ieee journal of emerging and selected topics in …...48 ieee journal of emerging and selected...

IEEE JOURNAL OF EMERGING AND SELECTED TOPICS IN POWER ELECTRONICS, VOL. 1, NO. 2, JUNE 2013 47

Evolving and Emerging Applications of PowerElectronics in Systems

John G. Kassakian, Life Fellow, IEEE, and Thomas M. Jahns, Fellow, IEEE

(Invited Paper)

Abstract— The continuing trend toward greater electrifica-tion and control of functions in consumer, commercial, indus-trial, transportation, and even medical applications promises adynamic and increasingly important role for power electronics.The growing penetration of power electronics in energy systemsis driven by new materials such as SiC and GaN, as well asnew packaging technologies which allow the physical integrationof electronics with powered and controlled devices such asmotors. Advances in very high-frequency conversion led by theapplication of SiC and GaN devices promises to make converter-on-a-chip technology possible, but concurrent advances in passivecomponent technology are necessary. The principal challenge toapplication penetration remains cost reduction.

Index Terms— Adjustable speed drives, data centers, energyconversion, flexible ac transmission system (FACTS), GaN,HVDC, integrated magnetics, power electronics, power supplies,semiconductor devices, SiC.

I. INTRODUCTION

POWER electronics refers to electronic circuits whose pri-mary function is to process energy. This is in contrast to

signal-level circuits that process information. Power electronicsystems comprise a marriage of these two circuit classesinterposed between sources and sinks of electric energy.Beyond conventional applications of power electronic systemsas power supplies for computers and telecommunication equip-ment, electrical actuation is rapidly replacing pneumatic andhydraulic actuation of mechanical systems, the Boeing 787aircraft being a very recent and visible example. This airplaneincorporates more electric-based actuation of control surfacesthan any commercial aircraft in history, in addition to a largenumber of variable speed drives for pumps and the cabin airconditioning system.

The electronics part of the power electronic system con-sists of semiconductor switches controlled and configured invarious topological forms to produce the desired conversionfunction. The increasing penetration of power electronics intocommercial, industrial, and military applications are drivenby three forces—decreasing cost making solid-state powertechnology more economically attractive, new semiconductormaterials allowing the control of higher voltages and operation

Manuscript received May 6, 2013; accepted June 3, 2013. Date ofpublication June 21, 2013; date of current version July 31, 2013.Recommended for publication by Associate Editor Don F. D. Tan.

J. G. Kassakian is with the Electrical Engineering and Computer ScienceDepartment, MIT, Cambridge, MA 02139 USA (e-mail: [email protected]).

T. M. Jahns is with the Electrical and Computer Engineering Department,UW-Madison, Madison, WI 53706 USA (e-mail: [email protected]).

Color versions of one or more of the figures in this paper are availableonline at http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/JESTPE.2013.2271111

at higher temperatures and frequencies, and the need for moresophisticated and energy-efficient control of processes.

Continued monolithic integration of functions on microcon-trollers and power devices are responsible for driving down thecost of semiconductor components, while new packaging andthermal control materials and technologies have made systemcosts more tractable. The continued application of Moore’sLaw to the evolution of integrated circuits has made controlprocessors smaller, faster, more efficient, and less costly, mak-ing possible the exploitation of the speed of power electronicsto control complex processes in response to multiple highbandwidth sensor inputs.

The continued progress in fabricating power devices inwide bandgap semiconductor materials, specifically SiC andGaN, promises new application regimes for power electronics.The much higher critical fields, compared with Si, for thesematerials results in the potential for a large increase in devicebreakdown voltage, providing new opportunities in high volt-age power system applications. Both their wide bandgap, andgreater thermal conductivity, especially GaN, provide accessto high temperature applications that are either denied to, ormade difficult to accommodate with, Si-based devices. Can-didate applications for these devices include automotive anddown-hole controls for oil and gas drilling. For conventionalapplications, the higher permitted temperatures for these widebandgap devices means greater latitude in thermal design andpackaging. In addition, the extremely fast switching speeds ofGaN and SiC power semiconductor devices open the door tomoving the operating frequencies of power converters upwardinto the RF band, exceeding 100 MHz in some cases.

Although the availability of new wide bandgap powersemiconductors deserve the credit for enabling RF power con-version, advances in the design and materials used to producecritical passive components (i.e., capacitors and inductors)have also played a vital role in achieving notable power densityincreases in the newest generation of RF power converters.Advances in a multidisciplinary combination of materials andpackaging technologies are also leading to power-supply-on-a-chip architectures that combine all of the components neededfor a regulated power converter into a single integrated circuit.More details about this integrated power converter technologyfor power supplies are provided in an upcoming section of thispaper.

The availability of smaller power converters that operateat higher temperatures is setting the stage for a growingtrend toward higher levels of power electronics integrationinto the associated loads or sources for a variety of appli-cations. One example of both the promise and limitations of

2168-6777 © 2013 IEEE

48 IEEE JOURNAL OF EMERGING AND SELECTED TOPICS IN POWER ELECTRONICS, VOL. 1, NO. 2, JUNE 2013

power electronics integration using current technology is solid-state lighting using LEDs. The LEDs require well-regulatedlow-voltage dc power to operate, meaning that every LED bulbrequires its own power electronic driver to convert the utility-provided ac voltage into the much lower dc voltage needed tosupply the LED array. The growing use of LEDs for lighting,motivated by United States Federal law mandating minimumefficacies, provides a substantial incentive for innovative driverdesigns.1 The reliability and lifetime of the driver is ofparticular concern because experience shows that the primarycause of spiral compact fluorescent lamp failure has beenfailures of their electronic ballasts [1].

