Improved DSP-controlled online UPS system with high real output power T.-J. Liang and J.-L. Shyu Abstract: A circuit topology is proposed for single-phase online UPS systems. The proposed circuit incorporates a built-in bidirectional AC/DC converter, which provides power factor correction and active power filtering. The input stage, based on a bidirectional AC/DC converter, functions as a rectifier with unity power factor correction for normal AC line mode operation, and as an active power filter for outage mode operation. During input power failure, the bidirectional AC/DC converter functions as an active power filter, providing reactive power to the load. Simultaneously, the DC/AC voltage inverter output stage supplies real power from the battery bank to the critical load. The real power capability is improved compared with that of a conventional online UPS system, and extra loads can be connected to the UPS system during outage. The power circuit of the proposed UPS system is presented and analysed. Circuit models are derived and a UPS digital control system using a digital signal processor (DSP) is developed. Finally, simulated and experimental results obtained from a laboratory prototype are presented to confirm the feasibility and features of the proposed UPS scheme. 1 Introduction Uninterruptible power supplies (UPS) ensure continuous power flow to critical loads in the event of disturbance surges or AC line failure, and have been used for a wide variety of loads, including telecommunication systems, medical systems, industrial control systems etc. Generally, an UPS system requires the normal utility AC line input current to have a low total harmonic distortion (THD) sinusoidal current with unity power factor. A UPS is required to deliver a well-regulated sinusoidal output voltage with low THD to the load, regardless of whether the utility AC power source is normal or not. Online and offline UPS systems are the two major UPS schemes [1–6] . Online UPS systems are generally preferred because of high reliability and good voltage regulation. Because AC lines are easily contaminated, a power factor correction circuit is usually employed to obtain high power factor from the UPS system, thereby preventing contam- ination of the AC lines [7–9] . In a conventional online UPS system, the input stage is disabled and kept idle during AC line outage. The conventional UPS system has an inherent real output power limitation for nonlinear loads, as the load current may contain undesirable harmonics and is not kept in phase with the output voltage. As a result, the output power factor of the conventional UPS systems is low while feeding nonlinear loads because the real power absorbed by the load is low. Nevertheless, a UPS system is expected to produce nearly sinusoidal output current at the output side and thus raise the output power factor of the UPS system. An active power filter (APF) is usually employed to compensate for line current harmonics and the reactive power of nonlinear loads, yielding a near-unity power factor and sinusoidal current waveform for the AC voltage source [10, 11] . This paper presents the hardware implementation and experimental results of a bidirectional AC/DC converter with power factor correction and active power filtering, a system that was presented previously as a simulation [12] . The input stage, based on a bidirectional AC/DC converter, functions as a power factor correction circuit for normal AC line mode operation to increase the AC line power factor and to reduce the AC line current harmonic components. During input power failure, the bidirectional AC/DC converter functions as an active power filter, providing reactive power to the load. Thus the real output power capability is improved compared with that of a conventional online UPS, and an efficient economic UPS system is achieved. 2 Proposed UPS configuration Figure 1 shows the proposed online UPS circuit configura- tion which consists of a bidirectional AC/DC converter and a DC/AC voltage inverter. An LC lowpass filter is used at the output side for eliminating unwanted harmonic AC line V DC DC/AC DC/AC voltage voltage inverter inverter L o C o bidirectional bidirectional AC/DC AC/DC converter converter L S v s v o S1 S2 rated load extra load C D AC + + − − Fig. 1 Proposed online UPS system T.-J. Liang is with the Department of Electrical Engineering, National Cheng Kung University, 1 University Road, Tainan, Taiwan 70101, ROC J.-L. Shyu is with the Department of Electrical Engineering, Kao Yuan Institute of Technology, 1821 Chung-Shan Road, Lu-Chu Hsian, Kaohsiung Hsian, Taiwan, ROC r IEE, 2004 IEE Proceedings online no. 20040020 doi:10.1049/ip-epa:20040020 Paper first received 20th September and in revised form 4th June 2003 IEE Proc.-Electr. Power Appl., Vol. 151, No. 1, January 2004 121
