Sensitivity of AC Adjustable Speed Drives to Voltage Sags and Short Interruptions

494 IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 20, NO. 1, JANUARY 2005

Sensitivity of AC Adjustable Speed Drives to VoltageSags and Short Interruptions

S. Ž. Djokic, K. Stockman, Member, IEEE, J. V. Milanovic, Senior Member, IEEE, J. J. M. Desmet, Member, IEEE,and R. Belmans, Senior Member, IEEE

Abstract—This paper discusses the sensitivity of adjustablespeed drives (ASDs) to voltage sags and short interruptions on thebasis of extensive test results. Existing standards and previouslypublished work are critically reviewed, and a description of testprocedures needed for appropriate assessment of ASD sensitivityis presented. The following tests were performed: sensitivity torectangular three-phase, two-phase, and single-phase voltage sagswith ideal and nonideal supply characteristics, as well as sensi-tivity to nonrectangular-balanced three-phase voltage sags similarto those caused by the starting of large motors. The results showthat although the behavior of this equipment has a rather complexpattern, a simple representation of ASD sensitivity to varioustypes of voltage sags and short interruption can be established.

Index Terms—Nonideal power supply characteristics, powerquality, rectangular and nonrectangular three-phase, two-phase,and single-phase voltage sags and short interruptions, voltage-tol-erance curve.

I. INTRODUCTION

AC adjustable speed drives (ASDs) are typical examplesof recently emerged, rather complex and sophisticated

nonlinear power electronic equipment. An ASD controls thespeed of an induction or synchronous motor by convertingfixed frequency/fixed magnitude ac mains supply voltage toa variable frequency/variable magnitude voltage at the motorterminals. Improved process control, energy savings in appli-cations with variable torque loads and reduced motor speeds,reduction of mechanical and thermal stresses through “soft”start, acceleration, and deceleration, remote communicationand control, simple maintenance and automated diagnostic aresome of the benefits which ASDs provide. The dominant typeof ASD in several hundreds up to the megawatt range is thepulsewidth-modulation (PWM)-controlled voltage-source in-verter. In essence, the PWM ac ASD is a converter, comprisingof a rectifier (or input converter), a dc link (or dc bus), aninverter (or output converter), and additional control, protectionand measurement circuits. All of these circuits respond tovarious power-quality disturbances both individually and as a

Manuscript received April 8, 2003; revised October 13, 2003. This workwas supported in part by the U.K. Engineering and Physical Sciences ResearchCouncil (EPSRC) under Grant GR/R40265/01, in part by the Copper Develop-ment Association (U.K.), and in part by Electrotek Concepts Inc. (USA). Paperno. TPWRD-00380-2003.

S. Ž. Djokic and J. V. Milanovic are with the School of Electrical and Elec-tronic Engineering, The University of Manchester, Manchester M60 1QD, U.K.

K. Stockman and J. Desmet are with the Department Provinciale IndustriëleHogeschool, Hogeschool West-Vlaanderen, Kortrijk, Belgium.

R. Belmans is with the Electrical Engineering Department, Div. ELECTA ofthe Katholieke Universiteit Leuven, Leuven B-3000, Belgium.

Digital Object Identifier 10.1109/TPWRD.2004.832353

complete assembly, leading to high and complex patterns ofdrive sensitivity.

Usually, the ASD is only one part of some system in whichother electrical components are involved. If a power-quality dis-turbance is severe enough to cause disconnection of at leastone of the critical system components, the whole process maybe stopped. Disconnection of the ASD during voltage sags andshort interruptions can be: i) internally driven, when some of thedrive protection systems commands disconnection, or ii) exter-nally driven, when some other component (e.g., motor or con-troller) commands the stoppage of the process and subsequentdisconnection of the drive. Different ASD applications and var-ious modes of drive operation may result in a number of param-eters and criteria that can be used for externally initiated dis-connection of the drives. For example, decrease in speed and/ortorque of the controlled motor during the sag may be criticalin some processes. However, all external malfunction/perfor-mance criteria are application specific, and their usage for drivesensitivity assessment should be considered on a “case-by-case”basis.

Sensitivity of ASDs to voltage sags is usually expressedas a voltage tolerance curve, in terms of only one pair ofsag magnitude/duration values. These two values are denotedas the threshold values—if the voltage sag is longer thanthe specific duration threshold and deeper than the specificvoltage magnitude threshold, the ASD will malfunction/trip.For ASDs, reported threshold values vary from 50–60% to80–90% of rated voltage for magnitude, and from cycle(or even less) up to 5–6 cycles for the duration [1]. The useof magnitude/duration threshold values is straightforward forsingle-phase equipment and balanced polyphase sags, butit cannot be applied for assessment of ASDs sensitivity topolyphase unbalanced voltage sags. The ASDs are three-phaseequipment and voltage sags with different combinations ofphase voltages will have different effects on their operation.These effects can be assessed only if the sensitivity of ASDsand voltage sags characteristics are expressed considering thethree-phase nature of power supply and drive itself.

This paper is concerned only with the internally drivenmalfunction of the ASD. Bypass mode of operation (whencontrolled motor continues to operate directly connected tothe power supply and without speed control by the drive) isconsidered as the drive disconnection condition. Similarly,automatic restart of the drive was considered as not possiblein general. (Some drives do not support that feature, and someprocesses do not allow arbitrary restart.) The paper summarizesresults of a comprehensive study of ASD behavior during

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DJOKIC et al.: SENSITIVITY OF AC ADJUSTABLE SPEED DRIVES TO VOLTAGE SAGS AND SHORT INTERRUPTIONS 495

voltage sags and short interruptions. After a critical review ofexisting standards and previously published work, the paperpresents extensive experimental results, demonstrating that thehigh sensitivity of ASDs to various types of voltage sags has arather complex nature.