The trend toward higher levels of power electronics integra-tion is appearing in a wide variety of equipment and applica-tions. One good example of the classes of applications that areexperiencing this trend is motor drives, particularly those inhigh-volume, low-power (<3 kW) applications. In such cases,the opportunity to eliminate the need for a separate dedicatedmechanical housing for the power electronics together withthe interconnecting power cable and connectors provides asignificant economic incentive to adopt an integrated driveconfiguration. However, the peak temperatures that the motorcan typically sustain (e.g., 180 °C) compared to the maximumsafe operation for the power electronics (<150 °C) createsspecial challenges for the integrated motor drive designer.Despite these challenges, many successful examples of inte-grated motor drives are found in a variety of automotive,white goods and industry applications, a few of which arehighlighted in the later sections of this paper.

A most unusual application of power electronics is gaininginterest recently—the extraction of energy from very lowpower sources. These range from mechanical vibrations toelectrochemical potentials within a tree [2]. One applicationis to power medical implants, such as chemical and molecularsensors and drug delivery actuators, by harvesting energyfrom electrochemical potentials within the body [3]. This isan extreme power electronics challenge, requiring conversionat very low voltages (10s of millivolts), with an efficiencythat produces useable energy, and a size that is biologicallyimplantable.

II. DEVICE AND COMPONENT ADVANCES

The devices and components that comprise the power elec-tronic circuit in a system impose constraints on conversionperformance. These include thermal problems because ofsemiconductor, transformer, or inductor dissipation, thermallimits and limited life for electrolytic capacitors, and voltage,current, and switching speed constrained by the power semi-conductor devices. New technologies, particularly new semi-conductor materials, are showing promise for relaxing someof these constraints. In addition, new packaging and thermalmanagement technologies are providing opportunities to phys-ically integrate the electronics into the application package.

The continued development of higher performance andlower cost IGBTs and MOSFETs is one of the primary

1A table of international regulations phasing out incandescent lamps arefound in [1].

drivers behind the expanding applications of power electronics.Silicon IGBTs are now available at voltages up to 6.5 kV andcurrents up to 1 kA, making practical PWM applications inhigh voltage utility environments and high power traction drivesystems, as discussed later.

One of the most significant recent developments influencingthe future applications of power electronics is the fabrica-tion of practical SiC and GaN power devices. Compared toSi, these wide bandgap materials exhibit very high criticalelectric fields, higher saturation drift velocities, and higherthermal conductivity. These attributes translate into very highvoltage, high temperature, high speed, and low forward dropMOSFETs, JFETs, BJTs, IGBTs, thyristors, and Schottky andbipolar diodes. For example, a 12 kV SiC IGBT with a forwarddrop of 6.1 V at a current density of 200 A/cm2 has beendemonstrated [4], as has a 21 kV, 50 A/cm2 BJT with a currentgain of 63 [5]. Fujitsu has recently published the results ofapplying their GaN high electron mobility transistor to thepower factor (PF) correction circuit of a 2.5 kW, 380 Vdcpower supply [6]. The challenges facing the widespreadapplication of these new devices is the cost of compoundsemiconductor material. Crystal growth, particularly for GaN,is a complex process which does not yield large defect-freesubstrates yet. Epitaxially grown GaN on Si, SiC, or sapphiresubstrates are the basis for GaN devices, including blue LEDsused for lighting. The resulting interface mismatch creates anumber of problems which are the subject of ongoing research.

Electrolytic capacitors remain one of the greatest reliabilitychallenge for systems that incorporate them, and althoughthere are attempts made to improve them and to monitortheir condition [7], they continue to impose limits on theoperating temperature and mass/volume of power convertersthat use them. However, circuit techniques employing activelyswitched capacitors that can accommodate larger ripple volt-ages are being developed, leading to the filtering requirementsbeing satisfied by solid dielectric capacitors. The result is asignificant improvement in lifetime and reliability [8], [9].

III. POWER SUPPLIES

The term power supply generally refers to a circuit thatsupplies power to a nonkinetic system, e.g., computers,telecommunication equipments, or radios. The future of theseapplications lies in their higher efficiency, smaller size, lighterweight, lower cost, and higher performance (THD, PF, etc.)The term power supply is also used to describe the primarysource of electrical energy for a large facility, such as theDD1000 Zumwalt class destroyer that is described later, wheresecondary conversion systems supply both kinetic and staticloads. In these cases, the system includes the conversion ofthe primary energy source—a gas turbine in the case of theDD1000.

The increasing electrification of processes and functionsprecipitates new requirements for power supplies in harshenvironments. The temperature limitations imposed by sili-con devices and conventional components have prevented ormade difficult the use of power electronics in a variety ofapplications, including electric and hybrid vehicles, down-hole gas and petroleum drilling and monitoring, and aircraft.

KASSAKIAN AND JAHNS: EVOLVING AND EMERGING APPLICATIONS OF POWER ELECTRONICS 49

Fig. 1. 6 W, 73% efficient Phi-2 boost converter operating at 75 MHz [12].

The challenge is fundamentally materials. Although SiC orGaN devices are promising in the abstract, the balance of sys-tem is also not up to the thermal challenge. However, progressis being made in developing high temperature die attachment,gate drives, control electronics, and passive components [10].

In the following sections, we limit the discussion to the firstcategory of power supplies, those known as off-line (ac/dc)or point-of-load (dc/dc) supplies with ratings up to a fewhundreds of watts. An off-line supply is the one that powersyour computer or television. A point-of-load supply provides,for example, dc power at low voltage from a higher voltagedc bus to a blade server in a large server farm.

Power supplies today that power equipment from the ac lineare invariably universal switching supplies, universal meaningthey can accommodate input voltages between 100 and 220 V50/60 Hz ac. This input flexibility is the direct result ofswitching circuits that can, through adaptive control, changeswitching patterns to keep the output voltage constant in theface of varying input voltage. The technology trajectory forthese systems has higher efficiencies in smaller profiles, andthe primary approach to moving up this trajectory has, andcontinues to be, higher switching frequencies.

