IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 55, NO. 4, APRIL 2008 1771

On the Efficiency of Voltage Source and CurrentSource Inverters for High-Power Drives

Eduardo P. Wiechmann, Senior Member, IEEE, Pablo Aqueveque, Student Member, IEEE,Rolando Burgos, Member, IEEE, and José Rodríguez, Senior Member, IEEE

Abstract—The energy performance of various types of voltage-source and current-source converters is examined. For fairnessand completeness, efficiency is calculated for three major battle-ground scenarios. The first is a low dynamic nonregenerativegroup of applications such as pumps, fans, and compressors.This group represents 85% of high power (∼2 MW) industrialapplications where energy savings are usually a primary con-sideration justifying investment. The second scenario considersapplications requiring good dynamic response and regenerativebraking. Finally, the third group considers very high power ap-plications (over 20 MW). The evaluation presented takes intoaccount semiconductor switching and conduction losses, losses inthe medium voltage feeding transformer (determined per IEEEStandard C57.18.10-1998), and the losses in ac and dc filters.For purposes of analysis, computer simulations validated againstmeasurements taken on a 1-MW voltage source inverter (VSI) anda 1.4-MW current source inverter (CSI) were used. The results ofthe first scenario show competitive efficiencies for VSI and CSIdrives, whereas voltage source-based solutions are more energyefficient in the second scenario considered. For the last group,the current source load-commutated inverter exhibits the bestperformance.

Index Terms—AC motor drives, efficiency, electric machines,motor drives, multilevel systems, power electronics, pulsewidthmodulation (PWM).

I. INTRODUCTION

R ECENT advancement in power semiconductors technol-ogy has seen insulated gate bipolar transistors (IGBT)

and integrated gate commutated thyristors (IGCT) rapidly re-place gate turn-off thyristors (GTO) in high-power applications[1]–[3]. IGBTs and GCTs presently have a voltage blockingcapability of up to 6 kV, with saturation voltages rangingbetween 3 and 4.5 V and 2 and 3 V, respectively. Thus, theycan switch at frequencies up to 1 kHz, enabling their usage inhigh power medium-voltage adjustable speed drives (ASD) innumerous industrial processes [4].

Manuscript received March 21, 2007; revised January 3, 2008. This workwas supported by Grant 1060902 from the Chilean Fund for Science andTechnology Development (CONICYT) and by ENAP Chile for facilitatingmeasurement of efficiency at its oil refinery installations in Concepción, Chile.This work used the Engineering Research Center Shared Facilities supportedby the National Science Foundation under NSF Award EEC-9731677 and theCPES Industry Partnership Program.

E. P. Wiechmann and P. Aqueveque are with the Department of Electri-cal Engineering, Concepción University, Concepción 160-C, Chile (e-mail:wiechmann@ieee.org).

R. Burgos is with the Center for Power Electronics Systems, VirginiaPolytechnic Institute and State University, Blacksburg, VA 24060 USA.

J. Rodríguez is with the Electronics Engineering Department, UniversidadTécnica Federico Santa María, Valparaíso 110-V, Chile.

Digital Object Identifier 10.1109/TIE.2008.918625

Nearly 85% of motors in industrial processes are used todrive pumps, fans, and compressors. These are relatively simpleapplications that do not require any advanced controls due totheir relatively benign dynamics—slow torque and speed tran-sitions, which can nonetheless achieve significant energy andcost savings, has been well proven in the past [1], [4]. Processeslike mining, metallurgy, and traction have therefore readilyincorporated ASD technology, benefiting from its advantages,pursuing further increases in production, product quality, andrevenues [4] as well. The remaining applications require moreadvanced ASDs, which usually employ regenerative brakingand demand a high dynamic performance [8], [9]. Finally, animportant family of applications not covered by this paperdemands low motor speeds usually less than 1200 rpm. Thepreferred solution is to use custom-made cycloconverter andslip-power recovery drives [1], [2].

ASD manufacturers have developed multiple types of drivesto answer the needs of multiple industrial applications; how-ever, the choice of standard equipments (not custom made)for high-power processes is not as broad and is actually quitelimited, as shown in Table I. These drives can be classifiedinto two main categories: voltage and current source converters.Past research has studied and analyzed control strategies as wellas pulsewidth modulation (PWM) schemes for voltage- andcurrent-source inverters and rectifiers (VSIs and CSIs) wheresignificant accomplishments have been achieved [10], [16],[21], [22], e.g., harmonic distortion minimization, high-inputpower factor, and reduced switching frequencies, among others.Power conversion efficiency has not been as thoroughly studied,however, or even compared with these two alternative VSIand CSI families of converters [10]–[12], [18], [20]. Naturally,comparison is of utmost importance at the high-power level andis often the tipping point for selecting one or the other.

VSIs have the advantage of requiring only a simple diodebridge as a front-end converter, featuring minimum costs,high efficiency, and high reliability for the rectifying stage.The main disadvantage of VSI converters is the generationof high dv/dt transients of harmful effects on the motorinsulation—depending on the feeder impedance, and thus dis-tance between converter and motor [12], which, if not filtered,can reduce the motor lifetime significantly. Another drawbackof this topology is the generation of common-mode voltages,also affecting motor insulation and motor bearings.

