Novel line-interactive uninterruptible power supply

H.-L. Jou, J.-C. Wu, C. Tsai, K.-D. Wu and M.-S. Huang

Abstract: In the paper, a novel uninterruptible power supply (UPS) is proposed with theperformance of input current harmonic suppression, input power factor correction andoutput voltage regulation. This UPS comprises two power converters, a shunt powerconverter and a series power converter. The shunt power converter is connected to theutility via a link inductor, for providing a compensating voltage, to suppress the inputcurrent harmonic and improve the input power factor. The series power converter is connectedserially to the load to provide a clean and regulated output voltage. A voltage-modecontrol algorithm with fast response and simple control circuit is proposed to control the shuntpower converter. The series power converter is controlled by a current-mode control to ensure thequality of output voltage. A single-phase prototype is developed and tested to verify theperformance of the proposed UPS. Experimental results show that the proposed UPS has expectedperformance.

1 Introduction

Power supply circuits are widely used in facilities such asuninterruptible power supplies (UPSs), motor drivesand other applications. Conventional UPSs use a varietyof different circuit topologies, including standby,line-interactive and online topologies. Generally, each ofthe three topologies has advantages and disadvantages.The selection of UPS depends on the requirement of theapplications [1].The major difference between these UPS topologies is

whether the power-load demand is supplied from the utilitydirectly or not under the normal utility condition. A typicalstandby UPS topology includes a switch that connects theload directly to the utility, under the normal utilitycondition, and that transfers the load to the inverter derivedfrom a battery, when the utility fails. As the switchingtransition time of the electromagnetic switch or relay isseveral milliseconds or more, such a standby UPS oftenhas a significant interruption before the backup power isdelivered to the load. In addition, standby UPSs cannotcompensate the power quality, e.g. voltage dip, harmonicdistortion and low power factor. Nevertheless, the standbytopology is often used for low-cost UPSs, because it ischeaper than other UPS topologies.A typical online UPS includes a series train of an AC/DC

converter and a DC/AC inverter. The AC/DC powerconverter converts the AC power of the utility to a DCpower on a DC busbar, and the DC/AC inverter convertsthe DC power to an AC power for supplying a clean and

regulated output voltage to the load. Typically, the onlineUPS includes the DC busbar that is used to isolate the loadfrom disturbance and other voltage sag or voltage dip of theutility. The DC busbar is coupled to an auxiliary source ofpower, such as a battery, which maintains the DC busbarvoltage when the utility fails. In addition, the AC outputvoltage of an online UPS is well regulated, and there is nopower interruption to the load when the utility fails. Athyristor-controlled rectifier often configures the AC/DCpower converter, and it will result in problems of high inputcurrent harmonic and poor input power factor. However,these problems can be solved if the power factor correction(PFC) circuit or the PWMAC/DC power converter is used[2, 3]. The online UPS is less efficient than the standby UPSdue to dual power conversion, but this UPS guarantees thatthe power to the load is clean and well regulated, in spite ofthe condition of the utility.The line-interactive UPS may contain one or two power

conversion stages [4–10]. The line-interactive UPS using onepower stage [4–6] can improve the problem of input currentharmonics. However, the performance of power factorcorrection and output voltage regulation cannot be satisfiedsimultaneously. The line-interactive UPS using two powerconversion stages can perform the input harmonic suppres-sion, the power factor correction and the output voltageregulation simultaneously. Compared with the online UPS,the overall power converter capacity of line-interactive UPSusing two power conversion stages can be reduced,evidently. Moreover, the efficiency of line-interactiveUPS using two power conversion stages is higher than thatof online UPS.In this paper, a novel line-interactive UPS, configured by

a shunt power converter and a series power converter, isproposed. The shunt power converter has the function ofbattery charger, and facilitates power factor correction andharmonic suppression. The series power converter acts asa voltage regulator and regulates the output voltage. Avoltage-mode control algorithm with fast response andsimple control circuit is proposed to control the shuntpower converter. The series power converter is controlled bya current-mode control to ensure the quality of outputvoltage. To demonstrate its performance, a single-phaseprototype has been developed and tested.