Transcript
Improved DSP-controlled online UPS system withhigh real output power
T.-J. Liang and J.-L. Shyu
Abstract: A circuit topology is proposed for single-phase online UPS systems. The proposed circuitincorporates a built-in bidirectional AC/DC converter, which provides power factor correction andactive power filtering. The input stage, based on a bidirectional AC/DC converter, functions as arectifier with unity power factor correction for normal AC line mode operation, and as an activepower filter for outage mode operation. During input power failure, the bidirectional AC/DCconverter functions as an active power filter, providing reactive power to the load. Simultaneously,the DC/AC voltage inverter output stage supplies real power from the battery bank to the criticalload. The real power capability is improved compared with that of a conventional online UPSsystem, and extra loads can be connected to the UPS system during outage. The power circuit ofthe proposed UPS system is presented and analysed. Circuit models are derived and a UPS digitalcontrol system using a digital signal processor (DSP) is developed. Finally, simulated andexperimental results obtained from a laboratory prototype are presented to confirm the feasibilityand features of the proposed UPS scheme.
1 Introduction
Uninterruptible power supplies (UPS) ensure continuouspower flow to critical loads in the event of disturbancesurges or AC line failure, and have been used for a widevariety of loads, including telecommunication systems,medical systems, industrial control systems etc. Generally,an UPS system requires the normal utility AC line inputcurrent to have a low total harmonic distortion (THD)sinusoidal current with unity power factor. A UPS isrequired to deliver a well-regulated sinusoidal outputvoltage with low THD to the load, regardless of whetherthe utility AC power source is normal or not. Online andoffline UPS systems are the two major UPS schemes [1–6].Online UPS systems are generally preferred because of highreliability and good voltage regulation.Because AC lines are easily contaminated, a power factor
correction circuit is usually employed to obtain high powerfactor from the UPS system, thereby preventing contam-ination of the AC lines [7–9]. In a conventional online UPSsystem, the input stage is disabled and kept idle during ACline outage. The conventional UPS system has an inherentreal output power limitation for nonlinear loads, as the loadcurrent may contain undesirable harmonics and is not keptin phase with the output voltage. As a result, the outputpower factor of the conventional UPS systems is low whilefeeding nonlinear loads because the real power absorbed bythe load is low. Nevertheless, a UPS system is expected toproduce nearly sinusoidal output current at the output side
and thus raise the output power factor of the UPS system.An active power filter (APF) is usually employed tocompensate for line current harmonics and the reactivepower of nonlinear loads, yielding a near-unity power factorand sinusoidal current waveform for the AC voltage source[10, 11].This paper presents the hardware implementation and
experimental results of a bidirectional AC/DC converterwith power factor correction and active power filtering, asystem that was presented previously as a simulation [12].The input stage, based on a bidirectional AC/DC converter,functions as a power factor correction circuit for normalAC line mode operation to increase the AC line powerfactor and to reduce the AC line current harmoniccomponents. During input power failure, the bidirectionalAC/DC converter functions as an active power filter,providing reactive power to the load. Thus the real outputpower capability is improved compared with that of aconventional online UPS, and an efficient economic UPSsystem is achieved.