On the other hand, high-power CSIs have the advantage ofrequiring a reduced number of semiconductors since symmetricIGCTs can block reverse voltage. Also, since output capacitorsare required for CSI operation, voltages and currents applied

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1772 IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 55, NO. 4, APRIL 2008

TABLE IHIGH-POWER DRIVES OFFERED BY MAJOR MANUFACTURERS

to the motor are quasi-sinusoidal because this capacitive filtereliminates the high di/dt of the inverter PWM action [17]. Thistype of drive also requires a controlled current source in its dclink, for which an active front-end converter is used to regulatethis current [19], [30]. Although this is a disadvantage for non-regenerative applications, since it increases drive complexity,the PWM CSI converter intrinsically offers sinusoidal currentsand a high-power factor at its input terminals [21]. Also, theconverter offers an additional short-circuit protection providedby the dc-link choke (current source). The latter, however,represents a drawback regarding size, volume, and weight be-cause this dc-inductor is usually quite heavy and bulky. Anotherdisadvantage of this converter type is the possible occurrence ofresonances between the output capacitive filter and the motorinductances [19].

As we can see, there are many trade-offs involved in choosingbetween the different ASD technologies. The final decision isusually made based on cost and economical factors, or simplyby following desired industrial practices. The objective of thispaper is to provide a fair analysis and comparison tool for high-power VSI and CSI converters, based on the power conversionefficiency analysis of the main set of industrial high-powermotor drives, including pumps, fans, and compressor applica-tions, motor drives requiring more advanced features such asregeneration or high dynamic performance, and converters usedin very high power applications (> 20 MW).

The efficiency comparison performed takes the semiconduc-tor conduction and switching losses [13] into account as well asthe losses on the feeding transformer [23], which is required forisolation purposes and for harmonic distortion minimization inmedium voltage converters.

This analysis methodology is easily applied to alternativehigh-power medium-voltage converters topologies.

II. HIGH-POWER DRIVE TOPOLOGIES

Different manufacturers currently participate in the medium-voltage market offering a variety of drives [4], [14]. Table Isummarizes the main series of products available, indicatingthe type of topology, semiconductor devices, power and voltage

range used as well as manufacturer and model name. The most-used inverter topologies are the two-level, three-level neutralpoint clamped (NPC) [5]–[7], and cascaded multilevel H-bridgeVSIs. All of these use diode front-end rectifiers, either 12 or18 pulses, depending on the specific harmonic requirements[15]. If the application at hand requires regeneration, thenPWM rectifiers are used to replace diode front-end converters.Because this paper focuses on comparing voltage- and current-source topologies, only two- and three-level topologies areanalyzed in detail. Cascaded multilevel H-bridge inverters werenot evaluated because there is currently no direct current-sourceequivalent, and the feasibility of such industrial development isvery low.

As for CSI converters, the most used topology is the three-phase bridge using series semiconductors to support higheroperating voltages. The rectifying stage for these convertersemploys PWM current-source rectifiers (CSRs), which have re-placed thyristor-phase-controlled rectifiers offering sinusoidalinput currents with a near-unity power factor. So far, RockwellAutomation is the only manufacturer producing current-sourceconverters. Very high power applications, on the other hand,use load-commutated inverters, which are implemented as CSIsdriving synchronous machines up to 72 MW. These convertersare well known for their high efficiency due to the reduced con-duction voltage drop of their thyristors, which usually presentsaturation voltages on the order of 1.4 V.

III. RELIABILITY AND FAULT TOLERANCE

The reliability and fault tolerance of medium-voltage con-verters is of interest, given the critical nature of their functionswithin industrial sites. In VSI topologies, the most commonand destructive fault by far is the shoot-through [31], for whichdifferent protection schemes have been developed based on thedesaturation protection that effectively opens the switches inthe phase leg before their destruction occurs [31], [32]. CSItopologies, on the other hand, are prone to destructive failuresunder open circuit faults—due to faulty semiconductor gatedriving—for which no solution exists. However, the use oflight-triggered thyristors has virtually eliminated this fault [33].

WIECHMANN et al.: ON THE EFFICIENCY OF VOLTAGE SOURCE AND CURRENT SOURCE INVERTERS FOR HIGH-POWER DRIVES 1773

For very high power drives, the use of redundant semi-conductors is a standard practice to increase the converterreliability. These can be implemented as paralleled devices orparalleled power modules, for instance redundant phase legs.Welchko et al. [31] show some usual practices to improveconverter availability.

IV. LOSSES IN HIGH-POWER DRIVES

A. Conduction Losses

Conduction losses of high-power semiconductors are a func-tion of the device saturation voltage (VSat) and the instanta-neous current passing through it, i(t). The device saturationvoltage can be modeled using a first-order linear approximationcomprised of a threshold voltage (VTO) and a series resistance(rT ) as follows:

VSat(t) = VT0 + rT · i(t). (1)

The device average conduction losses (Pcond) over a funda-mental frequency period may then be obtained multiplying (1)by i(t) and integrating accordingly. These are given as follows:

Pcond = VT0 · IAvg + rT · I2RMS (2)

where IAvg corresponds to the average current flowing throughthe device, and Irms to the rms value.

