energies

Concept Paper Modularized Three-Phase with Bidirectional Power Flow for Medium Application

Do-Hyun Kim, Byung-Moon Han * and Jun-Young Lee

Department of , Myong-ji University, 116 Myongji-ro, Yongin-si, Gyeonggi-do 449-728, Korea; [email protected] (D.-H.K.); [email protected] (J.-Y.L.) * Correspondence: [email protected]; Tel.: +82-31-330-6366

Academic Editor: Gabriele Grandi Received: 22 May 2016; Accepted: 17 August 2016; Published: 23 August 2016

Abstract: This paper describes a prototype of modularized three-phase semiconductor transformer that was developed in the lab for feasibility study. The developed prototype is composed of three units of single-phase semiconductor transformer coupled in Y-connection. Each single-phase unit has multiple units of high-voltage high-frequency resonant AC–DC converter, a low-voltage hybrid-switching DC–DC converter, and a low-voltage hybrid-switching DC–AC inverter. Also, each single-phase unit has two digital signal processor (DSP) boards to control converter operation and to acquire monitoring data. The monitoring system was developed based on LabView by using controller area network (CAN) communication between the DSP board and the personal computer (PC). Through diverse experimental analyses it was verified that the prototype operates with proper performance under normal and sag conditions. The developed prototype confirms the possibility of fabricating a commercial high-voltage high-power semiconductor transformer by increasing the number of series-connected converter modules in high-voltage side and improving the system efficiency with a new switching device such as an SiC device.

Keywords: modularized semiconductor transformer; high-frequency resonant AC–DC converter; hybrid-switching DC–DC converter; hybrid-switching DC–AC converter; digital signal processor (DSP) board; controller area network (CAN) communication; LabView

1. Introduction Electric Power Research Institute (EPRI) reviewed the feasibility of an intelligent semiconductor transformer that can replace the existing transformer consisting of iron core and coil. Based on this review, a prototype of 4.16 kV 20 kVA rating was fabricated to conduct diverse experiments [1–4]. This prototype has a full-bridge three- or five-level converter on the high-voltage side to change the high-voltage into the high-voltage . It also changes the high-voltage direct current into a high-frequency current using a half-bridge three- or five-level converter operating with soft switching, and again changes the high-frequency current into the low-voltage direct current using a high-frequency transformer and bridge. The low-voltage direct current is changed into the alternating current at the necessary voltage level by multiple parallel-connected full-bridge converters to supply power to loads. Although this prototype has a relatively higher system efficiency of 92%, it makes power flow only from the high-voltage side to the low-voltage side [5–7]. A semiconductor transformer with bidirectional power flow capability was proposed [8,9], in which a high-voltage multi-level converter is used on the input terminal to change the high-voltage AC into the high-voltage DC. Multiple units of resonant DC–DC converter, in which the input side is series connected and the output side is parallel connected, were used to convert the high-voltage DC

Energies 2016, 9, 668; doi:10.3390/en9090668 www.mdpi.com/journal/energies Energies 2016, 9, 668 2 of 17

intoEnergies the DC 2016 400, 9, 668 V. The DC 400 V is again converted into the AC 120 V or 240 V through the single-phase2 of 17 inverter. In this semiconductor transformer, the output voltage of high-voltage multi-level converter is reducedinto the due DC to 400 series V. connectionThe DC 400 in V the is input again side, converted while itsinto output the AC current 120 V is increasedor 240 V through due to parallel the connectionsingle‐phase in the inverter. output In side. this However, semiconductor this semiconductor transformer, transformerthe output hasvoltage a drawback of high‐ involtage that the high-voltagemulti‐level converter multi-level is converterreduced due causes to series relatively connection larger in switching the input side, loss. while its output current is increasedAnother due semiconductor to parallel connection transformer in the output with side. bidirectional However, this power semiconductor flow capability transformer was proposedhas a drawback[10–15] in, in that which the high the‐voltage input terminalmulti‐level of converter multiple causes units relatively of H-bridge larger converter switching is loss. series connected.Another The DCsemiconductor output voltage transformer from the with individual bidirectional H-bridge power is converted flow capability into the was DC proposed 400 V using the[10–15], resonant in which DC–DC the converter,input terminal and of the multiple DC 400 units V is of again H‐bridge converted converter into is theseries AC connected. 120 V or The 240 V throughDC output the single-phase voltage from inverter. the individual In this H semiconductor‐bridge is converted transformer, into the the DC high-voltage 400 V using input the resonant is divided DC–DC converter, and the DC 400 V is again converted into the AC 120 V or 240 V through the into the low-voltage output by the number of series-connected H-bridges and supplies a rectified single‐phase inverter. In this semiconductor transformer, the high‐voltage input is divided into the waveform to the resonant DC–DC converter, which is parallel connected for voltage reduction and low‐voltage output by the number of series‐connected H‐bridges and supplies a rectified waveform current increase. However, this semiconductor transformer demands separate balance control to to the resonant DC–DC converter, which is parallel connected for voltage reduction and current ensure that the high-voltage AC input to H-bridges is evenly divided. increase. However, this semiconductor transformer demands separate balance control to ensure that theIn high this‐voltage paper AC a new input structure to H‐bridges of semiconductor is evenly divided. transformer with bidirectional power flow capabilityIn this was paper proposed. a new Thestructure proposed of semiconductor bidirectional transformer semiconductor with bidirectional transformer power converts flow the single-phasecapability was AC proposed. 1.9 kV into The the proposed single-phase bidirectional AC 127 semiconductor V [16]. Three transformer units of this converts single-phase the semiconductorsingle‐phase AC transformer 1.9 kV into are coupledthe single in‐phase Y-connection AC 127 toV convert[16]. Three three-phase units of 3.3 this kV single input‐phase voltage intosemiconductor three-phase transformer 220 V output are voltage. coupled Thein Y‐ operationsconnection ofto convert proposed three three-phase‐phase 3.3 kV transformer input voltage in the forwardinto three power‐phase flow 220 and V output in the reversevoltage. power The operations flow were of checked proposed through three‐phase diverse transformer experiments. in the It is alsoforward checked power whether flow and the proposedin the reverse transformer power flow properly were checked operates through even when diverse the experiments. input voltage It is sag occurs.also checked In addition, whether in order the proposed to monitor transformer the operation properly of the operates entire even system, when a monitoring the input voltage system sag was developedoccurs. In that addition, connects in order the DSP to monitor boards withthe operation the PC through of the entire CAN system, communication a monitoring based system on LabView. was developed that connects the DSP boards with the PC through CAN communication based on LabView. 2. Single-Phase Semiconductor Transformer 2. Single‐Phase Semiconductor Transformer Figure1 shows the system configuration and operation principle of a single-phase bidirectional semiconductorFigure 1 shows transformer, the system which configuration has multiple and units operation of resonant principle AC–DC of a single converter‐phase bidirectional with insulated gatesemiconductor bipolar transistor transformer, (IGBT), which hybrid-switching has multiple units DC–DC of resonant converter, AC–DC and converter hybrid-switching with insulated DC–AC inverter.gate bipolar Three transistor units of (IGBT), half-bridge hybrid bidirectional‐switching DC–DC AC–DC converter, converter and are hybrid series‐switching connected DC–AC on the high-voltageinverter. Three side units so that of anhalf effective‐bridge valuebidirectional of 633 VAC–DC (maximum converter value are of 896series V), connected which is one-thirdon the ofhigh the‐ effectivevoltage side value so that of 1.9 an kV,effective is evenly value applied of 633 V for (maximum each unit value of AC–DC of 896 V), converters. which is one Three‐third unitsof ofthe AC–DC effective converter value of are1.9 kV, parallel is evenly connected applied onfor theeach low-voltage unit of AC–DC side converters. so that the Three effective units of value AC– of DC converter are parallel connected on the low‐voltage side so that the effective value of 1.05 A is 1.05 A is evenly shared. Each AC–DC converter changes the AC voltage with an effective value of evenly shared. Each AC–DC converter changes the AC voltage with an effective value of 633 V into 633 V into the full-wave-rectified waveform with an effective value of 320 V through resonance by a the full‐wave‐rectified waveform with an effective value of 320 V through resonance by a high-frequency transformer. This full-wave-rectified waveform is converted into the DC 600 V by a high‐frequency transformer. This full‐wave‐rectified waveform is converted into the DC 600 V by a bidirectional DC–DC converter operated with adjustable duty ratio and hybrid-switching. The DC bidirectional DC–DC converter operated with adjustable duty ratio and hybrid‐switching. The DC 600600 V isV convertedis converted into into the the single-phase single‐phase AC AC with with an an effective effective value value of of 127 127 V V by by a a bidirectional bidirectional DC–AC DC– inverterAC inverter that is that also is operated also operated with with hybrid-switching. hybrid‐switching.

