1338876302.482800339000_Voltage Source Converters - A Survey

7/29/2019 1338876302.482800339000_Voltage Source Converters - A Survey

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PWM Current Control Techniques

of Voltage Source Converters - A Survey

Luigi Malesani and Paolo Tomasin

Department of Electrical Engineering, University of Padova

Via Gradeniso 6a, 35 13 1, Padova, I taly

Tel. (39)(49) 8287507

Abstract - The paper gives a synthetic survey of available CurrentControl Techniques of the Voltage Source Inverters. These techniqueshave gained an increasing impo rtance with the widesp read application ofVoltage Source Inverters in high performance applications, such as ACdrives, AC Power Supplies and Active Filters, where fast response andhigh accuracy are needed.

A variety of methods have been proposed, which can be gathered intothree main categories: linear control (or ramp comparison), hysteresiscontrol and predictive control. The fundamentals of these methods aredescribed and the more recent developments are discussed.

The impact of the introduction of digital techniques and of theavailability of powerful microcon trollers on the developmen t of reliableand sophisticated current controls is put in evidence.

INTRODUCTION

In DC/AC conversion there is now a general preference to use

voltage-source rather than current-source inverters (Fig. I) [ 1 +3]. Thistrend, which grew in the last two decades, is mainly justified by the

introduction of power dev ices with self turn-off capability and by theadvantages of a capacitive DC storage, over an inductive one, in termsof weight, cost and efficiency. Additiona l advantages are determined bythe fact that this kind of converters is well matched with the inductivecharacteristic of usual AC loads, without the need of output filtercapacitors, and the majority of modern power devices have anti-parallelfree-wheeling diodes, deriving from their physical structure or includedin the package. As a result, v oltage source inverters (VSI) have become

a simple and reliable solution, and current-source converters (linecommutated or PWM controlled) are now used mainly wherebi-directional power flow with the AC supply is needed, or in AClDC

conversion with high-inductance DC loads , such as magnets or large DCmotors.

On the other hand, mo tors and other AC loads which are usually fedby converters exhibit in general better performance and faster responseif they are cu rrent-fed rather than v oltage-fed [ I ] . In AC motors, currentcontrol reduces the dependence on stator parameters and allows animmed iate action on the torque delivered by the m achine. In other ACloads, such in the case of UPS, current control results in an increasedstability of the control loop and in an intrinsic short-circuit protection.

These requirements can be fulfilled, while keeping the advantages ofthe VSI power structu re, by a closed-loop regulation of the AC currentsproduced by the inverter. This solution ensures several additionaladvantages. Among them, it gives the control of the current waveformwithin the AC period, which compensates also for load transients andnonlinearities and for commutation delays. T he feedback loop results alsoin some limitations: fast-response voltage modu lation techniques mustbe employed, such as Pulse Width Modulation (PWM) [ 4+ 7] or Discrete

Pulse Modu lation (DPM)[8- lo ]. Instead, Optimal Techniques, whichuse pre-calculated switching patterns within the AC period [11 ,12], canbe hardly used, as they are not oriented to ensure current waveformcontrol,

A number of VSI current-regulation techniques have been develop ed,

which differ for the kind of mod ulation and for the type of control used.As regards the modulation, excep t for DPM which is used ma inly in the

case of Resonant Link Converters (RD CL), PW M is generally used. Forthis latter, fixed or variable frequency or random modulation can besuccessfully employed . As regards th e control, three main categ ories canhe considered: Linear (or Ramp Comparison) Control [13+191,

0-7803-0891-3/93$03.00019931EEE

Hystere sis Control [20+37] and Predictive Control [38+6 1]. Alternativeand innovative kinds of current regulation, how ever, have been adopted,such as Delta an d Sigma Delta Modulation (AM, CAM)[62+66] and

Neural Nemrks and/or Fuzzy Logic Controls [67+69] . For each ofthese categories, a variety of modifications and improvements of thebasic principles have been developed, due to extensive work done and

to large interest on the application of this kind of converters.In today’s practice, voltage source inverters with Current

Regulation (CR) are employed in every ap plication where fast response,high accuracy and high-level performance are needed . As these featuresare required more and more, the interest toward cheap, reliable andhigh-quality current-regulation techniques is increasing too.

