solid-state transformers - on the origins and evolution of ... · seen as a (direct) matrix-type...

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IEEE Industrial Electronics Magazine, pp. 19-28, September 2016

Solid-State Transformers - On the Origins and Evolution of Key Concepts

J. Huber,J. W. Kolar

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Solid-State TransformersOn the Origins and Evolution of Key Concepts

JONAS E. HUBER and JOHANN W. KOLAR

During the past two decades, solid-state transformers (SSTs) have evolved quickly and have been con-sidered for replacing conventional low-frequency (LF) transformers in applications such as traction,

where weight and volume savings and substan-tial efficiency improvements can be achieved, or in smart grids because of their controllability. As shown in this article, all main modern SST topologies realize the common key characteris-

tics of these transformers—medium- frequency (MF) isolation stage, connection to medium volt-

age (MV), and controllability—by employing combi-nations of a very few key concepts, which have been

described or patented as early as the 1960s. But still, key research challenges concerning protection, isolation, and

reliability remain.SST technology has undergone a quick development during the

past two decades, and numerous different topologies, systems, and ap-plications have been—and are being—proposed and analyzed. Various projects [1]–[5] have been or currently are dedicated to developing SST technology as a key building block of future smart distribution grids, where SSTs could facilitate the integration of renewable energy sources or energy storage systems with ei-ther direct current (dc) or alternating current (ac) interfaces, e.g., photovoltaics and wind power, into the grid or where they could enable power routing and facili-tate energy management in local microgrids [6]–[10]. On the other hand, SSTs are well suited for applications where volume and weight restrictions apply and high-er efficiencies of the power conversion are targeted, as is the case, e.g., in traction applications. There, SSTs allow simultaneous improvement of power density and efficiency compared to conventional solutions based on LF transformers; this has attracted the interest of different manufacturers of traction equipment [11]–[18].

In this article, the key characteristics of today’s SSTs are identified, and the five main modern SST topologies (and therefore almost all contemporary SST topolo-gies) are shown to realize these characteristics by employing the same few key concepts in various combinations. The origins of these key concepts can be traced back to the 1960s. To understand why the same few key concepts are employed in most contemporary SSTs, first the defining characteristics of an SST must be done. Considering the aforementioned literature on SSTs, the common key characteris-tics can be identified as

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Date of publication: 21 September 2016

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20 IEEE IndustrIal ElEctronIcs magazInE ■ SEPTEMBER 2016

■ connection to MV [in contrast to other isolated power electronic converters that operate with a low-voltage (LV) input]

■ potential separation by means of MF transformers

■ controllability (as opposed to con-ventional LF transformers).First, an SST interfaces to MV lev-

els, be it ac or dc, at least with one in-put or output, which is a consequence of the high power levels involved with typical applications, such as traction or smart distribution grids. Therefore, any SST needs to handle the connec-tion of power electronics to MV by some means.

Second, increasing the operating frequency of a transformer allows for a reduced volume and weight without increasing the winding current densi-ty Jrms and/or the maximum core flux density Bmax

t (and therefore degrading the efficiency) as can be seen from the area product, i.e., the product of the core cross section area Acore and the winding window area ,AWdg which de-fines the dependency of the physical size and/or volume of a transformer

on the power to be transferred (e.g., [19]) as

( ) ,

A Ak J B f

P

V A Af

2

1

·

/

maxW

43

3 4

Core Wdgrms

Core Wdg& ? ?

