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Electric Power Systems Research 78 (2008) 276–289

Review

Three-phase, power quality improvement ac/dc converters

Abdul Hamid Bhat ∗, Pramod Agarwal

Electrical Engineering Department, Indian Institute of Technology Roorkee, India

Received 17 August 2006; received in revised form 8 February 2007; accepted 8 February 2007

Available online 19 March 2007

Abstract

Thegeneration of harmonicsand reactive power flow in a powersystem is greatly influenced by thewidespread useof power electronic converters

in addition to other sources of harmonics and reactive power. The design, development and successful application of single-phase, power quality

improvement converters in domestic, commercial and industrial environment has made possible the design and development of three-phase, powerquality improvement converters and their widespread use in different applications. This paper deals with a comprehensive review of three-phase,

power quality improvement converter configurations, control approaches, performance on supply and load sides in terms of input power factor,

THD and well-regulated, reduced-rippled dc output, power rating, cost and selection for specific applications. It also provides state-of-the-art of

power quality improvement converter technology to researchers, designers and engineersworking on three-phase, switched-mode ac–dc converters.

© 2007 Elsevier B.V. All rights reserved.

Keywords: Power quality improvement converters; ac/dc converters; Multilevel converters; Control techniques

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 276

2. Classifications of three-phase power quality improvement converters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2773. Three-phase, unidirectional and bi-directional boost converters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 277

4. Three-phase, unidirectional and bi-directional buck converters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 279

5. Three-phase unidirectional and bi-directional buck–boost converters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 281

6. Three-phase, unidirectional and bi-directional multilevel converters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 282

7. Control strategies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 284

8. Selection criteria of three-phase power quality improvement converters for different applications. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 286

9. State-of-the-art of three-phase power quality improvement converter technology. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 286

10. Comparative features of three-phase, POWER quality improvement converters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 286

11. Conclusions and future development . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 288

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 288

1. Introduction

AC/DC power converters are extensively used in various

applications like power supplies, dc motor drives, front-end

converters in adjustable-speed ac drives, HVDC transmis-

sion, SMPS, utility interface with non-conventional energy

sources, in process technology like welding, power supplies for

∗ Corresponding author. Tel.: +91 9412919556; fax: +91 133228523.

E-mail address: bhat [email protected] (A.H. Bhat).

telecommunications systems, aerospace, military environment

and so on. Traditionally, ac–dc power conversion has been

dominated by diode or phase-controlled rectifiers which act as

non-linear loads on the power systems and draw input currents

which are rich in harmonics and have poor supply power factor,

thus creating the power quality problem for the power distribu-

tion network and for other electrical systems in the vicinity of

rectifier. The other associated problems with these converters

include:

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A.H. Bhat, P. Agarwal / Electric Power Systems Research 78 (2008) 276–289 277

(i) large reactive power drawn by the rectifiers from the

power system which requires that the distribution equip-

ment handle large power, thus increasing its volt-ampere

ratings;

(ii) voltage drops at the buses;

(iii) higher input current harmonics resulting in the distorted

line current which tends to distort the line voltage wave-

form. This often creates problems in the reliable operation

of sensitive equipment operating on the same bus;

(iv) increased losses in the equipments (due to harmonics)

such as transformers and motors connected to the utility;

(v) electromagnetic interference with the nearby communi-

cations circuits;

(vi) blown-fuses on power factor correction capacitors due

to high voltages and currents from resonance with line

impedance and capacitor bank failures;

(vii) nuisance operation of protective devices including false

tripping of relays;

(viii) damaging dielectric heating in cables and so on.

Thus the reliable operation of a power system is greatly influ-

enced by the widespread use of ac/dc converters in different

applications. In view of this, regulatory agencies have issued

several strict standards such as IEEE-519, IEC-1000, and IEC

61000-3-2, etc. [8,18] and are being enforced on the consumers.