H.-L. Jou, K.-D. Wu and M.-S. Huang are with the Department of ElectricalEngineering, National Kaohsiung University of Applied Sciences, Kaohsiung80782, Taiwan, Republic of China

J.-C. Wu is with the Department of Electrical Engineering, Kun ShanUniversity of Technology, Tainan Hsien 710, Taiwan, Republic of China

C. Tsai is with the Delta Electronics Corporation, Taiwan, Republic of China

r IEE, 2004

IEE Proceedings online no. 20040481

doi:10.1049/ip-epa:20040481

Paper first received 26th March 2003 and in revised form 19th September 2003.Originally published online: 16th March 2004

IEE Proc.-Electr. Power Appl., Vol. 151, No. 3, May 2004 359

2 System configuration

The system configuration of the proposed line-interactiveUPS is shown in Fig. 1. This UPS consists of a linkinductor, a shunt power converter, a battery set, a seriespower converter, a transformer, a filter capacitor and threeswitches. When the utility voltage is normal, S1 and S3 areclosed. During this time, the shunt power converter is usedto generate a voltage, such that the input current is asubstantial sinusoidal wave and is in phase with the utilityvoltage, and the series power converter is serially connectedto the load via a transformer for providing a clean andregulated output voltage to the load. Hence, this UPS hasthe function of providing harmonic current suppression,power factor correction, battery charger and output voltageregulation. Moreover, the power demanded by the load isalmost supplied from the utility directly. The shunt powerconverter only charges the power of the battery and theseries power converter regulates the output voltage, hencethe efficiency is higher than that of an online UPS. Whenthe utility voltage is abnormal, S1 and S2 are opened todisconnect the utility power. The full-bridge powerconverter is applied in the shunt and the series powerconverters, and the operation of these power converters iscontrolled to be the bidirectional power flow [11]. The shuntand the series power converters convert the energy stored inthe battery of DC busbar to the load. Hence, this UPS hasthe function of supplying backup power. When this UPS ismaintained, S1 and S3 are opened to disconnect the shuntand series power converters from the utility and the load,and S2 is closed to form a bypass loop to supply the utilitypower to the load.

3 Operation principle

In the proposed UPS, the shunt power converter is used tocontrol the input current to be sinusoidal and in phase withthe utility voltage, and the series power converter is used tocompensate the output voltage of the UPS as a constantvalue with low harmonic distortion. The shunt powerconverter is controlled by the voltage-mode control, and theseries power converter is controlled by the current-modecontrol. Figure 2 is the equivalent circuit of Fig. 1 while theutility is normal.

3.1 Shunt power converterThe basic operation principle of the voltage–mode shuntpower converter, which is connected to the utility power viaa link inductor, can be explained by using the equivalentcircuit shown in Fig. 2. As can be seen, it consists of twovoltage sources, the utility voltage and the output voltage of

the shunt power converter. The utility voltage is uncontrol-lable, and the output voltage of the shunt power converter iscontrollable. The operation of the shunt power converter isbased on the concept that, if the voltage waveforms of twosuccessive nodes in a circuit were sinusoidal signals, thenthe current waveform between these two nodes would besinusoidal. As the utility voltage is uncontrollable, thedesired phase and amplitude of current waveform canbe obtained by controlling the phase and the amplitude ofthe output voltage of the shunt power converter.