Theoretically, higher frequencies reduce the size of neces-sary energy storage and resonating components, i.e., inductors,transformers, and capacitors. In practice, the trajectory isnonlinear in which a higher switching frequency is accom-panied by increased semiconductor switching losses, inductorcore and winding losses and dielectric losses, along with agreater influence of parasitic elements, such as bonding leadand trace inductances, on circuit behavior. However, progressalong the trajectory continues with the development of bettersemiconductor devices, fabrication and packaging technologiesthat are attempting to more closely approximate an integratedcircuit and control techniques that adaptively control circuitconfigurations in response to dynamic load requirements. Theultimate objective is a converter-on-a-chip [11].

The availability of newly developed GaN and SiC tran-sistors and Schottky diodes, and techniques for fabricatingmultilayer but planar inductors have already enabled thedemonstration of dc/dc converters using integrated circuit-related construction techniques and operating at frequenciesin excess of 50 MHz at efficiencies in the 80%–90% range.Fig. 1 shows a 75 MHz boost converter using an optimized

(a)

(b)

(c)

Fig. 2. (a) Schematic of the converter of Fig. 1 with the printed circuitboard (PCB) transformer indicated by the dashed box. (b) Transistor with itsresonant gate drive to be integrated. (c) Monolithically integrated transistorand gate drive with control. The inductor LV is off-chip [12].

LDMOS power transistor and integrated controls in a Si bipo-lar/CMOS/DMOS process [12]. Fig. 2 shows the schematicof the converter and a photo of the IC consisting of gatedrive and control. The construction details of the PCB trans-former used in the converter are also shown in Fig. 3.The fabrication of magnetic components compatible withthe planar construction of very high frequency convert-ers is the subject of considerable research [13]. Fig. 4shows another innovative approach to the problem whichapproaches compatibility with IC process technology. TheCo–Zr–O thin-film core is deposited in a V-groove etched intothe Si substrate and copper is plated into the groove to formthe conductor [14].

As GaN and SiC materials become less expensive andthese relatively exotic semiconductor devices continue to bedeveloped in more varied ratings and become available attractable costs, there is rapid movement on the trajectory toa converter-on-a-chip. For a given rating, the smaller size ofthese devices results in reduced packaging and interconnectparasitics and lower capacitance, both resulting in higherpractical switching frequencies. Development of appropriate


Fig. 3. Schematic showing the PCB construction of 1:3 transformer shownin the dotted box on the converter schematic of Fig. 2(a) [12].

Fig. 4. Anisotropic core of V-groove inductors. The green region isthe magnetic core, the light blue region is the insulating oxide layer, andthe orange region is the conductor. The current in the conductor goes into thepage [14].

passive components and packaging then become thechallenge.

IV. DATA CENTERS

Data centers, the large aggregation of computers and mem-ory to support high-intensity data processing, are large usersof electricity, accounting for 2% of the U.S. electrical load in2010 [15]. Reliability is of primary importance, so large unin-terruptible power supplies (UPSs) are always part of such cen-ters or server farms. To minimize cost, simplify the architec-ture and increase expansion flexibility, dc distribution withinthe center is being considered, though there is some contro-versy regarding whether such a system increases the alreadyhigh efficiency of the ac power supply and distribution archi-tecture [16]. Fig. 5 shows the typical architectures for ac anddc data center power distribution systems, including the UPS.

Historically, data centers are operated at full electrical load,independent of the variability in the quantity of data beingprocessed. This results in an energy for computation efficiencyin the vicinity of 10% and a large financial and energy expensefor air conditioning. Of great impact in this area has been thedynamic control of processors to match their activity and speedto the computational load. The resulting reduction in electricalload is then accommodated by dynamically controlling thepower supply, for example, by taking some supplies off-lineand maintaining the rest at their peak efficiency loads insteadof operating all supplies at reduced power and efficiency.

Fig. 6 shows the historical and projected (at 2006) U.S.annual data center electrical load assuming various levelsof energy use efficiency improvements. The mechanisms

Fig. 5. Typical configurations of ac and dc distribution networks for largedata centers [17].

Fig. 6. Data center energy use and projections incorporating varioustechnologies to reduce future consumption [18].

assumed for reducing energy use affect the loads such aslighting, fans, and computer room air conditioning. Alsoassumed are the dynamic control of servers and power supply,as mentioned earlier, and not the efficiency of the power supplyper se, which for modern supplies is already in excess of98%. Therefore except for conversion to a dc distributionsystem, advances in data center energy management have arelatively minor impact on power electronics. However, thecost of the UPS and distribution network respond to newdevice, packaging and control developments, and the expectedgrowth in data centers provide a growing market for powerelectronic equipment.

V. PROPULSION SYSTEMS

Nearly every major mode of transportation in the land,sea, and air is in the midst of undergoing significant changesinvolving the introduction of some combination of electric-based propulsion and electric-powered accessory systems.The value proposition that is driving these changes variesin striking ways among the different transportation modes.However, there are several shared underlying motivationsincluding the demand for higher fuel economy, improvedvehicle controllability, and a growing need for substantialamounts of processed electrical power for large auxiliary andaccessory loads.

For both on- and off-road vehicles, much of the impetusfor adopting hybrid combinations of electrical and internalcombustion engine propulsion, or electric-only propulsion, aredriven by the desire for greater fuel economy and reduced


dependence on petroleum-based fuels. Nearly every majorautomaker in the world offers at least one, and, in manycases, a significant number of models with hybrid-electric orbattery-electric powertrains. The combined sales of hybrid-electric vehicles (HEVs) and battery-electric vehicles (BEVs)exceed 3% of all automobile sales in the United States during2012 [19] and China has announced plans to build five millionelectric vehicles by 2020 [20].