IEEE-519-1992 Recommended Practices and Requirements for

Harmonic control in Electrical Power Systems provide guide-

lines for determining what are the acceptable limits. To meet

these standards and improve the power quality, use of pas-

sive filters, active filters and hybrid filters has been made along

with conventional rectifiers, especially in high power rating and

already existing installations.The fixedcompensation, bulkinessof the components, series and parallel resonance phenomena

in passive filters and large rating and complexity of active

power filters are the greatest drawbacks with these compensa-

tion techniques. To overcome these drawbacks, power quality

improvement ac/dc converters are included as an inherent part of

the ac–dc conversion system which produces higher efficiency,

reduced size, and well regulated dc output. The output volt-

age in these converters is regulated even under the fluctuations

of the input voltage and changes in the load. Since the con-

verter output voltage remains constant even under the supply

voltage fluctuations, the power quality improvement convert-

ers can solve the supply brownout problem. This new breed of

rectifiers has been made possible mainly because of the use of modern solid-state, self-commutating power semi-conducting

devices such as power MOSFETs, IGBTs, GTOs, etc. and are

specifically known as switched mode rectifiers (SMRs), power

factor correction converters (PFCs), high power factor con-

verters (HPFCs) and PWM rectifiers. The single-phase power

quality improvement converters are already in common practice

and the industrial applications of three-phase converters have

also emerged. It can be said that for medium to high power

applications, the input rectifier is fed from a three-phase ac

source.

This paper attempts to give a comprehensive review of

three-phase power quality improvement converters with spe-

cial attention paid to the modern state-of-the-art power factor

correction converters and their control strategies.

2. Classifications of three-phase power quality

improvement converters

Three-phase power quality improvement converters can be

classified on the basis of:

A. Converter topology as boost, buck, buck–boost and multi-

level converters with unidirectional and bi-directional power

flow [49].

B. Type of converter used as unidirectional and bi-directional

converters.

Three-phase unidirectional power factor correction convert-

ers (PFCs) are realized usinga three-phase diodebridge followed

by step-down chopper, step-up chopper, step-down/up chopper,

isolated, forward, flyback, push-pull, half-bridge, full-bridge,

SEPIC, Cuk, Zeta and multilevel converters. These convertersare implemented using single-stage conversion which reduces

the size, weight, volume, losses and cost. A high-frequency iso-

lation transformer offers reduced size, weight, cost, appropriate

voltage matching and isolation. On the other hand, three-phase

bi-directional PFCsconsist of basicconverterssuch as push-pull,

half-bridge, voltage source inverter (VSI) topology or current

source inverter (CSI) topology. Four-quadrant three-phase ac–dc

power converters are normally implemented using matrix con-

verters.

3. Three-phase, unidirectional and bi-directional boost

converters

In case of three-phase boost converters [1,4,5,7,19,23,25,

34,49], the output voltage is greater than the peak input volt-

age. Unlike a single-phase boost converter, the voltage across

the output capacitor does not have low-frequency ripple in bal-

anced conditions. Thus a wide bandwidth voltage feedback loop

can be used resulting in fast voltage control without distorting

the input current references.

Fig. 1 shows some of the topologies of three-phase unidi-

rectional boost converters [19]. High power-factor can be easily

obtained when three-phase unidirectional boost converters are

operated in discontinuous conduction mode (DCM) with con-

stant duty cycles [23]. This is because the basic types of dc–dcconverters, when operating in DCM, have self-power factor cor-

rection (PFC) property, that is, if these converters are connected

to the rectified ac line, they have the capability to give higher

power factor by the nature of their topologies. The peak of

the supply side inductor current is sampling the line-voltage

automatically, giving the boost converter the self-PFC property

because no control loop is required from its input side. This is

an advantage over continuous conduction mode (CCM) PFC cir-

cuit in which multi-loop control strategy is essential. However

the input inductor operating in DCM cannot hold the excessive

input energy because it must release all its stored energy before

the end of each switching cycle. As a result, a bulky capacitor



278 A.H. Bhat, P. Agarwal / Electric Power Systems Research 78 (2008) 276–289

Fig. 1. (a) Single-switch, unidirectional boost converter. (b) Interleaved, parallel-stage unidirectional boost converter.

is used to balance the instantaneous power between the input

and output. Also since the input current is normally a train of

triangular pulses with nearly constant duty ratio, an input fil-ter is necessary for smoothing the pulsating input current into a

continuous one.