r= t

(1)

where f denotes the transformer op -era ting frequency, and kW is the wind-ing window filling factor. This is espe-cially relevant in applications where space and weight are limited, as is the case in, e.g., traction applications [Fig-ure 1(a)]. Early efforts to increase the operating frequency of a locomotive’s main transformer above the low grid frequency (which can be as low as 16.6 Hz in several European national traction grids) by means of power con-verters, and, thereby, to reduce its vol-ume and weight, date back to 1978 [20]. Recent trends in traction systems, such as distributed propulsion, low-floor vehicles with the traction equipment mounted on the roof and high-speed trains where a higher power proce-ssing capability must be provided while maintaining the same maximum

axle weight limit, lead to tighter volume and weight constraints [21], [15], which renders MF isolation even more attra-ctive for these applications. In contrast to grid applications where weight and volume constraints do not usually ap-ply, the additional features that an SST could provide to a future smart grid, such as harmonic filtering, do require high control dynamics and therefore high switching frequencies, too. There-fore, independent of its application area, a typical SST employs power elec-tronic interfaces and an isolation stage that features a transformer operated in the MF range, i.e., several 100 Hz to sev-eral kilohertz.

Third, in contrast to a conven-tional transformer, an SST features controllability, i.e., it allows for con-trol of its input and output voltages and currents (and typically the out-put frequency) as well as the power flows, and it can protect loads from power system disturbances or the power system from load disturbanc-es. This is an important argument for the application of SSTs in future smart grids [ Figure 1(b)], which were first proposed around the turn of the millennium [6]–[10]. Another argument is the possibility of inter-facing renewable energy sources, such as photovoltaics and energy storage systems directly to an in-ternal dc bus available in many SST topologies.

(a) (b)

M

ac

acdc

acac

ac dc

dc

ac

dc ac

ac

ac

ac

SST

fT >> f1fT >> f1

v1, f1

v1, f1

f1

f1

v1

f1

v1

f1

f2 = f1

SST

M

v2 = v2∗

f2 = f2∗

v2 ≈ n ·v1

FIGURE 1 — The replacement of an LF transformer by an SST in (a) a traction application to comply with weight and space constraints and (b) a distribution grid application, which is motivated by increased functionality such as output voltage control or reactive power compensation. Note that v1 is in the MV range, fT is in the MF range, and f *

2 includes also the possibility of a dc voltage ( f *2 = 0 Hz).

ssts are well suited for applications where volume and weight restrictions apply and higher efficiencies of the power conversion are targeted.

SEPTEMBER 2016 ■ IEEE IndustrIal ElEctronIcs magazInE 21

Key ConceptsWith the key characteristics of an SST defined, the key concepts (and their origins) that are employed to realize these characteristics can be given a closer look. The two basic variants of isolated converters that allow the use of high-frequency or MF transformers to provide galvanic separation have been proposed by McMurray in 1968 [22], [23], as “converter circuits hav-ing a high frequency link.” Figure 2(a) shows the basic, non–resonant con-cept [23], where an input ac voltage is chopped by four-quadrant turn-off switches (13 and 15), implemented as antiparallel connections of transis-tors with series diodes or, alternative-ly, as antiparallel connections of re-verse-blocking gate turn-off thyristor, to generate a high-frequency voltage that is applied to the transformer. The output voltage can be regulated by

phase shifting the switching instants of the secondary side rectifier’s four-quadrant turn-off switches (16 and 17) with respect to the primary side switches—the system can then be seen as a (direct) matrix-type elec-tronic ac-ac transformer with volt-age regulation capability and high-frequency isolation.

In the 1970s, thyristor switching devices were typically employed in-stead of power transistors, especially

for higher power levels. To facilitate the commutation of these thyristors, a shaping of the transformer current into sinusoidal pulses with a natural zero-crossing by means of adding a se-ries resonant capacitor [Figure 2(b)] was additionally proposed by McMur-ray, also in 1968 [22], [24] (and, almost simultaneously, in a similar fashion by Schwarz [25]). Today, this type of reso-nant converter is known as the half-cy-cle discontinuous-conduction-mode

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t Vol

tage

Inpu

t Vol

tage

Out

put V

olta

geO

utpu

t Vol

tage

Transformer Voltage

TransformerVoltage Transformer

Current

Output ≈ Input Voltage

(for UnityTurns Ratio)

(a)

(b)