The peak power ratings for the electric machine drivesin these vehicles range from <10 kW in mild hybrid con-figurations to >175 kW in high-performance strong hybridand battery-electric drivetrains. Interior permanent magnet(PM) synchronous machines are, by far, the dominant typeof electric machine used in production HEVs and BEVs allover the world, reflecting the advantages of these machinesfor achieving high torque/power densities and high efficiency.However, the concerns about the cost and availability of rare-earth magnets primarily sourced from China have caused amajor reassessment of alternatives to PM machines includinginduction and switched reluctance machines to determine theirsuitability for use in future vehicles [21]. Although it isunlikely that the performance metrics of these other candidatemachines will exceed those of a well-designed PM machine,design studies may still tip in favor of one or more of thechallengers if their cost/performance tradeoffs look sufficientlyattractive.

Although these magnet issues are a serious concern, thecost and performance limitations of available electrical energysources and storage devices, including batteries and fuel cells,have presented a larger obstacle that has retarded the adop-tion of electric-only propulsion systems for on-road vehicles.Today, lithium-based batteries with a variety of chemistrieshave improved to the point that they dominate the batteryenergy storage market for HEVs and BEVs. Power electronicsis playing an increasingly important role in battery man-agement systems that closely coordinate the charge/dischargebehavior of all the battery cells. Power electronics also playsa critical role in the battery charger equipment whether itis onboard the vehicle or external, and the sophistication ofthis power electronics only increases when high-frequencywireless battery charging is desired. Despite the impressiveprogress of this battery-related technology, the cost, energydensity, and cycle lifetime characteristics of currently avail-able lithium batteries continue to present serious challengesto global automakers that are striving to develop electricvehicles that can successfully compete with conventionalvehicles on the basis of cost, driving range, and usefullifetime.

Hybrid-electric drivetrains are also proving to be appealingchoices for other on-road and off-road vehicle applicationsbeyond passenger vehicles, particularly in applications inwhich there is an opportunity for substantial energy recoveryfrom repetitive stop–start or lift-drop actions. Examples ofsuch applications include on-road hybrid urban buses anddelivery vans, as well as off-road hybrid bulldozers, as shownin Fig. 7, and front loaders. Typically, series hybrid configu-rations are the preferred choice for these applications, similarto the type of architecture used in diesel-electric locomotives.

Fig. 7. Series hybrid drivetrain configuration of Caterpillar D7E bull-dozer [22].

Fig. 8. Exploded view of Protean wheel motor drive [25].

The fuel economy improvements claimed for the hybrid off-road construction equipment is typically in the vicinity of25% [23]. It is also worth noting that the hybrid front-loaders provide one of the domains where large switchedreluctance traction drives with ratings >300 kW per machineare successfully deployed. Series hybrid drivetrains have alsobeen successfully applied in military truck applications in aconfiguration that makes it possible to deliver up to 375 kWof 450 Vac, 60 Hz power to off-board loads [24].

One of the features that most of these series hybrid propul-sion systems have in common is the use of a separate tractionmachine in each wheel. Although this architecture is not thepreferred approach to date for passenger vehicles, there areactive efforts under way to introduce new innovations withthe goal of making the wheel motor approach practical andappealing for this application. As shown in Fig. 8, one of theapproaches that is receiving attention combines an external-rotor PM machine with the power electronics inside each ofthe four wheel hubs [26] to yield compact wheel motor unitsthat draw on integrated motor drive concepts that are noted inthe introduction.

In a related development, at least one automaker has begunto introduce a steer-by-wire architecture into its production


vehicles that eliminates the mechanical linkage between thesteering wheel and the rotating wheels [27], providing furtherincentive to consider independent wheel motor drive con-figurations. However, the success of this approach depends,in part, on finding solutions to longstanding engineeringchallenges associated with the vehicle suspension causedby the increased unsprung mass represented by the wheelmotors.

The integrated wheel motor units described briefly inthe preceding paragraph are excellent examples of both theimpressive progress and the remaining technical challengesthat are associated with currently available power electronics.A wheel motor represents a particularly hostile environmentfor power electronics in terms of temperature extremes, vibra-tion, and the presence of corrosive and thermally obstruc-tive foreign matter. The fact that technology innovators arewilling to design power electronics into a such demandingenvironment attests to the improvements in power electronicsruggedness that are achieved so far. However, the continuedpresence of a second radiator in some hybrid vehicles toaccommodate the need of the inverter power electronics forlower temperature (75 °C) coolant than the rest of the pow-ertrain equipment also highlights the work that still remainsto be done to develop economical high-temperature powerelectronics.

Overall, there is considerable reason for confidence thatthe continued and growing popularity of HEVs and BEVsaround the world will spur the automakers and their sup-pliers to accelerate their aggressive campaign to drive downthe cost of future power electronics for use in high-volumeautomotive applications. Progress in driving down the cost ofautomotive power electronics yields additional dividends forother manufacturers of high-volume products such as whitegoods/appliances that also depend on the availability of low-cost power electronics to increase the penetration of thistechnology. This is a good example of a field where progresstoday begets even more progress tomorrow.

In ships, the growing adoption of rotatable propulsionpods powered by large multimegawatt electric motors inseries-hybrid power configurations deliver tremendousimprovements in ship maneuverability that is critical to newgenerations of cruise ships, tug boats, and a variety of othertypes of commercial and navy vessels. Fig. 9 shows oneexample of such a propulsion pod. These propulsion podsare designed using both induction and PM synchronousmachines with power ratings up to 30 MW. In addition tothe maneuverability advantages, the propulsion pods increaseship fuel economy by 5%–15%, providing added incentivesfor adoption [29].