3.1.1 Conventional voltage-mode controlalgorithm: Conventionally, the control algorithm ofthe voltage-mode line-interactive UPS contains two controlloops: a real power control loop and a reactive powercontrol loop [4–6]. The real power control loop is used tocontrol the phase of converter output voltage. The realpower of the utility must meet the real power demanded bythe load and the battery set. The reactive power controlloop is used to control the amplitude of converter outputvoltage to meet the input power factor requirement. As theconventional control algorithm contains two control loops,the above control loops may interact each other. This mayresult in oscillation around the steady-state operation point.To suppress the interaction between these two controlloops, the response speed of one control loop must be fasterthan that of the other control loop. Hence, the transientresponse of the overall system is very poor. Moreover, thecontrol circuit of this algorithm is very complicated. Insome applications, the constant amplitude of the loadvoltage is to be expected. It uses only the real power controlloop. However, the power factor cannot be close to unity inthis application.

3.1.2 Proposed voltage-mode control algor-ithm: A voltage-mode control algorithm for the voltage-source voltage-control active power filter proposed in [12]has been modified and applied in the shunt power converterof the proposed line-interactive UPS. The salient point ofthe proposed voltage-mode control algorithm for the shuntpower converter is that the phase and amplitude of theconverter output voltage is combined into one control loop,rather than two individual control loops, in the conven-tional voltage-mode control algorithm. As the unity powerfactor is expected, the utility current must be sinusoidal andin phase with the utility voltage. If the utility voltage isrepresented as

vsðtÞ ¼ Vs sin ðotÞ ð1Þthen the utility current must be

isðtÞ ¼ Is sin ðotÞ ð2ÞFrom Fig. 2, the converter output voltage can be derived as:

vconðtÞ ¼ vsðtÞ � ZLisðtÞ ð3Þ

utilitylink inductor

S1

S2

S3load

battery set

shunt powerconverter

series powerconverter

Fig. 1 System configuration of proposed UPS

is(t ) iL(t )

ic(t )ZL(t )

vs(t ) vconv(t )i conv1(t )

iconv2(t )

load

Fig. 2 Equivalent circuit

360 IEE Proc.-Electr. Power Appl., Vol. 151, No. 3, May 2004

where ZL is the impedance of the link inductor. Figure 3is the vector diagram showing the relation between theutility voltage and the output voltage of the shunt powerconverter. As seen in Fig. 3, the compensating voltagecontrolled by the shunt power converter is a vector additionof the utility voltage (vs(t)) and a negative voltage across thelink inductor (�ZLis(t)). As the utility current is in phasewith the utility voltage, the voltage across the link inductoris orthogonal to the utility voltage. The output voltage ofthe power converter can be obtained by summing the utilityvoltage and the voltage of the link inductor voltage(�ZLis(t)). The utility voltage can be detected directly.Hence, the unknown parameter for determining the outputvoltage of the shunt power converter is only the amplitudeof the link inductor voltage (�ZLis(t)). However, theamplitude of the link inductor voltage is proportional tothe amplitude of the utility current. As the input powerfactor is unity, the real power supplied from the utility isproportional to the amplitude of the utility current. Thismeans that the real power supplied from the utility isproportional to the amplitude of the orthogonal vector ofthe utility voltage directly. The supplied or consumed realpower among the utility, the load and the battery set mustbe balanced. In the steady state, the real power suppliedfrom the utility is matched to the real power demanded bythe load and the battery set. The steady state will be lostunder the condition that the load or the utility voltage isvaried. The difference in real power between the utility andthe load will be injected into or supplied from the batteryset. In this condition, the power charging into the battery setwill be away from its set value. The real power charging intothe battery set will be larger than its set value when thesupplied real power of the utility is larger than the realpower demanded by the load and the battery set. On thecontrary, the real power charging into the battery set will besmaller than its set value. Hence, comparing the real powercharging into the battery set with the set value can help todetermine the amplitude of voltage drop on the linkinductor.

From this, it can be concluded that only one controlloop is used in the proposed control algorithm, and theinteraction problem of the conventional control algorithmcan be avoided. Hence, the proposed algorithm canimprove the response of voltage-controlled algorithms.Also, the control circuit is simplified and no current sensoris required.