FIGURE 2 — (a) A non–resonant ac-ac power converter with a high-frequency link as proposed in 1968 by McMurray [23], with key converter wave-forms illustrating the output voltage regulation by delaying the firing of the secondary side bridge. (b) A resonant variant (also shown in an ac-ac configuration) employing the HC-DCM operating mode, also from a patent filed by McMurray in 1968 [22]. (The colors were added by the authors.)

the Hc-dcm src features the very interesting property of maintaining a constant ratio between its input and output dc voltages without requiring control and (largely) independent of the transferred power.


series resonant converter (HC-DCM SRC), due to the discontinuous shape of the transformer current pulses. The HC-DCM SRC features the very interest-ing property of maintaining a constant ratio between its input and output dc [or, in case bidirectional switches are employed as in Figure 2(b), also LF ac] voltages without requiring control and (largely) independent of the trans-ferred power. Therefore, it acts as an ac-ac or dc-dc transformer with a fixed voltage transfer ratio defined by the turns ratio of the MF transformer. Furthermore, in contrast to the non–resonant concept, the HC-DCM SRC allows for soft-switching of the power semiconductors, thereby reducing

the switching losses and improving the converter efficiency. Therefore, even though today’s semiconductors do not need a zero crossing of the cur-rent to turn off as did McMurray’s thy-ristors, the simplicity of the HC-DCM SRC’s autonomous operation as an iso-lation stage and the ability to achieve load-independent soft-switching ren-ders it highly advantageous for many modern SST topologies.

Whereas the converter circuits shown in Figure 2 are suitable for in-creasing the operating frequency of the isolation transformer, they can-not be connected directly to MV, i.e., several kilovolts to several tens of ki-lovolts, because the blocking voltage

ratings of readily available power semiconductors are limited to a few kilovolts. Therefore, either a series connection of semiconductors or a multilevel converter topology must be employed; the latter offers signifi-cant benefits in terms of reducing the filtering effort due to a multilevel out-put voltage. The use of neutral-point clamped (NPC) converters (1981) [26] is typically feasible up to five levels, but they are not arbitrarily scalable to higher voltages. Flying ca-pacitor converters (1992) [27] can be realized with more voltage levels and can therefore support higher volt-ages; however, they do not provide a modular system structure. Therefore, most contemporary SSTs employ a cascade configuration of several con-verter cells. Such a topology is shown in Figure 3, which is an excerpt from a patent filed in 1969—also by Mc-Murray—describing a “fast response stepped-wave switching power con-verter” for driving a sonar transducer [28]. Even though the focus at that time was not on high voltage (HV) levels but on fast transient response, the structure shown in Figure 3 cor-responds to the cascaded H-bridges (CHB) topology as it is employed in one form or another in most of to-day’s multicell SSTs: each converter cell features an isolated dc interface and a dc-ac conversion stage. The cells are connected in series at their ac terminals and in parallel at their (isolated) dc terminals, thereby form-ing an input-series output-parallel (ISOP) structure. Today, the supply of a cells’ isolated dc bus is typically realized by a (bidirectional) isolated dc-dc converter instead of a unidirec-tional diode rectifier as shown in the figure. Note, however, that already McMurray mentioned the possibility of using a high-frequency transform-er to supply the cells to reduce the system’s size and weight. It should also be noted that ISOP structures typically feature a beneficial self-bal-ancing mechanism that ensures the balancing of the floating capacitor voltages, essentially because all cells are connected in parallel to the same LV voltage [29].

MultilevelOutput Voltage

FIGURE 3 — The “fast response stepped-wave switching power converter circuit” patented in 1971 by McMurray [28]. (The colors were added by the authors.)

most contemporary ssts employ a cascade configuration of several converter cells.


