A novel high performance voltage regulator for single phase AC source

A Novel High-Performance V oltage Regulator for
Single-Phase AC Sources
Han-Woong Park,Member,IEEE,Sung-Jun Park,Member,IEEE,Jin-Gil Park,Member,IEEE,and
Cheul-U Kim,Member,IEEE
Abstract—Regulation of load voltage in single-phase applica-tions is becoming an important issue for critical loads.This paper presents a novel high-performance single-phase voltage regulator which has a common arm between the rectifier and inverter, and adopts an appropriate switching strategy.The proposed voltage regulator employs six switches and can be implemented by only one three-phase inverter module.The proposed voltage regulator has the capability of delivering sinusoidal input current with unity power factor,good output voltage regulation,and bidirectional power flow.For these purposes,a fully digital controller is designed and implemented using a TMS320F240 digital signal processor.In addition,a novel low-cost ac capacitor is also presented.This type of capacitor requires two dc capacitors and two diodes,enabling low-cost and compact manufacturing. Consequently,the complete voltage regulator system,which is mainly suitable for an uninterruptible power supply as well as reactive or nonlinear loads,can be constructed compactly and inexpensively.Experimental results are presented to verify the feasibility of the proposed voltage regulator system.
Index Terms—Low-cost ac capacitor,simple and compact ac source,voltage regulator.
I.I NTRODUCTION
R EQUIREMENTS on the power quality of ac sources are becoming more significant in voltage-sensitive equipment.This results in a greater demand for high-quality voltage sources,increasing concerns about harmonic content, noise,and voltage stability in electrical supply systems.In particular,owing to the proliferation of computer systems and the increasing awareness to protect critical information,the demand for uninterruptible power supplies(UPSs)has grown tremendously.Many circuit topologies have been developed so far for single-phase voltage regulators(SVRs).Among them,ac choppers[1]–[3]have been used to obtain a variable ac voltage from a fixed ac source.The phase-controlled ac choppers have been extensively employed in the ac power control applications such as industrial heating,light dimming,and ac motor control. These converters offer the advantages of simplicity and the ability to control a large amount of power economically.
Manuscript received December15,1999;revised December10,2000.Ab-stract published on the Internet February15,2001.An earlier version of this paper was presented at IEEE ISIE’99,Bled,Slovenia,July11–16,1999.
H.-W.Park is with the Department of Electrical Engineering,Korea Naval Academy,KyungNam645-797,Korea.
S.-J.Park is with the Department of Electrical Engineering,Tongmyung Col-lege,Pusan608-749,Korea.
J.-G.Park is with the Department of Marine System Engineering,Korea Mar-itime University,Pusan606-791,Korea.
C.-U Kim is with the Department of Electrical Engineering,Pusan National University,Pusan609-735,Korea.
Publisher Item Identifier S
0278-0046(01)03374-3.
(a)
(b)
Fig.1.Conventional circuit configurations of SVR.(a)Discontinuous input current.(b)Continuous input current.
However,significant harmonics in both the output voltage and current are introduced,and a discontinuity of power flow appears at both the input and output sides.Although these problems can be partially improved by using more advanced control methods,it is not still suitable for high-quality voltage sources.Therefore,an ac/dc/ac-type voltage regulator has been traditionally used in most single-phase power supplies. The main required features of an SVR are as follows:minia-turization,active power-factor correction(PFC)at the input terminals,good output voltage regulation,low cost,and high efficiency.
In recent years,development of the SVR having such capa-bilities has been attempted in several papers[4]–[6].The two most typical configurations of SVR are shown in Fig.1.In the
figure,
switched-mode rectifiers(SMRs)have been adopted in both SVR types.Although the SMR of the SVR shown in Fig.1(a) has advantages regarding the reduction of self-turnoff switch count(and,hence,cost),it also has disadvantages such as a sub-stantial increase in current stresses of the switching devices due to the discontinuous nature of the rectifier input current and the high ripple content of the switching frequency in the SMR input current.On the other hand,the SMR shown in Fig.1(b)oper-ating in continuous input current mode can eliminate the dis-advantages just mentioned above,and it has an additional ca-pability of bidirectional power flow.Therefore,from the theo-retical point of view,the SVR topology as shown in Fig.1(b) is preferred to that shown in Fig.1(a).However,from the prac-
tical and commercial points of view,it has several disadvantages such as high cost,large size,and control complexity due to the large number of self-turnoff switches.
