IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 36, NO. 1, JANUARY/FEBRUARY 2000 33

Automatic Voltage Regulator Using an ACVoltage–Voltage Converter

Steven M. Hietpas, Member, IEEE,and Mark Naden, Student Member, IEEE

Abstract—Voltage sags and extended undervoltages are one ofthe main concerns of industry today. These voltage sags could causehigh negative impact on productivity, which is certainly an undesir-able aspect in industrial and commercial applications. Current tap-changing transformers used in distribution systems have proven tobe inadequate in solving these problems related to line regulation.A solution to these problems is to install an ac voltage–voltage con-verter that has been developed primarily for voltage-sag correc-tion. This system incorporates high-speed insulated gate bipolartransistor switching technology and was designed to provide thespeed and efficiency required by industrial customers. Further-more, the system will provide the flexibility of installation with orwithout the incorporation of tap-changing transformers. Simula-tion results show the operations involved in voltage-sag correctionof the ac voltage–voltage converter. A single-phase system can bereadily developed into a three-phase converter system based on theexact principle of operation.

Index Terms—AC–AC buck, ac voltage–voltage converter, insu-lated gate bipolar transistors, power quality, tap-changing trans-formers, voltage sags.

I. INTRODUCTION

POWER quality describes the quality of voltage and currenta facility has, and is one of the most important considera-

tions in industrial and commercial applications today. It is es-sential that processes, in particular, in industrial plants, operateuninterrupted where high productivity levels are an importantfactor. Power quality problems commonly faced by industrialoperations include transients, sags, swells, surges, outages, har-monics, and impulses that vary in quantity or magnitude of thevoltage [1]. Of these, voltage sags and extended undervoltageshave the largest negative impact on industrial productivity, andcould be the most important type of power quality variation formany industrial and commercial customers [1]–[6].

Voltage sags are momentary voltage variations, usuallycaused by a short circuit or fault somewhere in a powerdistribution system, as shown in Fig. 1.

Recent survey results show that a typical distribution systemcustomer experiences an average of 70 events per year wherethe voltage drops below 70% of the nominal [7]. Interruptions

Paper ICPSD 99-C5, presented at the 1999 IEEE Rural Electric Power Con-ference, Indianapolis, IN, May 2–4, and approved for publication in the IEEETRANSACTIONS ONINDUSTRYAPPLICATIONSby the Rural Electric Power Com-mittee of the IEEE Industry Applications Society. This work was supported inpart by the American Power Producers Association under a DEED Grant. Man-uscript submitted for review May 4, 1999 and released for publication August27, 1999.

The authors are with the Electrical Engineering Department, South DakotaState University, Brookings, SD 57007 USA (e-mail: hietpass@mg.sdstate.edu;naden@brookings.net).

Publisher Item Identifier S 0093-9994(00)00039-6.

of a few cycles could affect complex processes that use precisionelectronic equipment and computerized control. The Computerand Business Equipment Manufacturers Association (CBEMA)also found that sags below 70% are likely to impact data pro-cessing equipment [7].

Some major problems associated with unregulated line volt-ages (in particular, long-term voltage sags) include equipmenttripping, stalling, overheating, and complete process shutdowns[8]. These subsequently lead to lower efficiencies, higher powerdemand, higher cost for power, electromagnetic interference tocontrol circuits, excessive heating of cables and equipment, andincreased risk of equipment damage. The need for line voltageregulation still remains a necessity to meet demands for high in-dustrial productivity.

There are several conditioning solutions to voltage regula-tion, which are currently available in the marketplace. Amongthe most common are tap-changing transformers, which are thetypes of voltage regulators used in today’s power distributionsystems. However, these methods have significant shortcom-ings. For instance, the tap-changing transformer requires a largenumber of thyristors, which results in highly complex opera-tion for fast response [8]. Furthermore, it has very poor transientvoltage rejection, and only has an average response time.

Some solutions have been suggested to encounter the effectsof voltage sag in [7], [9], and [10], but have not been devel-oped fully towards replacement of tap-changing transformers.An ac voltage–voltage converter prototype is under develop-ment at South Dakota State University, Brookings, to providesolutions to problems related to line regulation, which will meetthe power quality preferred by industrial customers [11].

The remainder of this paper is divided into three major sec-tions. Section II provides details of the design and descriptionof the ac voltage–voltage converter model. Section III containssimulation results showing the functionality of the design.Section IV illustrates several options that incorporate theconverter into tap-changing transformer configurations, as wellas for complete replacement. The paper ends with concludingremarks.