(a)

Figure 1. Cont.

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(b)

FigureFigure 1. 1.Configuration Configuration of of bidirectional bidirectional intelligent semiconductor semiconductor transformer: transformer: (a () aconceptual) conceptual diagram; diagram; (b) circuit diagram. (b) circuit diagram.

The step‐down ratio between input and output is primarily determined by the turn The step-down ratio between input and output voltages is primarily determined by the turn ratio of the high‐frequency transformer and secondarily determined by the modulation index of the ratiobidirectional of the high-frequency DC–AC inverter. transformer The input and side secondarily of series‐connected determined high‐ byfrequency the modulation AC–DC converters index of the bidirectionalcan have an DC–AC advantage inverter. of easy The multiple input sideconnections of series-connected because it can high-frequency easily form voltage AC–DC balance. converters In canaddition, have an the advantage entire circuit of easy can multiple be simplified connections without because any additional it can easily form by voltage making balance. the Inconverter’s addition, theoutput entire into circuit the full can‐wave be‐rectified simplified waveform. without The any control additional of the entire rectifier system by making of a high the converter’sfrequency outputresonance into AC–DC the full-wave-rectified converter is simple waveform.because this converter The control operates of the at a entire fixed systemduty ratio of a highwith frequency soft switching resonance and its AC–DC power converter factor is controlled is simple through because a this bidirectional converter DC–DC operates converter at a fixed and duty ratioa DC–AC with soft inverter. switching Table and 1 itsshows power the factorspecification is controlled of the throughthree‐phase a bidirectional semiconductor DC–DC transformer converter andfabricated a DC–AC as inverter. a prototype. Table 1 shows the specification of the three-phase semiconductor transformer fabricated as a prototype. Table 1. Specification of bidirectional intelligent semiconductor transformer. Table 1. SpecificationItem of bidirectional intelligent semiconductorSpecification transformer. Power Rating 3‐Phase, 6 kVA Item Specification Input Voltage 3‐Phase, 60 Hz, AC 3.3 kV OutputPower Rating Voltage 3‐Phase, 3-Phase, 60 Hz, 6 kVA AC 220 V ResonantInput‐switching Voltage Frequency 3-Phase, 6050 Hz, kHz AC 3.3 kV Output Voltage 3-Phase, 60 Hz, AC 220 V Hybrid‐switching Frequency 25 kHz Resonant-switching Frequency 50 kHz Hybrid-switchingHigh frequency Transformer Frequency EE6565 core, 25 kHz Turn Ratio 2:1 HighResonant frequency Transformer ܥ୰ EE6565 core,0.4 Turn μF Ratio 2:1 ResonantResonant Capacitor Reactor Cܮr୰ PQ26250.4 core,µF 25 μH HybridResonant‐switching Reactor ReactorLr OD610PQ2625 toroidal core, core, 25 µH 2 mH Hybrid-switchingResonant‐switching Reactor IGBT FGL40N150D, OD610 toroidal 1500 core, V/40 2 mH A Resonant-switching IGBT FGL40N150D, 1500 V/40 A Hybrid‐switching IGBT BSM50GB120DLC, 1200 V/50 A Hybrid-switching IGBT BSM50GB120DLC, 1200 V/50 A HybridHybrid-switching‐switching MOSFET MOSFET FQA13N80, 800 800 V/12.6 V/12.6 A A Hybrid-switchingHybrid‐switching Diode Diode STTH3012D, 1200 1200 V/30 V/30 A A