3.2 Series power converterThe series power converter is connected to the primarywinding of a transformer, and the secondary winding of thetransformer is connected in series with the load. The DCbusbar used in the series power converter is the same as theshunt power converter. To obtain a high-quality output

voltage, the series power converter is controlled by thecurrent-mode control, rather than voltage-mode controldue to its fast response. For a failed UPS, the load powermust be supplied by the utility. To obtain a better transientperformance, the output voltage of the UPS must be inphase with the utility and be represented as

voðtÞ ¼ Vo sin ðotÞ ð4ÞFrom Fig. 2, the capacitor current can be represented as

icðtÞ ¼VOXCsin ðot þ 90�Þ ð5Þ

where Xc is the impedance of filter capacitor. To obtain thedesired output voltage shown as (4), the desired outputcurrent of the series power converter can be represented as

icon2ðtÞ ¼iLðtÞ þ icðtÞ

Ktð6Þ

where Kt is the turn ratio of the transformer.The amplitude of the utility voltage varies at any time.

From (2), it can be found that the amplitude of the shuntpower converter output voltage also varies with the utilityvoltage. However, the amplitude of the proposed UPSoutput voltage is expected to be a constant, and the voltagedrop on the secondary winding of series transformer is thevoltage difference between the output voltage of the UPSand the shunt power converter. In a general distributionpower system, the voltage variation in the utility is not morethan725% of the specified utility voltage. Thus, the powercapacity of the series power converter is only about 25% ofthe load capacity. Hence, the total capacity of both shuntand series power converters of the proposed UPS is smallerthan that of the conventional online UPS. Therefore, theefficiency, volume and hardware cost of the proposed UPSare superior to that of the conventional online UPS.From this, it can be found that the shunt power converter

in the proposed UPS is used to generate a compensatingvoltage, such that the input current is a substantialsinusoidal wave and is in phase with the utility voltage.The shunt and series power converters are controlled bythe voltage-mode control and the current-mode control,respectively, under both normal and abnormal conditions.Hence, the performance of transition transient at the instantof the utility failure is very good and the control circuit canbe simplified.

4 Control block diagram

The shunt power converter is controlled by the voltage-mode control, and the series power converter is controlledby the current-mode control, described in the following.

4.1 Control block diagram of shunt powerconverterFigure 4 shows the control block diagram of the shuntpower converter. The switch w2 is placed at point ‘c’ underthe normal utility and at point ‘d’ as the utility fails. FromFig. 2, it can be found that the converter output voltage canbe obtained by the vector summation of the utility voltageand the link inductor voltage (�ZLis(t)) under the normalutility. The utility voltage is often distorted, due to theneighbouring nonlinear loads in the industrial distributionsystem. To block the spike, dip and harmonic voltage of theutility, the detected utility voltage is fed into the bandpassfilter I to extract its fundamental component. The output ofthe bandpass filter I is fed into a phase shift circuit I togenerate a sinusoidal signal which is lagging to the utilityvoltage 901, then the waveform and the phase of the

iS (t )

vconv(t )

vs(t )

(−ZLiS(t ))

Fig. 3 Vector diagram of shunt power converter

IEE Proc.-Electr. Power Appl., Vol. 151, No. 3, May 2004 361

negative link inductor voltage (�ZLis(t)) is obtained. Theamplitude of the link inductor voltage can be determined bythe charging power control. The charging power control ofthe battery set is divided into two paths. The switch w1 isused to select the battery charging to be the constant-current charge mode or the constant-voltage charge mode,and the setting place of w1 depends on the voltage of thebattery set (DC busbar voltage). If the voltage of battery setis smaller than the preset threshold value, the switch w1 isplaced at point ‘a’ for the constant-current charge mode.The voltage of the battery set will rise during the process ofconstant-current charge. When the voltage of battery set ishigher than the preset threshold value, the switch w1 ischanged into ‘b’. Then, the battery set is changed into theconstant-voltage charge mode. The output of lowpass filterI or II is compared with the set value, then, and thecompared result is then sent to a PI controller. The responsetime of charging current and DC busbar voltage is notthe same, however this difference can be designed in thelowpass filters I and II.Hence, the control of charging current and DC busbar