II. EXISTING STANDARDS AND PREVIOUS RESEARCH

A. Overview of Existing Standards

1) General Power Quality Standards: Detailed overviewand critical assessment of the PQ standards related to voltagesags and short interruptions is given in [2], [3]. Due to the spacelimitations, only a few points will be highlighted in this section.

Existing power-quality standards [4]–[12] define voltage sagsand short interruptions as a short duration variation of voltagemagnitude of any or of all phase voltages of a single-phase ora polyphase power supply at a point in the electrical system. In[10], voltage sag is described as a “ two dimensional electro-magnetic disturbance, the level of which is determined by both(rms value of) voltage and time (duration)”. Other characteris-tics of voltage sags (e.g., phase shift during the sag, or point onwave of sag initiation), are generally ignored, or it is eventuallysuggested ([12]) not to consider them for the equipment compat-ibility evaluation. However, the behavior of certain equipment isinfluenced by phase shift and/or point on wave of sag initiation[2], and without these parameters equipment sensitivity cannotbe fully assessed.

All current standards assume that the voltage magnitude isconstant during the sag, i.e., that sag has a “rectangular shape”.Other voltage sag “shapes”, e.g., two-stage voltage sags, orvoltage sags due to starting of large motors are, at the best,just mentioned. Practical instructions about quantification andcharacterization of nonrectangular sags are not given, nor howto assess their influence on equipment sensitivity.

2) Standards Related to Testing: Testing procedures in ex-isting standards are in accordance with the “two-dimensional”definition of voltage sags and short interruptions [13]–[15].Recommended tests are related to generation of simple rect-angular (or “step”) sags and interruptions, in which onlymagnitude and duration are controlled. Standard [15] considersonly some (few) discrete values of voltage magnitude andduration for testing, while the most recent standard [13] rec-ommends testing with incremental changes in sag magnitude,not larger than 5%. For each sag magnitude, maximum (orcritical) sag duration before the equipment malfunction occursshould be identified. That way, [13] introduces the conceptof voltage-tolerance curve as a graphical representation ofequipment sensitivity.

Regarding testing of three-phase equipment, only two proce-dures are described in standards. The first is “phase-by-phase”testing, in which only one phase of the three-phase equipmentis exposed to sags and interruptions [13], [15]. The rated condi-tions are in all aspects maintained in two other, unsagged phases.After testing of one phase, tests should be repeated with thesame conditions two more times, for both other phases. How-ever, standards do not give precise instructions how to representequipment sensitivity if different responses were identified fordifferent phases (adopt response of the most sensitive phase, or

average of tested phases, or show the results for all three phases).In the second procedure [15], three-phase equipment should betested with the same sag duration and magnitude applied to allthree phases simultaneously. In both standards, these two pro-cedures are indicated as sufficient for assessment of three-phaseequipment sensitivity.

Standards give no suggestions related to testing of equipmentto nonrectangular voltage sags. Allowed deviations in magni-tude, frequency, and total harmonic distortion from the idealvoltage supply conditions (e.g., in [9]) are not considered as apart of the testing procedure. Pre- and post-sag voltage wave-forms used in tests should be the ideal sine waves with ratedvoltage and frequency. Tests should be performed preferably at0 point on wave [14]. Standard [15] suggests testing for ad-ditional angles (from 0 to 360 in steps of 45 ) only if theyare “ considered critical by product committees or individualproduct specifications”.

Standard [16] recognizes that the voltage sags could havedetrimental effects on ASDs. It does not recommend, however,testing of drives against sags: “According to the state of the art,the behavior of the drive is predictable with simple and reli-able calculations, and depends on its operating mode and onits rating. Therefore, tests against voltage sags (which are un-economic) need not be performed.” Unfortunately, there are noinstructions in [16] provided about “simple and reliable calcu-lations” that should be used instead of testing. Standard [16],however, considers testing of drives to short interruptions: “where it is possible and not dangerous, the behavior of the driveduring short interruptions may be verified by switching off andon the mains supply during the standard operating conditions ofthe drive.” Finally, it concludes that: “ EMC characteristicsof the drive are normally not affected by the amount of load onthe motor” and instructs that testing with unloaded motor is ac-ceptable, and that full load testing should be considered only ifit is required.

B. Overview of the Previous Research

There are a lot of published papers and reports concerned withvarious power-quality aspects of operation and application ofASDs. Only a relatively small number of existing reports dis-cussing experimental or simulation results of drive sensitivityto voltage sags and interruptions is discussed here.

In [17]–[21], a simplified simulated model of an ac ASD wasanalyzed regarding balanced and unbalanced sags, with em-phasis on changes in dc link voltage and reduction in motorspeed and/or torque. Voltage-tolerance curves of drives werenot obtained, simply because the protection systems of the drivewere not modeled and duration of sags was neglected as a pa-rameter of influence. It was concluded that testing of three-phase equipment is more complicated than testing of single-phase equipment, and suggested that testing of drives shouldbe performed against sags classified in several (seven) types, re-lated to different fault types. Only results of simulations againstthese several sag types were presented, without any comparisonwith the experimental ones. It was concluded that point on waveof sag initiation and phase shift during the sag do not influencethe behavior of ASDs during the sags.


A test bed for ASDs is described in [22] and [23]. Results oftesting of various ASDs against different sag types (accordingto the classification in [21]) are presented, and also comparedto results obtained in simulations of the drives. The identifiedvoltage-tolerance curves have flat or slightly inclined horizontalpart, flat vertical part and a “knee” between them. Lower sen-sitivity of drives was identified for operation of the controlledmotor with lower torque and lower speed. Voltage-tolerancecurves are also used for the illustration of drive ride-through ca-pability improvements, when an active front end or a front-endboost converter are applied.