Thethree-phasePFCs arenow in thestageof being developed

using single-stage power conversion for transformer-coupled dc

power supplies as the applications for three-phase systems are

likely to increase, particularly in areas where high-performance

and/or high power density are critical, like the telecommunica-

tions industry or in autonomous system such as aerospace and

marine. Moreover, in order to increase the power handling capa-

bility of the system, single-switch boost converter stages can be

connected in parallel, thus giving rise to interleaved parallel-

stage circuits as shown in Fig. 1(b), which improves the line

currents, reduces the L–C input filter, output capacitor volume

andreduces thecurrent stressof the switchingdeviceswith inter-

leaved switching topology. Three-phase, unidirectional boostconverters are widely used nowadays as a replacement of con-

ventional diode rectifiers to provide unity input pf, reduced THD

at ac mains and constant, regulated dc output voltage even under

fluctuations of ac voltage and dc load.

Fig. 2 depicts some of the topologies of bi-directional boost

converters. In case of the bi-directional boost converters operat-

ing in CCM [25], since the input current is the inductor current,

it can be easily programmed by current-mode control. Various

current control techniques are available for controlling the input

current so as to make theinputcurrent THD negligible associated

with a unity input pf. In high power applications especially when

high performance is required, the CCM three-phase boost recti-




Fig. 2. (a) VSI-bridge-based bi-directional boost converter. (b) Four-legged bi-directional boost converter.

fier is usually used due to high efficiency, good current quality.

The voltage source converter topology is used for the construc-

tion of a three-phase, bi-directional boost converter [1,4,49,50].

We know that VSIs can reverse the power flow from load to dc

link as a rectifier. However a standalone voltage source rectifier

requires a special dc bus able to keep voltage constant with-

out the requirement of a voltage supply. This is accomplished

with a dc capacitor and a feedback control loop. As shown

in Fig. 9(a), the basic operating principle of a three-phase bi-

directional boost converter consists on keeping the load dc-link

voltage at a desired reference value, V 0 ref using a feedback con-

trol loop. This reference value has to be high enough to keep

the diodes of the converter blocked. The dc link voltage, V 0 is

measured and compared with the reference voltage, V 0ref and

the error signal, e generated is used to switch ON and OFF the

devices of the converter. In this way, power can come and return

to the ac source in accordance with the dc-link voltage value.This means that when the dc load current, I 0 is positive (rec-

tifier operation), the capacitor, C 0 is being discharged and the

error signal becomes positive. Under this condition, the con-

trol block generates appropriate PWM signals for switches of

the converter so that power is taken from the supply. In this

way, current flows from ac to dc side and capacitor voltage is

recovered. In the inverter operation, the error signal causes the

control to discharge capacitor returning power to the ac mains.

The modulator switches the devices ON and OFF following a

pre-established template which is a sinusoidal waveform of volt-

age or current. The four-wire boost converter topology as shown

in Fig. 2(b) is employed to reduce the dc link voltage ripple and

balancing the supply currents even under unbalanced supply

voltage conditions.

The electromagnetic interference (EMI) emissions are a great

concern in power quality improvement converter applications.

The high-speed switching action of a PFC converter generates

both differential-mode and common-mode noises at the input of

converter at high frequencies. Passive filtering has been widely

used to reduce the EMI emissions into the utility. Boost convert-

ers operating in CCM give low dv/dt stress and hence produce

low EMI emissions as compared to those operating in DCM.

The softswitching techniques reduce the di /dt and dv/dt and

hence improve the performance of bi-directional boost convert-

ers by causing low EMI emissions [25]. Recently the extensive

research on the multilevel converters has also proved low dv/dt

stress in these converters and hence low EMI noise emissions

without using the passive filtering techniques [21].

The three-phase, bi-directional, boost PFCs are suitable forhigh power applications with improved performance as front-

end converters with regeneration capability for variable-speed

ac motor drives and also for hoists, cranes, lifts, battery energy

storage systems (BESS), and so on.

4. Three-phase, unidirectional and bi-directional buck

converters

Three-phase, buck converters [2,3,9,10,13,14,38,49] produce

output voltages less than the converter input voltage. They have

some attractive features compared to boost rectifiers such as

meeting the requirement of varying controllable output dc volt-




Fig. 4. (a) GTO-based bi-directional buck converter. (b) IGBT-based bi-directional buck converter.

to high dv/dt , and mechanical failure due to bearing currents

[47].