voltage can be combined in a PI controller. The output ofthe PI controller is the amplitude of the value of the linkinductor voltage (�ZLis(t)). The output signals of the PIcontroller and the phase shift circuit are sent to a multiplier,to obtain the link inductor voltage (�ZLis(t)). The referencesignal of the power converter is obtained by summing themultiplier output and detected utility voltage. If the utilityvoltage fails, the switch w2 is changed into ‘d’. The PLLcircuit will be operated in the free-running mode, and this

mode will automatically generate a square signal with afrequency of 60Hz. The reference signal is a sinusoidalsignal with constant amplitude from the output of thebandpass filter II.When the utility voltage is recovered, the PLL circuit can

force the frequency and phase of converter output voltage

to trace the utility. The switch w2 will be set to position ‘c’when the phase and frequency are locked. Finally, thereference signal and the detected output voltage of the shuntpower converter is sent to the waveform controller andPWM circuit I. The outputs of PWM circuit I are fed intothe gate driver I to generate the driving signals for switchingdevices of the shunt power converter.

4.2 Control block diagram of series powerconverterFigure 5 shows the control block diagram of the seriespower converter. The desired output current of the seriespower converter is the summation of the load currentand the filter capacitor current shown in (5). The loadcurrent is detected directly by a current sensor. The filtercapacitor current can be obtained by dividing the desiredoutput voltage by the impedance of the filter capacitor.The bandpass filter II in the control block diagram ofFig. 4 generates a reference sinusoidal signal v�LðtÞ withthe constant amplitude waveform and in phase with theutility voltage. The reference signal v�LðtÞ is used asthe desired output voltage of the UPS. The referencesignal v�LðtÞ is sent to the phase shift circuit II to generatea signal, which leads the reference signal v�LðtÞ 901.The output of the phase shift circuit II is sent to anamplifier I to obtain the desired capacitor current i�cðtÞ. Inthe ideal condition, the output voltage waveform of theproposed UPS is a sinusoidal waveform with constantamplitude, if the series power converter can supply thecurrent shown as (6).

However, an error may be generated in the current-modecontroller. Hence, the output voltage of the UPS may bedistorted and its amplitude may be varied due to loadvariation. This problem can be overcome by using an extrafeedback control loop of output voltage [4]. The outputvoltage of the proposed UPS is detected and compared with

+

−

+

load current

output voltage

series convertercurrent

vL*(t ) phase shift

circuit IIamplifier I

amplifier II

currentcontroller

gatedriver II

PWMcircuit II

ic*(t)

Fig. 5 Control block diagram of series power converter

+

a

b

w1

+

−

c

d

w2

vL*(t)

−Z Li S(t)

converter output voltage

utility voltage

DC busbar voltage

charging current

PLLcircuit

bandpass filter II

lowpass filter II

lowpass filter I

bandpass filter I

phase shift circuit I

set value

PI controller

waveformcontroller

PWMcircuit I

gatedriver I

Fig. 4 Control block diagram of shunt power converter

362 IEE Proc.-Electr. Power Appl., Vol. 151, No. 3, May 2004

the reference signal v�LðtÞ. The compared result is sent toamplifier II to generate a current correction component tocorrect the error caused by the current-mode controller. Theoutput of this amplifier II is added to i�cðtÞ and the loadcurrent iL(t) to obtain the desired output current of theseries power converter. The desired output current and thepractical current of the series power converter are sent tothe current–mode controller. The output of current-modecontroller is then sent to the PWM circuit II to generate thedriving signals of power devices in the series powerconverter. The outputs of PWM circuit II are fed into thegate driver II to obtain the driving signals of the seriespower converter.