In [24], a programmable ac power source with integrated ar-bitrary waveform generator is described and used for testing ofseveral configurations of two ASDs. Some tabulated informa-tion about the changes in speed of the controlled motor, dc linkvoltage and input current are presented for tests with single, twoand three-phase sags and different loading torques. No voltage-tolerance curve is identified for the tested drives. It is concludedthat the way of supplying the drive control logic has a largeimpact on drive sensitivity (higher sensitivity is identified forsupply by ac mains voltage than by dc link voltage).

Voltage unbalance and phase angle shift were reported as twoadditional sag parameters in [25], but their influence on drivesensitivity was assessed only regarding changes in drive inputcurrent, dc link voltage and motor speed. It is not shown howthese two parameters actually change drive sensitivity. However,in [26] is concluded that: “ any phase shift during a voltagesag affecting the zero crossing of the supply voltage does notappear to greatly impact the operation of an ac PWM drive.”Load dependent behavior of tested ac drives was documentedwith two voltage-tolerance curves [25]. Curves are obtained forthree-phase balanced sags and interruptions with duration from2 to 20 cycles, indicating a decrease in drive sensitivity withdecrease in both speed and load of the controlled motor.

A test protocol and results of tests on twelve 3-hp drives andten 20-hp drives were presented in [27]. Drives were tested onlyagainst three pairs of magnitude/duration values (i.e., againstone balanced three-phase interruption and two balanced three-phase sags) (0, 50, and 70% of rated voltage with -s, -sand 1–s duration, respectively). Four malfunction criteria wereused: i) only momentary change in speed, ii) drop in speedto zero or reversing, but drive recovers with loaded motor, iii)drop in speed to zero or reversing, but drive recovers if motorload is removed, and iv) disconnection of the drive. A constanttorque load type and two loading conditions (full and half ofrated value) were applied. Results show that 3 hp are less sen-sitive than 20–hp drives, and that the loading of the motor doesnot have a significant influence on drive sensitivity. However,it is suggested that additional tests with unbalanced sags, dif-ferent load types, loading conditions and speeds of the motor, aswell as with introduction of the phase shifts and nonideal supplycharacteristics are necessary for full assessment of drive sensi-tivity.

Results of testing of one 15 kW ASD against single, two andthree-phase sags were presented in [28]. Tests were conductedwith two different load conditions (25 and 75% of rated load)and three different pre-sag voltages (95, 100,, and 105% of rated

voltage). The malfunction criterion used in tests was disconnec-tion of the drive. Families of voltage-tolerance curves were ob-tained, showing the highest sensitivity for three-phase and thelowest for single-phase sags. Lower value of pre-sag voltage in-crease sensitivity, and vice versa. Also, slightly lower sensitivitywas identified for lower loading conditions.

Voltage sag measurements and their analysis performed intwo industries for a period of 17 months were presented in[29]. Voltage sags recorded in the system were correlated withthe malfunction of the drives installed at two industrial sites.That way, instead of direct testing, sensitivity of the drives wasassessed against actual voltage sags. Magnitude and durationthresholds for drives with the highest sensitivity were identifiedas 80% of rated voltage and a duration of six cycles. However,information about the recorded sag types was not provided(only one-point sag representation was given). Although it isstated that a detailed database was developed with all relateddata of the drives (brand, model, capacity, installed number andnumber of malfunction), this information was not presented.

A controllable dynamometer was used in testing of driveswith different load characteristics in [30] and [31]. Three dif-ferent load types with different inertia constants were simulatedin tests: constant torque, constant mechanical power and vari-able (quadratic) torque versus speed. A decrease in motor speedduring the sag was the only parameter monitored. No infor-mation on changes in drive ride-through capability was given,nor were voltage-tolerance curves provided. The maximum sen-sitivity (i.e., change in speed) is obtained for constant torqueload type, and the minimum for quadratic load type. However,presented results show relatively small changes in speed fordifferent load types and different inertia constants, especiallyduring the time between sag initiation and moment of tripping.

III. TESTING OF ADJUSTABLE SPEED DRIVES

A. List of Tests

The ASDs are three-phase equipment and different combi-nations of three phase voltages during the sags have differenteffects on their operation. However, in realistic power systems,not all combinations of phase voltages during polyphase voltagesags are likely to occur. In tests presented here, it was assumedthat voltage sags and short interruptions caused by different faulttypes (line to ground, double-line to ground, line-to-line andthree-phase faults) propagate in power systems in such a waythat at least two phase voltages during the sag have almost equalmagnitudes. More precisely, testing of ASDs described in thispaper was conducted with the following three types of voltagesags:

1) Three-phase balanced voltage sags (i.e., during-sagvoltage magnitudes in all three phases are equal).

2) Generalized two-phase voltage sags (i.e., during-sagvoltage magnitudes of two sagged phases are equal;voltage in the third, “unsagged” phase is used as param-eter, and it can be either rated or below the rated).

3) Generalized single-phase voltage sags (i.e., during-sagvoltage magnitude of one (sagged) phase is below therated value; voltage magnitudes in two other phases are


TABLE ITEST CONDITIONS

used as a parameter—they are always equal and rated, orequal and below the rated value).

Note: Strictly speaking, generalized two-phase voltagesags and single-phase voltage sags with one or two “un-sagged” phase voltages below the rated are three-phase un-balanced sags. In this paper, however, these sags are stillcalled single- and two-phase sags in order to simplify iden-tification and maintain consistency in presentation of var-ious sag types.Two different pre- and post-sag voltage waveforms were

used in tests: a) supply from an ideal three-phase voltage sourceat 50 Hz, and b) supply from a nonideal three-phase voltagesource with up to % variation in voltage magnitude, up to

% variation in frequency, and different harmonic contentwith total harmonic distortion (THD) up to 20%. In all cases,the during-sag voltage waveform was an ideal sine wave at ratedfrequency. Preadjusted during-sag voltage magnitudes werekept constant for rectangular sags. In tests with nonrectangularsags, during-sag voltage magnitude was changed graduallyand equally in all three phases. Voltage sags were initiated atdifferent points on the voltage sine wave (one phase of thethree-phase supply was used as a reference). Duration of allsags produced in tests was the same in all sagged phases. If aphase shift was introduced during the sag, the same value wasapplied to all sagged phases. Table I summarizes the conductedtests.

