Modular Hybrid Solid State Transformer (H-SST) for Next Generation Flexible and Adaptable Large Power Transformer (LPT)

TRAC Program ReviewUS Department of Energy, Office of Electricity

Presented at Oak Ridge National LaboratoryOak Ridge, TN

8/13/2019

Professor Alex Q. HuangDula D. Cockrell Centennial Chair in Engineering Director, Semiconductor Power Electronics Center (SPEC)University of Texas at Austinaqhuang@utexas.edu

2

Project Overview• Project summary:

• Develop and demonstrate a modular Hybrid Solid State Transformer (H-SST) for next generation Flexible and Adaptable large power transformer (LPT).

• Demonstrate advanced control functions of the H-SST that is currently not available in traditional transformers.

• Total value of award (federal + cost share): $2.16m($1.73m/$433k)• Period of performance: 3/18/2019-3/217/2021• Project lead and partners

500kVA HSST

ACAC

20kV, 60Hz

160:45LFT

4.5kV, 60Hz

HFSST

6MVA 138kV/35kV LPT based on 500kVA HSST

Project Plan

4

[1] Electricity grid simple- North America" by United States Department of Energy, SVG version by User:J Jmesserly - http://www.ferc.gov/industries/electric/indus-act/reliability/blackout/ch1-3.pdf Page 13 Title:"Final Report on the August 14, 2003 Blackout in the United States and Canada" Dated April 2004. Accessed on 2010-12-25. Licensed under Public domain via Wikimedia Commons http://commons.wikimedia.org/wiki/File:Electricity_grid_simple-_North_America.svg#mediaviewer/File:Electricity_grid_simple-_North_America.svg

Electricity grid in North America. Source: [1]

Transformer History:invented in 1886 by William Stanley

Split phase 120/240V

Distribution grid transformer (pole mounted)

• Designed for unidirectional power flow and century-old transformer technology with little controllability• Requires a wide spectrum of products for power quality improvement (SVC, active filter, voltage regulator, DVR,

etc.)• Strong coupling and won’t isolate harmonics/other disturbances • Not friendly for integration of renewable energy source (DC-typed sources need more conversion stages,

synchronization), EV, electronic load

William Stanley

Need for advanced control function and flexibility

MVAC

Type A-1

LVAC

Type B

Type C

Type D

Type E

MVAC LVDC

MVAC

MVAC

MVAC

LVDC

MVDC

LVRAC

LVAC

LVAC

LVDC LVAC

LVDC

MF

MF MF

MF MF

MF

MF

MF

MF

MF

Type A-2

MVAC LVHAC LVACMF LFLFMVHAC

Power Electronics Solutions: Solid State Transformer5

Cell 1

Cell 2

Cell N

MVAC

LVAC

ISOP

Cell 1

Cell 2

Cell N

MVAC LVAC

IPOP

Cell 1

Cell 2

Cell N

MVAC LVAC

ISOS

Xu She, Alex Huang, “Review of Solid state Transformer in the Distribution system: From components to Field application,” in Energy Conversion Congress and

Exposition (ECCE), Raleigh, NC, 2012, pp. 4077-4084.

Modular Based Configuration, in which low

voltage converters or devices are connected in

series to share the voltage and power. Each of the

cell can be type A to type E.

Previous SST Prototypes: Distribution grid focus

60Hz Transformer

Gen- 1 SST: Si-based (6.5 kV IGBT 3kHz)

Gen- 2 SST: SiC-based (15 kV SiCMOSFET 10 kHz)

Gen- 3 SST:SiC @ 40 kHz

Wide Bandgap DevicesControls

36"

25

"

17"

Topology/magnetics

Fuse+Relay

Rectifier

CrpTransformer

MV MOS

LV MOS

Li

MV AC Input

LV AC Output

Control board

Unfolding Bridge

Lo

MOV

Crs

24"

30

"

10"

7.2 kV single phase transformer for residential node (US market)