For larger naval vessels, there is a major commitment by theU.S. Navy to the adoption of series-hybrid propulsion systemsfor its new DDG 1000 Zumwalt Class destroyer. This vesselfeatures direct-drive propellers that eliminate the need for largegearboxes and reversing-pitch propellers. The ship is suppliedwith large amounts of electric power exceeding 78 MW at4160 Vac that can be used either for propulsion or for agrowing arsenal of electric-based infrastructure and weaponry[30]. The ship was originally designed to use large 36.5 MW

Fig. 9. Cross-section of ABB Azipod XO podded ship propulsion sys-tem [28].

PM machines for propulsion, but technology readiness issuescaused the Navy to switch to induction machines in 2007 forthe DDG 1000 system. The first of these ships is due to becompleted before the end of 2013.

The impact of power electronics and electric machineson air transportation is also significantly expanding.In particular, more-electric architectures are being increasinglyembraced by the world’s major commercial aircraft manufac-turers. Although jet engines are still retained for propulsion,electric power is used to minimize or eliminate the need forhydraulic and pneumatic accessory systems in aircraft, signif-icantly improving the serviceability of new more-electric air-craft. The Boeing 787 Dreamliner is the most comprehensiveexample to date of a new generation of more-electric commer-cial airliners that, among other innovations, uses electric-basedcabin climate control equipment to eliminate the need for bleedair from the engine, providing a significant boost to the aircraftfuel economy in the process. This increased electrificationresults in an electric power system capable of delivering upto 1.45 MW of variable-frequency three-phase power at 235Vac [31]. In combination, the new engines and other featuresof the 787 are designed to deliver a 20% reduction in fuelconsumption. However, Boeing’s experience with the first useof Li-ion batteries in this aircraft underscores the cautionrequired in assessing the reliability and failure modes of newtechnologies applied in critical applications [32].

New military aircraft such as the Lockheed Martin F-35 Joint Strike Fighter are also adopting many more elec-tric architectural features to support their increased depen-dence on electrical power for accessories and weapon con-trol systems. For example, all of the primary flight controlactuation in the F-35 fighter is accomplished using electro-hydrostatic actuators, eliminating the need for high-pressurehydraulic lines to be distributed around the aircraft [33].Major R&D efforts are under way that makes it possi-ble for the next generation of high-performance fighter air-craft to move even further in the direction of using elec-tric power to replace traditional pneumatic and hydraulicsecondary power sources and infrastructure, as shown inFig. 10.


Fig. 10. More-electric aircraft concept developed by U.S. Air Force [30].

VI. POWER SYSTEMS

Power electronics is playing a role in power systems as con-verter stations for high-voltage dc (HVDC) transmission lines.Its role is now expanded to include devices that produce flex-ible ac transmission systems (FACTS) that provide adaptiveline compensation and controlled routing of power flow [34].The future availability of fully controllable semiconductordevices at higher voltage and current ratings will increasethe penetration of power electronics as controlling elementsin the grid. Nearly all alternative energy sources require apower electronic interface between the source and the grid,providing the foundation for new power systems architecturesbased on microgrids and distributed energy resources (DER).Also, the idea of using plug-in electric vehicles as a demand-side resource depends on power electronics communicatingwith the grid. These applications are driving the developmentof converters capable of adapting to the changing requirementsof the interfaces.

HVDC transmission lines are constructed ever more fre-quently around the world because of the large distances thatpower can be transmitted as dc, and the availability of greatercontrol of power than is possible using ac lines. Dc linesare in operation up to ±800 kV (dc) [i.e., 1.6 MV (dc)line-line] and have used solid-state converters to interfacewith the ac system since 1972. Historically using exclusivelyline-commutated thyristor valves, converters in recent yearsare designed using IGBTs for low to moderate powers. Theessential differences between these two converter circuits areshown in Fig. 11. The high voltage and current ratings ofavailable IGBTs make it possible to employ voltage sourceconverter (VSC) circuits at converter terminals, providing adegree of control and flexibility not available with thyristors.In particular, multiterminal dc lines become possible, and theuse of PWM and multilevel converter techniques ease theproblem of filtering the current harmonics on the ac sideof the converter. Both of these characteristics are significantadvantages, the first making it possible to tap a dc line,and the second reducing expense in terms of both dollarsand real estate, because the filters necessary for a thyristor-based converter station occupy a substantial fraction of thestation footprint. A further advantage of the VSC-based HVDCline is that it is capable of providing black start capacitybecause the presence of a stiff ac source at the receiving end,

(a) (b)

Fig. 11. Essential elements of (a) line commutated thyristor HVDC converterand (b) voltage source HVDC converter [36].

Fig. 12. Functional representation of ABB HVDC circuit breaker(Source: ABB).

necessary for the operation of thyristor-based line commutatedconverters, is not required. Currently available devices andtechnology limit the capability of these VSC-based dc linesto ±150 kV (dc) and 400 MW. In contrast, thyristor-basedHVDC lines are operating at ±800 kV (dc) and 3 GW with a2000 km, 7.2 GW line connecting JinPing and SuNan in Chinaunder construction [35]. With the continuing and aggressivedevelopment of the IGBT, HVDC using VSCs will becomemore common at higher voltages and currents, making dclines a more attractive option for shorter lines providing tightercontrol of flow paths on the grid.

The recently announced development of a fast HVDCbreaker by ABB [37] in combination with the new voltagesource HVDC converters now makes it possible to create anHVDC grid. An HVDC grid is challenging because a faultreducing the dc voltage below a threshold value causes agrid collapse. A fast breaker is therefore essential for thepractical realization of a dc grid. The ABB breaker, shownschematically in Fig. 12, employs IGBTs in combination withmechanical switches to provide a disconnect within 5 ms.When a fault is detected, the Load Commutation Switch isopened and the main breaker closed thus commutating thefault current to the main breaker. The mechanical Ultra FastDisconnector is then opened, after which the main breakeris opened interrupting all but a small leakage current. Thecurrent is finally brought to zero by opening the mechanicalResidual Current Disconnecting Circuit Breaker. Because theLoad Commutation Switch is protected from the high dcvoltage, it’s insertion loss is on the order of 0.01%.