B. Test Procedure

Testing is probably the most efficient and reliable way to iden-tify equipment sensitivity to voltage sags and short interrup-tions. However, different brands of the same equipment type,and even different models of the same brand, often have verydifferent sensitivity thresholds. Hence, assessment of upper andlower limits of sensitivity is connected with testing of largenumber of items of a particular equipment type. That was notthe aim of the tests described in this paper. Instead, testing wasconducted in order to determine what sag and equipment param-eters may have influence on equipment sensitivity, and to as-sess general, characteristic responses and behavior of the testedequipment. In order to maintain full and precise assessment ofequipment sensitivity, testing of equipment was conducted in aconsistent manner, with high degree of repeatability. To illus-trate this, the test procedure used with rectangular voltage sagsis described:

1) The drive was first connected to the voltage sag generator(VSG) which supplies the drive with rated input charac-teristics. Controlled motor was started unloaded.

2) The motor was loaded with one of three load types andthe selected torque. The speed of the motor was adjusted

to one of the pre-selected values. One of three voltage sagtypes was selected for testing. The point on wave of thesag initiation and phase shift during the sag were set.

3) Voltage sags were applied in steps of 1% of rated value,starting from 0 V. For each sag magnitude, the duration ofthe sag was prolonged until some of drive protection sys-tems activate the disconnection of the drive, or, if thereis no disconnection, up to a few seconds. The critical sagduration was ascertained by up to 10 repeated measure-ments. A 5–10 s recovery time was allocated between theconsecutive voltage sags.

4) In testing with sags for which voltage in the unsaggedphase(s) was used as a parameter, this value was changedin 10% steps from 0–100% of the rated value, and mea-surements described in step 3 were repeated.

5) The point on wave of voltage sag initiation was adjusted insteps of 15 (from 0 to 360 ) and measurements describedin step 3 were repeated.

6) The phase shift during the sag was changed in steps of 15(from 0 to ) and measurements described in step 3were repeated.

7) The speed of the motor was changed to the next value andmeasurements described in step 3 were repeated (motorspeeds selected for testing were: 100, 75, 50, and 25% ofrated motor speed).

8) Loading torque was changed to the next value, andmeasurements described in step 3 were repeated. Torquevalues selected for testing were: 100, 75, 50, 25, and 0%of rated motor torque.

9) The load type was changed to next of three types (i.e., con-stant torque load, load with linear relationship betweenthe speed and torque, and load with quadratic relation-ship between the speed and torque) and measurements de-scribed in step 3 were repeated.

10)The voltage sag type was changed to the next of thethree types and measurements described in step 3 wererepeated.

C. Testing Equipment

The test bed used for testing of ASDs consisted of the fol-lowing main parts [23]:

1) Computer controlled voltage sag generator (SchaffnerProfline 2100), consisting of three synchronizedsingle-phase power source units with 3 5 kVA max-imum power output [32].

2) Independent real-time data acquisition system (Dranetz658 power-quality analyzer), with four-channel input (7.2kHz/channel rms and 1.85-MHz/channel impulse sam-pling rates) [33].

3) Loading machine and loading controller (dc machine withdSpace DS1103 PPC controller with Matlab/Simulinksoftware interface for real-time torque control).

D. Multiplicity of the Voltage-Tolerance Curves

Testing with rectangular sags resulted in several sets ofvoltage-tolerance curves. Each voltage-tolerance curve corre-sponds to one particular voltage sag type, one particular load


TABLE IIBASIC DATA OF TESTED ASDS

type, one particular value of loading torque and one particularvalue of motor speed. If the voltage in the unsagged phase(s)was used as a parameter, each parameter value then producedone additional voltage-tolerance curve. Obtained voltage-toler-ance curves were then grouped in sets, in order to illustrate theeffects of various parameters on ASD sensitivity. For example,grouping for generalized two-phase voltage sags, constanttorque load type, rated torque and rated speed results in a set of11 curves. Each voltage-tolerance curve in the set correspondsto one of 11 different values of voltage magnitude in the third,“unsagged” phase (changed in 10% steps from 0% to 100%rated voltage). This set, then, clearly illustrates the influenceof the third phase voltage magnitude on drive sensitivity togeneralized two-phase voltage sags.

E. Tested Adjustable Speed Drives

In order to assess and quantify the effects of voltage sags andshort interruptions on ASDs in a more general way, differentdrives, from different manufacturers, with different characteris-tics and manufactured during last five years were tested. The listof ASDs used in tests is given in Table II.

IV. TEST RESULTS

A. Testing of Drives With Rectangular Voltage Sags

The results of the tests are presented graphically as voltage-tolerance curves and grouped in related sets.

1) Supply From Ideal Voltage Source: It was found that thevoltage-tolerance curves of all tested ASDs have three distinc-tive parts: flat or slightly inclined vertical part, slightly inclinedhorizontal part, and a smooth “knee” between them. It was alsofound that different points on wave of sag initiation and differentphase shifts during the sag do not have any noticeable influenceon ASDs behavior. This is due the fact that all tested ASDs havean uncontrolled three-phase full bridge diode rectifier, not af-fected by the variations in phase shift and point on wave.

a) Three-Phase Voltage Sags: Firstly, balanced rectan-gular three-phase sags and interruptions were applied to thedrives. Fig. 1 illustrates the basic shape of the voltage-tolerancecurve of drive ASD1 with the motor operating at rated speed,

Fig. 1. Sensitivity of drive ASD1 to balanced three-phase voltage sags.

constant torque load type and rated torque. Two different pro-tection systems are responsible for disconnection of the drive,as indicated in Fig. 1. Undervoltage protection is activatedafter the dc link capacitor discharges its energy and the dc linkvoltage drops below the minimum allowed value .This minimum value can be adjusted as low as 50% of rated dclink voltage, but usual settings are in the range from 65–70%up to 85–90%. Fig. 1 shows that the undervoltage protectionsetting determines the duration threshold (i.e., this protectionis activated by interruptions and deep sags). The overcurrentprotection system is triggered either by increased current drawnduring the sag, or by high inrush current immediately afterthe sag (or voltage recovery) ending (due to charging of thedischarged dc link capacitor) It is related to the maximumallowed current (usually from 120% to 170% of rated drivecurrent). This protection determines magnitude threshold ofdrive sensitivity. Both protections can be implemented asuser-settable or hard-wired setting options.