The Five Classes of Modern SST TopologiesUntil today, a myriad of SST topolo-gies have been proposed by both aca-demia and industry—far more than could be covered in a short article (e.g., see [30]–[33] for a more com-prehensive overview). However, most of these are variations of five main modern SST topologies that can be derived as combinations of the key concepts described above. Note that the topologies discussed here on the example of an ac-dc configuration can also be realized with an ac output voltage, i.e., as ac-ac converters, by adding an additional inverter stage on the LV side.

Matrix TypeIn applications where a direct ac-ac con-version with MF isolation is required, e.g., to reduce the overall weight and vol-ume of the isolation transformer, but no adaption of the output frequency (which is the same as the input frequency) or any other more advanced controllabil-ity that would rely on an intermediate dc bus, an ISOP arrangement of McMur-ray’s ac-ac isolation stages (Figure 2) can be used to interface to MV systems. Instead of employing a direct matrix-converter approach with bidirectional switches as in the original McMurray patent, it is also possible to first place a folding/unfolding stage to convert an ac voltage into a ac (rectified ac) voltage, which can then be further processed. This concept has its origin in the field of power supplies and dates back to 1979 [34]. If the further processing is simply a chopping of the ac voltage to oper-ate an isolation transformer at a higher frequency, the SST topology shown in Figure 4(a) is obtained, where the con-version stages are of the indirect matrix-converter type. This modern (indirect) matrix-type ac-ac SST topology has recently been patented by General Elec-tric (GE) in 2008 [35], and later a 1-MVA single-phase prototype system was real-ized [Figure 5(a)]. This solid-state power substation (SPSS) prototype employs an ISOP configuration of four cells based on 10 kV of silicon carbide (SiC) devices to interface a 13.8-kV grid, which allows for an MF transformer operating frequency

FIGURE 4 — (a) A typical modern matrix-type AC-AC SST topology, which consists of an ISOP arrangement of ac-ac isolation stages as shown in Figure 2 (resonant or non–resonant variants are possible); an industrial prototype is shown in Figure 5(a). (b) A typical modern IBE topology, realizing a multilevel output voltage by means of CHB (Figure 3) and employing a resonant dc-dc isolation stage [Figure 2(b)]; an industrial prototype is shown in Figure 5(b). (c) A typical modern IFE topology (Figure 6), using also cascaded converter cells and a resonant ac-|ac| isolation stage [Figure 2(b)].

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of 20 kHz, an overall ac-ac conversion ef-ficiency of 97%, and a claimed reduction of the weight by 75% and of the size by 50% compared to conventional LF trans-formers [36], [37], even if this is not eas-ily visible from published material. Note that the planned full three-phase system would connect three single-phase sys-tems in a delta configuration on the MV side and in a star configuration on the LV side [38].

Isolated Back EndIf, in contrast, a dc output and/or a higher degree of controllability are required, McMurray’s stepped-wave converter (Figure 3) can be combined with HC-DCM SRC [Figure 2(b)] dc-dc

(b)(a)

FIGURE 5 — Examples of the industrial realizations of SST systems. (a) GE’s all-SiC 1 MVA single-phase SPSS prototype featuring a matrix-type ac-ac topology as shown in Figure 4(a). (Photo courtesy of [37].) (b) ABB’s single-phase 1.2 MVA ac-dc PETT employing an IBE topology as shown in Figure 4(b). (Photos courtesy of [59].)

A

B

B

D E Boost Inductor Current

A

C DE

Inpu

t Vol

tage

1985

(a)

(c)

(b)

C

FIGURE 6 — A topology to “eliminate the 16.6-Hz main transformer on electrical traction vehicles” proposed by Weiss in 1985 [40], where the con-trol stage (boost converter, E) is placed on the secondary side of the MF isolation stage (B-C-D)—i.e., an IFE approach. (Image courtesy of [40]; the colors were added by the authors.)