Figure 2 shows the structure of a bidirectional high frequency AC–DC converter that converts Figure2 shows the structure of a bidirectional high frequency AC–DC converter that converts single‐phase AC 1.9 kV into the 320 V full‐wave‐rectified waveform. The circuits on the input and single-phaseoutput sides AC are 1.9 basically kV into configured the 320 V based full-wave-rectified on half‐bridge units. waveform. Three half The‐bridge circuits units on on the the input input and outputside are sides series are connected basically configured because the based voltage on on half-bridge the input side units. is high. Three Three half-bridge half‐bridge units modules on the inputon sidethe are output series side connected are parallel because connected, the voltage considering on the input the sidefact that is high. the Threevoltage half-bridge on the output modules side on thevoltage output is side low. are The parallel switching connected, element considering of the input the fact side that of thethe voltagehalf‐bridge on the unit output was sidemade voltage by is low.connecting The switching two IGBTs element in the reverse of the input direction. side Bidirectional of the half-bridge high‐frequency unit was madeLLC resonant by connecting circuit is two IGBTs in the reverse direction. Bidirectional high-frequency LLC resonant circuit is used to reduce the

Energies 2016,, 9,, 668 4 of 17 Energies 2016, 9, 668 4 of 17 used to reduce the size of the entire system. The system is operated with lower switching loss usedsizebecause of to the thereduce entire resonant the system. size circuit Theof theis system operated entire is operatedsystem. at a fixed withThe duty system lower ratio switching [17].is operated loss with because lower the resonantswitching circuit loss becauseis operated the atresonant a fixed circuit duty ratio is operated [17]. at a fixed duty ratio [17].

Figure 2. Circuit diagram of bidirectional high‐frequency AC–DC rectifier. FigureFigure 2. 2. CircuitCircuit diagram diagram of of bidirectional bidirectional high high-frequency‐frequency AC–DC AC–DC rectifier. rectifier. Figure 3a shows the gate pulses supplied to the individual IGBT switch according to the AC inputFigureFigure voltage 33aa shows inshows a single the the gate gateunit pulses ofpulses a suppliedbidirectional supplied to theto high the individual ‐individualfrequency IGBT AC–DCIGBT switch switch converter, according according toand the towhere AC the input ACthe inputvoltagesame gatevoltage in apulses single in a are unitsingle generated of aunit bidirectional of regardlessa bidirectional high-frequency of power high ‐flowfrequency AC–DC direction. converter,AC–DC Detailed converter, and operation where and the modeswhere same gate theare samepulsesdescribed gate are in generatedpulses Figures are 4 regardlessgenerated and 5. Figure ofregardless power 3b shows flow of powerthe direction. operation flow Detailed direction. analysis operation resultsDetailed of modes operationthe proposed are described modes AC/DC are in describedFiguresconverter4 and inthrough Figures5. Figure computer 4 and3b shows 5. Figuresimulations. the 3b operation shows The the analysisupper operation two results waveforms analysis of the results proposed show of the the AC/DC input proposed‐side converter AC/DCvoltage converterthroughand current, computer through which simulations.computer are sinusoidal simulations. The with upper negligible The two upper waveforms distortion. two waveforms show The the second input-side show waveform the voltage input ‐describesside and voltage current, the andwhichresonant current, are current sinusoidal which through are with sinusoidal transformer negligible with distortion. in negligiblewhich Thethe distortion.expanded second waveform waveformThe second describes is waveformshown the on resonant thedescribes right current side. the resonantthroughThe third transformercurrent two waveforms through in which transformer show the the expanded output in which‐side waveform the voltage expanded is and shown waveformcurrent, on the which is right shown look side. on like Thethe a right thirdfull‐ waveside. two Thewaveformsrectified third shape. two show waveforms the output-side show the voltage output and‐side current, voltage which and look current, like a which full-wave look rectified like a full shape.‐wave rectified shape.

(a) (a) Figure 3. Cont.

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(b) (b) Figure 3. Gating pulses and operation analysis results of AC/DC converter: (a) gating pulses for six Figure 3. Gating pulses and operationoperation analysisanalysis resultsresults ofof AC/DCAC/DC converter: ( a) gating pulses for six switches; (b) operation analysis results. switches; (b)) operation analysis results.results. Figure 4 shows equivalent circuits in the forward power flow by each operation mode Figure4 shows4 shows equivalent equivalent circuits circuits in the in forward the forward power flow power by each flow operation by each mode operation concentrated mode concentrated on the current path due to the gate driving pulses according to the AC input voltage in onconcentrated the current on path the due current to the path gate due driving to the pulses gate according driving pulses to the according AC input voltageto the AC in Figureinput 3voltage. Mode in 1 Figure 3. Mode 1 is an equivalent circuit at the positive input voltage as shown in Figure 4a. The isFigure an equivalent 3. Mode 1 circuit is an atequivalent the positive circuit input at voltagethe positive as shown input involtage Figure as4a. shown The transistor in Figure of 4a. M1 The is transistor of M1 is turned on and the current that passes through the transistor of M1 and the diode turnedtransistor on of and M1 the is currentturned on that and passes the current through that the passes transistor through of M1 the and transistor the diode of of M1 M2 and flows the on diode the of M2 flows on the primary side of the high‐frequency transformer. The current that passes through primaryof M2 flows side on of the the primary high-frequency side of the transformer. high‐frequency The currenttransformer. that passes The current through that the passes diode through of M5 the diode of M5 flows on the secondary side. Mode 2 is an equivalent circuit at the positive input flowsthe diode on the of secondaryM5 flows side.on the Mode secondary 2 is an side. equivalent Mode circuit2 is an at equivalent the positive circuit input at voltage the positive as shown input in voltage as shown in Figure 4b. The transistor of M3 is turned on and the current that passes through Figurevoltage4 b.as Theshown transistor in Figure of M34b. isThe turned transistor on and of theM3 currentis turned that on passesand the through current the that transistor passes through of M3 the transistor of M3 and the diode of M4 flows on the primary side of the high‐frequency andthe thetransistor diode ofof M4 M3 flows and onthe the diode primary of M4 side flows of the on high-frequency the primary transformer. side of the Thehigh current‐frequency that transformer. The current that passes through the diode of M6 flows on the secondary side. passestransformer. through The the current diode that of M6 passes flows through on the secondary the diode side.of M6 flows on the secondary side. Mode 3 is an equivalent circuit at the negative input voltage as shown in Figure 4c. The Mode 33 is is an an equivalent equivalent circuit circuit at the at negativethe negative input input voltage voltage as shown as inshown Figure in4c. Figure The transistor 4c. The transistor of M2 is turned on and the current that passes through the diode of M1 and the transistor oftransistor M2 is turned of M2 on is andturned the on current and the that current passes that through passes the through diode of the M1 diode and the of transistorM1 and the of transistor M2 flows of M2 flows on the primary side of the high‐frequency transformer. The current that passes through onof M2 the flows primary on sidethe primary of the high-frequency side of the high transformer.‐frequency transformer. The current thatThe passescurrent through that passes the diodethrough of the diode of M6 flows on the secondary side. Mode 4 is an equivalent circuit at the negative input M6the flowsdiode on of theM6 secondary flows on the side. secondary Mode 4 is side. an equivalent Mode 4 is circuit an equivalent at the negative circuit input at the voltage negative as shown input voltage as shown in Figure 4d. The transistor of M4 is turned on and the current that passes through involtage Figure as4 d.shown The transistor in Figure 4d. of M4 The is transistor turned on of and M4 the is turned current on that and passes the current through that the passes diode through of M3 the diode of M3 and the transistor of M4 flows on the primary side of the high‐frequency andthe thediode transistor of M3 of and M4 flowsthe transistor on the primary of M4 side flows of the on high-frequency the primary transformer.side of the Thehigh current‐frequency that transformer. The current that passes through the diode of M5 flows on the secondary side. passestransformer. through The the current diode that of M5 passes flows through on the secondary the diode side.of M5 flows on the secondary side.