In this paper, the conduction power losses were calculatednumerically using computer simulations; specifically, using (2)to continuously calculate the total conduction losses of eachconverter considered.

B. Switching Losses

Power semiconductor switching losses are determined by thetotal commutation time when the device is turned on or off, andby the voltage (V ) and current (I) across the device during theprocess. The energy dissipated during commutation is Eon andEoff for turn-on and turn-off, respectively, and is provided bythe device manufacturers on their datasheet; it corresponds tothe integral of the instantaneous power during the commutationprocess.

This energy value is given by manufacturers at a specifiedvoltage and current. The following: linear interpolation is usedat every switching instant to determine the energy dissipated atthe operating points throughout a line cycle

Eon/off(t) = Eon/off−datasheet ·V (t) · I(t)

Vdatasheet · Idatasheet. (3)

In (3), the average switching power losses (Psw) over a com-plete fundamental-frequency cycle may be determined usingthe following summation, where fsw corresponds to the switch-ing frequency used, T to the fundamental frequency period, andk to each of the discrete switching instants (Tfsw/cycle)

Psw =1T

T ·fsw∑

k=1

(Eon(k) + Eoff(k)) . (4)

TABLE IISEMICONDUCTORS PARAMETERS

The switching losses in this paper are calculated numericallyusing computer simulations just as for conduction losses, whichin this case evaluate (4) to calculate the total switching lossesof each converter considered (per switch). The semiconductorparameters used for the different topologies and devices aregiven in Table II.

For high-power VSI and CSI, the use of semiconductors inseries requires snubbers to balance voltage. Their losses havebeen computed and added as switching losses [1].

C. DC-Link Losses

The losses on the dc-link choke for CSI converters areproduced by its equivalent series resistance; these are usually2%–4% of the rated power of the inductor. A disadvantage ofthis type of converter is that when the inverter operates withnominal dc current, losses on the inductor will be the maxi-mum regardless of the load demand. However, control schemesregulating the dc-link current, such as those for drives with lowdynamic requirements, will present fewer losses, being morefavorable from an efficiency point of view. On the other hand,VSIs present lower losses when compared to CSI converters,since capacitors are very efficient devices in general. In fact,the total loss of a dc-link capacitor bank rarely exceeds 0.5% ofits rated value.

D. Transformer Losses

High-power transformers feature very high efficiency(premium manufacturing category) due to the considerableeconomic impact that their operation has on industrial processes[25]. Thus, an efficiency of 98.5% is common for medium-voltage transformers. Still, the different transformer configu-rations used by medium-voltage drives could easily tilt theefficiency performance from one to another, a reason why thelosses in the feeding transformers have also been taken into ac-count in this study. Specifically, IEEE Standard 57.18.10–1998presents a method for calculating all factors affecting the deter-mination of transformer losses under the presence of harmoniccurrents [24], which are the main contributors to transformerpower losses in motor drives. This method has been chosen inthis paper, since the currents circulating through the transformerwindings may be easily determined from the detailed switchingmodels used.

1774 IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 55, NO. 4, APRIL 2008

Fig. 1. ASD with diodes rectifier and PWM VSI.

For example, this section discusses the loss calculation forthe delta-wye isolation transformer of the diode front-end PWMVSI motor drive, although the method has been applied to allthe topologies under consideration.

Transformer losses are proportional to the square of thevoltage (iron losses) and proportional to the square of the loadcurrent (copper losses); therefore, the power factor seen at theinput of the drive will have a direct impact on the total losses.Additionally, the harmonic content of the converter currents—measured by the k factor—will further increase losses on thetransformer. It is clear, then, that the better the power factoris and the more sinusoidal the currents flowing through thetransformer are, the lower the total losses will be in the trans-former. Further loss reduction requires optimizing transformermechanical design and construction. For instance, increasingthe conductor section reduces losses by reducing the conductionlosses and allowing the transformer to operate at a lower tem-perature, since it is known that operating at lower temperaturesimproves the transformer efficiency. This type of solution, how-ever, should be weighed carefully to determine the optimumbalance between increased efficiency and the additional initialinvestment required in the modified transformer.

As an example, losses of the six-pulse motor-drive trans-former are shown in Appendix A. This procedure is in accor-dance with IEEE Standard 57.18.10–1998.

E. Motor Losses

The use of PWM inverters for the speed control of inductionmachines generates additional losses due to the harmonicscurrents that circulate through their windings. These lossesdepend on the types of modulation schemes and machines thatare used [27]. Specifically, voltage source inverters increaseiron losses, which has been thoroughly studied, as seen in [28],[29]. It is found that the use of sinusoidal PWM and spacevector modulation increase losses in approximately 1.5% of thecases. CSIs, on the other hand, offer a much better motor lossperformance due to the mandatory capacitive filter at its outputterminals.

Losses induced on induction machines by these convertersare then perfectly comparable to motors fed from sinusoidalthree-phase voltages [30] and are significantly lower than thosegenerated by PWM VSI converters. A comparison of totallosses is presented in Section IX.