Due to the wide variety of techniques proposed, there is the need ofsummarize and compare the characteristics of the available solutions[70,71]. Some authoritative works describing the status of the art, bothin VSI current regulation [72,73] and, more in general, in PWM

techniques [74] have been presented recently. In this paper, a furthercontribution is given to an overall view of this vast subject.

BASICCONCEFTS

In order to compare so many different approaches, it may be usefulto recall some common basic concepts.

A three-phase Voltage Source Inverter (Fig. l) , generates at eachoutput phase i a voltage U, with two-level, rectangular waveform(neglecting the commutation times) (Fig.2a). In conventional VSI, thereare not mutual constraints between phase switching instants, so that thepulse length can be varied continuously (PWM ). In some cases however,commutation mechanism (RDCL, D P M ) or control system (AM) allowcommutations only at f ixed times. Usually, losses put a limitation on theaverage switching frequency of each phase. Filter, Control or otherneeds may also require the switching frequency to be constant.

Modulation process controls the phase switching sequence accordingto a given reference U,*, so that the phase voltage low-order harmonicsresult in a voltage ui (mean average) whose waveform should follow U,*

as close as possible. Modulation generates also high order harmonics,located around the switching frequen cy. If this latter is high enoug h, the

two groups are quite separated each other.A translation of pulse position within the modulation period does not

affect appreciably mean average value u, ( F i g . 2 ~ ) .

If a three-phase load with no neutral connection is fed by theinverter, phase currents depend only on voltage difference betweenphases, so that only two degrees of freedom are allowed: a comm on termcan be added to the phase voltages, thus shifting their mean value U

without affecting load currents. This shift is often used to extend themaximum phase voltage which can be produced by the inverter. At this

1:T -

Fig.1 - Voltage Source Inverter

670



T I

: .. .. . .a

*...*’

b

Fig.2 - Pulse patterns and vectors

purpose, when sinusoidal phase voltages are produced, a suitable thirdharmonic term can be adde d, with no effect on the load. In other cases,voltages U, are shifted so that the upper (the lower) of them coincideswith the voltage of the upper (lower) DC rail. Thus, no commutations

are needed for that leg (discontinuous switching), so that the average

switching frequency is reduced [6,7,27].While phase voltages can be controlled independently, phase currentsare determined not only by their ow n phase voltage, but also by those of

other phases. Thus, a phase interference results.Vector representation of inverter states (Figs.2b,2d) is particularly

suitable to treat as a whole the phase voltage effects on the load [1,5].Vector sequences with the same resultant give equal mean voltages U,,

i.e. equal mean average current i, on an inductive load (Fig.2b andFig.2d). On the other hand, different vector paths produce differentcurrent ripples. A sensible ripple reduction, mainly at high modulationindex, is obtained when phase pulses are centered and symm etrical, witha choice of i& corresponding to F ig .2~.This condition results in amaximum of zero states duration and, in vector representation (Fig.2d),in an equal length for states 0 and 7 (5,741. With this choice, also themaximum phase voltage is increased by a 15% with respect to thecentered PWM, where & is kept at the middle of DC supply.

Vector representation, however, has also some drawbacks: thecorrespondence with phase commutations is less immediate and some

aspects, e.g. the determination of phase voltage mean value, loseevidence.Current control of a VSI requires to determine the phase voltages

which will produce the desired mean average load currents. This is

‘l c PI

’&

generally obtained by means of a closed-loop control. A feed-forwardaction can help if load structure is determined enough. The control loopis more or less affected by low frequency error term s, which degrade themean average current precision. These errors add to low frequencyerrors resulting from voltage modulation and to the high frequency

ripple, thus determining the overall system accuracy.While the low frequency errors (tracking errors) influence the

performance of the drive, or other system where the VSI operates, thecurrent ripple is usually related to losses, noises and vibrations. Som eaccuracy evaluation criteria have been proposed [39,73,74], which arerelated each other. Among them, the most closely connected to current

control seem to be the r m s . harmonic value and the distortion factor.This latter is usually referred to the current harmonic content given by

a square wave voltage waveform at fundamental frequency [74].Other im portant performance figures are the transient response time

and the degree of utilization of DC-link voltage.