Changes in speed (thick dashed line in Fig. 1), identified forlong sags that are not deep enough to cause disconnection of thedrive, are shown in Fig. 1 only for illustration purpose. This isnot shown in following graphs, because this paper is not con-cerned with any externally driven malfunction criteria.

Magnitude and duration values of balanced rectangularthree-phase sags and short interruptions recommended fortesting in [15] are marked by “x” in Fig. 1 (magnitude: 70, 40,and 0%, with , 1, 5, 10, and 25 cycles duration). If testingis performed only with these values, duration threshold ofsensitivity of drive ASD1 could be assessed roughly as beingbetween 10 and 20 ms for short interruptions and sags withmagnitude below 40% of the rated voltage, and somewherebetween 10 and 100 ms for sags with magnitude between40–70% of the rated voltage. Magnitude threshold of drivesensitivity can be assessed only as somewhere above the 70%of the rated voltage.

Fig. 2 presents voltage-tolerance curves for drive ASD1 withthree different load types: i) constant, ii) linear, and iii) quadratictorque. There are very small, if any, differences between thethree voltage-tolerance curves for the different load types. Asmall influence is identified only for the horizontal parts of thevoltage-tolerance curves (magnified part in Fig. 2). The vertical


Fig. 2. Influence of different load types on sensitivity of drive ASD1 tobalanced three-phase voltage sags.

Fig. 3. Influence of different torque values on sensitivity of drive ASD1 tobalanced three-phase voltage sags.

parts for all three load types are identical. The most sensitivevoltage-tolerance curve is related to the constant torque loadtype. This load type is regarded as the “worst load type con-dition” in subsequent tests. The results in Fig. 2 are obtained forthe rated speed and torque of the motor, so the active power is thesame for all applied load types. This explains the identical be-havior for interruptions (“vertical part” of the sensitivity curve).The slightly different horizontal parts are the consequence of thedifferent speed drops during the sag, which in turn results in alower torque for the linear and quadratic torque profiles.

Voltage-tolerance curves obtained in tests with different loadtorque (for constant torque load type and rated motor speed)are shown in Fig. 3. It can be seen that the loading of the motorhas significant influence on drive sensitivity. Decreasing loadingtorque results in lower sensitivity (i.e., better ride-through capa-bilities). It should be noted, however, that even if the motor iscompletely unloaded, the drive ASD1 still remains very sensi-tive.

Fig. 4 presents voltage-tolerance curves when motor speedwas used as a parameter (for constant torque load type and

Fig. 4. Influence of different motor speeds on sensitivity of the drive ASD1 tobalanced three-phase voltage sags.

Fig. 5. Sensitivity of all tested drives to balanced three-phase voltage sags andrunning at rated speed and rated torque, constant torque load type.

rated torque). It can be seen that the maximum sensitivity re-garding the vertical part of the voltage-tolerance curve was notobtained for rated motor speed. However, the dominant effect isrelated to the horizontal part of the voltage-tolerance curve, i.e.,to decrease of voltage magnitude threshold with the reductionin speed.

It is interesting to note that several references (e.g., [34] and[35]) treat the operation of a drive with its motor running at re-duced speed and/or with reduced load as a measure for improve-ment of drive ride-through capabilities. Furthermore, operationof the motor with lower torques at lower speeds is often recom-mended as a measure for preventing overheating of motor (der-ating). Results presented in Figs. 3 and 4 indeed confirm lowersensitivity of the drive for reduced motor speed and/or torque.However, operating the motor in such conditions highly dependson the process requirements.

Similar voltage-tolerance curves were obtained for all testedASDs and are shown in Fig. 5 (rated speed and torque, constanttorque load type). The voltage sag magnitude threshold for alltested drives varies between 73–91% of rated voltage; durationthreshold varies between 5–20 ms. Regarding the fact that rmsvalues cannot be calculated for periodic quantity shorter thana cycle period, sensitivities of the drives ASD4 and ASD5


Fig. 6. Sensitivity of the drive ASD1 to generalized two-phase voltage sags.

Fig. 7. Influence of different load types on sensitivity of the drive ASD1 totwo-phase voltage sags with rated voltage in unsagged phase.

should be referenced to undervoltage transients, not voltage sagsnor short interruptions.

b) Generalized Two-Phase Voltage Sags: During the testswith rectangular generalized two-phase voltage sags, voltagemagnitudes in the two sagged phases were always kept equal.Voltage magnitude in the third, “unsagged” phase was used as aparameter. That way, a set of voltage-tolerance curves was ob-tained. Results for ASD1 are shown in Fig. 6 (rated speed andtorque, constant torque load type). As the voltage magnitude inthe third phase decreases from rated to zero value, sensitivity ofthe drive increases.

Comparison between different voltage-tolerance curves fordifferent load types in the case of two-phase voltage sags withrated voltage in the third phase for ASD1 is shown Fig. 7. Theinfluence of different load types on sensitivity is even smallerthan in the case of balanced three-phase sags.

Fig. 8 compares voltage-tolerance curves for two-phase sagswith rated voltage in the third phase and different motor speeds.Again, for some speeds lower than rated, drive is slightly moresensitive. The voltage magnitude threshold, however, decreaseswith the decrease in speed.