V. HIGH-POWER DRIVE EFFICIENCY FOR

CONVENTIONAL APPLICATIONS

As mentioned previously, the vast majority of industrialapplications employing medium-voltage drives are fans (30%),pumps (40%), and compressors, extruders, and conveyors(15%). The remaining 15% corresponds to applications requir-ing more advanced and specific drive types. These are analyzedin Section VI. Conventional applications normally operate withlow speed dynamics and do not regenerate energy into the grid.The most common converters currently available in the mar-ket are 12-pulse rectifiers with PWM-VSI, 12-pulse rectifierswith three-level NPC VSI, and PWM current-source converters(CSR + CSI). The following sections show the analysis andloss calculation for each of these three converters driving a1-MW motor. The losses considered are power transformers,semiconductors, and filters.

A. Diode Rectifier and PWM VSI

The most-used converter for low-voltage (< 1 kV) industrialapplications consists of a six-pulse diode rectifier, an LC dc-busfilter, and a PWM VSI. This topology has been extended to in-clude medium-voltage applications, thanks to the developmentof high-voltage IGBTs, which, when used by series-connecteddevices, are capable of feeding motors of up to 4160 V asshown in Fig. 1. The purpose of the dc inductor is to smoothout the dc-link current and improve the input current distortion,reducing the fifth and seventh current harmonics at the input.Diode commutation losses vary with the actual current and thegrid impedance. Because the reverse recovery of fast powerdiodes is very small, switch losses are neglected in this study.The parameters VT0 and rT for the diodes used are 0.94 V and0.147 mΩ as per Table II.

The VSI, on the other hand, has eight possible active states.Six of them transfer energy to the load, and two are zerovoltage or freewheeling states where the motor phases areshorted. Regardless of the state, only the top or bottom IGBTs(highlighted in Fig. 1) are turned on per phase leg at any giventime to avoid shorting the dc bus—a fatal failure.

Semiconductor thermal characteristics restrain the high-power VSIs (> 1 MW) to use switching frequencies of up to1 kHz, which also limits the switching losses of the motor drive.To match experimental data in this paper, a switching frequency

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Fig. 2. ASD with 12 pulses diodes rectifier and NPC multilevel VSI.

Fig. 3. CSI converter with PWM rectifier and inverter.

of 1 KHz is used at a nominal load to calculate the power lossesof the VSI for a 1-MW motor drive fed by a six-pulse dioderectifier. The losses of the delta-wye isolation transformer arealso calculated.

B. 12-Pulse Rectifier and Three-Level NPC VSI

The higher power demand of industrial applications usingac motors has driven the increase of dc-bus voltages to limitcurrent amplitude and achieve high-power conversion efficien-cies. To compensate for the lack of high-voltage semiconduc-tor devices, multilevel converters have been developed; NPCtopology is by far the most utilized for medium-voltage VSIsas shown in Fig. 2. The main effect of multilevel voltagewaveforms is to reduce the dv/dt voltage stress on the mo-tor insulation, thus extending its lifetime. Also, the harmonicspectrum is improved for the same switching frequency whencompared to a two-level VSI, reducing the losses and torqueripple in the motor. Use of a 12-pulse diode rectifier as a front-end converter also improves the harmonic distortion at the inputlines, increasing the power factor and reducing the losses on the12-pulse isolation transformer. Its main drawback is that thedc-link current is now conducted by four diodes, although thishas a small overall effect because the voltage level for this typeof converter is usually higher. The dc-link current magnitude inconsequence is lower for the same power level when comparedto a 6-pulse rectifier. Overall, the reduced input and outputharmonic distortion will contribute to a higher efficiency of thismedium-voltage drive topology.

The NPC-VSI has two IGBTs in the turned-on state at anygiven time per converter phase-leg. Consequently, the outputphase-voltage of the converter can take any of three possiblelevels: −Vdc/2, 0 or +Vdc/2. Energy is drawn or injected intothe dc-link clamped capacitors depending on the converter stateapplied. As a result, the capacitor voltages need to be con-trolled to avoid any unbalance between them. Multiple controlschemes have been developed effectively for this purpose, thussolving this problem [6], [7]. As we can see, the two centerIGBTs in the phase leg (Sw1b and Sw4a for phase leg a, Fig. 2)conduct continuously for nearly half the fundamental frequencyperiod, whereas the top and bottom phase leg IGBTs conductin a pulse fashion during the same period. This implies thatthe predominant losses for the center and outer IGBTs areconduction and switching losses, respectively.

C. PWM CSR-CSI Converters

Fig. 3 shows the circuit schematic of a current-source con-verter comprised of a PWM CSR and CSI using symmetricGCT semiconductors. These GCTs have high saturation volt-ages peaking at 8 V for currents of 1 kA (ABB 5SHZ 08F6000,Mitsubishi GCU04AA-130), which correspond to twice the sat-uration voltage of asymmetric IGCTs used for voltage-sourceconverters. The advantage of the symmetric device is its highervoltage blocking capacity (up to 6 kV), which, if combined withthe use of series-connected arrangements of GCTs, allows thistopology to drive 6.6 kV motors at high-power levels. As seenin this figure, both rectifier and inverter use the same bridge

1776 IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 55, NO. 4, APRIL 2008

Fig. 4. PWM AFE and VSI.

topology. Both converters are also controlled using space vectormodulation, a natural fit for current-source topologies [19].