(a) (a)

Figure 4. Cont.

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(b)

(c)

(d)

Figure 4.4. CurrentCurrent path path of of AC–DC AC–DC converter converter in forwardin forward power power flow: flow: (a) Mode (a) Mode 1: positive 1: positive input voltage input voltage(M1: on, (M1: M2: on, M5:M2: on);on, M5: (b) Mode on); ( 2:b) positiveMode 2: input positive voltage input (M3: voltage on, M4: (M3: on, on, M6: M4: on); on, (c )M6: Mode on); 3: (negativec) Mode input 3: negative voltage input (M1: on,voltage M2: on,(M1: M6: on, on); M2: ( don,) Mode M6: 4:on); negative (d) Mode input 4: voltage negative (M3: input on, voltage M4: on, (M3:M5: on).on, M4: on, M5: on).

Figure5 5shows shows equivalent equivalent circuits circuits in the in reverse the reverse power flow power by each flow operation by each mode operation concentrated mode concentratedon the current on path the current due to path the gate due drivingto the gate pulses driving according pulses toaccording the AC to input the AC voltage input in voltage Figure in3. FigureMode 53. is Mode an equivalent 5 is an equivalent circuit at the circuit positive at the input positive voltage input as shown voltage in as Figure shown5a. in The Figure transistor 5a. The of transistorM2 is turned of M2 on, is and turned the current on, and that the passes current through that passes the diode through of M1 the and diode the transistorof M1 and of the M2 transistor flows on ofthe M2 primary flows on side the of primary the high-frequency side of the high transformer.‐frequency The transformer. current that The passes current through that thepasses transistor through of theM5 transistor flows on the of M5 secondary flows on side. the Mode secondary 6 is an side. equivalent Mode 6 circuit is an equivalent at the positive circuit input at the voltage positive as shown input voltagein Figure as5 b.shown The transistorin Figure 5b. of M4 The is transistor turned on of and M4 theis turned current on that and passes the current through that the passes diode through of M3 theand diode the transistor of M3 ofand M4 flowsthe transistor on the primary of M4 side flows of the on high-frequency the primary transformer.side of the Thehigh current‐frequency that transformer.passes through The the current transistor that ofpasses M6 flows through on thethe secondary transistor of side. M6 flows on the secondary side. Mode 77 isis an an equivalent equivalent circuit circuit at the at negativethe negative input input voltage voltage as shown as inshown Figure in5 c.Figure The transistor 5c. The transistorof M1 is turned of M1 is on, turned and the on, currentand the that current passes that through passes through the transistor the transistor of M1 and of M1 the and diode the ofdiode M2 offlows M2 onflows the on primary the primary side of side the of high-frequency the high‐frequency transformer. transformer. The The current current that that passes passes through through the thetransistor transistor of M6 of M6 flows flows on on the the secondary secondary side. side. Mode Mode 8 8 is is an an equivalent equivalent circuit circuit at at thethe negative input voltage as shown in Figure 55d.d. TheThe transistortransistor ofof M3M3 isis turnedturned onon andand thethe currentcurrent thatthat passespasses throughthrough the transistortransistor of of M3 M3 and and the the diode diode of M4 of flows M4 onflows the primaryon the sideprimary of the side high-frequency of the high transformer.‐frequency transformer.The current that The passes current through that passes the transistor through the of M5 transistor flows on of theM5 secondaryflows on the side. secondary side.

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(a)

(b)

(c)

(d)

Figure 5. Current path of AC–DC converter in reverse power flow: (a) Mode 5: positive input voltage Figure 5. Current path of AC–DC converter in reverse power flow: (a) Mode 5: positive input voltage (M1: on, M2: on, M5: on); (b) Mode 6: positive input voltage (M3: on, M4: on, M6: on); (c) Mode 7: (M1: on, M2: on, M5: on); (b) Mode 6: positive input voltage (M3: on, M4: on, M6: on); (c) Mode 7: negative input voltage (M1: on, M2: on, on, M6: M6: on); on); ( (dd)) Mode Mode 8: 8: negative negative input input voltage voltage (M3: (M3: on, on, M4: M4: on, on, M5: on). on).