LINEARONTROL

Linear Current Control, called also Ramp Comparison orSine-Triangle Current Regulator, uses three independent PI erroramplifiers to produce voltage references for a three-phase triangularPWM modulator (F ig.3 ) [13+ 191. This reg ulator is directly derivedfrom the original triangular suboscillation method [4]. However, thebehaviour is quite different, as the output current ripple is fed back andinfluences the switching times.

Integral amplifier characteristic minimizes errors at low frequency,while proportional gain and zero placement are related to the amount ofripple: maximum slope of error signals U, should never exceed the

triangle slope. Additional problems may a rise from multiple crossing oftriangular boundaries.

As a consequence, the regulator performance is satisfactory only ifthe significant harmonics of current references and of load e.m .f. arelimited at a freque ncy well below than that of triangle carrier (less than1/9) (51. Moreover, in drive applications, current phase error may be ofparticular concern for the correct b ehaviour of field-oriented controls athigh speed. Feed-forward error correction has been employed, especiallyin case the load e.m.f. can be determined [14,17,18].

A modification of linear control, which is well suited to drives and

to all cases where sinusoidal current and voltage waveform s are required,is that of the Rotating-Frame Current Regulator (Fig.4)[16]. It employsd-q transformation on error signals, so that their fundamentalcomponents become DC quantities. PI amplifiers reduce to zero the

steady state errors due to fundamental components of current and ofe m f . The response remains limited, however, in case of transients andof distorted waveforms. An additional benefit of this regulator is that,being sensitive to the fundamental component of current, it ensures thecorrect phase relationship of the load current w ith its reference, even incase of six step operation of the inverter. This feature is of particular

Current References

F ig3 - Linear Current Regulator

6 7 1

Fig.4 - Rotating frame linear current regulator



b a d

Actual currents

Fig.5 - Basic hysteresis current control

importance for drive applications, were such an operating condition ismet very often.

Linear current regulator structure is well suited to be implem ented byanalog schemes, and this solution has been extensively used for a long

time. Quite early, ho wever, attem pts were made to translate the schemeto a digital implemen tation [15]. This latter is particularly convenient forthe Rotating Frame Regulator, as d-q transforms are more easilyperformed by digital means [19]. Despite to its limitations, LinearRegulator is satisfactory for a large num ber of applications, particularlyfor drives of low and medium performance. It is simple, robust andinsensitive to load parameters, and its performance is improving as th eswitching frequency of modern power devices increases. For theregulator, a variety of dedicated ICs are available, which perform at least

the task of PWM modulation, and can be associated with the today's fastand powerful microcontrollers.

HYSTERESIS ONTROL

Hysteresis Current Control is an instantaneous feedback system whichdetects the current error and produces directly the drive commands forthe switches when the err or exceeds an assigned band (Fig.5)[20*37].The advantages of this technique are high simplicity, good accuracy,outstanding robustness and a response speed limited only by switchingspeed and load time constant. Some characteristics, like the variableswitching frequency and the random operation, are considered in manyapplications as unfavourable. The origin of the main drawbacks ofthree-phase hysteresis regulator, due to the interference between phasecontrols, has been clarified in [23].

A quantity of proposals has been made to overcome the hysteresisregulator limitations: variable band amplitude or shape to get constantfrequency [21,22,25,26,27,28,34], introduction of zero-states or delays

to avoid limit-cycle operation [20,24,25], injection of common-modesignal to synchronize the operation and reduce interference 1361.

An approach which eliminates the interference, an its consequences,

is that of decoupling error signals by subtracting an interference signalderived from mean inverter voltage U, (Fig.6)[26]. Similar results areobtained in case of "discontinuous switching" operation, wheredecoupling is more easily obtained without estimating load impedance[27]. Once decoupled, regular operation is obtained and phasecommutations may (but not need to) be easily synchronized to a clock.