2 Proposed UPS configuration
Figure 1 shows the proposed online UPS circuit configura-tion which consists of a bidirectional AC/DC converter anda DC/AC voltage inverter. An LC lowpass filter is used atthe output side for eliminating unwanted harmonic
T.-J. Liang is with the Department of Electrical Engineering, National ChengKung University, 1 University Road, Tainan, Taiwan 70101, ROC
J.-L. Shyu is with the Department of Electrical Engineering, Kao Yuan Instituteof Technology, 1821 Chung-Shan Road, Lu-Chu Hsian, Kaohsiung Hsian,Taiwan, ROC
r IEE, 2004
IEE Proceedings online no. 20040020
doi:10.1049/ip-epa:20040020
Paper first received 20th September and in revised form 4th June 2003
IEE Proc.-Electr. Power Appl., Vol. 151, No. 1, January 2004 121
components from the inverter output voltage. Switch S1 isincorporated in the power circuit to provide isolationbetween the UPS and the input AC power. When an inputpower failure occurs, power is transferred to the critical loadfrom the battery bank through the DC/AC voltage inverter.The system has two operating modes. One is the normal
mode (Fig. 2a), and the other is the outage mode (Fig. 2b).In the normal mode, switch S2 is opened and thebidirectional AC/DC converter functions as a power factorcorrection rectifier, so that the line current is sinusoidal andin phase with the line voltage. The DC/AC voltage inverteris modulated such that the output voltage amplitude is heldconstant with low THD. During outage mode operation,switch S2 is closed and the bidirectional AC/DC converteris linked with the output of the DC/AC voltage inverter.The bidirectional AC/DC converter functions as an activepower filter, i.e. the bidirectional converter provides reactivepower and non-sinusoidal current to the load. The DC/ACvoltage inverter can supply high real output power to theload. Thus the output power factor of the DC/AC voltageinverter is near-unity and the overall real output powercapability of the UPS system is improved. For outageoperation, this additional real power capability allows theaddition of significantly extra load.
3 System analysis and controller design
The proposed UPS system comprises a bidirectional AC/DC converter and a DC/AC voltage source inverter.
3.1 Power factor correction rectifierFigure 3 shows the circuit schematic and the controller ofthe power factor correction (PFC) rectifier [7–9]. Whenutility AC power is normal, the bidirectional AC/DCconverter operates as a PFC rectifier to improve inputpower factor. The control scheme consists of two parts: aninner current controller and outer voltage controller. Thecontrol strategy of the gating signals involves comparing theoutput voltage vcp of the current controller with triangularcarrier vtri. Under this gating control scheme, the linecurrent is follows the current command i�s such that near-unity power factor and low current harmonics are obtained.
If the control voltage vcp is kept constant over a switchingperiod, the duty ratios of the PFC rectifier can be expressedby:
d1p ¼ 12þ vcp2vtri
d2p ¼ 12� vcp2vtri
ð1Þ
where d1p and d2p denote the duty ratios of lower switchesSW2 and SW4 at every switching period Ts. The amplitudeof the triangular carrier is vtri. Resistance of the AC line isneglected to simplify circuit analysis. The equations of thePFC circuit model shown in Fig. 3a can be obtained asfollows:
Lsdisdt
¼ vs � vp ð2Þ