5. Three-phase unidirectional and bi-directional

buck–boost converters

In three-phase boost converters, the output voltages lower

than the supply voltage cannot be achieved. Also in three-

phase buck converters, the output voltages higher than the

supply voltage cannot be achieved. However, it has the inherent

dc short-circuit current and inrush current limitation capabil-

ity. The three-phase buck–boost type ac–dc converters have

step-up or step-down output voltage characteristics and alsothe capability of limiting the inrush and dc short-circuit cur-

rents. Therefore this type of converter is convenient for several

power supplies and is highly suitable for input pf correction

[12,15,16,22,31,41,44,49]. Fig. 5 depicts some of the topolo-

gies of three-phase unidirectional buck–boost converters. They

may have either isolated or non-isolated dc output from input

ac mains. These converters are also realized by combination

of three-phase diode bridge with filter and buck–boost dc–dc

converters such as Cuk [22], SEPIC [31], flyback [44], etc. For

isolated dc output with a high-frequency transformer to reduce

the size, a diode rectifier in conjunction with flyback, isolated

Cuk, Zeta and SEPIC converters is used. A three-phase flyback

converter has all the advantages of buck–boost converter without

any limitation. What is more, input–output isolation can be pro-

vided by three-phase flyback converter. These advantages make

flyback converter more preferable for power factor correction

with DCM input technique.

The DCM self-PFC property of different three-phase, PFCs

is summarized in Table 1.

There are some applications which require a wide range of

output dc voltage with bi-directional dc current as four-quadrant

operation and bi-directional power flow. The simplest way of

realizing a three-phase bi-directional buck–boost converter is

by using a matrix converter as shown in Fig. 6. The three-phase

bi-directional buck–boost converters can be used for medium

Table 1

DCM self-PFC property of different three-phase power quality improvement

converters

Basic converter DCM self-PFC property

Buck Poor

Boost Good

Buck–boost Excellent

Flyback Excellent

Forward No

Cuk, Sepic, Zeta Poor




Fig. 5. (a) Isolated Cuk-derived unidirectional buck–boost converter. (b) SEPIC-derived unidirectional buck–boost converter.

power applications in telecommunications and also for motor

drive control.

6. Three-phase, unidirectional and bi-directional

multilevel converters

Multilevel converters (MLCs) are gaining widespread pop-

ularity because of their excellent performance and better

line-side and load side power quality such as reduced THD

of input current, high supply power factor, ripple-free reg-

ulated dc output voltage, reduced voltage stress of devices,

reduced dv/dt stresses, and hence lower EMI emissions

[6,20,21,28,36,40,42,45,49,51,52]. They also avoid the use of

transformers in some applications which results in higher effi-

ciency of these converters. The sinusoidal line currents at unity

power factor are produced in these converters at reduced switch-

ing frequencies in comparison with their two-level counterparts.

Moreover since an MLC itself consists of series connection

of switching power devices and each device is clamped to the

dc-link capacitor voltage through the clamping diodes, it does

not require special consideration to balance the voltages of the

powerdevices. On the other hand, the series connection of power

devices is a big issue in two-level converters. Moreover, in case

of a multilevel converter, each device is stressed to a voltage

V dc /(n−1), where V dc is the dc-bus voltage and n is the num-

ber of levels. Hence the device stress is considerably reduced as

the number of levels increases. This makes multilevel converters

the best choice for the high-voltage and high-power applications

and they have invited a lot of attention for high-power industrial

applications. Nevertheless, the neutral point of the neutral point

Fig. 6. Matrix-converter-based bi-directional buck–boost converter.




Fig. 7. (a) Six-switch unidirectional three-level converter. (b) Three-switch unidirectional three-level converter.

clamped converter is prone to fluctuations due to the irregular

charging and discharging of the output capacitors. Thus the ter-

minal voltage applied at the switches on dc side can exceed that

imposed by the manufacturer and an excessively large capaci-

tor voltage unbalance causes distortion of source current, thus

offsetting the advantage of obtaining a low distortion source

current in these converters. A lot of research is being done to

address this burning issue in multilevel converters and attempts

have been successfully made to balance the dc-bus capacitor

voltages with simple control algorithms. Moreover the device

count is large in multilevel converters and complex control is

involved.