In response to these concerns,this paper focuses on the prac-tical and commercial feasibility of the SVR as shown in Fig.1(b) by reducing the number of self-turnoff switches,designing a simple and low-cost controller,and decreasing the cost of the filter.Therefore,a novel single-phase voltage regulator which has a common arm between the rectifier and inverter is proposed and investigated in this paper.The structure of the proposed reg-ulator is composed of three arms using six self-turnoff switches. Therefore,it can reduce the number of switches and can be im-plemented by only one three-phase inverter module,which en-ables the reduction of cost and size,maintaining the capabilities of the circuit configuration in Fig.1(b).
The configuration incorporates a fully digital control scheme. In point of view of commercial use,utilization of a high-speed floating-point digital-signal-processor(DSP)-based controller in switching power converters is still expensive in spite of its outstanding performance.Considering the cost and computa-tion time,a relatively low-cost fixed-point TMS320F240(F240) DSP can be a proper choice for the feasibility of the proposed SVR.In addition,a novel ac capacitor is also presented.This type of capacitor requires two dc capacitors and two diodes enabling low-cost and compact manufacturing.Consequently, the complete voltage regulator system,which is mainly suitable for a UPS as well as reactive or nonlinear loads,can be con-structed compactly and inexpensively.Experimental results are presented to verify the feasibility of the proposed voltage regu-lator system.
II.P ROPOSED V OLTAGE R EGULATOR
A.Main Circuit and Operation
Fig.2shows the configuration of the main circuits of the pro-posed single-phase voltage regulator that consists of three arms.
A center arm,which is called the common arm and employs the two self-turnoff
switches
and inverter can be written
as
(1) As mentioned above,the polarities and duty ratios of the rec-tifier and inverter can be determined by upper switches of each arm,only the states
of
Fig.3.Switching patterns.
TABLE I
S WITCHING M ODES A CCORDING TO ON 1OFF S TATES OF E ACH S
WITCH
rectifier,such as high input current harmonics,low input power factor,lower rectifier efficiency,ac source voltage distortion,and high reactive-component sizes.Therefore,the optimal rec-tifier would be one in which the input would draw a pure sinu-soidal current at unity power factor.One of the most important research topics in power electronics at present time is the PFC of rectifiers [7]–[10].These kinds of rectifiers are mostly utilized in industrial and commercial applications,and one of them is adopted in this paper,as shown in Fig.2.However,because the proposed voltage regulator has the common arm between the two converters,an appropriate switching strategy must be care-fully designed to realize the PFC scheme in the rectifier.It can be described as follows.
If the rectifier is operating under PFC control and the rectifier input
voltage
and
and
lie under
the
always
advances
.If the resistance can be ignored,the phase angle
difference
.Consequently,in order to control
the input current being in phase with supply
voltage,
and
are written as
follows:
(4)
where if the initial angle of the source
voltage
denotes the delay angle of the rectifier input voltage.
Therefore,
phasor
.Then,the duty
ratio
(5)
With this duty ratio and the reference current command,the rec-tifier can obtain unity power factor and can deliver sinusoidal input current.From the above equation,to assure the stable op-eration
for
should be confined to be equal to or less
than
(6)
Therefore,because the phase angle
of
(a)
(b)
(c)
(d)
(e)
(f)
(g)(h)
Fig.4.Switching modes of the proposed single-phase voltage regulator.(a)Mode 1.(b)Mode 2.(c)Mode 3.(d)Mode 4.(e)Mode 5.(f)Mode 6.(g)Mode 7.(h)Mode 8.
D.Low-Cost AC Capacitor
The output filter of the inverter of the proposed voltage regu-lator is used to eliminate the harmonic components of the output voltage.It employs
inductor
of the electrode and
permittivity
between electrodes
as
(7)
Among those parameters,the main factor to determine the ca-pacitance is,in
general,
Fig.5.Phasor
diagram.
(a)(b)
Fig. 6.Conventional and proposed ac capacitors.(a)Conventional.(b)Proposed.
the electrodes are made of metals,dielectrics should be required to insulate two electrodes and the effective distance is also in-creased.On the other hand,as the dc capacitor can be made of p-n junction semiconductors,its effective distance between two electrodes can be much smaller than that of an ac capacitor,which results in great size and cost reductions.However,it is not allowed to apply reverse voltage in a dc capacitor.There-fore,by presenting how to construct an ac capacitor using two dc capacitors,this paper attempts to reduce the cost and size of the conventional ac capacitor for the filter at the inverter output terminals.