where vp is the pulse width modulation (PWM) PFCvoltage, vs is the supply voltage and is is the line current.PFC voltage vp is represented in terms of duty ratios d1p andd2p as:
vp ¼ ðd1p � d2pÞVDC ¼ kpwmvcp ð3Þwhere kpw¼ vDC/vtri is the gain of the PFC rectifier. If thebattery bank is fully charged and neglecting the power lossof the converter, the DC-side current of the PFC circuit canbe expressed as:
id ¼ CDdvDCdt
þ vDCRDC
ð4Þ
vo
VDCAC
DC/AC voltage inverter
LoCo
vo
Lo
Co
AC line
LS
iois
vs
on
off
PFC rectifier
+
if
ioif
S2
S2
ilrated load
extra load
CD
S1
S1
active power filter (APF)
VDCAC
DC/AC voltage inverter
AC line
LS
ia
vs
off
on
ilrated load
extra load
CD
−
+
− −
+
−
+
a
b
Fig. 2 UPS operation modesa Schematic of proposed online UPS system during normal modeb Schematic of proposed online UPS system during outage mode
is
CDvDC
SW1
SW4
SW1SW2
SW3SW4
SW2
SW3
id
icRDC
iL
AC
LS
vs vp
*sivcp
vtricurrent controller
voltage controller
is
sin �t DCv
*vDC
Gsc(s)
Lss1kpwm
vs
(k )is* isvcp (k)
kcp
kif
vpcurrent current
controllercontroller
fi
vfk
kif
1v DC (k )
ev (k )
vpv (k )Gpv (z)*
vDC
sin (k )
voltage voltage controllercontroller
current current looploop
is
is(k)*kad RDCCDs+1
RDCid
fv
+
+
+
−
−
+ −
+ + +
−
−−
+
+−
−
−
+
−
a
b
c
�
�
Fig. 3 Circuit configuration and control strategy of bidirectionalAC/DC converter during normal modea PFC circuitb Current loopc Voltage loop
122 IEE Proc.-Electr. Power Appl., Vol. 151, No. 1, January 2004
Using the power balance relation between the inputand output of the PWM converter at unity power factorgives:
kad9idis¼ vs
vDCð5Þ
where kad denotes as power transfer gain. Figs. 3b andc show the controllers using circuit modelling equations(1)–(5).To eliminate the disturbance of the line voltage vs shown
in Fig. 3b, the disturbance compensator is set to:
GscðsÞ ¼ 1=kpwm ð6ÞA proportional controller kcp is chosen as the currentcontroller. In this case, the transfer function between i�s andis can be derived as:
isi�s¼ kcpkpwm
Lssþ kcpkpwmkif¼ 1
kifð7Þ
where kif is current feedback scaling gain and i�s is the linecurrent command. The digital current control algorithm canbe written as:
vcpðkÞ ¼ ½i�s ðkÞ � isðkÞ�kcp � vsðkÞkpwm
ð8Þ
The digital proportionalFintegral (PI) controller Gpv(z)shown in Fig. 3c is used to regulate the DC voltage and isexpressed as:
GpvðzÞ ¼ kpp þ kpiTvz� 1 ð9Þ
where Tv is the sampling period of the voltage loop, and kpp
and kpi are the proportional and integral gains of the PIcontroller. Therefore, the finite difference equation of the PIcontroller can be obtained as:
where ev(k�1) and vpv(k�1) are the voltage error andcontroller output at the (k�1) sampling instant. Multiplyingvpv(k) by the synchronising signal sin(k) of measured supplyvoltage vs(k), the resulting line current command i�s ðkÞ isthen obtained as:
i�s ðkÞ ¼ vpvðkÞ sin ðkÞ ð11ÞSubstituting (10) into (11) gives the line current commandi�s ðkÞ. Therefore, the desired duty ratios d1p(k) and d2p(k)can be calculated by substituting (8) into (1).
3.2 Active power filterWhen utility power failure is detected, the APF isimmediately connected in parallel with the loads throughswitch S2. The input stage bidirectional AC/DC converter isthen switched from PFC mode to APF mode andconnected in parallel with the DC/AC voltage inverter.The APF circuit configuration and controllers are shown inFig. 4 [10, 11]. Multi-loop controls, an inner current loopfor regulating the DC/AC inverter output current io and anouter voltage loop for regulating the DC bus voltage, areused as the APF control system. The APF is designed toprovide compensatory reactive energy to load current il sothat the DC/AC voltage inverter output current io is inphase with the sinusoidal output voltage vo regardless ofload nonlinearities.Using a procedure similar to the above PFC circuit