Fig. 7 depicts some of the topologies of three-phase, unidi-

rectional multi-level converters [49]. These converters also offerboost operation for the output voltage with unidirectional power

flow. These converters can be developed for a high number of

levels to offer reduced THD and improved pf of supply current

and reduced ripple and regulated dc output voltage under vary-

ing load conditions [20,40]. However the regeneration is not

possible with these converters which reduces their scope for a

few applications only.

Different topologies for bi-directional, three-phase MLCs are

diode-clamped MLC, flying capacitor MLC, and cascaded MLC

as shown in Fig. 8 [21]. When a multilevel bi-directional con-

verter is used for reactive power compensation as a static var

generator (SVG), the phase voltage and current are 90

◦

apart and

the capacitor charging and discharging can be balanced [21]. In

fact, all the three types of multilevel bi-directional converters

as shown in Fig. 8 can be used in reactive power compensation

without having voltage unbalance problem. In case of diode-

clamped multilevel converters, the reactive power flow control

is easier. But the main drawback of this converter is that exces-

sive clamping diodes are required when the number of levels

is high. In case of flying capacitor multilevel converters, large

amount of storage capacitors provides extra ride through capa-

bilities during power outage. But the main drawback is that a

large number of capacitors is required when the number of lev-

els is high which makes the system less reliable and bulky and

thus more difficult to package. In case of cascaded multilevel

converters, least number of components is required and mod-ularized circuit layout and packaging is possible because each

level has the same structure and there are no extra clamping

diodes or voltage balancing capacitors. But the main drawback

is that it needs separate dc sources, thus making its applica-

tions somewhat limited. Table 2 gives the comparison of three

types of MLCs in terms of power components required in each

type of converter. In this table, n specifies the number of lev-

els.

Multilevel bi-directional converters are used at high power

ratings at high voltages with boost voltage for bi-directional

power flow. Thus they are recommended for high power and

bi-directional power flow applications such as battery energy




Fig.8. (a) Three-level diode-clampedbi-directional converter.(b) Five-levelfly-

ing capacitor bi-directional converter. (c) Three-level converter using H-bridge

(cascade) modules.

Table 2

Comparison of power components required per phase per legamong threeMLCs

Converter type Diode-clamped

MLC

Flying-capacitor

MLC

Cascaded

MLC

Main power switches (n−1)× 2 (n−1)× 2 (n−1)× 2

Clamping diodes (n−1)× (n−2) 0 0

DC-Bus capacitors (n−1) (n−1) (n−1)/2

Balancing capacitors 0 (n−1)× (n−2)/2 0

storage systems [6], four-quadrant variable-speed ac motor

drives [28], HVDC transmissions, FACTS [45], and static var

compensation [21] to offer high efficiency and low THD of volt-

age and currents in the absence of PWM switching. A number

of control techniques have been reported in the literature for

the control of multilevel converters with emphasis given to the

balance of neutral-point potential.

7. Control strategies

The issues of control of PFCs have recently been receiving

significant attention of researchers. Control strategy of power

converters usually is the heart of three-phase PFCs. All stan-

dard modulation techniques like SPWM, SVPWM and HCC

developed for inverters can be used in rectifier applications.

Normally dc output voltage of converters is the system out-

put used as feedback in outer closed-loop control and various

control approaches such as PI controller, PID controller, sliding-

mode control (SMC), also known as variable structure control

(VSC), fuzzy logic controllers (FLCs), neural network (NN)

based controllers areemployedto provide fast dynamic response

while maintaining the stability of the converter system over wide

operating range.

In the classic solution, the voltage-oriented control (VOC)

scheme, the unity pf condition is met when the line current vec-

tor is aligned with the phase voltage vector of the power line

supplying the rectifier. The control of dc-link voltage requires

a feedback control loop. The dc voltage, V 0 is compared with

a reference, V 0 ref and the error signal, e obtained is used to

generate a template waveform. The template should be a sinu-

soidal waveform with the same frequency of the mains supply.