isolation stages supplying the cascad-ed cells’ floating dc buses to obtain the topology shown in Figure 4(b). This topology, which is likely the most common of the basic modern SST topologies, was patented in 1996 [11], [12]. Because the controlled ac-dc stages that generate the multi-level output voltage and are used to control the grid current are located on the MV side of the isolation bar-rier, this kind of SST topology can be referred to as an isolated back-end (IBE) system [39]. Since the initial pat-ent at the end of the 1990s, the IBE topology has gained interest from different companies [13], [16]–[18], recently resulting in ABB mounting a fully functional SST prototype on a shunting locomotive that has been field-tested on the Swiss railroad [Figure 5(b)]. This 1.2-MW single-phase power-electronic-traction transformer (PETT) interfaces the 15-kV, 16-Hz Swiss traction grid using an ISOP con-figuration of nine cascaded converter cells (one of which is redundant) based on 6.5-kV silicon insulated-gate bipo-lar transistors (IGBTs) on the input ac side, operating the MF transform-ers with 1.8 kHz, and realizing a high overall ac-dc conversion efficiency of 96% and an improved power to weight ratio of up to 0.75 kVA/kg compared to a standard LF transformer realization (#0.35 kVA/kg) [17].

Isolated Front EndAnother option for arranging the isola-tion and the controlling stage, which is a third key concept, was proposed by Weiss in 1985 for a traction application [40]: as shown in Figure 6, the output voltage of a folding stage, i.e., the rec-tified (folded) grid voltage, is directly chopped by a thyristor inverter to op-erate the isolation transformer with a voltage of higher frequency, which is similar to McMurray’s approach [Figure 2(a)]. However, the pulsating LV side dc bus is then interfaced by an ad-ditional controlling stage, which, back in the 1980s, Weiss realized as a boost converter based on forced-commutat-ed thyristors. This boost converter is controlled to draw a current that is proportional to the (rectified) ac

transformer output voltage, which in turn is proportional to the folded input voltage on the primary side. Doing this realizes unity power factor operation on the grid side and provides a dc volt-age at the boost converter’s output. Since, in contrast to the IBE approach, the controlling stage is placed on the LV side of the isolation barrier, i.e., af-ter the isolation stage, this approach can be referred to as an isolated front-end (IFE) system.

Recently, this concept has been combined with the cascading of con-verter cells to obtain SST topologies for traction [41] or multipurpose ap-plications [42]. However, as in the original concept from Weiss, these systems feature hard-switched ma-trix-type isolation stages as shown in Figure 2(a). In contrast, Han et al. [43], [44] employed the resonant (and soft-switched) version of McMurray’s high-frequency link isolation stage [Figure 2(b)], thereby realizing an IFE topology similar to, but slightly more complicated than, the one shown in Figure 4(c) [45], [39]. Here, the HC-DCM SRC isolation stage operates as an ac- ac converter, which is similar to the ac-ac operating mode shown in McMurray’s 1968 patent [Figure 2(b)]. This is in contrast to the IBE approach, where it operates as a dc-dc convert-er, and therefore large dc capacitors on the cascaded cells’ MV sides, i.e., on floating potential, are required due to the pulsation of the input side rec-tifier stage output power with twice the mains frequency. In the IFE topol-ogy, the capacitors of the isolation stage are only small commutation or resonant capacitors, i.e., there is no significant energy storage in the isolation stage. Therefore, the power flow defined by the controlled boost converter on the LV side is directly translated to the MV grid side, allow-ing for unity power factor operation

without requiring MV side measure-ments, thereby reducing the overall system complexity. (However, one has to note that operation with power fac-tors differing from unity is possible but results in disturbances in the grid current [39].) Currently, the Power Electronic Systems Laboratory of ETH Zurich is working on an all-SiC realiza-tion of an IFE-based ac-dc SST [45], [39]—the Swiss SST—in the scope of a research program funded by the Swiss government aiming at the promotion of renewable energy and the imple-mentation of smart grid concepts [4].