analysis, the APF circuit modelling equations can now bewritten. The duty ratios d1a and d2a of the switches SW1
(SW4) and SW3 (SW2) are expressed as:
d1a ¼1
2þ vca2vtri
d2a ¼1
2� vca2vtri
ð12Þ
where d1a and d2a are the APF duty ratios and vtri is theamplitude of the triangular carrier. The voltage equation ofAPF can be derived as:
Lsdiadt
¼ va � vo ð13Þ
where ia is the APF output current, vo is the output voltageof the DC/AC voltage inverter and va is the PWM APFvoltage. The APF voltage va can be obtained in terms ofduty ratios d1a and d2a as:
va ¼ ðd1a � d2aÞVDC ¼ kpwmvca ð14Þwhere kpwm¼ vDC/vtri denotes the APF converter gain. Thecurrent equations on the DC side and AC side can bewritten as:
ic ¼ CDdvDCdt
ð15Þ
il ¼ ia þ io ð16ÞThe current controller and voltage controller are illustratedin Figs. 4b and c, where kif and kvf are the current andvoltage sensor scaling gains. The current controller shownin Fig. 4b is a proportional controller with proportional
ia
sin ωt
DC/AC DC/AC voltage voltage inverterinverter
CD vDC
icLS
vavo
ilrated load
extra load
ioif
vDC
SW5 SW7
SW8SW6
SW5
SW6
SW7
SW5
SW8
SW8
SW7
SW6
Lfvi
APFAPF
current controller
voltage controller
io
i*
vDC
*vDC
vcavtri
kif
Lss1kpwm
ilvo
io (k)* kca ioiavca(k)
Gos(s)
current current controllercontroller
va
fi
vDC
kvf
li
v* (k)
i * (k)
sin (k)
ev (k) vav (k)
Gav (z)
voltage voltage controllercontroller
current current looploop
iaiokif
1kad
icCD s
1
fv
+
−
+
+
+ ++
++
+ + +
+
+
−−
−
−
−−
−
−
−
−
+
− −
+
−
+ +
−
o
Gis (s)
a
b
c
o DC
�
�
Fig. 4 Circuit configuration and control strategy of bidirectionalAC/DC converter during outage modea APF circuitb Current loopc Voltage loop
IEE Proc.-Electr. Power Appl., Vol. 151, No. 1, January 2004 123
gain kca. The closed-loop transfer function of current loop isexpressed as:
io ¼kpwmkca
Lssþ kif kpwmkcai�o þ
1
Lssþ kif kpwmkcail ð17Þ
The second term of (17) shows that il is viewed as adisturbance. If the feed-forward compensator Gos(s) shownin Fig. 4b is set to 1/kpwm and the current controller kca is setto high proportional gain, then disturbance is eliminated forvo and il. Thus the resulting current loop can be furthersimplified to a constant gain:
ioi�o¼ 1
kifð18Þ
The control voltage vca(k) generated from the current loopcan be derived as:
vcaðkÞ ¼ ½i�oðkÞ � ioðkÞ�kca þvoðkÞkpwm
ð19Þ
The outer voltage loop is shown in Fig. 4c. A digital PIvoltage controllerGav(z) is used. Using procedures similar tothose described previously in the PFC control system, thecomposite voltage loop control algorithm can also beobtained as:
where kap and kai are the proportional and integral gain ofthe PI controller. By letting feed-forward disturbancecompensator Gls(s)¼ kif, disturbance il is cancelled. Then,the output current command i�oðkÞ of the DC/AC voltageinverter is computed by multiplying i�oðkÞ with unit vectorsin(k) of the output voltage vo(k). This gives:
i�oðkÞ ¼ vavðkÞ sinðkÞ � kif ilðkÞ ð21ÞEquation (20) is used to determine i�oðkÞ. Duty ratios d1aand d2a are obtained by substituting (19) into (12).