Fig.6(a)shows the commercially available ac capacitor and Fig.6(b)shows the proposed ac capacitor using two dc capac-itors and two diodes with the same capacitance as Fig.6(a).In the figure,two dc
capacitors
(8)
If the voltage polarity and current flow of the proposed ac ca-pacitor is assumed to be positive
and
=(+);
V =(+);
V =(+);
V =(+);
V =
(+);
V
=0;
V =(0);
V =(+);
V =(+);
V
=(0);
V =(+);
V =0;
V
i i i i
i
Fig.8.Schematic diagram of the DSP
controller.
Fig.9.Flowchart of the main program.
controlled for the input current to follow up the reference com-mand using a PWM technique.The output voltage of the in-
verter,,is controlled by the duty ratio which is calculated from the fundamental component of the measured load
voltage.
Fig.10.Gate drive circuit using self-charging method.
The CPU implements the desired control algorithms for each power converter and calculates the required PWM duty ratios.The integrated PWM modules use the duty ratios to generate six PWM outputs to control the switches.
Fig.9shows the flowchart for the main routine of the pro-posed voltage regulator.The control routine is activated with a period of
200
TABLE II
E XPERIMENTAL C
ONDITIONS
Fig.11.Switching patterns.
III.E XPERIMENTAL R ESULTS
The experiments of the proposed converter are carried out for various loads.The instantaneous power-balanced control for the rectifier stage should be ensured so that the dc-link capacitor can be reduced to a suitable value.A 1-kV A prototype was con-structed using a 600-V 30-A insulated gate bipolar transistor (IGBT)inverter module.The proposed converter can deliver the output voltage to the load ranging from 0to 100V .In the experi-ments,the load voltage is set to 100V .The main key parameters for the experimental verification are listed in Table II.
Figs.11–14show the experimental waveforms of voltages and currents of the proposed voltage regulator system illus-trating the steady-state characteristics for a resistive load.An
11-
and rectifier input current I
(upper)and load voltage
V
It
(a)
(b)
Fig.14.Waveforms of the two dc capacitor voltages and inverter output
voltage.(a)Two dc capacitor voltage V and V.(b)Lower capacitor
voltage V and load voltage V
IV .C ONCLUSION
In this paper,a novel single-phase voltage regulator,which has a common arm between the ac/dc and dc/ac power con-verters and,consequently,being composed of a three-arm bridge,has been presented.In addition,a new low-cost ac capacitor using two dc capacitors was also presented.The fully digital controller was designed using a high-speed cost-ef-fective DSP.The validity of the proposed system was proven through experiments and their results are as follows.1)The number of switching devices can be reduced.
2)The control strategy is very simple by controlling the vari-
ables
.3)High-performance PFC control and output voltage regu-lation are achieved.
4)The feasibility of the compact and low-cost voltage regu-lator has been verified.
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Han-Woong Park (S’85–M’89)was born in KyungNam,Korea,in 1959.He received the B.S.,M.S.,and Ph.D.degrees in electrical engineering from Pusan National University,Pusan,Korea,in 1983,1987,and 1997,respectively.
Since 1990,he has been with the Department of Electrical Engineering,Korea Naval Academy,KyungNam,Korea.His research interests are motor design,power electronics,motor control,mechatronics,intelligent control,and
robotics.
Sung-Jun Park (M’01)was born in KyungPook,Korea,in 1965.He received the B.S.,M.S.,and Ph.D.degrees in electrical engineering from Pusan National University,Pusan,Korea,in 1991,1993,and 1996,respectively.
Since 1996,he has been with the Department of Electrical Engineering,Tongmyung College,Pusan,Korea.His research interests are power electronics,motor control,mechatronics,micromachine automa-tion,and
robotics.
Jin-Gil Park (M’01)was born in Changwon,Korea,in 1939.He received the B.S.,M.S.,and Ph.D.degrees in marine engineering from Korea Maritime University,Pusan,Korea.
From 1963to 1972,he was an Engineer with Korea Power Company.Since 1972,he has been a Professor in the Department of Marine System Engineering,Korea Maritime University.His current interests include the control of variable-speed motors and their drive
systems.
Cheul-U Kim (S’85–M’87)was born in Kyung-Nam,Korea,in 1942.He received the B.S.degree in electrical engineering from Pusan National University,Pusan,Korea,the M.S.degree from the University of Electro-Communication,Tokyo,Japan,and the Ph.D.degree from Chung-Ang University,Seoul,Korea,in 1969,1974,and 1986,respectively.
Since 1975,he has been a Professor at Pusan National University.His research activities are in the area of power electronics and motor control,
including cyclo-converter design,drive systems,and high-efficiency SMPS.Dr.Kim is a member of the Korea Institute of Electrical Engineers,Korea Institute of Power Electronics,Korea Institute of Illuminating and Electrical Installation Engineers,and Japan Institute of Electrical Engineers.。