An advantage of the current-source converter over voltage-source types is that the dc-link current may be controlled atwill if the load dynamic allows it. Thus in fan, pump, andcompressor applications with low dynamic requirements, thiscurrent may be reduced according to the operating point tomaintain a high modulation index of the CSI and reduce theswitching and conduction losses of the semiconductor devices.CSRs or CSIs have nine electrical states, six active ones wherepower is transferred to the load and three zero states where thedc-link current is freewheeled through the inverter back intothe dc-link. The switching losses for these two converters arenaturally different since the CSR blocks the line voltages whilethe CSI blocks the operating point motor voltages.

VI. EFFICIENCY OF HIGH-PERFORMANCE CONVERTERS

High-performance drives require front-end converters withregenerative capabilities as well as inverters with fast dynamicresponses to accomplish precise motion controls and com-pensate for sudden load changes. The previously presentedmedium-voltage drives do not meet these requirements, andthus modified topologies have been developed. Specifically,VSI-based topologies replace the diode front end with an activefront-end voltage-source converter with which they can readilyregenerate energy into the grid. Current-source converters, onthe contrary, are intrinsically regenerative (by dc-link voltageinversion) and do not require any hardware modification. How-ever, to achieve a high dynamic response, the dc-link currentmust be kept at its nominal value to provide an instantaneousfull-torque capability. The following: sections present high-performance medium-voltage converters currently available.

A. PWM VSI Converters With Active Front Ends (AFE)

The main features of the AFE-VSI converter are its regen-erative capability and high-input power factor attained by azero displacement angle and low harmonic distortion of itsinput currents. Fig. 4 shows the simplest topology embodyingthis drive. The AFE is simply a VSI connected to the grid,

thus its modulation and operation are the same as that of aVSI. The losses of this topology are slightly higher than itspassive front-end counterpart due to higher losses of IGBTswhen compared to power diodes. The AFE and VSI also presentsimilar losses, although these losses are distributed differentlybetween IGBTs and diodes, because the diodes conduct morethan the diodes of the VSI for the front-end converter due tothe near-unity displacement power factor at the input terminals.This converter topology, with series-connected IGBTs, canoperate up to voltage levels of 4 kV; however, its usage in high-power applications is still limited due to the reduced reliabilityof the series connection of devices. For the same reason, manu-facturers normally recommend the NPC back-to-back voltage-source converter for high-performance applications.

B. Back-to-Back NPC Voltage-Source Converter

The three-level NPC-VSI has been successfully used in theindustry in past years. The same multilevel topology has beenused to realize the AFE converter for this inverter, providingthe required regeneration capability for the drive. The use of athree-level NPC AFE (Fig. 5) has the additional advantage ofreducing the input current harmonic distortion. The operationand control of this converter is the same as that of its invertercounterpart—just as for the two-level voltage-source converter.Therefore, the loss distribution is similar among each phase-legIGBTs, with the outer IGBTs having higher switching lossesand the center IGBTs higher conduction losses. Also, the lossdistribution between IGBTs and diodes is different for the AFEand VSI due to the near unity input displacement power factorof the AFE.

C. CSI Converter With PWM Rectifier

For high-performance applications requiring a fast dynamicresponse, the CSR-CSI is another topology available in themarket. This is exactly the same topology used for conventionalmedium-voltage drives; therefore, the dc-link current is alwaysconducted by two switches per converter (Fig. 3). The differ-ence lies in the fact that to achieve a high dynamic response, thedc-link current is always kept at its nominal value. By doing so,

WIECHMANN et al.: ON THE EFFICIENCY OF VOLTAGE SOURCE AND CURRENT SOURCE INVERTERS FOR HIGH-POWER DRIVES 1777

Fig. 5. Multilevel NPC AFE and VSI.

Fig. 6. Five-level VSI converter for high-power applications.

the load current may be changed by the PWM modulator fromzero to its nominal value almost instantaneously if required.

The downside of this scheme is a significant increase in bothconduction and switching losses of the rectifier and inverter,since the dc-link current is now at maximum regardless of theoperating point. This affects the efficiency of this converter,which when operating at 10% nominal power, presents effi-ciencies lower than 90%, for example. The results presentedin Section VII verify this.

VII. DRIVES FOR VERY HIGH-POWER APPLICATIONS

For very high power applications, the previously describedtopologies cannot be used, which is why manufacturers havedeveloped numerous topologies appropriate for motor drivesoperating at high-power ratings, such as in excess of 20 MW.In the case of voltage-source inverters, five-level inverters fedfrom 24- or 36-pulse diode rectifiers are a standard solution.Current source topologies, on the other hand, use thyristor-based rectifiers and inverters, such as phase-controlled rectifiersand load commutated inverters (LCI).