3.3 DC/AC voltage inverterThe circuit diagram and control block diagram of a voltageinverter are shown in Fig. 5 [13–15]. The inverter has twocontrollers: one is for the inner current loop and the other isfor the outer voltage loop. The multi-loop control techniquewith feed-forward control is used to make the outputvoltage vo follow the sinusoidal voltage command v�o closely.The duty ratios d1i and d2i of upper switches SW5 (SW8) andSW7 (SW6) are:
d1i ¼1
2þ vci2vtri
d2i ¼1
2� vci2vtri
ð22Þ
Applying Kirchhoff’s law on the AC output side gives:
Lodifdt
¼ vi � vo ð23Þ
where vi is the PWM inverter output voltage, which may beobtained from duty ratios d1i and d2i as:
vi ¼ ðd1i � d2iÞVDC ¼ kpwmvci ð24Þwhere kpwm¼ vDC/vtri denotes the inverter gain. FromFig. 5a, the current equation can be obtained as:
Codvodt
¼ if � io ð25Þ
where if is the inductor current and io is the output current.Using (22)–(25), the multi-loop control system of theinverter output voltage can therefore be developed as
shown in Figs. 5b and c. kvf and kif are the voltage andcurrent sensor scaling gains. The voltage controller andcurrent controller can be realised by proportional gains kvi
and kci to simplify the control algorithm. As shown inFig. 5b, if feed-forward disturbance compensator Gvs(s)satisfies:
GvsðsÞ ¼ 1=kpwm ð26Þthe disturbance of output voltage vo can be cancelled. Thetransfer function of the current loop can be simplified as:
ifi�f
¼ 1
kifð27Þ
The desired control voltage vci(k) is
vciðkÞ ¼ ½i�f ðkÞ � if ðkÞ�kci þvoðkÞkpwm
ð28Þ
The output current io also acts as a disturbance to thevoltage loop shown in Fig. 5c. Likewise, io can be eliminatedby setting:
GisðsÞ ¼ kif ð29ÞTo produce an output voltage with zero steady state error, acommand feed-forward controller Gff(s) is used, derived as:
Gff ðsÞ ¼kifCo
kvfð30Þ
ioif
vDC
SW5
SW5
SW6
SW7
SW8
SW7
SW6 SW8
ilLo
vi
currentcontroller
voltage controller
if
i * (k)
vo
vci
vo loadloadextra extra loadload
vtri
Co
a
b
c
kif
Gvs (s)
Los1kpwm
vo
kci ifvci (k) vicurrent current
controllercontroller
fi
kvf
kvi Cos1
kif
1ev (k)vo
voltage voltage controllercontroller
current current looploop
v* (k)
if
Zload
1Gff (z ) Gis (s)io
fv
−
+
−
+
−+
−
−
+−
−
+
+ +
+++
+
−
+
f
i * f
v*o
o
i* (k)f
v* (k)o
−+
+++
�
�
Fig. 5 Circuit configuration and control strategy of DC/ACvoltage invertera Inverter circuitb Current loopc Voltage loop
124 IEE Proc.-Electr. Power Appl., Vol. 151, No. 1, January 2004
Thus the inductor current command i�f ðkÞ is represented by:
i�f ðkÞ ¼½v�oðkÞ � voðkÞ�kvi
þ kcfCo
kvfv��ðkÞ þ kif ioðkÞ ð31Þ
where v�oðkÞ is the sinusoidal voltage command.Substituting (31) into (28), the control voltage vci(k) can
be obtained. The duty ratios d1i and d2i at every switchingperiod can then be calculated by substituting (28) into (22).Note that the DSP software design is easy to implement
as the multi-loop control algorithms during normal andoutage modes have similar control structures.
4 Simulated and experimental results
The closed-loop UPS system has been tested by bothsimulation and experiment. An experimental prototypeusing a digital signal processor TMS320F240 digitalcontroller was built. The sampling frequency fc and fv ofthe current controller and voltage controller were set to18kHz and 9kHz, respectively. The PWM switchingfrequency fs was 18kHz. The UPS specifications and circuitparameters are:
AC input voltage : 110 Vrms; 60HzACoutput voltage : 110 Vrms; 60Hz
DCbus voltage : VDC ¼ 210VPower factor : � 0:98
Input filter inductance : Ls ¼ 3:0mHSwitching frequency fs: 18 kHz
Output filter inductor : Lo ¼ 1:0mHDCLoad :RDC ¼ 80O
In the design of these controllers, the controller parameterswere chosen to ensure that satisfactory dynamic responsewas obtained over the operating range of the UPS system.The controllers were tested systematically in the operatingconditions given in Table 1.