The ability to practically implement an HVDC grid hasparticular appeal to off-shore wind farms where the outputs ofseveral hundred wind turbines are aggregated and transmitted


(a) (b)

Fig. 13. Schematic examples of two representative FACTS devices.(a) Static compensator, or STATCOM. The reactive component of loadcurrent, IR , is compensated by the current Icomp injected by the voltagesource converter so the generator sees only the in-phase (active) current IA .(b) SSSC which injects a voltage VR in quadrature with the line current IL .

to land through undersea cables. Dc interconnection of theturbines avoids the problem of cable reactance and provides(unidirectional) access to turbine inertia, and dc transmissionto land relieves the constraint on off-shore distance. A furthersubtle, but important advantage of the VSC HVDC designis that because power reversal is accomplished by reversingthe current instead of the voltage (as in the case of theline-commutated thyristor converter) a more environmentallybenign solid dielectric cable (e.g., XLPE) can be employedinstead of cable with an oil impregnated dielectric.

FACTS has been a commercially available technology sinceearly 2000, but the cost has deterred widespread application.The advent of high-power IGBTs make FACTS devices moreeconomically attractive. The technology is similar to the appli-cation of the IGBT in HVDC systems in that both employ aVSC. Two forms of FACTS devices are the static compensator(STATCOM) shown conceptually in Fig. 13(a) and the staticseries synchronous compensator (SSSC) shown in Fig. 13(b).The VSCs tie a dc source providing no real power to the acsystem by controlling either the injected ac current (STAT-COM) or voltage (SSSC) to be ±90 degrees electrical withrespect to the ac side voltage or current. In locations wherethe dynamics of the power system require rapid compensationwith a wide swing between + and − VARS, the STATCOMprovides an excellent solution. Additionally, depending onthe bandwidth of the inverter, the STATCOM is capable ofcompensating for harmonics in the load current. There aremany flavors of FACTS devices performing a variety of realand reactive power controls and all benefit from the continueddevelopment of IGBT device and control technology.

The heightened attention to improving the functionality andefficiency of the grid has resulted in proposals for a solid-statetransformer to replace the conventional passive transformersused in today’s distribution systems [38]. The concept is toprovide isolation and voltage conversion at high frequencies asshown in Fig. 14. There are a variety of topologies which havebeen proposed to achieve the transformer implementation,each with its own advantages and disadvantages. Although theconcept presents opportunities for voltage and load control,and the potential for significantly reducing size and weight, itis accompanied by a number of challenging obstacles.

To achieve the benefits of small size and low weight, thedesirable switching frequency is in excess of 10 kHz. At this

Fig. 14. Conceptual block diagram of a solid-state transformer for distributionsystems.

Fig. 15. 13.8 kV: 465 V, 1 MVA solid-state transformer using SiC MOSFETsoperating at 20 kHz. Size is reduced by 50% and weight by 75% relative toa conventional 60 Hz transformer [39].

frequency and for application at distribution level voltages,e.g., 13.8 kV or 6.9 kV, applicable silicon power devices suchas the IGBT exhibit unacceptable losses. The high-frequencytransformer also requires special attention to both core andwinding losses. These challenges are overcome with the useof SiC MOSFETs or IGBTs with their high voltage ratingsand acceptable switching losses, and specialized low-loss corematerials, such as amorphous steel alloys. The challenge thenbecomes benefits versus cost, as it does for most applicationsof power electronics. In addition to size and weight, the solid-state transformer is capable of providing important informationabout system conditions through SCADA, and rapid controlof the distribution voltage to compensate for various systemconditions, such as low voltage. A further advantage of thesolid-state transformer is the potential access it provides fordistributed generation sources, such as solar arrays or fuelcells, or battery storage, at its dc link. Fig. 15 shows thecomplexity of a solid-state transformer, a leading factor con-tributing to its high relative cost.

The growing popularity of distributed renewable energysources and energy storage components, including rooftop


Fig. 16. Basic microgrid configuration including sources, energy storage,and loads connected to the utility grid using fast controllable switch.

solar panels, natural gas gensets, battery banks, and fuel cells(collectively known as distributed energy resources or DERs),is giving rise to new power system architectures based onmicrogrids that depend on the availability of power electronicgrid-tie inverters to couple these sources and storage compo-nents (dc in many cases) to the 50 or 60 Hz ac utility grid. Onedefinition of a microgrid whose elements are shown in Fig. 16is an “integrated energy system consisting of interconnectedloads and DERs that can operate in parallel with the grid or inan intentional island mode” [40]. Although much of the workis focused on ac microgrids, there is also considerable interestand research on dc microgrids that seek to reap the advantagesof the microgrid concept while minimizing the number ofdc-to-ac power conversion stages. Hybrid architectures thatcombine both ac and dc microgrid architectures are also beingseriously pursued for some applications. The high-efficiencycharacteristics desired for most microgrids encourages theintroduction of combined heating & power (CHP) configura-tions that can boost the average efficiency of the microgrid by20%–30% [41].

One of the most widely adopted versions of the microgridconcept is the Consortium for Electric Reliability TechnologySolutions (CERTS) microgrid [42]. The CERTS microgridis distinguished by its use of both voltage droop and fre-quency droop to give this architecture rugged plug-and-playcharacteristics that do not depend on the presence of central-ized controllers. Investigations have shown that the CERTSmicrogrid architecture can be configured to successfully com-bine microsources that require grid-tie inverters with othertypes of microsources such as gensets that do not need apower electronic dc-to-ac inverter. The CERTS microgrid hasbeen successfully tested in scaled-up field tests supported bythe United States Department of Energy with power ratingsexceeding 3 MW at 4.17 kV, demonstrating both the growingmaturity and readiness of this architecture [43].