5 Experimental results

To verify the performance of the proposed algorithm, asingle-phase scale-down prototype (500VA) is developed.The full-bridge power converters are used in the shunt andthe series power converter. The parameters used in theprototype are shown in Table 1. Figure 6 shows theexperimental result under the normal utility condition.The load is a rectifier. As seen in Fig. 6b, the output voltageof the UPS is a pure sinusoidal waveform and is in phasewith the utility voltage. Figure 6c shows that the utilitycurrent is nearly sinusoidal and in phase with the utilityvoltage, and the unity input power factor can be obtained.Figure 7 shows the regulation performance of UPS outputvoltage. As seen in From Fig. 7, the output voltagevariation of the proposed UPS is not more than 1% underthe utility voltage with725% variation. Figure 8 shows theoutput voltage under the variation of the utility voltage. As

seen in Fig. 8b, the output voltage is not affected by thevariation in the utility voltage.Figure 9 shows the performance of the proposed UPS

under the distorted utility voltage. Figure 9a shows theutility voltage with 15% total harmonic distortion (THD).As seen in Fig. 9b, the utility current is still sinusoidal and inphase with the utility voltage, and Fig. 9c shows that the

Table 1: The parameters of prototype

DC busbar voltage 190V Link inductor 5mH

Switching frequencyof shunt powerconverter

20KHz Switching frequencyof series powerconverter

20KHz

Transformer 4:1 Output filter capacitor 20mF

Fig. 6 Experimental result under normal utility voltagea Utility voltageb Output voltagec Utility currentd Load current

114

113

110

109

108

107

106

111

112

load

vol

tage

, V

85 90 95 100 105 110 115 120 125 130 135

mains voltage, V

Fig. 7 Regulation performance of UPS output voltage

Fig. 8 Experimental result under variation of the utility voltagea Utility voltageb Output voltage

Fig. 9 Experimental result under distorted utility voltagea Utility voltageb Utility currentc Output voltage

IEE Proc.-Electr. Power Appl., Vol. 151, No. 3, May 2004 363

output voltage of the proposed UPS is still nearly sinusoidaland in phase with the utility voltage, regardless, whether theutility voltage is distorted or not. Figure 10 shows theperformance of the proposed UPS under the voltage dipcondition. Figure 10a shows the utility voltage with thevoltage dip. As seen in Fig. 10b, the utility current is stillnearly sinusoidal and in phase with the utility voltage.Figure 10c shows that the output voltage of the proposedUPS is still nearly sinusoidal and in phase with the utilityvoltage regardless whether the utility voltage is dip or not.Figure 11 shows the experimental result at the instant ofthe utility failure. As can be seen, the transition transientperformance of the proposed UPS at the instant of theutility failure is excellent. These experimental results showthat the proposed UPS has the expected performance.

6 Conclusions

The online UPS can supply clean power to the critical loads.However, it has the disadvantages of high input currentharmonic, poor input power factor and low efficiency. Thestandby UPS has high efficiency, but it cannot improvepower quality of the load.In this paper, a new line-interactive UPS is proposed. The

proposed UPS has the performance of low input currentharmonic distortion and high input power factor. Inaddition, the overall power rating of the power conversionstages and the power loss are smaller than that of onlineUPS. The salient contributions of the proposed UPS arethat the control modes of both the series and shunt powerconverters remain the same under normal and failed utilityconditions. It has the advantages of simple control circuitand excellent performance at the transient transition. Toverify the performance of the proposed UPS, a single-phaseprototype is developed and tested. The experimental resultsshow that the proposed UPS has the expected performance.

7 Acknowledgment

The authors would like to express our acknowledgmentto the DELTA Electronics Corporation for the financialsupport of this paper. Also, the authors express theiracknowledgement to the students of National KaohsiungUniversity of Applied Sciences who helped to set up thehardware circuit.

8 References

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Fig. 10 Experimental result under utility voltage with dipa Utility voltageb Utility currentc Output voltage

Fig. 11 Experimental result at instant of utility failurea Output voltageb Utility current

364 IEE Proc.-Electr. Power Appl., Vol. 151, No. 3, May 2004