As shown in Figure 6, a low‐voltage side converter has a DC–DC converter that converts the 320 As shown in Figure6, a low-voltage side converter has a DC–DC converter that converts the V full‐wave‐rectified voltage into the 600 V DC voltage, and the DC–AC inverter that converts the 320 V full-wave-rectified voltage into the 600 V DC voltage, and the DC–AC inverter that converts the DC 600 V into the single‐phase 127 V, connected with the DC–DC converter in a two‐stage structure. DC 600 V into the single-phase 127 V, connected with the DC–DC converter in a two-stage structure. Both DC–DC converter and DC–AC inverter use hybrid switches in which an IGBT and a Both DC–DC converter and DC–AC inverter use hybrid switches in which an IGBT and a metal-oxide metal‐oxide semiconductor field effect transistor (MOSFET) are parallel connected. The DC–DC semiconductor field effect transistor (MOSFET) are parallel connected. The DC–DC converter in the converter in the front stage performs control and DC link voltage control, and the DC– front stage performs power factor control and DC link voltage control, and the DC–AC inverter in the AC inverter in the rear stage controls the output voltage supplied to the load. rear stage controls the output voltage supplied to the load.

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Figure 6. Configuration of the low‐voltage side of a bidirectional converter. FigureFigure 6. 6. ConfigurationConfiguration of of the the low low-voltage‐voltage side of a bidirectional converter. An IGBT has a drawback in that switching loss increases in the turn‐off period due to An IGBTIGBT has has a drawbacka drawback in that in switchingthat switching loss increases loss increases in the turn-off in the periodturn‐off due period to tail-current. due to tail‐current. As a method of minimizing this disadvantage, a hybrid switch made by Astail‐ acurrent. method ofAs minimizing a method this of disadvantage, minimizing a hybridthis disadvantage, switch made bya parallel-connectedhybrid switch made IGBT andby parallel‐connected IGBT and MOSFET was used [18–20]. Figure 7a shows turn‐off voltage and parallel‐connected IGBT and MOSFET was used [18–20]. Figure 7a shows turn‐off voltage and MOSFETcurrent wascharacteristics used [18– 20of]. IGBT Figure and7a showsMOSFET. turn-off The voltagetail‐current and currentof IGBT characteristics subsists for a of long IGBT time, and current characteristics of IGBT and MOSFET. The tail‐current of IGBT subsists for a long time, MOSFET.indicating The that tail-current relatively oflarger IGBT switching subsists for loss a longis induced time, indicatingin the IGBT that compared relatively to larger the MOSFET. switching indicating that relatively larger switching loss is induced in the IGBT compared to the MOSFET. lossFigure is induced 7b shows in the a IGBT timing compared diagram to theto MOSFET.apply gating Figure signals7b shows in a timinghybrid diagramswitching. to applyThe Figure 7b shows a timing diagram to apply gating signals in hybrid switching. The gatingparallel signals‐connected in hybrid MOSFET switching. is turned The on parallel-connected a little earlier than MOSFET the time point is turned at which on a the little IGBT earlier should than parallel‐connected MOSFET is turned on a little earlier than the time point at which the IGBT should thebe timeturned point off. atThe which IGBT the is turned IGBT shouldoff thereafter, be turned and off.the TheMOSFET IGBT is is turned turned off off at thereafter, the time point and at the be turned off. The IGBT is turned off thereafter, and the MOSFET is turned off at the time point at MOSFETwhich the is IGBT turned is offto be at turned the time off. point Considering at which the the IGBT IGBT tail is tocurrent, be turned the duration off. Considering time of MOSFET the IGBT whichtailswitching current, the IGBT was the duration isdetermined to be turned time to of off.be MOSFET 2Considering μs. In switching hybrid the switching, IGBT was determinedtail thecurrent, inside to the diode be duration 2 µ s.of InMOSFET hybridtime of switching,does MOSFET not switchingthehave inside a good diodewas reverse determined of MOSFET recovery to does becharacteristic; not2 μ haves. In ahybrid good the additional reverseswitching, recovery diode the blocksinside characteristic; thediode conduction of the MOSFET additional through does diode the not haveblocksMOSFET a thegood conductionbody reverse diode. recovery through The resistor characteristic; the MOSFET works tothe body remove additional diode. the The dioderinging resistor blocks effect works the during conduction to removethe free through the‐wheeling ringing the MOSFETeffectperiod during that body occurs the diode. free-wheeling when The both resistor IGBT period andworks that MOSFET occursto remove whenturn off.the both ringing IGBT andeffect MOSFET during turn the off.free‐wheeling period that occurs when both IGBT and MOSFET turn off.

Vsw i Vsw sw MOSFET tail-current isw MOSFETIGBT tail-currenttail-current IGBT tail-current

MOSFET switching Loss Ploss MOSFETIGBT switchingswitching Loss Ploss IGBT switching Loss

t0 t1 t2 t3

t0 t1 t2(a) t3 (a)

(b) (b) FigureFigure 7. 7.Tail Tail current current in in turn-off turn‐off and and hybridhybrid switching: ( a)) turn turn-off‐off tail tail current; current; ( (bb) )hybrid hybrid switching. switching. Figure 7. Tail current in turn‐off and hybrid switching: (a) turn‐off tail current; (b) hybrid switching.