An alternative approach is that of controlling the inve rter as a whole,

according to the vector control concept. At this end, an hexagonal [25]or square hystere sis field [29,32,3 5,37] is define d in the a-A plane, and

a suitable vector voltage is selected, in dependence of error vector, bymeans of a look-up table .The method gives good results. Attempts aremade to reduce the asymmetry between phase behaviours (371.

In its various implementation, hysteresis current regulator is the

fastest and more stable control av ailable. Even in the case a PI amplifieris introduced in the loop to eliminate steady state errors due to

commu tation delays, it can be ma de fast enough as not to deteriorate theregulator speed. In high accuracy applications, such as fast drives foractuators or precision AC power supplies, it is the only control which isable to face the load and inverter nonlinearities due to commutationdelays. Moreover, even if the more sophisticated modifications are

Fig.6 - Decoupled, constant frequency hysteresis current control

introduced, the system complexity should not be overestimated. It

remains in many cases well below than that of many digital controls, andcould be greatly reduced by the developm ent of dedicated ICs.

However, hysteresis approach is hardly compatible with the alldi gita ltrend which is now taking place. Its instantaneous response would bedeteriorated by A/D conversion or microprocessor interrupt delays.Vector control would remain in any case only a part of the totalhardware required. Thus, hysteresis control application is likely to beadopted mainly in high-performance, high speed applications where itsfeatures cannot be replaced by o ther solutions.

PREDICT~VEONTROL

Predictive Current Control is a technique which predicts at thebeginning of each modulation period the evolution of the current errorvector on the basis of the actual error and of the load parameters andother load variables; the voltage vector to be generated by PWM duringthe next modulation cycle is thus determined so as to minimize theforecast error (Fig.7) [38+61].

The advantage of this technique is that, by giving to the regulatormore informa tions than the mere error signal, a faster and more accurateresponse can be obtained. However, by detecting the error only at fixedintervals, and by performing calculations often complex, delays andinaccuracies which limit the regulator speed and performance are

introduced. Thus, predictive regulators are particularly suitable fordigital implementation, where signal acquisition is discrete in time andin amplitude, and where it is easier to improve calculation power thanto increase sampling rate and reduce conversion and calculation times.

Within the class of predictive regulators a large variety of solutions

can be included [72], which m ake use of more or less sophisticated plantmodels for the prediction. When the choice of the voltage vector is made

in order to null the error at the end of the cycle, the predictive regulationis often also called "dead beat control" [40,42,45,46,51,53].

Among the additional informations given to the control, non availablestate variables (such as flux or speed) can be include. Theirdetermination can require the use of observers or other high performance

Inverter-

h

Rekrerlce I I I l lCu r r e n tVectnr

ActwlCur ran t --w k r

Fig.7 - Basic predictive current control

672



control blocks, which often may be shared with the control of the entire

system, as in case of drives [41,50,57,59].

In some specific applications, system informations allow to simplifythe prediction process, leading to simple and effective regulators. Thisis the case of generation of a set of symm etrical three-phase sinusoidalcurrents, such as those employed in AC drives in steady state. For thiscase, vector sequence can be very easily determined by the help ofprecalculated look-up tables [52].

The sampling rate is typically limited by inverter switching speed orby calculation time. In these situations, a constant switching frequencyis usually adopted, or some synchronization with the fundamental

component is introduced, to prevent the generation of subharm onics.In some cases, mainly if the control is applied to large power

systems, the switching frequency allowed by pow er semicond uctors, suchas GTOs, is quite low. In this instance, not only the sampling rate islimited, but it is often so close to the required fundamental frequencythat the optimization of current trajectories and of switching instants is

of major importance to obtain acceptable results [39,49,61]. At the samepurpose, the adoption of pre-calculated switching paths has beenproposed, introducing suitable procedures to ensure adequate responseto transients and smooth transition from one steady operation to the other[44,56,60,61].

The interest for predictive current regulators is growing with theincreasing trend for totally digital controls. T he performance of this c lassof regulators, and their sophistication, is rising together with the speedand calculation power of today’s digital electronics [ S I . Thus, fo rclassical applications such as induction motor AC drives, the availabilityon the market of cheap, reliable and high performance solutions can beenvisaged in a next future.