II. AC V OLTAGE–VOLTAGE CONVERTERMODEL

The ac voltage-voltage converter model is based on a buckconverter ( ) configuration, with a step-up injectiontransformer used at its output. The converter incorporatesfast-switching insulated gate bipolar transistor (IGBT) tech-nology, and controls involving pulsewidth modulation (PWM)techniques. The model block diagram is shown in Fig. 2.

Letting (Fig. 2) represent the nominal line voltage, avoltage sag is present when (the true line voltage) is less than

0093–9994/00$10.00 © 2000 IEEE

34 IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 36, NO. 1, JANUARY/FEBRUARY 2000

Fig. 1. Typical voltage sag.

Fig. 2. Model block diagram of the ac voltage–voltage converter system.

. The objective is to regulate to when a voltagesag is present. In Fig. 2, the converter output voltage,, isdetermined by the duty cycle based on negative feedback ofthe output line voltage. The value for is given by

(1)

Through closed-loop feedback, the duty cycle changes ac-cording to the degree of source voltage drop (sag). The outputvoltage of the converter is injected in series with the linethrough the injection transformer, where the turns ratioisgiven by

(2)

Let the output voltage represent the line-to-line voltage onthe load side of the injection transformer. Assuming negligiblephase shift from to , as well as negligible phase shiftfrom to , the magnitude of this output line voltage is

and can further be simplified to

(3)

We define the percentage voltage sag as

(4)

The required turns ratio is based on the worst case an-ticipated. Under worst case conditions (maximum ), notingthat the objective is to regulate to , and using (1)–(4),two design equations result and are given in(5) and(6). The de-sign equation for is given by

(5)

Fig. 3. Square-wave PWM signal.

TABLE IVALUES OF a FOR A RANGE OF EXPECTED

VOLTAGE SAGS

Fig. 4. Turns ratioa as a function ofP .

and for a given transformer turns ratio, the duty cycle for aregulated output is given by

(6)

is a function of the PWM signal, which is essentially asquare wave with a variable pulsewidth (shown in Fig. 3). Basedon Fig. 3, is given by

(7)

where is the time that switches and are on in a givenswitching period, . The complementary pair and areon for the remaining period of time, (see Fig. 10).

Table I shows the required value of transformer turns ratiofor a range of expected worst case voltage sags.

Fig. 4 shows a plot for versus based on (5).Fig. 5 is a schematic of a single-phase ac voltage–voltage

converter.Table II shows the various values of for a range of voltage

sags, assuming a worst case voltage sag of 75%, hence, the re-quirement for a 1 : 3 step-up transformer ( ).

Fig. 6 shows various plots for as a function of fora range of specified worst case sag conditions and transformerturns ratios.

III. SIMULATION RESULTS AND ANALYSIS

SPICE simulation results demonstrate the operation of theconverter model. Simulation results use ideal characteristics ofcircuit components.

The design example considered is based on the assumptionthat the distribution system may experience as much as 75%voltage sag due to certain disturbances. Fig. 7 shows how theconverter corrects a voltage sag of 60% from the nominal line

HIETPAS AND NADEN: AUTOMATIC VOLTAGE REGULATOR 35

Fig. 5. Single-phase ac voltage–voltage converter schematic.

TABLE IIVALUES OFD FOR A RANGE OF VOLTAGE SAGS ASSUMING A

WORSTCASE SAG OF 75%

Fig. 6. D as a function ofP for a range of expected worst case voltage sagsand selected transformer turns ratio.

voltage ( V ) between ms (onset of disturbance)and ms (fault is cleared). The line voltage is reduced to83.2 V for a period of approximately 4 cycles. Using (1),(2), and (6), we note that during the fault time, the duty cyclefrom the PWM signal is adjusted to 50%, providing an outputvoltage of 41.6 V from the converter.

Fig. 8 (curve from Fig. 6 where [Max P , a] = [75,1/3]) shows the steady-state duty cycle that is requiredfor voltage-sag correction under these specified conditions.Changes in the duty cycle only take place half a cycle after thesag occurs, as this is the minimum time required to sense and

Fig. 7. Ideal characteristics of the voltage-sag correction, whereV(RL:2)is theoutput line voltage andV(V1:+) is the input line voltage.

Fig. 8. D = 50% for 60% sag correction, (P (max) = 75% anda =1=3).

36 IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 36, NO. 1, JANUARY/FEBRUARY 2000

Fig. 9. Output line voltage during fault condition.