Energies 2016, 9, 668 9 of 17

Energies 2016, 9, 668 9 of 17 3. Computer Simulation 3. Computer Simulation Figure8 shows simulated waveforms of a three-phase 6 kVA bidirectional intelligent Figure 8 shows simulated waveforms of a three‐phase 6 kVA bidirectional intelligent semiconductor transformer. Figure8a are the input voltage and current, the rectified voltage and semiconductor transformer. Figure 8a are the input voltage and current, the rectified voltage and current,current, the the DC DC link link voltage, voltage, and and the the output output voltagevoltage and current current appeared appeared in in forward forward power power flow flow whenwhen three-phase three‐phase 3.3 3.3 kV kV was was applied applied to to the the high-voltagehigh‐voltage resonant resonant converter. converter. Figure Figure 8b8 showsb shows the the samesame waveforms waveforms for for the the case case of of reverse reverse power power flow. flow. In theseIn these figures, figures, it can it becan seen be thatseen thethat input the input voltage voltage and current and current are in theare samein the phase, same indicatingphase, thatindicating the power that factor the power is corrected factor is and corrected the rectified and the rectified voltage voltage and current and current are also are in also the in same the same phase. Thephase. DC link The voltage DC link is voltage maintained is maintained at 600 V at with 600 V negligible with negligible ripples ripples less than less 0.8%,than 0.8%, and theand outputthe voltageoutput and voltage current and waveforms current waveforms of the DC–AC of the DC–AC inverter inverter are sinusoidal are sinusoidal with with a relatively a relatively low low level of harmonics.level of harmonics. FigureFigure9 shows 9 shows the the waveforms waveforms obtained obtained through through simulationssimulations conducted conducted to to examine examine circuit circuit operationsoperations when when sag sag occurs occurs in in the the input input voltage. voltage. Figure Figure 99aa shows the the waveforms waveforms of of the the circuit circuit operation in the case of forward power flow and Figure 9b shows the waveforms of the circuit operation in the case of forward power flow and Figure9b shows the waveforms of the circuit operation in the case of reverse power flow. It can be seen that when sag occurs, the rectified voltage operation in the case of reverse power flow. It can be seen that when sag occurs, the rectified voltage becomes lower and the current increases in order to supply constant power to the output side. becomes lower and the current increases in order to supply constant power to the output side. However, However, the DC voltage is maintained at 600 V through the voltage control of the DC–DC the DCconverter. voltage It iscan maintained also be seen at that 600 Vthe through output voltage the voltage and current control are of the maintained DC–DC converter.at constant Itvalues can also be seenregardless that the of the output sag, even voltage though and the current voltage are maintained is 2.5%, atwhich constant is higher values than regardless the ripple of 0.8% the sag, evenin though normal theoperation. voltage ripple is 2.5%, which is higher than the ripple of 0.8% in normal operation.

High‐voltage side Voltage AC 3.3kV

High‐voltage side

Current

Rectified voltage

Rectified voltage

DC voltage 600V

Low‐voltage side

Voltage AC 220V

Low‐voltage side

Current

(a)

Figure 8. Cont.

Energies 2016, 9, 668 10 of 17 Energies 2016, 9, 668 10 of 17 Energies 2016, 9, 668 10 of 17

High‐voltage side VoltageHigh‐voltage AC 3.3kV side Voltage AC 3.3kV High‐voltage side High‐voltage side Current Current

Rectified voltage Rectified voltage

Rectified voltage Rectified voltage

DC voltage 600V DC voltage 600V

Low‐voltage side Low‐voltage side Voltage AC 220V Voltage AC 220V Low‐voltage side Low‐voltage side Current Current (b) (b) Figure 8. Simulation results in normal operation: (a) forward power flow; and (b) reverse power flow. Figure 8. Simulation results in normal operation: (a) forward power flow; and (b) reverse power flow. Figure 8. Simulation results in normal operation: (a) forward power flow; and (b) reverse power flow.

High‐voltage side VoltageHigh‐voltage AC 3.3kV side Voltage AC 3.3kV High‐voltage side High‐voltage side Current Current

Rectified voltage Rectified voltage

Rectified voltage Rectified voltage

DC voltage 600V DC voltage 600V

Low‐voltage side Low‐voltage side Voltage AC 220V Voltage AC 220V Low‐voltage side Low‐voltage side Current Current (a) (a)

Figure 9. Cont.

Energies 2016, 9, 668 11 of 17 EnergiesEnergies 2016 2016, 9, ,9 668, 668 1111 of of 17 17

HighHigh‐voltage‐voltage sideside VoltageVoltage ACAC 3.3kV3.3kV

HighHigh‐voltage‐voltage sideside CurrentCurrent

Rectified voltage Rectified voltage

Rectified voltage Rectified voltage

DC voltage 600V DC voltage 600V

Low‐voltage side Low‐voltage side Voltage AC 220V Voltage AC 220V

Low‐voltage side Low‐voltage side Current Current

(b) (b) FigureFigure 9. 9.Simulation Simulation resultsresults in input input voltage voltage sag: sag: (a) ( aforward) forward power power flow; flow; (b) reverse (b) reverse power power flow. flow. Figure 9. Simulation results in input voltage sag: (a) forward power flow; (b) reverse power flow. 4. Hardware Experiment 4. Hardware Experiment 4. Hardware Experiment Figure 10 shows a prototype of single‐phase semiconductor transformer with a rating of 1.9 Figure 10 shows a prototype of single-phase semiconductor transformer with a rating of kV/127Figure V 210 kVA shows that a was prototype fabricated of singleand experimented‐phase semiconductor on in the lab.transformer A prototype with of a threerating‐phase of 1.9 1.9kV/127 kV/127semiconductor V V 2 2kVA kVA transformer thatthat was was fabricated fabricatedwith a rating and and ofexperimented experimented 3.3 kV/220 V on 6 on kVAin in the thewas lab. lab. fabricated A Aprototype prototype by coupling of ofthree three-phase ‐threephase semiconductorsemiconductorsingle‐phase transformerunits transformer in Y‐connection with with aa rating ratingand mounted ofof 3.33.3 kV/220 in a rack V V as6 6 kVAshown kVA was was in Figurefabricated fabricated 10. The by by couplinginput coupling voltage three three single-phasesingleand current,‐phase units units rectified in in Y-connection Y voltage‐connection and and current,and mountedmounted DC link in voltage a rack onas as shownthe shown high in in‐voltage Figure Figure side,10. 10 The. and The input the input voltage voltage voltage andandand current, current, current rectified rectified on the low voltage voltage‐voltage and and side current, current, were measured DCDC linklink voltage in real timeon on the the and high high-voltage indicated‐voltage on side, side,the topand and of the the voltage rack voltage andandfor current current monitoring on on the the purpose. low-voltage low‐voltage side side were were measuredmeasured in real real time time and and indicated indicated on on the the top top of ofthe the rack rack forfor monitoring monitoring purpose. purpose.