A. 24-Pulse Rectifier and Five-Level VSI

The five-level diode clamped VSI uses eight series-connectedsemiconductors per converter phase leg, enabling operationwith voltages of up to 8 kV, supported by four series capacitorsin the dc-bus. This high voltage translates into relatively lowcurrent levels for the motor drive so that the semiconductor

losses remain low. Fig. 6 shows a circuit diagram of this con-verter consisting of 24 high-voltage IGBTs. Different vendorsindistinctively offer IGBT- or IGCT-based configurations. Thisconverter can operate motors up to 20 MW, usually encounteredin grinding mills or low-speed applications. As the figureshows, the front-end rectifier is a 24-pulse rectifier with fourseries-connected three-phase bridges [15]. The input currentTHD of this rectifier is less than 3%; however, dependingon the harmonic requirements, this configuration is increasedto 36 pulses. The input power factor is also very high, be-ing close to unity throughout the complete operating range.Although this converter is nonregenerative, few high-powerapplications have these requirements, so the topology is stillsignificantly used.

B. Phase-Controlled Rectifier and Load-Commutated Inverter

This motor drive is only used for very high-power appli-cations driving synchronous machines. Fig. 7 shows a circuitdiagram of the converter depicting its full-thyristor realization.The use of thyristors enables this converter to reach voltagesas high as 10 kV, featuring saturation voltages in the order of1.3 and 1.5 V, respectively, for currents up to 4 kA. Conse-quently, this converter is highly efficient and presents losses sig-nificantly lower than any other converter using IGBTs, IGCTs,or symmetric GCTs. The major drawback of this converter is itsrelatively high harmonic distortion due to the intrinsic six-pulseoperation as well as its poor input power factor due to the phasecontrol of the rectifying stage.

1778 IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 55, NO. 4, APRIL 2008

Fig. 7. Load-commutated current-source motor drive.

The inverter works similarly to the rectifier under naturalload voltage commutation—instead of line voltage commuta-tion, only synchronous motors can be driven because thesemachines can have a displacement angle between their terminalvoltage and current adjusted to ensure the safe commutationof thyristors. Both the inverter and rectifier have six activestates, just as a diode bridge does; therefore, two thyristorsconduct in each converter at a time. The conduction losses canthen be determined by calculating the losses on four thyristorsconducting the dc-link current.

For higher power applications, multiple three-phase bridgescan be connected in series-driving motors up to 72 MW.

VIII. EXPERIMENTAL VERIFICATION

To validate the results obtained through simulation, severalmeasurements to calculate the efficiency of medium-voltageVSI and CSI were conducted. For VSI a 1 MVA, 2.4 kVpump operating in an oil refinery was considered. Efficiencymeasurements include a transformer, a six-pulse diode frontend, a dc-link filter, and an IGBT-based two-level VSI. Theefficiency measured in this converter operating at 56% nominalpower was 96.0%, and was increased to 96.2% at nominalpower. The second converter was a CSR-CSI driving a blowerin the grinding plant of a mining site. The converter wasfed from 3300 V 50 Hz and drove a load of 1250 HP. Thismeasurement included transformer losses, rectifier and invertersemiconductor losses (SGCT), a dc-link inductor, and outputfilter. The efficiency measured at nominal power was 94.9%.

The measurements were taken simultaneously at the inputof the transformer and the output terminals of the converter,capturing the input and output three-phase voltages and cur-rents. For purposes of accuracy, two sets of measurements wereconducted on each converter, one using oscilloscopes (Fluke199C and Tektronix TPS2014), and one using a power qualityanalyzer (Dranetz-BMI Power Guide 4400). These results arein good agreement with the efficiency predicted through simu-lation in the proposed method as shown in Tables IV and V andFigs. 8 and 9.

IX. GENERAL EVALUATION

The topologies presented in this paper for medium-voltagehigh-power converters have been simulated to calculate semi-conductor, transformer, and filter losses and establish the ad-vantages and disadvantages that each of them presents. Tothis end, all motor drives were run from 10% to a full load,

Fig. 8. Efficiency of VSI and CSI converters for full range of operation(Case 1).

Fig. 9. Efficiency of VSI and CSI converters for full range of operation(Case 2).

TABLE IIISIMULATION PARAMETERS

calculating the losses throughout the operating range using theparameters in Table III. The evaluation has been split threeways according to the paper’s structure—conventional, high-performance, and very high power applications. The results arepresented as follows.

WIECHMANN et al.: ON THE EFFICIENCY OF VOLTAGE SOURCE AND CURRENT SOURCE INVERTERS FOR HIGH-POWER DRIVES 1779

TABLE IVLOSSES AND EFFICIENCY FOR VS- AND CSI-BASED DRIVES IN FULL RANGE OF OPERATION (CASE 1)

TABLE VLOSSES AND EFFICIENCY FOR VSI- AND CSI-BASED DRIVES IN FULL RANGE OF OPERATION (CASE 2)

A. Case 1

Table IV shows the results obtained for conventionalmedium-voltage converters. All converters drove the same1 MW motor rated at 2.4 kV, shown in Table IV. As wecan see, voltage-source topologies present a slightly higherefficiency due to the higher saturation voltage of the symmetricGCTs and the dc link inductor used by the PWM CSR-CSIconverter. However, this 2% higher efficiency exhibited byVSIs becomes negligible when motor losses are included. Asmentioned previously, CSI converters are 1.5% more efficientin the motor section of the drive.