At present, the only limitations for these regulators are in theresponse speed and in the accuracy of system parameters used for theplant model. The latter problem, which cannot be neglected also for theother regulator types, is of particular relevance in this case, as theamount of feedback employed is limited and typically affected byappreciable delays. At this regard, particular attention is given to systemsincluding some self-tuning and self-adapting capability [43]. Thesesolutions can be m ore easily adopted as the calculation power and speedof modern digital systems increase.

DELTAMODULATION

The basic structure of a current regulated Delta Modulator is shownin Fig.8 [6 2+ 66 ]. It looks quite similar to that o fa hysteresis regulator,but operating principle is quite different. In fact, only error sign is

detected by comparators, whose outputs are sam pled at a fixed rate sothat inverter status is kept constant during each sam pling interval. Thus,no pulse width modulation is performed, but only hasic voltage vectorscan be generated by the inverter for a fixed time. This m ode of operation

gives a discretization of inverter output voltage, that opposes to the

capability of continuous variation of output voltages which is peculiar ofPWM.

A first effect of this discretization is that, w hen synthesizing periodicwaveforms, a non negligible amount of sub-harmonics is generated[9,10,71]. Thus, to o btain comparable results, a delta modulator shouldswitch at a frequency about ten times higher than a PWM modulator[71I. As a counterpart, delta modulators are very simple and insensitive

to the load parameters.When applied to three-phase inverters w ith insulated-neutral load, the

mutual phase interference and the increased degree of freedom in the

voltage vedor choice must he taken into account. Therefore, according

to some proposals, instead of performing inde pendent delta modulation

in each phase control, output vectors are chosen in dependence not onlyof the error vector, but also in function of the previous status, so that the

change rate is limited and zero vector states become possible [63]. A

further proposal is to employ a transformation into a d-q rotating frame,in order to perform a PI control of fundamental components, whichreduces error of these components and eases the operation of the inverterin saturated conditions [66). These features are of pa rticular importancein drive applications.

Delta control has been mainly studied for those resonant linkinverters, which allow commutations only at fixed time intervals,

. .. H /H

Actual cur rents

Fig.8 - Delta modulation current regulator

although at a frequency much higher than that of conventional inverters[62]. However, some proposals have been made for delta modulators,where switching instant of three ph ases are shifted by one third of period[65]. Therefore, a reduced discretization is obtained, thus limiting the

subharmonic content. This solution may be interesting, due to itssimplicity, for inverters, like MOSFET inverters, capable of operationat very high frequency.

NEURAL ND FUZZY CONTROLS

The introduction of innovative control techniques in currentregulation looks attractive, in order to overcome the limitations of the

classical control methods.There are some proposal to take advantage of learning capability andnon-linear nature of neural nenuorks, to improve the performance both

of linear regulators [68] and of hysteresis controls [67]. In the first case,neural network replaces PI amplifier, and adjusts itself in order tocompensate for steady state errors at various loading situations. In thesecond case, the network, trained off line, avoids the interference andlimit cycle problems. No proposals are known of application of neural

networks in predictive regulators. The desired robustness and accuracyare obtained with quite simple structures. T he main lim itation resides inthe time consuming design procedure 1731.

An application offuzzy logic in linear current regulators is describedin (691. Its implementation requires a limited amount of hardware, butthe design calls for a large amount of expertise [73]. The results seemto be appreciable, especially if fuzzy control includes also other systemvariables, such as flux and torque in AC drives.

CONCLUSIONS

Current Control Techniques for Voltage Source Inverters can he

subdivided as Linear. Hysteresis, Predictive an d Delta ModulationControls. The basic principles and the latest developments of these

techniques have been synthetically described in the paper. Theadvantages and limitations have been examined, indicating the applicationfield where each technique is particularly suited.

In particular, for low performance applications with large diffusion(pumps, blowers & fans, retrofit applications), digitally implementedlinear regulators are well adequate.

Predictive controllers applications are quickly growing in

medium/high performance systems, especially for traction and highpower units.

Hysteresis controls, in their improved version, are well suited to fast,

accurate conversion systems.lnteresting perspectives seem to come also from innovative controls,

such as neural networks and fuzzy logic.

6 7 3



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