Fig. 10. Ripple content on the peak output voltage during voltage sag period.

convert the ac voltage into an rms value. The duty cycle canbe changed either instantly after the half-cycle rms calculation,or ramped up for the same period of time over the remaininghalf cycle (less transient disturbance effect). As the duty cycleis adjusted, the converter output voltage is boosted by theseries injection transformer and added to the line voltage. Thisprocess yields a regulated output voltage V .

Fig. 9 shows the converter output voltage based on the voltagesag conditions shown in Fig. 7. Small transients are observeddue to the instantaneous change of the duty cycle. Fig. 10 showsthe ripple on the output voltage and how it increases during thevoltage sag or fault condition.

Further discussion on these issues concerning the ripple con-tent and mathematical expressions used for the calculation ofcomponent values involved are available in Fig. 10.

IV. A PPLICATION OF THEAC VOLTAGE–VOLTAGE CONVERTER

The ac voltage–voltage converter was designed to providegreat flexibility to the industrial customer in choosing themethod of installation that will apply in voltage regulation fora power distribution system. Some options have been studiedbased on various facts about how feasibly this model will fit in

Fig. 11. Converter coupled in series with the tap-changing transformer.

Fig. 12. Converter coupled in series with tap-changing transformer withS1

normally closed at nominal line voltage.

Fig. 13. Replacement of tap-changing transformers with the acvoltage–voltage converter.

a distribution system, with or without the use of tap-changingtransformers. Three possible options are briefly describedbelow.

A. Option 1—Combined Tap-Changing Transformer andConverter

In order to achieve a well-regulated voltage, the ac voltage-voltage converter is coupled in series at the output of the taptransformer, as shown Fig. 11.

In this case, the converter will be continuously switching, i.e.,converting voltage, constantly monitoring the tap transformeroutput voltage, and regulating the line voltage to the load. Whena voltage sag occurs at any instance, the tap-changing trans-former automatically selects a tap that would provide a voltageclosest to the nominal (usually ±3% [7]). If a tap transformerwith fewer tap settings is in place, with correspondingly lessthan adequate regulation, the regulation can be greatly enhancedwith the converter in series with its output. Not only is the regu-lation significantly enhanced, but also the response time. As-suming the tap-changing transformer has its own local feed-back, it will slowly increase its output while the duty cycle ofthe converter decreases in value. The primary advantage to re-taining the tapped transformer is that, during steady-state op-eration, the voltage converter enters close to a quiescent state,

HIETPAS AND NADEN: AUTOMATIC VOLTAGE REGULATOR 37

Fig. 14. Three-phase ac voltage–voltage converter schematic.

resulting in more acceptable efficiencies. A disadvantage is con-tinued maintenance of the tapped transformer. Another disad-vantage is simply the increased complexity of the system.

B. Option 2—Combined Tap-Changing Transformer andConverter with Ability to Disconnect Converter

When the converter enters a quiescent state as described inOption 1, the duty cycle of the converter approaches 0, de-pending on the regulation capability of the tapped transformeras well as that of the converter itself. If regulation is a secondaryrequirement to response time, yet the response time is still re-quired to be greater than the tapped transformer, a switch canbe placed in parallel across the secondary side of the injectiontransformer, as shown in Fig. 12.

The switch can be comprised of appropriate power semicon-ductors, such as the gate-turn-off thyristor. When a voltage sagis detected, the switch is closed at the next line zero-currentcrossing. Once is opened, the converter’s duty cycleis in-creased from a value of 0.0 to the necessary value to restore lineregulation, with slightly slower response compared toOption 1.

Once regulation is achieved and the voltage sag does not exist,can be closed on the next zero-voltage crossing of the in-

jection transformer’s output. Since the converter is essentiallytaken out of the regulation loop when is closed, regulationis only as good as the rating of the tapped transformer. A pri-mary advantage of this option compared toOption 1is improvedefficiency, reduced maintenance and increased life expectancy

of the converter. If the response time specification can be fur-ther relaxed, can be of mechanical variety with resulting in-crease in system efficiency. This approach is not recommendedif response time does not surpass that of the tapped transformeralone. The complexity of this option is slightly greater thanOp-tion 1.

C. Option 3—AC Voltage Converter Without Tap-ChangingTransformer

The third option replaces the tap-changing transformer withthe ac voltage-voltage converter, as shown in Fig. 13. The con-verter is continuously in operation unless a similar switch asdiscussed inOption 2 is employed. This method of applica-tion eliminates the need for maintenance on tap changing trans-formers, and subsequently reduces maintenance and operatingcosts associated withOptions 1and2.