VII. INDUSTRIAL MOTOR DRIVES

Electric machines are ubiquitous in industry for a widerange of applications, consuming between 43% and 46% of allelectricity that is generated in the world [44]. Although somemachines are used for high-performance applications, such asrobots and machine tools, the majority are used in industrialprocesses for pumps, compressors, fans, conveyors, and otherslower-dynamic applications. The primary economic value ofa motor drive is derived from its ability to control the speedof its attached load (e.g., pumps or fans), making it possibleto slash the high mechanical losses that are incurred whenthe process is controlled using inefficient throttling techniques

while the machine spins continuously at its full speed. It isestimated that 92%–95% of the life cycle costs of electricmotors are associated with the energy they consume, leadingto typical payback periods of <2 years for the installationof an adjustable-speed drive. It is rather surprising to learnthat, despite overwhelming evidence of the attainable savings,only 10%–15% of all industrial motors presently use electronicadjustable speed drives [44], [45]. However, there is wideagreement that the potential for future growth in the sales ofindustrial drives is still very substantial.

The large majority of motor drives sold today for industrialapplications are variable-frequency drives for ac machines.Nearly all of the ac drives with ratings between 1 kW and2 MW use IGBT power switches that have evolved to becomethe predominant choice for commercial ac inverters duringthe past 25 years. Today, the competitive regime for invertersbuilt with thyristor-based devices including gate turnoff (GTO)devices and insulated gate-commutated thyristors (IGCTs)is restricted to high-power ratings >2 MW in the mediumvoltage range (>1 kVac). At the other end of the powerand voltage spectrum, silicon-based power MOSFETs becomeprogressively more competitive with IGBTs when the powerratings drop below 10 kW with voltage ratings of 300 Vacor less. Small automotive accessories operating from 14 Vdcis a good example of the class of applications where powerMOSFETs are widely adopted.

Major improvements in all of the key components needed tobuild industrial drives ranging from sensors to capacitors haveled to impressive reductions in the volume, mass, and cost ofcommercially available drives during the past 30 years. Forexample, the volume of general-purpose ac drives with powerratings in the range 10–100 kW has decreased by ∼95% sincethe 1980s [46]. It is likely that the future availability of widebandgap power devices will make it possible to further reducethe mass and volume of industrial drives by increasing theinverter efficiency while making it possible to further increasetheir PWM switching frequencies. The opportunities for futureindustrial motor drives almost certainly will grow as drivetechnology continues to improve while energy costs graduallyrise in many parts of the world.

Induction motors comprise the most widely adopted typeof ac machines in use today for industry process applications.However, some of the same features described previouslythat have led to the dominant position of PM synchronousmachines in automotive traction drives are responsible forgiving them a growing foothold in selected industrial appli-cations. The best example of these market opportunities arehigh-torque, direct drive applications that eliminate the needfor mechanical gearboxes to step down the machine speedto boost the machine torque to match the load requirements[47]. Despite the aforementioned problems associated withthe cost and availability of rare-earth magnets, PM machinescontinue to be the preferred choice for demanding industrialand commercial applications in which they are difficult toreplace because of their special power density and efficiencyadvantages.

As the price of high-performance digital signal processorsand microcomputers continues to decrease, modern industrial


drives continue to add controls and diagnostic features thatwere not economically feasible in the past. Increasingly, low-performance constant volt-per-hertz control algorithms arebeing replaced by some form of field-oriented control or directtorque control algorithms, even though the load applications(e.g., pumps and fans) do not require fast dynamic response.Drive manufacturers compete in providing control features thatare designed to make the machine drive easier to set up andoperate, including self-commissioning and diagnostic featuresto recognize the signs of problems in either the motor or itsconnected load.

For those applications that require tighter control ofthe machine speed or dynamic performance characteristics,so-called sensorless control algorithms that eliminate theneed for an encoder or resolver are becoming increas-ingly robust and popular. In the subset of applicationsfor which the machine is custom-designed in combinationwith the drive (e.g., elevators), there is growing interestin applying self-sensing techniques in which the machineis specifically designed to operate well as a position sen-sor in addition to its electromechanical energy conversionresponsibilities [48].

As noted earlier in this paper, the trend toward increasinglevels of power electronics integration with the sources andloads is appearing in many forms, including industrial drives.One of the most notable examples is the integrated industrialservo drive where multiple suppliers have developed integratedmotor/controller units with continuous power ratings up to5 kW that are capable of operating in hostile factory-floorconditions including water jet washdowns (i.e., ingress pro-tection class IP65 [49]). These integrated motor drives requireonly 50% of the volume of a conventional servomotor plus itsseparate controller and offer significant simplifications of theinterconnection wiring by combining the power supply andcommunications into a single cable.

VIII. RESIDENTIAL AND CONSUMER APPLIANCE

APPLICATIONS

The presence of power electronics and variable-speed motordrives in residential and consumer appliance applications hasbeen expanding significantly all around the world during recentyears. As previously mentioned, the drivers for this expansionare a familiar combination including performance advantages,energy savings, and reductions in the cost of power elec-tronics. Japan has long been an international leader in devel-oping and commercializing inverterized appliances, reflectingmotivation provided by its traditionally higher energy costs.In the United States, tightening government regulations on theenergy efficiency of appliance and heating, ventilating, andair conditioning (HVAC) equipments have stimulated productevolution in the same direction.

It has been well-known for a long time that the replace-ment of ON–OFF cycling control of motors in refrigerationand HVAC equipment by variable speed drives can leadto significant energy savings as well as improved comfortlevels for building inhabitants. An example is the replace-ment of a traditional ON–OFF blower for a furnace or air

Fig. 17. Fractional-hp electronically commutated motor for furnace blowerapplications [50].