Figure 6 shows the line voltage vs and line current iswaveforms for the PFC rectifier during normal mode. Itconfirms that, under normal mode condition, the PFCrectifier can obtain high power factor line current with lowcurrent harmonics. The line current is controlled to besinusoidal and in phase with the AC mains voltage while innormal mode. Line current harmonic components show aTHD of 3.16% and an input power factor of 0.995. TheUPS output voltage shows good regulation with low THDwhen feeding nonlinear loads, as shown in Fig. 7. Despite adistorted current io, the measured THD of inverter outputvoltage vo is 4.53%. Figure 8 shows that the bidirectionalAC/DC converter acts as an APF and produces the currentrequired by a nonlinear load in outage mode. Inverter
output voltage vo, load current il, active filter current ia andoutput current io are presented in Fig. 8. Active filter currentia shows almost the same harmonic components as loadcurrent il. Thus inverter output current io becomessinusoidal with nearly unity power factor at the DC/ACvoltage inverter side. The THD of the DC/AC voltageinverter output current io is 6.89%.Table 2 lists the measured values during normal and
outage modes. It also shows that line current THD, reactivepower Q and UPS apparent power S are significantlydecreased when the UPS operates at outage mode, clearlyshowing the effectiveness of the proposed UPSsystem. Consequently, the proposed UPS system ensures353W real output power to the existing load, but is alsocapable of delivering an additional 143W real power to anyadditional load. Thus the proposed UPS exhibits excellentperformance for nonlinear loads. The transitions fromnormal mode to outage mode and vice versa are shown inFigs. 9 and 10. The above experimental results all giveconsistent agreement with simulated results, confirming
Fig. 6 Simulated and experimental results of AC line voltage vs
and current is in normal mode (vs 50 V/division, is 5 A/division, time5 ms/division)a Simulated resultb Experimental resultc Harmonic spectrum of line current is; THD¼ 3.16%
IEE Proc.-Electr. Power Appl., Vol. 151, No. 1, January 2004 125
good performance compared with the conventional UPSapproach.
5 Conclusions
This paper has presented a online UPS system with built-inbidirectional AC/DC converter with improved performancerelative to that of a conventional UPS system. ADSP-basedUPS prototype has been implemented to verify the multi-loop digital control schemes. The control systems aredescribed and simulations presented to verify the perfor-mance of the proposed system. Simulation results fornormal mode show a near-unity input power factor and awell-regulated output voltage to the load. The UPS systemshows a well-regulated output voltage and an outputcurrent in phase with the output voltage during outage.Experimental results are in good agreement with simulatedresults, confirming the effectiveness of the proposed UPSand improved operating characteristics compared withthose of conventional online UPS systems. Advantages ofthe proposed system include high real output power
vo
io
a
b
vo
io
100
00 2.0k
dist
ortio
n, %
frequency, Hzc
Fig. 7 Simulated and experimental results of DC/AC inverteroutput voltage vo and output current io in normal mode (vo 50 V/division, io: 5 A/division, time 5 ms/division)a Simulated resultb Experimental resultc Harmonic spectrum of voltage vo; THD¼ 4.53%
vo
vo
il
ia
io
il
ia
io
100
00 2.0k
dist
ortio
n, %
frequency, Hzc
b
a
Fig. 8 Simulated and experimental results of UPS system duringoutage mode (inverter output voltage vo, 100 V/division; load currentil, 10 A/division; bidirectional AC/DC output current ia, 5 A/division;inverter output current io, 5 A/division; time 5 ms/division)a Simulated resultb Experimental resultc Harmonic spectrum of inverter output current io; THD¼ 6.89%
Table 2: Operating parameters at outage mode
ConventionalUPS
Proposed UPSsystem
Output current io THD % 71.62 6.89
Output power factor 0.707 0.991
Real power P 353W 353W
Reactive power Q 357 VAr 45 VAr
Apparent power S 500VA 356VA
Inverter voltage vo RMS 110V 110V
Inverter output current io RMS 4.48A 3.21A
126 IEE Proc.-Electr. Power Appl., Vol. 151, No. 1, January 2004
capacity, full utilisation of the power converter and lowsystem cost per unit of real output power.