The efficiency of the system could possibly be less than inOptions 1and2, since the converter is processing power contin-uously. The actual values of efficiency are highly dependent onthe design of the converter and can likely be optimized for a par-ticular system. The response time of this configuration is equalto that ofOption 1and better thanOption 2. Overall regulationis equal to or better than eitherOptions 1and2. Complexity ofdesign is also improved.

A single-phase system is currently being developed and canreadily be made into a three-phase system by either combiningthree single-phase units or using a three-phase design shown in

38 IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 36, NO. 1, JANUARY/FEBRUARY 2000

Fig. 14. Using three single-phase units provides the advantageof independent line regulation, but results in a higher cost (moreIGBT’s required).

V. CONCLUSIONS

The simulation results provided in this paper have illus-trated the feasibility of the ac voltage–voltage converter forvoltage-sag correction. The efficiency and flexibility of usingsuch a system in power distribution stations were discussed,and have been shown to provide a workable solution to currentpower quality problems. Three options discussed providegreater flexibility in voltage regulation systems at distributionsubstations and ensure adequate efficiency and power quality.Future work will include the hardware design and implemen-tation of the system, including closed-loop feedback controlnecessary to achieve response and regulation requirements.

REFERENCES

[1] P. B. Steciuk and J. R. Redmon, “Voltage sag analysis peaks customerservice,”IEEE Comput. Appl. Power, vol. 9, pp. 48–51, Oct. 1996.

[2] C. Beckeret al., “Proposed Chapter 9 for predicting voltage sags (dips)in revision to IEEE Std. 493, the Gold Book,”IEEE Trans. Ind. Applicat.,vol. 30, pp. 805–821, May/June 1994.

[3] M. F. McGranaghan, D. R. Mueller, and M. J. Samotyj, “Voltage sagsin industrial systems,”IEEE Trans. Ind. Applicat., vol. 29, pp. 397–403,Mar./Apr. 1993.

[4] M. H. J. Bollen, “The influence of motor reacceleration on voltage sags,”IEEE Trans. Ind. Applicat., vol. 31, pp. 667–674, July/Aug. 1995.

[5] M. H. J. Bollen, “Characterization of voltage sags experienced by three-phase adjustable-speed drives,”IEEE Trans. Power Delivery, vol. 12,pp. 1666–1671, Oct. 1997.

[6] H. G. Sarmiento and E. Estrada, “A voltage sag study in an industry withadjustable speed drives,”IEEE Ind. Applicat. Mag., vol. 2, pp. 16–19,Jan./Feb. 1996.

[7] D. Divan, P. Sutherland, and T. Grant, “Dynamic sag corrector: A newconcept in power conditioning,”Power Quality Assurance, pp. 42–48,Sept./Oct. 1998.

[8] N. Kutkut, R. Schneider, T. Grant, and D. Divan, “AC voltage regulationTechnologies,”Power Quality Assurance, pp. 92–97, July/Aug. 1997.

[9] G. Venkataramanan, B. K. Johnson, and A. Sundaram, “An AC–ACpower converter for custom power applications,”IEEE Trans. Power De-livery, vol. 11, pp. 1666–1671, July 1996.

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[11] U. K. Tejwani and S. M. Hietpas, “Modified PWM technique for reduc-tion of voltage harmonic distortion using AC–AC converter,” inProc.29th Annu. Frontiers of Power Conf., Oct. 1996, pp. IX-1–IX-6.

Steven M. Hietpas(S’83–M’84) received the B.S.degree in electrical engineering and the M.S. andPh.D. degrees from Montana State University,Bozeman, in 1984, 1991, and 1994, respectively.

From 1984 to 1989, he was with the Space EnergyGroup, Space Systems Division, General Dynamics,San Diego, CA, where he worked on research anddevelopment of power processing/power electronicsfor the Shuttle Centaur Program and the InternationalSpace Station Program. He has been a member of thefaculty at South Dakota State University, Brookings,

since 1994 and currently holds the position of Associate Professor and alsoserves as the Coordinator for the Center for Power System Studies. His researchinterests include power electronics, power quality, and control.

Mark Naden (S’99) received the Business andTechnology Education Council (BTEC) HigherNational Diploma in electrical engineering in 1996from Nottingham Trent University, Nottingham,U.K., and the B.S. degree in electrical engineeringfrom South Dakota State University, Brookings,where he is currently working toward the M.S.degree.

His current research interests include powerelectronics and applications of digital electronics inpower systems.

Mr. Naden is a recipient of an American Power Producers Association DEEDGrant Scholarship.