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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:
0378-7796/$ – see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.epsr.2007.02.002
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(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
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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-
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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-
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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
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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.
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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
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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.
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(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
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286 A.H. Bhat, P. Agarwal / Electric Power Systems Research 78 (2008) 276–289
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,
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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
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288 A.H. Bhat, P. Agarwal / Electric Power Systems Research 78 (2008) 276–289
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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