Similar sets of voltage-tolerance curves were obtained for alltested ASDs (rated speed and torque, constant torque load type,

Fig. 8. Influence of different motor speeds on sensitivity of ASD1 to two-phasevoltage sags with rated voltage in unsagged phase.

Fig. 9. Sensitivity of all tested drives to two-phase voltage sags with voltagemagnitude in the third phase equal to rated value.

Fig. 9). The magnitude and duration thresholds vary between65–90% of rated voltage, and 6–20 ms.

c) Generalized Single-Phase Voltage Sags: Results oftesting against single-phase voltage sags are of particularinterests, because some of the standards recommend testing ofthree-phase equipment only to this sag type (“phase-by-phase”testing procedure). However, the results presented in this sec-tion exceed those that would be obtained if the recommendedprocedure was strictly followed. In tests performed, during-sagvoltage magnitude in two unsagged phases was used as aparameter and varied in tests. Consequently, a set of differentcurves was produced, clearly illustrating the effect of differentphase voltage combinations on drive sensitivity. Results forASD1 are presented in Fig. 10. The reduction of voltagemagnitude in two “unsagged” phases rapidly increases drivesensitivity for this sag type.

Fig. 11 again shows a very small influence of different loadtypes on ASD sensitivity in the case of single-phase voltage sagswith rated voltage in the two unsagged phases.

Only one voltage-tolerance curve for speed different fromrated is shown in Fig. 12. If two unsagged phase voltages areequal to the rated voltage and the speed of the induction motor


Fig. 10. Sensitivity of ASD1 to generalized single-phase voltage sags.

Fig. 11. Influence of different load types on sensitivity of the drive ASD1 tosingle-phase voltage sags with rated voltage in two unsagged phases.

Fig. 12. Influence of different motor speeds on sensitivity of ASD1 tosingle-phase voltage sags with rated voltage in two unsagged phases.

is lower than 90% of rated value, the drive will not discon-nect the motor even when permanently supplied by only twophases. This situation, however, raised the question about thephase missing protection setting of ASD1. The manufacturer ofthe drive ASD1 is contacted and it is clarified that the phasemissing protection is based on the actual value of the dc link

Fig. 13. Sensitivity of all tested drives to single-phase voltage sags withvoltage magnitude in two unsagged phases equal or below the rated value.

Fig. 14. Sensitivity of ASD2 to various types of voltage sags and two operationmodes: closed-loop (thick lines) and open-loop (hairlines).

voltage ripple, instead of the monitoring of the actual phase volt-ages at the drive terminals.

Similar curves are obtained for all tested ASDs (rated speedand torque, constant torque load type, Fig. 13). Drives ASD2and ASD3 can operate indefinitely with one phase missing in thethree-phase supply, if the other two phase voltages are equal tothe rated value. However, if voltage in the two unsagged phasesdecreases only by a few percents below rated voltage, these twodrives will become sensitive to single-phase sags. Voltage-tol-erance curves for drives ASD2 and ASD3 correspond to twounsagged phase voltages of 95% and 98% of rated voltage, re-spectively.

d) Operation in Closed-Loop Mode: All results presentedin the previous sections are related to so called “open-loop” op-eration mode. An ASD working in “closed-loop” mode uses aspeed or position sensor to detect the difference between thereference and actual speed of the motor. The speed control loopthus contributes to more precise (faster) response of the drive.Fig. 14 illustrate a slight increase in sensitivity of ASD2 to var-ious voltage sags when its operation changes from open-loop toclosed-loop mode.

e) Single-Graph Representation: Voltage-tolerancecurves for generalized two- and single-phase voltage sags


Fig. 15. Single-graph representation of drive sensitivity to various types ofvoltage sags: (a) drive ASD1, (b) drive ASD3.

(Figs. 6 and 10 for ASD1) in fact describe the same region ofthe drive sensitivity. The only difference between them is infixed parameter used. They are re-drawing of the same familyof curves related to the unbalanced three-phase voltage sagswith two phases always having the equal magnitudes. Thesetwo sets of curves are interchangeable and they can be merged.If the curve for balanced three-phase sags is also added, a“single-graph” representation of ASD sensitivity is obtained.This representation is made possible assuming that the voltagemagnitude in the unsagged phase(s) is always larger than, orequal to the magnitude in the sagged phase(s). Single-graphrepresentations for ASD1 and ASD3 (rated torque and speed,constant torque load type) are shown in Fig. 15(a) and (b).

It can be seen from Fig. 15(a) that the sensitivity of ASD1to various single-phase and two-phase sags is inside the areabounded by the lowest and the highest voltage-tolerance curve.The lowest curve corresponds to single-phase sags with ratedvoltage in the two unsagged phases, and the highest to bal-anced three-phase sags. ASD3, however, is completely insen-sitive to single-phase sags and interruptions if the voltage intwo unsagged phases is higher than 98% of the rated voltage[Fig. 15(b)]. Hence, its sensitivity varies in a wider range, be-tween the curve for balanced three-phase sags and “no trippingat all”.

With the single-graph representation, one graph can be usedfor assessment of drive sensitivity to all types of sags and inter-ruptions. Although this graph eliminates the need for separatesets of curves for different sag types, three sets of voltage-toler-ance curves related to three general voltage sag types still can beused for fast, convenient and simple identification of the effectsof sags on drive sensitivity. Moreover, these three sets supportthe presented intuitive classification of various sags in only threegeneral types (for an alternative sag classification see [21]).