Isolated Modular Multilevel ConverterConsidering the cascaded cells ar-rangement shown in Figure 3, the topology could be simplified if the in-dividual cells did not need a dc sup-ply. Such a structure has been basi-cally shown in 1981 [46]; however, only about 20 years later, in 2002, Mar-quardt et al. proposed a full converter including a control method that does not require these individual dc sup-plies for the cells: the modular mul-tilevel converter (M2LC) [47], [48]. A year later, the first SST topology based on a single-phase M2LC and a single MF transformer (Figure 7) was pro-posed for a traction application [49], [50]. In contrast to the fully modular topologies shown in Figure 4, here only the power electronics is modu-lar; there is only a single, fully rated MF transformer. The effort in terms of power semiconductors is compara-tively high, but on the other hand the single transformer might facilitate the implementation of the isolation system.

Single-Cell ApproachFinally, recent advances in SiC power semiconductor technology provide pow-er semiconductors, e.g., a SiC metal– oxide–semiconductor field-effect

the Power Electronic systems laboratory of EtH zurich is working on an all-sic realization of an IFE-based ac-dc sst—the swiss sst.


transistor, with significantly higher blocking voltages than available silicon devices. These could enable the use of single-cell converter topologies such as a standard two-level converter or more advanced approaches such as the NPC converter mentioned earlier, for interfacing at least lower MV levels.

While with these approaches, which are pursued, e.g., by the Future Renewable Electric Energy Delivery and Manage-ment (FREEDM) project employing 15-kV SiC devices in an NPC configuration to interface a 13.8-kV MV grid [3], the cascading of converter cells might not be necessary anymore, an MF isolation stage is still employed. The isolation stage is realized either as an HC-DCM SRC or as a dual active bridge (DAB) converter, which was patented 25 years ago by DeDoncker et al. in 1989 [51]. The

DAB offers a higher degree of controlla-bility but also complexity, which is less critical in single-cell systems.

Discussion and Future ChallengesInterestingly, a common characteris-tic of four of the five main SST topolo-gies is that they feature a modular

structure, i.e., they employ multiple converter cells, which allows them to achieve high reliability by means of adding redundant converter cells. High reliability is a key requirement of applications where SSTs may replace conventional transformers in the fu-ture, such as in traction and distribu-tion grid applications; future MV DC grids in marine [52], naval [53], and subsea applications [54]; and pos-sibly even future aircraft employing distributed propulsion concepts [55].

Furthermore, a modular approach is scalable to HV levels and also benefits from economies of scale. Therefore, it can be expected that, even with the availability of HV SiC devices, multi-cell SST topologies and the concept of cascading converter cells, in addition to MF isolation, will continue to be an integral characteristic of modern SST topologies.

In addition to the high reliabil-ity requirements, other major chal-lenges in the further development of SST technology include the protec-tion of comparably sensitive power electronic systems against mains overvoltages, e.g., lightning strikes, and short circuit currents that are to be expected in harsh (traction) grid environments; and the design of the MF transformers, where the trad-eoff between high isolation require-ments and the thermal management needs to be mastered (low thermal conductivity of electrically isolating materials) and where the impact of the mixed-frequency electrical field stress on isolation materials needs to be understood better.

Regarding the applicability of SST technology, ac-ac applications in the

a common characteristic of four of the five main sst topologies is that they feature a modular structure that allows them to achieve high reliability by means of adding redundant converter cells.

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MF-ac

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Multilevelac Voltage

tTg /2iggii

FIGURE 7 — An M2LC-based SST topology [50]. Note that there is only a single, fully rated MF transformer, i.e., only the power electronics is modular.


