. 4.

Controlled-speed-based system configuration.

(2)

microhydroelectric plants and fuel engine plants. Therefore,

this approach is not adequate for systems where the speed

variation is the basis to achieve profitable energy conversion,

such as wind systems.

The speed governor role is to set rotor speed so that the IG

produces enough power to supply the ac loads, the system

losses, and the PWM inverter control circuits, as well as to

keep properly charged.

In this system, the rotor speed is variable and has to be set

to suit the IG power requirements, conversely to synchronous

generator systems where the rotor speed is kept constant. Conse-

quently, the governor speed reference value (

III. D ESCRIPTION OF THE P ROPOSED S YSTEMS

Two distinct structures which are able to produce balanced

three-phase regulated voltages with constant frequency are pre-

sented. Both structures employ induction generator associated

with voltage-fed pulsewidth modulation (PWM) inverters, in

order to establish constant frequency at the IG stator ends.

One of the proposed configurations does not include a speed

governor, as the elimination of the speed control yields a quite

significant economy in the overall cost of the system. In this

case, the IG voltage regulation is attained by consuming all ex-

ceeding power, as the speed-governor absence does not allow

control of the amount of the generated power. In this case, the

excess of energy, which is not consumed by the ac load, is sent

to the utility grid via a single-phase line. This configuration is

able to be applied in sites where there is availability of enough

hydraulic energy source and a single-phase line connection to

grid.

The other proposed configuration employs the speed gov-

ernor, in order to control the amount of the generated energy.

This structure is more suitable to be applied in small fuel-en-

gine-driven generation systems.

The main goal of both proposed configurations is to feed

the ac loads with satisfactory energy quality, which means pro-

viding three-phase balanced voltages, with constant frequency,

sinusoidal waveform, and regulated amplitude.

) is made vari-

able in the present system.

In case the electric power produced by the IG is not enough

to match the consumed power, the PWM inverter dc capacitor

( ) is the only source from where the ac loads can take power.

Thus, the consumption of part of the energy stored in would

produce a decrease in the dc-link voltage ( ) up to the system

collapse. Similarly, an excess of generated power with relation

to the ac load power would be stored in , causing the unlim-

ited increase of . Therefore, is a suitable parameter to

indicate the system power balance and it can be employed as the

control variable of the speed governor. Thus, the speed-governor

control operates to maintain tracking a reference value, in

order to attain the system’s power balance.

Assuming the synchronous frequency at the induction gen-

erator stator is kept constant by the PWM inverter action, the

speed governor affects the voltage amplitude as well as the

generator terminal voltage at the proposed system (Fig. 4).

As acts as a voltage source to the PWM inverter, a good

voltage regulation is obtained at the IG leads by keeping in-

variable, since the only difference between the voltages at the IG

and at the PWM inverter ac terminals is the voltage drop at the

series inductance ( ). Provided is assessed to filter voltage

components at the switching frequency and higher frequencies,

the voltage drop at 60 Hz is quite small. Hence, a good voltage

regulation and the system power balance are both achieved when

the speed governor maintains constant .

Considering the PWM inverter allows bidirectional power

flow, the capability to compensate for reactive power is a natural

consequence of the system configuration and operation mode.

Therefore, when is kept constant, the generator voltage is

regulated, even when feeding dominantly reactive loads. Nev-

ertheless, the PWM inverter should be properly rated to support

the reactive power load flow.

A. Controlled-Speed-Based System

The controlled-speed-based system configuration is in-

herently composed of an induction generator excited by a

three-phase capacitor bank ( ), connected to the ac side of

a voltage-fed PWM inverter through series inductances ( ).

The rotor shaft speed is controlled by a speed governor, as

presented in Fig. 4.

The system is isolated from grid and the starting is accom-

plished from the self-excitation produced by the interaction be-

tween the residual flux voltage and the ac capacitive bank (

MARRA AND POMILIO: IG-BASED SYSTEM

911

This kind of system is suitable to be employed mainly in mi-

crohydroelectric plants whose rated power is lower than 50 kW,

such as rural sites where there are both enough hydraulic energy

source and a single-phase grid connection available. This at-

tributes are normally found in the north-central region of Brazil

and other Latin America rural areas.

The system rated power is limited by the availability of

low-cost turbines suitable to operate with nonregulated shaft.

Furthermore, the single-phase line has to be rated to receive all

the generated power if necessary.

IV. S IMULATION R ESULTS

The controlled-speed-based system simulation was carried

out for a 50-hp induction generator, assisted by the PSpice pro-

gram, using a three-stationary-axes model ( model) to rep-

resent the induction machine. This system experiences a more

critical dynamic response than the ungoverned-speed-based

system, due to the closed-loop speed-control dynamics in-

volved. Thus, the controlled-speed-based system simulation is

a more suitable method to probe the system feasibility.

The 50-hp cage-rotor induction machine parameters referred

to the stator are presented in Table I [15], where , , ,

and are the stator and rotor windings respective resistances

and leakage inductances, is the air-gap magnetization induc-

tance, and is the rotor inertia.

The system was simulated using proportional constant equal

to 0.5 and integral constant equal to 5 ms ( k ,

k , and nF, in Fig. 4), the inverter switching

frequency was 5 kHz, mH, F, and

mF. The dc-link reference voltage ( ) was set to 650 V.