Two curves corresponding to two testing procedures recom-mended by standards [13], [15] cannot be used individuallyfor precise assessment of drive sensitivity. The least sensitivevoltage-tolerance curve corresponds to the “phase-by-phase”testing procedure (i.e., to testing against single-phase voltagesags with rated voltage in two other phases). If only that voltagetolerance curve is obtained in tests and used for assessment, sen-sitivity of drives would be underestimated (that curve is “the bestcase” sensitivity). This situation is clearly illustrated in the caseof ASD3. Testing with only phase-by-phase procedure indicatesmisleadingly that this drive is completely immune to sags andinterruptions. On the other hand, the most sensitive curve cor-responds to testing with balanced three-phase sags. If only thatcurve is used for assessment, sensitivity of the drive would beoverestimated (that curve is the “worst case” sensitivity). Over-estimation is additionally driven by the fact that three-phasefaults (which only produce balanced three-phase sags) are themost rare of all system faults. And finally, even if both curveswere obtained in tests (but only these two), only upper and lowerlimits of drive sensitivity could be estimated, without any infor-mation about drive sensitivity inside these limits. Again, ASD3is a good example—a lower limit of its sensitivity does not exist,because it is immune to single-phase sags with rated voltage intwo unsagged phases. Therefore, precise assessment of equip-ment performance is possible only if sensitivity of the equip-ment is identified with regard to all sag types.

Finally, the proposed single-graph representation can beestablished for any other three-phase equipment and usedfor sensitivity assessment. Generally, three-phase equipmentshould have maximum and minimum sensitivity curves. (Max-imum sensitivity curve is likely to be related to balancedthree-phase sags, and minimum to single-phase sags with ratedvoltage in the unsagged phases). Such representation of sen-sitivity is also suitable for estimation of the expected numberof equipment malfunctions/tripping. Estimated maximum andminimum number of equipment malfunctions can be easilyrelated to the maximum and minimum sensitivity curves, andthen elaborated further regarding the voltage-tolerance curvesfor various sag types.

2) Supply From Nonideal Voltage Source: During thesecond stage of testing, ASDs were supplied from a nonidealvoltage source before and after the initiation of balanced rectan-gular three-phase sags and interruptions. Deviations in nonidealsupply were: voltage magnitude variations up to % of ratedvoltage, frequency variations up to % of rated frequency,and different harmonics with THD not exceeding 20%.

The influence of % voltage magnitude variation on ASD1sensitivity to balanced three-phase voltage sags (rated torqueand speed, constant torque load type) is shown in Fig. 16. It can


Fig. 16. Sensitivity of ASD1 to balanced three-phase sags regarding thevariations in pre-sag and post-sag supply voltage magnitude.

Fig. 17. Sensitivity of drive ASD1 to balanced three-phase sags regarding thevariations in harmonic contents of pre-sag and post-sag supply voltage.

be seen that the sensitivity threshold changes for both horizontaland vertical part of the related voltage-tolerance curves.

Frequency variations inside the limit of % rated value donot influence the sensitivity of all tested ASDs.

The influence of different harmonic content (3rd, 5th and 7thharmonics with up to 20%THD) is illustrated in Fig. 17 forASD1. Sensitivity threshold is most influenced if the supplyvoltage contains 5th harmonic (dominant harmonic in harmonicspectrum “emitted” during normal drive operation).

Note: Maximum phase voltage output of used voltagesag generator is limited to 380 V peak per phase, andtesting with different harmonic contents was done withslightly lower voltage magnitude (98% of the rated 230 Vphase voltage).Furthermore, the influence of voltage magnitude and phase

angle unbalance was investigated. It was found that up to 3–5%of voltage magnitude unbalance does not have any significantinfluence. However, – phase angle unbalance (phaseangle displacement) between the three phase voltages (pha-sors) does not allow normal operation of some drives, even ifall other conditions are rated. Surprisingly, disconnection ofdrives is activated by the phase missing protection system. As

Fig. 18. Representation of voltage sags due to the startup of large motors.

TABLE IIITIME OF ASD1 DISCONNECTION (IN ms) AS A FUNCTION OF INITIAL

VOLTAGE DROP AND VOLTAGE GRADIENT

stated before, the reason is that phase missing protection isimplemented regarding the ripple on the dc link voltage.

B. Testing of Drives to Nonrectangular Voltage Sags

The influence of balanced three-phase voltage sags similar tothose that occur during the startup of large motors was investi-gated in the last part of testing. These sags have basically twoparts: initial instantaneous voltage drop at the sag beginning,followed by a gradual voltage recovery. One example is shownin Fig. 18. At sag initiation, the voltage drops to 30% of ratedvalue, and then gradually recovers up to the pre-sag value of100% of rated voltage.

During the tests, five linear gradients of voltage recovery forvarious initial drops were applied. Results for ASD1 are shownin Table III. Results from Table III show that behavior of thedrive for this type of nonrectangular sags can be successfullypredicted. This is illustrated in Fig. 19 for two initial drops andtwo gradients. Times related to disconnection of the drive ob-tained in measurements can also be identified if envelopes ofapplied nonrectangular sags are drawn against the voltage-tol-erance curve for balanced three-phase voltage sags. This means


Fig. 19. Illustration of ASD1 sensitivity to balanced nonrectangularthree-phase voltage sags caused by starting of large induction motors.

that the response of the drive to voltage sags is mainly deter-mined by the sag type, not by the sag shape (see also [2], [3]).

V. CONCLUSION

The main reason for voltage sags and short interruptions at-tracting significant attention lately is because they are frequentcauses of malfunctions or even damage of electrical equipment.The characterization and presentation, of voltage sags and shortinterruptions therefore, should be done with regard to responsesof different electrical equipment. Partial information about thesags and interruptions are of limited value for the assessment ofequipment sensitivity. Current “two-dimensional” (2-D) defini-tion and description of voltage sags with a pair of magnitude/du-ration values is inadequate and insufficient in particular in thecase of three-phase equipment. This paper demonstrates that fulland clear description of sag type with sag magnitude/durationvalues provided for all sagged phases is the minimum informa-tion required for precise assessment of three-phase equipmentsensitivity.