6 Acknowledgments
The authors wish to thank Dr. J.F. Chen for offeringmany useful suggestions concerning the research in thispaper.
7 References
1 Hirachi, K., Sakane, M., Matsui, T., Kajima, A., and Nakaoka, M.:‘Cost-effective practical developments of a high performance andmulti-functional UPS with new system configurations and theirspecific control implementations’. Proc. IEEE Power electronicsspecialist conf. (PESC), 1995, pp. 480–485
2 Chen, J.F., and Chou, C.L.: ‘Combination of voltage-controlled andcurrent-controlled inverter for UPS parallel operation’, IEEE Trans.Power Electron., 1995, 10, (5), pp. 547–558
3 Krishnan, R.: ‘Design and development of a high frequencyon-line uninterruptible power supply’. Proc. IEEE Industrialelectronics, control, and instrumentation Conf. (IECON), 1995,pp. 578–583
4 Kamran, F., and Habetler, T.G.: ‘A novel on-line UPS with universalfiltering capabilities’, IEEE Trans. Power Electron., 1998, 13, (3),pp. 410–418
5 Joos, G., Lin, Y., Ziogas, P.D., and Lindsay, J.F.: ‘An on-line UPSwith improved input-output characteristics’. Proc. IEEE Appliedpower electronics Conf. (APEC), 1992, pp. 598–605
6 Von Jouanne, Enjeti, P.N., and Lucas, D.J.: ‘ DSP control of high-power UPS systems feeding nonlinear loads’, IEEE Trans. Ind.Electron., 1996, 43, (1), pp. 121–125
7 Kazerani, M., Ziogas, P.D., and Joos, G.: ‘An AC-to-DCactive current waveshaping technique for solid-state input powerfactor conditioners’, IEEE Trans. Ind. Electron., 1991, 38, (1),pp. 72–78
8 Salmon, J.C.: ‘Techniques for minimizing the input current distortionof current-controlled single-phase boost rectifiers’, IEEE Trans. PowerElectron., 1993, 8, (4), pp. 509–520
9 Spiazzi, G., and Lee, F.C.: ‘Implementation of single-phase power-factor-correction circuits in three-phase application’, IEEE Trans. Ind.Electron., 1997, 44, (3), pp. 365–371
10 De Souza, F.P., and Barbi, I.: ‘ Power factor correction of linear andnonlinear loads employing a single phase active power filter based on afull-bridge current source inverter controlled through the sensor of theAC mains current’. Proc. IEEE Power electronics specialist Conf.(PESC), 1999, pp. 387–392
11 Singh, B., Haddad, K.A., and Chanadra, A.: ‘A review of active filtersfor power quality improvement’, IEEE Trans. Ind. Electron., 1999, 46,(5), pp. 960–971
12 Liang, T.J., Shyu, J.L., and Chen, J.F.: ‘High real output poweron-line UPS system with built-in reactive power compensation’.Proc. IEEE Power electronics specialist conf. (PESC), 2001,pp. 1555–1560
14 Abdel_Rahim, N., and Quaicoe, J.E.: ‘Analysis and design of amultiple feedback loop control strategy for a single-phase voltage-source UPS inverter’, IEEE Trans. Power Electron., 1996, 11, (4),pp. 532–541
15 Ito, Y., and Kawauchi, S.: ‘Microprocessor-based robust digitalcontrol for UPS with three-phase PWM inverter’, IEEE Trans. PowerElectron., 1995, 10, (2), pp. 196–204
vs
vo
io
il
is / ia
is ia
Fig. 9 Experimental results of UPS system during suddentransition from normal to outage mode
ia / is
vs
vo
io
il
ia is
Fig. 10 Experimental results of UPS system during suddentransition from outage to normal mode
IEE Proc.-Electr. Power Appl., Vol. 151, No. 1, January 2004 127