Figure 10. Single‐phase hardware prototype.

FigureFigure 10. 10. Single-phaseSingle‐phase hardware prototype. prototype.

Energies 2016, 9, 668 12 of 17 Energies 2016, 9, 668 12 of 17

Figure 11 showsshows a configurationconfiguration of thethe entireentire experimentalexperimental set-upset‐up forfor analyzinganalyzing thethe operationoperation characteristics of the three-phasethree‐phase 3.33.3 kV/220kV/220 V V 6 6 kVA kVA semiconductor transformer. ①1 is the three-phasethree‐phase semiconductor transformer fabricated in the laboratory. Y Y-connected‐connected three units of single-phasesingle‐phase semiconductor transformer are mounted on the lower part and a monitoringmonitoring system is mountedmounted onon thethe upperupper part;part. ②2 isis aa conventionalconventional 380380 V/3.3V/3.3 kV three-phasethree‐phase four-wirefour‐wire transformer forfor supplying supplying 3.3 3.3 kV kV power power to to the the semiconductor semiconductor transformer; transformer. 3 is ③ the is main the main PC and PC three and notebooksthree notebooks necessary necessary to download to download control control algorithms algorithms programmed programmed with C with code C for code the DSPfor the boards, DSP whichboards, are which mounted are mounted on each single-phase on each single semiconductor‐phase semiconductor transformer; transformer. 4 is a 16-channel ④ is a oscilloscope 16‐channel frequentlyoscilloscope used frequently for measurements used for measurements that enables checking that enables whether checking the data whether obtained the from data DSP obtained boards andfrom LabView DSP boards are accurate.and LabView are accurate.

Figure 11. Experimental set set-up‐up for 3-phase3‐phase hardware prototype.

The designeddesigned monitoring monitoring system system based based on LabView on LabView should should display atdisplay least six at items least of six information items of includinginformation the including input voltage the and input current, voltage rectified and voltagecurrent, and rectified current, voltage and output and voltagecurrent, and and current output of thevoltage semiconductor and current transformer. of the semiconductor If three-phase transformer. operation If is three involved,‐phase it operation should display is involved, at least it 18 should items ofdisplay information. at least So, 18 curve-fittingitems of information. technique wasSo, curve utilized‐fitting to display technique all the was data utilized on the monitoringto display screen.all the data Sinceon the the monitoring DSP board screen. currently being used in the lab does not have enough pulse width modulationSince the (PWM) DSP channels,board currently each single-phase being used semiconductorin the lab does transformer not have enough has two pulse DSP boardswidth formodulation the high-voltage (PWM) sidechannels, and the each low-voltage single‐phase side, semiconductor respectively. For transformer control and has monitoring two DSP boards purposes, for twothe high DSP‐voltage boards inside a single-phaseand the low‐voltage semiconductor side, respectively. transformer For should control be and connected monitoring through purposes, CAN communication.two DSP boards in One a single DSP‐ boardphase insemiconductor a single-phase transformer semiconductor should transformer be connected transmits through data CAN to communication. One DSP board in a single‐phase semiconductor transformer transmits data to LabVIEW through CAN communication. LabVIEW through CAN communication. Figure 12 shows the experimental waveforms of a three-phase 6 kVA bidirectional intelligent Figure 12 shows the experimental waveforms of a three‐phase 6 kVA bidirectional intelligent semiconductor transformer. The waveforms in Figure 12a are the three-phase input voltage and semiconductor transformer. The waveforms in Figure 12a are the three‐phase input voltage and current, three-phase rectified voltage and current, DC link voltage, and three-phase output voltage current, three‐phase rectified voltage and current, DC link voltage, and three‐phase output voltage and current in forward power flow when three-phase 3.3 kV was applied to a high-voltage side of and current in forward power flow when three‐phase 3.3 kV was applied to a high‐voltage side of resonant converter. The waveforms in Figure 12b are for the case of reverse power flow. resonant converter. The waveforms in Figure 12b are for the case of reverse power flow. In these figures, it can be seen that the input voltage and current are in the same phase, indicating In these figures, it can be seen that the input voltage and current are in the same phase, that the power factor is corrected and the rectified voltage and current are also in the same phase. indicating that the power factor is corrected and the rectified voltage and current are also in the same The DC link voltage is maintained at 600 V with negligible ripples, and the output voltage and current phase. The DC link voltage is maintained at 600 V with negligible ripples, and the output voltage of the DC–AC inverter are sinusoidal with relatively low level of harmonics. These experimental and current of the DC–AC inverter are sinusoidal with relatively low level of harmonics. These results are consistent with the simulation results shown in Figure8. experimental results are consistent with the simulation results shown in Figure 8.

Energies 2016, 9, 668 13 of 17 Energies 2016, 9, 668 13 of 17

(a)

(b)

FigureFigure 12. 12.Experimental Experimental results results in in normalnormal operation: ( aa)) forward forward power power flow; flow; (b (b) )reverse reverse power power flow. flow.

FigureFigure 13 13 shows shows the the waveforms waveforms obtainedobtained throughthrough simulations conducted conducted to to examine examine circuit circuit operations when sag occurs in the input voltage. Figure 13a shows the waveforms from the circuit operations when sag occurs in the input voltage. Figure 13a shows the waveforms from the circuit operation in the case of forward power flow. Figure 13b shows the waveforms from the circuit operation in the case of forward power flow. Figure 13b shows the waveforms from the circuit operation in the case of reverse power flow. It can be seen that when sag occurs, in order to supply operation in the case of reverse power flow. It can be seen that when sag occurs, in order to supply constant power to the output side, the rectified voltage becomes lower and the current increases. constant power to the output side, the rectified voltage becomes lower and the current increases. However, the DC voltage is maintained at 600 V through the voltage control of the DC–DC However, the DC voltage is maintained at 600 V through the voltage control of the DC–DC converter. converter. It can also be seen that the output voltage and current are maintained at constant values It can also be seen that the output voltage and current are maintained at constant values regardless regardless of the input voltage sag. The above results obtained through experiments are consistent of the input voltage sag. The above results obtained through experiments are consistent with the with the simulation results shown in Figure 9. simulation results shown in Figure9.