Table IV also shows that the multilevel NPC-VSI has thehighest efficiency, achieved by the better harmonic spectrumof its currents, which reduces harmonic losses in the system.Due to lower conduction losses, the NPC with IGCT exhibits a0.2% higher efficiency than the NPC with IGBT. However,for higher commutation frequencies, the efficiencies are similardue to the extra commutation losses of the IGCT.

B. Case 2

Table V shows the results obtained for the high-performanceconverters under discussion. Simulation parameters are thesame as for Case 1. Fig. 9 shows the efficiency plots for thesemotor converters. The current-source topology is comparable inefficiency with the voltage-source topologies at nominal load;however, the voltage-source topology presents a better perfor-mance even when considering 1.5% motor losses at a lowerload level. This performance is due to the control scheme usedfor the current-source topology that keeps the dc-link currentat a nominal value to maintain its instantaneous nominal torque

Fig. 10. Efficiency of VSI and LCI converters. 20 MW (Case 3).

capability. Consequently, the efficiency of this converter is re-duced to 60% at light load conditions—a significant drop com-pared to the 93% efficiency of the voltage-source counterparts.

Also, the multilevel back-to-back topology presents aslightly higher efficiency than the two-level back-to-backarrangement. Specifically, the NPC IGCT shows a slightlybetter efficiency thanks to the lower conduction losses of thistype of device.

C. Case 3

This evaluation used a 20-MW motor as the load, rated at6.6 kV and 2.2 kA. As shown in Fig. 10, the losses of thevoltage-source topology are nearly 50% higher than those ofthe current-source thyristor-based topology, which represents adifference of 250 kW at full load. Both converters use outputfilters, and therefore motor losses are quite similar. Also, the

1780 IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 55, NO. 4, APRIL 2008

TABLE VILOSSES AND EFFICIENCY FOR VSI- AND LCI-BASED DRIVES IN

FULL RANGE OF OPERATION FOR 20 MW NOMINAL (CASE 3)

current-source topology in this case operates with minimumdc-link current, given the low dynamics of the load. Results aresummarized in Table VI.

For higher power converters (above 20 MW), LCI thyristordrives are the only choices for medium- to high-frequencyapplications because of their high efficiency and reliability.

X. CONCLUSION

This paper presented the efficiency evaluation of voltage-source and current-source ac-to-ac medium-voltage high-powerconverter topologies, taking into consideration conventionalnonregenerative low-dynamic applications, regenerative high-performance applications, and very high power applications.The analysis took into account semiconductor conduction andswitching losses as well as the power losses of medium-voltageisolation transformers and filters. Results show that VSIs andCSIs are competitive for nonregenerative low dynamic require-ment drives. For regenerative applications, the three-level NPCVSI achieves a higher efficiency in comparison to the CSIconverter. For very high power applications, the thyristor-basedcurrent-source topology presents a significantly better perfor-mance due to the low-voltage drop of the semiconductors used.

The proposed methodology may also be readily used forthe analysis and evaluation of alternative high-power medium-voltage topologies.

APPENDIX

EXAMPLE FOR LOSS CALCULATION IN

POWER TRANSFORMERS

The resistance values measured for the transformer’s differ-ent windings are as follows:Primary winding 366.8 mΩSecondary winding 9.39 mΩMeasured impedance 6.18%Tested core loss 5328 WTested load loss 28 452 W

The transformer used in this example has the same har-monic current spectrum in its primary and secondary windings(parameters given in Table VII). This is not valid naturallyfor multiwinding transformers because the current distributionfor this case can vary significantly depending on the numberof rectifier pulses, the transformer configuration, and the type

TABLE VIITRANSFORMER PARAMETERS

TABLE VIIIHARMONIC LOSS FACTORS FOR DELTA-WYE TRANSFORMER

of rectifier being fed. The harmonic spectrum used for thisexample is given in Table VIII.

These data are used to calculate the harmonic loss factorsdefined in IEEE Standard C57.18.10-1998 and are shown inTable VIII, where FHL−WE is defined by (5) and corresponds tothe harmonic factor that increases the eddy current losses PEC,and FHL−OSL is defined by (6) and corresponds to the harmonicfactor multiplying the stray inductance losses POSL

FHL−WE =n∑

1

Ih(pu)2h2 (5)

FHL−OSL =n∑

1

Ih(pu)2h0.8. (6)

The kilovolt-ampere ratings of the transformer are as follows:

Primary rms kVA = 1.0304 × 2700 = 2782 kVA

Then the measured dc resistance losses at nominal load andat the fundamental frequency may be determined as follows,with iprim = 113 A and isec = 650 A:

Primary winding I2R = 3 × (113)2 × 0.3668 = 14 041 W

Secondary winding = 3 × (650)2 × 0.00939 = 11 884 W

Total dc resistance load loss = 25 925 W.

So far, the losses calculated for this transformer at thefundamental frequency are 25 925 W. Taking the new data intoconsideration, total stray losses POSL and winding eddy currentPEC loss are then equal to

28 452 − 25 925 = 2527 W. (7)

WIECHMANN et al.: ON THE EFFICIENCY OF VOLTAGE SOURCE AND CURRENT SOURCE INVERTERS FOR HIGH-POWER DRIVES 1781

In this case, PEC data per winding were obtained from thetransformer manufacturer as follows:

Primary winding = 447 W

Secondary winding = 1644 W

Total windings PEC = 2091 W.