This template is used to produce a PWM pattern and allows

controlling the rectifier in two different ways:

(a) as a voltage-source current-controlled PWM rectifier or(b) as a voltage-source voltage-controlled PWM rectifier.

The first method controls the input current and the second

controls the magnitude and phase of voltage, V mod.

The above two methods of controlling the rectifier can also

be categorized as the direct current control (DCC) [1] and the

indirect current control (ICC) [5] methods as in the former case,

a direct control of input current is achieved which yields a sinu-

soidalcurrent with negligible THDand nearly unity powerfactor

whereas in the latter case, the input current is indirectly con-

trolled by controlling the magnitude and phase of the voltage,

V mod. This method avoids the use of current sensors.




(a) In case of current-controlled PWM rectifier (direct cur-

rent control), as shown in Fig. 9(a), the control is achieved

by measuring the instantaneous phase currents and forcing

them to follow a sinusoidal current reference template, I ref

whose amplitude is given by

I = Gce = Gc(V 0 ref − V 0) (1)

where Gc represents P, PI, Fuzzy or other controller.

Fig. 9. (a) Three-phase, current-controlled PWM rectifier. (b) Implementation

of three-phase, voltage-controlled rectifier for unity pf operation.

According to stability criteria (assuming a PI controller),

the following two relations are obtained [4]:

I x ≤CV 0

3KpLs(2)

I x ≤KpV x

2RKp + LsKi

cos ϕ (3)

where V 0 is the dc-link voltage, V x the rms supply voltage, R

and Ls are the input resistance and inductance respectively,

cosψ the input pf and I x is the rms input current.

These two relations are useful for the design of current-

controlled rectifier. With these relations, K p and K i can be

calculated to ensure stability of the rectifier.

The current-controllers are broadly classified into two

groups, linear and non-linear controllers [32]. Linear

controllers include PI stationary and synchronous, state-

feedback and predictive controller with constant switching

frequency. The sinusoidal PWM, optimal PWM and space-

vector modulation are used in these controllers which yieldconstant switching frequency. Non-linear current control

group includes hysteresis current controller (HCC) [1],

fuzzy logic based controller [27,29] and neural network

based controller [17,32,46], etc. The main advantages of

neural network based controllers are parallel processing,

learning ability, robustness, and generalization. In basic

applications, the fuzzy logic controller is used as a sub-

stitute for conventional PI compensator. The HCC has a

fast dynamic response, good accuracy, no dc offset, and

high robustness. The major problem with this current con-

troller is that the average switching frequency varies with

the dc load (very high frequency at very heavy loads) whichmakes the switching pattern uneven and random, thus caus-

ing excessive stresses on the switching power devices. The

predictive current control with fixed switching frequency

(PCFF) [7] shows a fast dynamic response and has a good

switching pattern that reduces switching device stresses.

However it is sensitive to parameter variations. In [33], a

space-vector modulation(SVM)based HCC for three-phase

PWM rectifier has been presented. This technique utilizes

all the advantages of HCC and SVM techniques. It reduces

significantly the number of switchings compared to conven-

tional HCC and at the same time gives the same maximum

voltage as the SVM technique, almost negligible response

time of current error, and insensitivity to line voltage andload parameter variations. In the case of buck converters,

due to topological restrictions, the three phases cannot oper-

ate independently. This prevents the direct use of hysteresis

input current controllers. Many high-frequency PWM tech-

niques such as six-step sinusoidal PWM [2], space vector

modulation [14], and delta modulation [10] have been pro-

posed and implemented for the single-stage, three-phase

buck-type PWM rectifiers. For high-power applications, a

GTOcurrent source converter (CSC)using SHE-PWM tech-

nique has been proposed [11].

(b) In the voltage-controlled PWM rectifier (indirect current

control), as shown schematically in Fig. 9(b), the control




is achieved by creating a sinusoidal voltage template, V mod

which is modified in amplitudeand angle to interact with the

mainsvoltageV . In this way, theinputcurrentsare controlled

without measuring them.