Furthermore, the paper demonstrates that there are at leasttwo reasons why procedures recommended for testing of three-phase equipment to voltage sags in current standards shouldbe revised and extended. First, these procedures do not includetesting against most common types of voltage disturbances inpower systems (i.e., unbalanced voltage sags). Second, sensi-tivity of three-phase equipment cannot be fully and preciselyassessed if only currently recommended procedures are used intesting. The paper therefore, first proposes simple and efficientclassification of voltage sags, and then presents results of testsobtained in corresponding testing procedures.

Required extensions of the existing test procedures are de-scribed in detail in the paper and illustrated on the example ofac adjustable speed drives. Simple and efficient “single-graph”representation of drive sensitivity is proposed.

The following specific conclusions can be drawn from thetests performed.

• Very few fundamental differences in the general behaviorof drives were identified in tests.

• The voltage-tolerance curves of all tested drives have analmost rectangular shape, with three distinctive parts: flat

or slightly inclined vertical part, slightly inclined hori-zontal part, and a smooth “knee” between them.

• Magnitude and duration thresholds of drive sensitivityvary in wide ranges, depending on the sag type anddrive/motor operating condition.

• Sensitivity of drives increases for some motor speedsdifferent from rated, but generally, if speed is lower thanrated, ride-through capabilities improve. If the loadingtorque is lower than rated, sensitivity of the drive de-creases, but the drive still has a high sensitivity to somesag types even if the controlled motor is completelyunloaded.

• Frequency deviations of % of rated frequency do nothave a noticeable influence on ASDs sensitivity.

• Different load types have only slight influence on drivesensitivity during the voltage sags. That influence is al-most unnoticeable for short interruptions.

• Point on wave of sag initiation and phase shift during thesag do not influence drive sensitivity.

• Variations in supply voltage magnitude and differentharmonic contents influence both vertical and horizontalparts of voltage-tolerance curves. If pre-sag and post-sagvoltage is 110% of rated, magnitude threshold decreasesand duration threshold increases, resulting in a decreaseof overall drive sensitivity. On the other hand, magnitudethreshold increases and duration threshold decreases (i.e.,overall drive sensitivity increases), if pre-sag and post-sagvoltage is 90% of rated.

• Highest increase and decrease in sensitivity was identifiedfor fifth harmonic with 0 and 180 phase angle, respec-tively. Influence of other harmonics (with the exceptionof the seventh harmonic with 180 angle, which also in-creases the sensitivity) on drive sensitivity is shallow.

• Drive sensitivity in closed-loop operation mode is slightlyhigher than its sensitivity in open-loop mode.

• During the tests, different protection systems of ASDswere activated. This suggests that settings of protectionsystems and their coordination should be carefully con-sidered. Important issues highlighted in tests are activa-tion of the phase missing protection in presence of phaseangle unbalance and “ignorance” of actual values of phasevoltages by that protection.

• Testing with rectangular and nonrectangular sags indi-cated that response of the drive is mainly determined bythe sag type, not by its shape.

ACKNOWLEDGMENT

The authors are grateful to the European accredited labora-tory “LEMCKO,” Kortrijk, Belgium, for the access and use oftheir test facilities. The authors also acknowledge contributionsto this project by M. T. Aung, Dr. C. P. Gupta, Prof. D. Kirschen,and Prof. G. Strbac.

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[35] R. A. Epperly, F. L. Hoadley, and R. W. Piefer, “Considerations whenapplying ASD’s in continuous processes,” IEEE Trans. Ind. Appl., vol.33, no. 2, pp. 389–396, Mar./Apr. 1997.

Sasa Ž. Djokic received the Dipl.Ing. and M.Sc. degrees in electrical engi-neering from the University of Nis, Nis, Yugoslavia, and the Ph.D. degree fromthe University of Manchester Institute of Science and Technology, Manchester,U.K.

Currently, he is a Research Associate with the School of Electrical and Elec-tronic Engineering, The University of Manchester, where he has been since2001.

Kurt Stockman (M’02) was born in Kortrijk, Belgium, on September 24, 1972.He received the industrial engineer degree in electrical engineering from Provin-ciale Industriële Hogeschool, Kortrijk, Belgium, in 1994 and the Ph.D. degreefrom Katholieke Universiteit Leuven, Leuven, Belgium, in 2003.

Currently, he is with the Department of Electrical Engineering of theHogeschool West-Vlaanderen, Kortrijk, Belgium.

Jovica V. Milanovic (M’95–SM’98) received the Dipl.Ing. and M.Sc. degreesfrom the University of Belgrade, Belgrade, Yugoslavia, and the Ph.D. degreefrom the University of Newcastle, Newcastle, Australia.

Currently, he is a Reader with the School of Electrical and Electronic Engi-neering, The University of Manchester, Manchester, U.K., where he has beensince 1998.

Jan J. M. Desmet (M’01) received the Polytechnical Engineer degree from thePolytechnic, Kortrijk, Belgium, in 1983, and the M.Sc. degree in electrical en-gineering from the Vrije Universiteit Brussels, Brussels, Belgium, in 1993.

Currently he is a Professor with the Department Provinciale IndustriëleHogeschool, Hogeschool West-Vlaanderen, Kortrijk, Belgium, where he hasbeen since 1984.

Ronnie J. M. Belmans (S’77–M’84–SM’89) received the M.S. degree in elec-trical engineering and the Ph.D. degree from the Katholieke Universiteit Leuven,Leuven, Belgium, in 1979 and 1984, respectively, and the special Doctorate andthe Habilitierung from RWTH Aachen, Aachen, Germany, in 1989 and 1993,respectively.

Currently, he is a Full Professor at the Katholieke Universiteit Leuven and aVisiting Professor at Imperial College, London, U.K.

Dr. Belmans is Chairman of the Board of Elia, the Belgian transmissionsystem operator.

Sensitivity of AC Adjustable Speed Drives to Voltage Sags and Short Interruptions

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