With these values, the total stray losses can be calculated asfollows:

POSL = 2527 − 2091 = 436 W.

The eddy current losses are measured through experimentaltests and are normally provided by the manufacturer. If thisinformation is not available, a series of measurements sup-plying the transformer with varying frequency voltages maybe performed as indicated in [26], which presents a detaileddescription of this procedure. Nonetheless, IEEE Standard57.18.10–1998 indicates that in case PEC and POSL are notavailable, a good approximation is to split the losses by 60%and 40%, respectively.

The total service I2R loss is the fundamental I2R loss ×(Irms/I1)2.

The service winding eddy current loss is the fundamentaleddy loss × FHL−WE.

The additional stray loss is the fundamental additional strayloss × FHL−OSL

Primary service I2R loss

= 14 041 × 1.03042 = 14 908 W

Primary service winding eddy current loss : P ′EC−P

= 447 × 4.2190 = 1886 W

Total primary winding service loss : PR−P = 16 794 W

Secondary service I2R loss

= 11 884 × 1.03042 = 12 618 W

Secondary service winding eddy current loss : P ′EC−S

= 1644 × 4.2190 = 6936 W

Total secondary winding service loss : P ′R−S

= 19 554 W

Service additional stray loss : P ′OSL

= 436 × 1.2783 = 557 W.

Total service load loss for the main transformer with distortedload current

P ′R−m = 16 794 + 19 554 + 557 = 36 904 W.

To obtain the total service loss, the tested core losses areadded to the total service load loss calculated above, whichyields

P ′TOTAL = 5328 + 36 904 = 42 232 W.

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[27] Y. Wu, R. A. Mcmahon, Y. Zhan, and A. M. Knight, “Impact of PWMschemes on induction motor losses,” in Conf. Rec. 41st IEEE IAS Annu.Meeting, Tampa, FL, Oct. 2006, vol. 2, pp. 813–818.

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Eduardo P. Wiechmann (S’81–M’86–SM’94) re-ceived the E.E. degree from Santa Maria University,Valparaiso, Chile, in 1975, and the Ph.D. degreein electrical engineering from Concordia University,Montreal, P.Q., Canada in 1985.

Since 1976, he has been with the University ofConcepción, Chile. He is now a Professor in theElectrical Engineering Department of the Univer-sity of Concepción. His research interests are powerconverters, high-current rectifiers, ac drives, UPSsystems, harmonics, and power factor control in in-

dustrial power distribution systems. His industrial experience includes morethan 6000 hours in engineering projects and consulting.

Dr. Wiechmann has published numerous technical papers and has coauthoredtechnical books. He received the year 2000 Concepción City Award for Out-standing Achievements in Applied Research.

Pablo Aqueveque (S’05) was born in Santiago,Chile, in 1976. He received the B.S. and E.E. de-grees from the University of Concepción, Chile, in2000 and 2002, respectively. He is currently workingtoward the Ph.D. degree in power electronics at theUniversity of Concepción.

His research interests include high-current recti-fiers, power converters, and modern digital devices.

Rolando Burgos (S’96–M’03) received the B.S.,E.E., M.S., and Ph.D. degrees from the University ofConcepción, Chile, in 1995, 1997, 1999, and 2002,respectively.

In 2002, he joined the Center for Power Electron-ics Systems (CPES) at Virginia Polytechnic Instituteand State University, Blacksburg, VA, as a Post-doctoral Fellow. Since 2005 he has been ResearchAssistant Professor in the Bradley Department ofElectrical and Computer Engineering, Center forPower Electronics Systems. His research interests

include multiphase power conversion, stability of ac power electronics systems,hierarchical modeling, control theory, and the synthesis of power electronicsconversion systems for sea, air, and land vehicular applications. He is Secretaryof the Committee on Simulation, Modeling, and Control of the IEEE PowerElectronics Society.

José Rodríguez (S’81–M’83–SM’94) received theEng. and Dr.-Ing degrees from the UniversidadTécnica Federico Santa Maria in Valparaíso, Chile,and the University of Erlangen in Germany in1977 and 1985, respectively, both in electricalengineering.

Since 1977, he has been a professor at the Univer-sity Federico Santa María. From 2001 to 2004 he wasDirector of the Electronics Engineering Department.From 2004 to 2005 he was Vice-Rector of AcademicAffairs, and in 2005 he was elected Rector at the

same university, a position he holds to this day. During his sabbatical leave in1996, he was responsible for the mining division of the Siemens Corporation inChile. He has a large consulting experience in the mining industry, especiallyin the application of large drives like cycloconverter-fed synchronous motorsfor SAG mills, high-power conveyors, controlled ac drives for shovels, andpower quality issues. His main research interests include multilevel inverters,new converter topologies, and adjustable speed drives. He has directed over40 R&D projects in the field of industrial electronics, he has coauthored over50 journal and 130 conference papers. His research group has been recognizedas one of the two centers of excellence in engineering in Chile in 2005and 2006.