The expression for V mod can be derived (for unity input pf)

as

V xmod =

V − RI − Ls

dI

dt

sin ωt −XsI cos ωt (4)

It canbe observed fromFig.9(b)thatthereisnoneedtosensethe

input currents. However to ensure stability limits as good as the

limits of current-controlled rectifier, the blocks, - R-s Ls and− X shave to emulate and reproduce exactly the real values of R, X sand Ls of power circuit. However these parameters do not remain

constant and this fact affects the stability of this system making

it less stable than the current-controlled system of Fig. 9(a).

Other advantages of current-controlled PWM converters are the

control of instantaneous current waveform and high accuracy,

peak current protection, overload rejection, and extremely gooddynamics.

Other control techniques reported in the literature are:

voltage-based direct power control (V-DPC) [30], virtual flux

oriented control (VFOC) [48], and virtual flux based direct

power control (VF-DPC) [43]. Compared to VOC, there is a

simpler algorithm andno current control loops,coordinate trans-

formation and separate PWM voltage modulator are required.

Moreover there is no need for decoupling between control of the

active and reactive components, and there are better dynamics

in VF-DPC.

A number of control techniques and modulation algorithms

have been reported for the control of multilevel converters

[51,52]. In addition, space vector pulse width modulation

(SVPWM) techniques are being extensively researched for

the control and improved performance of these converters

[35,36,42,52].

8. Selection criteria of three-phase power quality

improvement converters for different applications

Selection of three-phase power quality improvement ac/dc

converters for a particular application is an important and crit-

ical decision to be taken by the application engineers and

involves a deep knowledge of the subject and experience in

the field. Some important factors to be taken into considera-tion for selection of right converter configuration for specific

applications are permitted THD, input power-factor, required

level of power quality in dc output (voltage-ripple, voltage

regulation, sag, swell), type of output dc voltage (constant, vari-

able), nature of dc output (isolated, non-isolated), requirement

of dc output (buck, boost, and buck–boost), power flow (uni-

directional, bi-directional), number of quadrants (one, two, or

four), voltage rating (low, medium, high), power rating (low,

medium, high), efficiency, size and weight, reliability and cost

of converter system. These factors act as a guiding factor

for the proper selection of a converter for a specific applica-

tion.

9. State-of-the-art of three-phase power quality

improvement converter technology

Three-phase, power quality improvement converters are gain-

ing widespread use from low to high power levels due to

their improved performance and better power quality such as

nearly unity input power factor, negligible line-current THD

and regulated, reduced-rippled dc output voltage. There are new

developments in the PFC technology for further improvement

in their performance.

The concepts of interleaved and multi-level converters are

being experimented in three-phase PFCs. It further improves

their performance and eliminates the use of passive filters. Fur-

ther, more emphasis is laid on the development of single-stage,

three-phase PFCs which further reduces the size and increases

the efficiency and reliability of three-phase PFC circuits. Mul-

tilevel converters offer higher efficiency, reduced stress on the

devices, and reduced high-frequency noise and are being devel-

oped for use in high voltage, high power applications. High

current rectifiers [53] are also being researched and developedfor certain applications like in electrometallurgical, electro-

chemical, plasma torches, and arc furnaces where currents can

go as high as 130 kA.

Progress in the direction of solid-state device technology in

terms of low conduction losses, higher permissible switching

frequency, ease in gating process, and low voltage drop will

further give a boost to the ever growing technology of three-

phase PFCs.

Parallel with other developments, sensors reduction or elim-

ination techniques have also been attempted and successfully

implemented for many three-phase PFC systems which have

helped to further reduce the cost and increase the reliabil-ity of converter systems. The sensorless techniques can make

technical and economical contributions to the system simplifi-

cation, isolation between the power circuit and control circuit,

and cost-effectiveness. Both current sensorless techniques [24]

and voltage sensorless techniques [26,30,37,39,43] have been

reported in the literature.

High-speed micro-controllers, dedicated DSPs, and FPGA

haveaddeda new dimensionin this technology andverycomplex

control algorithms can be implemented with least effort and

high processing speed in these processors. Research work is

on for complete integration of control, interfacing and power

module of three-phase PFCs which will provide compactness,

cost-effectiveness, reliability, reduced size and weight, and highefficiency ac–dc converters. Thus the three-phase, PFCs seem

to be having a very brighter future in many ac–dc applications.