distribution grid seem difficult be-cause of the “efficiency challenge”: even if an MF transformer could be built with a slightly higher efficiency than an LF transformer, the losses of the additional conversion stages [Figure 1(b)] result in a lower over-all efficiency of the ac-ac SST system compared to an LF transformer. In addition, compatibility with existing grid infrastructure might be an issue, e.g., an SST cannot deliver several times its rated current to trip fuses. Possibly required (basic) controlla-bility could be provided to the distri-bution grid also by other means such as LF transformers equipped with highly efficient automatic on-load tap changers or active series voltage reg-ulators, where only a fraction of the total power needs to be processed by the power electronics. Also, hy-brid SSTs, i.e., combinations of an LF transformer and a power electronic converter rated only at a fraction of the power to provide certain regu-lation functionalities (e.g., reactive power compensation or active filter-ing of harmonics), are an interesting approach that follows the same line of thought [56], [57]. On the other hand, there is no alternative to an SST if galvanic separation is required in future dc grids, e.g., in dc collec-tion grids for wind parks [58]. In such isolated dc-dc conversion systems, the operating frequency of the iso-lation transformer can be selected arbitrarily without increasing the number of power processing stages, which is in contrast to the previously discussed ac-ac application, where a direct (i.e., without intermediate conversion stages) connection of a mains frequency (i.e., LF) transform-er to the available ac voltages gener-ally provides higher efficiency. If, on the other hand, as in traction appli-cations or also in future naval, sub-sea, or aerospace applications, space and weight constraints do apply, an SST might be the only solution to si-multaneously achieve high volumet-ric and gravimetric power densities and high efficiency for MV-connected isolated ac-ac or ac-dc power conver-sion systems.

ConclusionsBy combining three key concepts that emerged 30–50 years ago, the five main modern SST topologies can be derived. Matrix-type ac-ac topolo-gies are well suited for pure ac-ac applications in weight and volume constrained environments because, in contrast to the other topologies, they do not require bulky dc capacitors. IBE topologies have been widely analyzed and are a suitable choice for ac-dc (or mixed ac-dc and ac-ac) applications, such as in traction vehicles. IFE to-pologies offer reduced complexity but also reduced flexibility, and therefore they could find use in niche applica-tions, e.g., as high-power MVac-LVdc auxiliary power supplies in weight and volume restricted environments. Topologies based on the M2LC could benefit from the single, fully rated transformer (as compared to indi-vidual transformers for each cell in case of cellular systems), which may facilitate the challenging isolation sys-tem design and should therefore be in-vestigated further. Finally, single-cell topologies employing HV SiC devices feature very low system complexity; however, they might suffer from reli-ability challenges because of the dif-ficulty of implementing redundancy in a nonmodular system.

These five main SST topologies, and almost all modern SST topologies, have evolved from combinations, and pos-sibly adaptions, of only very few key concepts, whose origins can be traced back to the late 1960s. However, the evo-lution of SST topologies that are suitable for specific applications (e.g., in a future smart distribution grid, but especially also in applications where weight and volume constraints apply such as in traction or future marine environments) and that offer desirable charac teristics such as high efficiency, high power den-sity, high reliability, ro bustness, fault tolerance, and modularity, is a highly in-teresting and ongoing process.

BiographiesJonas E. Huber ([email protected]) received his M.Sc. (with distinction) degree from the Swiss Federal Institute of Technology in Zurich, Switzerland,

in 2012, where he is currently work-ing toward a Ph.D. degree at the Power Electronic Systems Laboratory. His research interests include solid-state transformers, focusing on the analysis, optimization, and design of high-power multicell converter systems, reliability considerations, control strategies, and grid integration aspects. He is a Stu-dent Member of the IEEE.

Johann W. Kolar ([email protected]. ethz.ch) is a full professor and the head of the Power Electronic Systems Labo-ratory at the Swiss Federal Institute of Technology in Zurich, Switzerland. He has published more than 650 scientific papers in international journals and conference proceedings and has filed more than 120 patents. He has received more than 20 IEEE transactions and conference prize paper awards, and he has received the ETH Zurich Golden Owl Award for excellence in teaching. The focus of his current research is on ultracompact and ultraefficient silicon carbide and gallium nitride converter systems, wireless power transfer, solid-state transformers, power supplies on chip, and ultrahigh-speed and bearing-less motors. He is a Fellow of the IEEE.

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