Fig. 6 presents the ac voltage at the IG terminals, the rotor

speed in radians per second, and the ac-load line current ob-

tained from simulation of an ac-load step transient connection.

After the startup process and an interval running under no

load, the system was submitted to an ac-load step at 800 ms.

The ac load was composed of a -connected resistance bank,

rated at about 40% of the generator rated power. The ac load

was kept connected up to 1.5 s, when the system returned to the

previous no-load condition. It was verified that the system was

able to maintain the generator terminal voltage during a severe

load transient. The closed-loop speed control acted in order to

adjust the rotor speed so that the generator could suit the ac-load

power requirements.

As the prime mover is not able to produce negative torque to

brake the rotor, a dc-link resistance was employed to avoid over-

voltages during the occurrence of disconnections of ac loads

rated at significant power values, similarly to what is done in

motor drives. A 5- resistance was then set to be switched on

when the DC voltage exceeds 670 V and, once connected, to

be switched off when the dc voltage returns to 650 V. Since the

purpose of the dc-link resistance is to avoid overvoltages under

transient episodes, this does not operate under normal circum-

stances, when the nondissipative speed control is intended to

maintain constant dc voltage.

It was observed that the system simulation demanded a

long computation time due to the concurrent high switching

frequency (5 kHz) and mechanical time constants involved.

Fig. 5.

Variable-speed-based system configuration.

The energy stored in is vital to improve the system’s ca-

pability to support extreme transient conditions, such as induc-

tion motor startups and high-power load steps. As a result, the

system’s transient behavior becomes more robust when

suitably rated.

Although Fig. 4 presents a proportional–integral (PI) gain for

the dc-voltage-loop error amplifier, other compensators can be

employed as an alternative to improve the system’s phase and

gain margins.

B. Ungoverned-Speed-Based System

Similarly to the controlled-speed-based system, the un-

governed-speed-based system also relies on a voltage-fed

PWM inverter to improve the induction generator electrical

characteristics, as presented in Fig. 5. This system does not

include a speed governor, hence, the generated power is

fully determined by the prime mover and the energy source

availability.

In this case, the system startup can be accomplished either

from the self-excitation produced by the rotor residual flux or

charging , with energy obtained from the utility grid via a

single-phase diode rectifier in series with a resistor ( ), con-

nected in parallel with the current inverter, as shown in Fig. 5.

Since there is no direct control upon the amount of the gener-

ated power, the control is accomplished by means of sending

the excess of energy, which is not consumed by the ac load, to

the utility grid through the current inverter and a single-phase

line.

This sort of system is intended to be driven by nonregu-

lated-shaft-speed hydraulic turbines. Therefore, it is necessary

to guarantee the existence of a coordination between the IG and

the turbine torque characteristics, so that the shaft speed does

not cause a rotor slip frequency higher than the rated value, at

the point relative to the maximum generated power.

The PWM inverter dc side is asynchronously connected to the

single-phase utility grid through a current inverter (CI) (Fig. 5).

Thus, the system works as a cogenerator for the utility.

A buck dc–dc converter operates as a high-power-factor reg-

ulator, ensuring that the current sent to grid is properly phased

with the utility terminal voltage, and attains practically sinu-

soidal waveform.

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IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 47, NO. 4, AUGUST 2000

TABLE I

I NDUCTION M ACHINE P ARAMETERS

Fig. 7.

(a) PWM-inverter line voltage. (b) IG terminal line voltage.

Fig. 6.

(a) IG terminal line voltage. (b) Rotor speed. (c) AC-load line curren.

V. E XPERIMENTAL R ESULTS

Both controlled-speed and ungoverned-speed IG-based

systems previously described were implemented, employing

a three-phase 1/2-hp induction machine with four poles, and

rated voltage of 220 V in delta connection. Moreover, the PWM

inverter switching frequency was 5 kHz, uF , and

mH, while is rated to produce satisfactory dynamic

behavior during both steady-state and transient conditions.

A. Controlled-Speed-Based System Results

The controlled-speed-based system was set up experimen-

tally, employing a dc motor as the system prime mover. The

dc motor was independently excited and driven by a controlled

rectifier. The system also attained

Fig. 8. Experimental relation between the IG rotor speed (r/min) and the

ac-load power {pu).

F at the PWM

inverter dc side.

Fig. 7 shows the IG and the PWM inverter terminal line volt-

ages in steady state. Observe that the filter was ef-

fective in preventing the IG line voltage from the presence of

high-frequency components.

The variation of the IG rotor speed with the ac-load active

power is indicated in Fig. 8. Notice that the speed governor

raises the rotor speed, as the ac load power increases, causing

an augment in the rotor slip frequency, so that more power is

produced by the IG to suit the ac load demand.

Fig. 9 indicates the startup of an eight-pole induction machine

whose rated values are 220 V (delta connection) and 70% of the

IG rated power. The induction motor was directly connected to

the IG leads at the startup. During the motor starting, part of

the energy stored in is employed in the motor acceleration.

This causes a voltage sag which is subsequently eliminated due

to the speed controller action ( control).

The maximum voltage sag allowed at the ac loads should be

a decisive guideline to assess the rated value of

Fig. 9. (a) IG terminal line voltage and (b) induction motor line current, during

the motor startup.

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