10. Comparative features of three-phase, POWER

quality improvement converters

The various three-phase, PFC circuits discussed have alto-

gether different features to suit a number of applications. A

designer should decide a configuration of a particular three-

phase PFC on the basis of a trade-off between performance and

cost. In certain situations, if a three-phase diode bridge recti-

fier is already working at sites, then the use of passive, active,



Table 3

Comparative features of different types of three-phase power quality improvement converters

Type of three-

phase HPFC

Comparative features

Size, weight, and

volume

Number of switches Switch stresses Conducted EMI Output voltage levels Power levels Efficiency

Boost converter Large but can be

reduced by reduc-

ing/eliminating

the sensors, using

soft-switching and

single-stage

conversion

techniques

Large in some topologies

(two-stage conversion),

but small in some other

topologies (single-stage

conversion)

Large in DCM but

less in CCM and

even lesser with

soft-switching

techniques and

paralleled

interleaved

topologies

Less with

soft-switching

techniques

Output voltage

greater than the peak

input voltage, so

suitable for high

voltage application as

with NPCRs

Low with DCM

but can be high

with CCM

Good and

with senso

topologies

reduced si

magnetics

Buck converter Large but can be

reduced by reduc-ing/eliminating

the sensors, using

soft-switching and

single-stage

conversion

techniques

Large in some topologies

(two-stage conversion),but small in some other

topologies (single-stage

conversion)

Low, can be even

lesser withsoft-switching

techniques

Low Output voltage Less

than peak inputvoltage, so suitable

for low voltage

applications

Usually low,

can be highwith

GTO-based

converter

Good, eve

withsoft-switch

techniques

h.f. tranfor

isolation

Buck–boost

converter

Less with h.f.

transformer

isolation and

soft-switching

Less Low, can be even

lesser with

soft-switching

techniques

Low Low to high Medium Fairly goo

Multilevel

converter

Reduced Large Least Less due to low

dv/dt and even

lesser withsoft-switching

techniques

High High Very high




or hybrid filter may be the right choice. However if a designer

is at the decision design stage, then power quality improvement

converters are the better option as these provide improvedperfor-

mance both at the input as well as output side of the converter

without the need for bulky and costly filters. The main com-

parative features of various types of three-phase, power quality

improvement converters are given in Table 3.

11. Conclusions and future development

This paper has attempted to give a comprehensive review of

three-phase, power quality improvement ac/dc converters. The

converters are classified into different topologies with differ-

ent converter configurations. The three-phase, PFCs are gaining

popularity in a variety of applications ranging from low to high

powerlevelsduetotheirimprovedpowerqualitybothattheinput

as well asthe outputterminals. Theuse of these converters results

in equipment behaving as a linear load at three-phase ac mains

and solves the power quality problems due to ac/dc converters.

An exhaustive comparative study of various types of three-phase PFCs has been done in this paper in terms of different

characteristics of the converter on input as well as output side

which will help the application engineer to select an appropriate

converter topology to fit a specific application. Various factors

which can act as guideline to the engineers for proper selection

of converter have also been discussed. It can be expected that

the use of three-phase, PFC circuits will grow substantially in

future. The growth will be more prominent in applications such

as telecommunications, autonomous ac power systems (e.g., air-

craft and ships), front-end converters for variable-speed drives,

dc motordrive control and otherapplications whereperformance

and/or power density are critical. In the commercial and indus-trial environments, three-phase PFC circuits will thus gain wide

popularity. For low-power applications, the simple three-phase,

PFCs may provide a cost-effective solution with satisfactory per-

formance. For high-power applications, the trade-off between

passive harmonic filtering and active PFC tends to be in favour

of active circuits in future.

Mostof the three-phase inverter topologiesand control strate-

gies can be easily adapted for PFC applications. Significant

effort is also needed in the area of modeling and control design,

particularly for operation under unbalanced and distorted input

voltage conditions. The development of multilevel converters

has to be followed with more analysis to be done in this field.

Moreover the new developments in device technology, proces-

sors, magnetics, and control algorithms will give a real boost

to three-phase, power quality improvement converters in near

future.

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if des topologies redresseurs

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