Fig. 18. Exploded view of Samsung DD washer tub assembly [51].

Fig. 19. Cutaway view of 100 kr/min PM motor assembly in Dyson AirbladeTap [52].

conditioner by a controllable variable-speed unit. Recognitionof this opportunity has led to the development of integratedmotor/controller units for these applications that use either PMor induction motors, one example of which is shown in Fig. 17.Unfortunately, the initially high manufacturing cost of theseunits stretched out their payback periods and significantlyslowed their acceptance in North America until recent years,when market acceptance has significantly increased becauseof the factors noted earlier.

At the same time, there is a growing trend in some of thelarger white goods appliances to replace conventional belt-driven powertrains with direct drive configurations [53]. Thisevolution is particularly apparent in laundry washing machinesthat traditionally have used a belt-drive and transmission forthe large majority of washing machines manufactured in NorthAmerica as well as other parts of the world. Fig. 18 shows oneexample of a direct drive tub motor where the external PMrotor is rigidly attached to the tub and fits around the stator


that provides variable-frequency excitation. The adoption ofa low-speed, high-torque machine (typically PM) to directlydrive the tub under inverter control, providing programmableagitation and spin cycles, first became popular in Australiaand New Zealand in the 1990’s. This approach has graduallygained popularity among white goods manufacturers aroundthe world during the past 20 years.

Other innovative implementations of direct drive motortechnology appear at the other end of the speed spectrum inapplications such as high-speed (∼100 krev/min) air blowersfor vacuum cleaners and hand dryers using switched reluctanceand PM motors, respectively. Fig. 19 shows the high-speedPM motor and its integrated power electronics used in acommercial hand dryer.

IX. CONCLUSION

Penetration of new technologies into the market requiresthat they be cost effective for the application. The market formany potential applications are strongly price-dependent, e.g.,white goods, lighting, and automotive. Wide bandgap devices,for example, will not be practical in these applications untiltheir cost becomes competitive with Si unless there is anoverwhelming performance benefit. Industrial applications aremore likely to be valued based on a life-cycle cost analysis,and in these a case can be made based on efficiency, volume,weight, or performance considerations. However, very highfrequency operation made possible by these wide bandgapdevices can not be really exploited until there are advancesin the high frequency performance of passive components.But the continuing trend toward greater electrification andcontrol of functions in consumer, commercial, industrial, trans-portation, and even medical applications, some of which werepresented here, promises a dynamic and increasingly importantrole for power electronics.

ACKNOWLEDGMENT

The authors would like to thank the following col-leagues for their valuable contributions, inputs, and sugges-tions: Dr. M. Jovanovic, Delta Electronics; Prof. G. Hurley,U. C. Galway; Dr. E. Landsman, and Mr. N. Rasmussen, Emer-son Electric; Prof. T. P. Chow, RPI; Prof. H. Akagi, TokyoInstitute of Technology; Prof. D. Perreault, Prof. J. Lang, andMr. S. Bandyopadhyay, MIT; Prof. C. R. Sullivan, Dartmouth;and Mr. C. Rytoft, ABB. The authors are also grateful toMr. R. Zhang, MIT, for his valuable assistance in formattingthis paper.

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John G. Kassakian (S’65–M’73–SM’80–F’89–LF’10) received his undergraduate and graduatedegrees from the Massachusetts Institute of Tech-nology (MIT), Cambridge, MA, USA.

He is currently Professor Emeritus of ElectricalEngineering and Computer Science at MIT. From1991 to 2009, he directed the interdepartmentalMIT Laboratory for Electromagnetic and ElectronicSystems. Prior to joining the MIT faculty, he serveda tour of duty in the US Navy. He has publishedextensively in the areas of power electronics, edu-

cation and automotive electrical systems, and is a coauthor of the textbookPrinciples of Power Electronics. He is the Founding President of the IEEEPower Electronics Society.

He is the recipient of the IEEE Centennial Medal, the IEEE William E.Newell Award, the IEEE Power Electronics Society’s Distinguished ServiceAward, and the IEEE Millennium Medal. He is a Member of the NationalAcademy of Engineering. He serves on the boards of several public companiesand for 12 years was a Member of the Board of Directors of ISO-NewEngland, the operator of the New England electricity grid.

Thomas M. Jahns (S’73–M’79–SM’91–F’93)received the S.B., S.M., and Ph.D. degrees in elec-trical engineering from the Massachusetts Instituteof Technology, Cambridge, MA, USA, in 1974 and1978, respectively.

He joined the Department of Electrical andComputer Engineering, University of Wisconsin(UW)-Madison, Madison, WI, USA, in 1998, as aGrainger Professor of Power Electronics and ElectricMachines, where he is currently a Co-Director of theWisconsin Electric Machines and Power Electronics

Consortium. Prior to joining UW, he was with GE Corporate Research andDevelopment (now GE Global Research Center), Niskayuna, NY, USA, for 15years, where he pursued new power electronics and motor drive technology ina variety of research and management positions. From 1996 to 1998, he waswith the Massachusetts Institute of Technology, where he directed researchactivities in the area of advanced automotive electrical systems and accessoriesas a Codirector of an industry-sponsored automotive consortium. His currentresearch interests include high-performance permanent-magnet synchronousmachines, electric traction drives, wind power systems, and distributed energyresources, including microgrids.

Dr. Jahns received the 2005 IEEE Nikola Tesla Technical Field Award, theIAS Outstanding Achievement Award in 2011, and the William E. NewellAward from the IEEE Power Electronics Society (PELS) in 1999. He servedas a Distinguished Lecturer for both IAS and PELS. He is a Past President ofPELS and served as a Division II Director on the IEEE Board of Directorsfrom 2001 to 2002.

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