Energies 2016, 9, 668 14 of 17 Energies 2016, 9, 668 14 of 17

(a)

Figure 13. Cont.

Energies 2016, 9, 668 15 of 17 Energies 2016, 9, 668 15 of 17

(b)

Figure 13. Experimental results in input voltage sag: (a) forward power flow; (b) reverse power flow. Figure 13. Experimental results in input voltage sag: (a) forward power flow; (b) reverse power flow. The entire efficiency of a 6 kVA three‐phase semiconductor transformer was measured using a powerThe analyzer, entire efficiency which shows of a 6 approximately kVA three-phase 83% semiconductor in both forward transformer and reverse was power measured flows. Figure using a power14 shows analyzer, loss analysis which shows results approximately according to bidirectional 83% in both power forward flow, and where reverse “A” power shows flows.total losses Figure in 14 showsthe primary loss analysis side of results high‐voltage according AC–DC to bidirectional converter, “B” power shows flow, total where losses “A”in the shows secondary total side losses of in thehigh primary‐voltage side AC–DC of high-voltage converter, AC–DC“C” shows converter, total losses “B” of shows the DC–DC total losses converter, in the “D” secondary shows total side of high-voltage AC–DC converter, “C” shows total losses of the DC–DC converter, “D” shows total losses

Energies 2016, 9, 668 16 of 17 Energies 2016, 9, 668 16 of 17 lossesof the DC–ACof the DC–AC inverter, inverter, “E” shows “E” total shows losses total (adding losses A, B,(adding C, and D),A, “F”B, C, shows and theD), transformer“F” shows loss,the “G”transformer shows the loss, inductor “G” shows loss, and the “H” inductor shows loss, entire and losses “H” of shows the semiconductor entire losses transformerof the semiconductor (adding E, transformerF, and G). The (adding switching E, F, losses and G). of the The high-voltage switching losses AC–DC of converter,the high‐voltage and the AC–DC DC–DC converter, converter and theDC–AC DC–DC inverter converter are dominant and DC–AC due inverter to high switching are dominant frequency. due to high switching frequency.

Conduction loss Switching loss Conduction loss Switching loss 1250 1250 Forward power flow Reverse power flow

1000 1000

750 750 [W] [W] loss loss P 500 P 500

250 250

0 0 ABCDEFGH ABCDEFGH

Figure 14. LossLoss analysis analysis result for major element.

5. Conclusions Conclusions This paper describes experimental results results of a prototype of a three-phasethree‐phase 3.3 kV/220 V V 6 6 kVA kVA bidirectional semiconductor to analyze its performance. The The prototype prototype was configured configured by Y Y-connection‐connection of three units ofof single-phasesingle‐phase 1.91.9 kV/127kV/127 V V semiconductor semiconductor transformer. transformer. Each single single-phase‐phase unit unit has has three three units units of bidirectional of bidirectional half‐bridge half-bridge high‐frequency high-frequency resonant resonant AC–DC converter,AC–DC converter, one hybrid one hybrid-switching‐switching bidirectional bidirectional DC–DC DC–DC converter, converter, and and one one hybrid-switchinghybrid‐switching bidirectional DC–AC inverter. Three units of half half-bridge‐bridge high high-frequency‐frequency resonant AC–DC converter are series connected on the high high-voltage‐voltage side to evenly divide the input voltage into thirds, and are parallel connected on the low-voltagelow‐voltage side to increase the current. A hybrid-switchinghybrid‐switching scheme was utilized to reduce the switching loss in the DC–DC converter and the DC–AC inverter. In order to monitor thethe entire entire system system operation, operation, DSP boardsDSP andboards PCs wereand connectedPCs were with connected CAN communication with CAN communicationbased on LabView based to create on LabView a display systemto create to indicatea display the system input voltage to indicate and current, the input rectified voltage voltage and current,and current, rectified and outputvoltage voltage and current, and current. and output voltage and current. Through the results of diverse experiments, it could be seen that the developed three three-phase‐phase semiconductor transformertransformer operatedoperated properly properly during during normal normal operation operation and and under under input input voltage voltage sag. sag.However, However, the system the system efficiency efficiency is rather is lowerrather at lower approximately at approximately 83%. Therefore, 83%. Therefore, the system the efficiency system efficiencyshould be should enhanced be toenhanced at least 90%to at by least adopting 90% by optimum adopting designs optimum and replacingdesigns and the existingreplacing IGBT the existingwith a new IGBT switching with a new device switching such as device an SiC such device. as an SiC device. The fabricated prototype of a three three-phase‐phase semiconductor transformer confirms confirms the possibility of commercialization withwith a highera higher voltage voltage rating rating by increasing by increasing the number the number of converter of converter units connected units connectedin series at in high-voltage series at high side‐voltage and in side parallel and at in low-voltage parallel at low side.‐voltage In addition side. In the addition proposed the three-phase proposed threesemiconductor‐phase semiconductor transformer hastransformer advantages has in advantages fabrication in and fabrication maintenance and because maintenance it is designed because with it is designeda modular with structure. a modular structure. Acknowledgments: This work was supported by the National Research Foundation (NRF) grant funded by the Acknowledgments: This work was supported by the National Research Foundation (NRF) grant funded by the Korea Ministry of Science, ICT and Future Planning (NRF-2015R1A2A2A01004102).(NRF‐2015R1A2A2A01004102). Author Contributions: ByungByung-Moon‐Moon Han and Jun Jun-Young‐Young Lee proposed the original idea and carried out the main research tasks. Do Do-Hyun‐Hyun Kim carried out computer simulations and experiments. Byung Byung-Moon‐Moon Han and Jun-Young Lee wrote the full manuscript and supervised the simulations and experiments. Jun‐Young Lee wrote the full manuscript and supervised the simulations and experiments. Conflicts of Interest: The authors declare no conflict of interest. Conflicts of Interest: The authors declare no conflict of interest.

Energies 2016, 9, 668 17 of 17

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