Modular Type D Type D Type A-2

SST based on Direct AC-AC Conversion (Type A-2)

7

Advantages High efficiency: one stage of high frequency power conversion High power density: 40kHz~100kHz+no dc capacitors Only two MF MV MOSFETs and ZVS guaranteed Current limit capability under over load conditions Minimized system stored energy

P2

P1

Crp2

S1

S2

D1

D4

D3

D2

Lr

ir

vMV

Lg

Co

vin

vin

Crp1

Diode Rectifier Bridge MV MF Series Resonant Converter

n:1 Crs2

Crs1

Lm

Lo

R5

R8

R7

R6

imvLV

LF Unfolding Bridge

vo

vo

D1

D2

MV LV

Li

Reliability + Efficiency

Efficiency

Power Density

FunctionalitySafety

Type A-2: (Uni-directional)• Use diode bridge to replace MOSFET bridge: cost effective if only unidirectional power

flow is needed. Only two HV SiC MOSFETs needed

Q. Zhu, L. Wang, A. Huang, K. Booth and L. Zhang, "7.2 kV Single Stage Solid State Transformer Based on Current Fed Series Resonant Converter and 15 kV SiC MOSFETs," in

IEEE Transactions on Power Electronics.

Measured Efficiency (MV AC- LV AC)

8

Input: 7.2 KVOutput: 240V

60 Hz transformer efficiency

9

Conclusion: single phase SST is approaching similar LFT efficiency!Challenge: How to sale this to transmission application

10

Hybrid SST: trade-off between power level and functionality

ACAC

20kV, 60Hz

160:45LFT

4.5kV, 60Hz

HFSST

ACAC

20kV, 60Hz

160:45LFT

4.5kV, 60Hz

HFSST

ACAC

20kV, 60Hz

160:45LFT

4.5kV, 60Hz

HFSST

ACAC

20kV, 60Hz

160:45LFT

4.5kV, 60Hz

HFSST

C1: Input parallel and output parallel

C2: Input parallel and output series

C3: Input series and output parallel

C4: Input series and output series

Flexible partial power control V1/V2=n1/n2 No voltage regulation 20kV devices 100kV insulation

Partial power Voltage regulation No 20kV devices 100kV insulation

Partial power No 20kV devices V1/V2=n1/n2 No voltage regulation 100kV insulation

Partial power Voltage

regulation 20kV devices 100kV insulation

ACAC

20kV, 60Hz

160:45LFT

4.5kV, 60Hz

HFSSTC5: Primary side cascaded

C6: Primary side input parallel and output series C7: Secondary side

cascaded

C8: Secondary side input parallel and output series

20kV, 60Hz

160:45LFT

4.5kV, 60Hz

AC

AC

20kV, 60Hz

160:45LFT

4.5kV, 60Hz

AC

AC

20kV, 60Hz

160:45LFT

4.5kV, 60Hz

ACAC

HFSST

Voltage regulation No transformer 20kV devices No partial power

Partial power Voltage

regulation 20kV devices 100kV

insulation

Voltage regulation No 20kV devices No transformer No partial power

Partial powerLow voltage insulationVoltage regulationNo 20kV devices

11

Preferred Hybrid-SST Solution: Secondary side IPOS configuration

ACAC

20kV, 60Hz

20:3.5LFT

3.0~4.0kV, 60Hz

HFSST

3.5kV, 60Hz

-0.5~0.5kV, 60Hz

Vout(kV)

Vout_SST(kV)

Vout(kV)

Pout_SST(kW)

83kW

62.5kW

Proposed DAB SST

Partial powerLow voltage insulation (buck and Boost)Voltage regulationNo 20kV devices

Low SST output voltage

Partial power @500 kVA output power

12

HSST Control Strategy: Constant Frequency with Single Phase-Shift

irefiref_ m

iout

vg Phase_ shift

sin(Ɵ)

Gci

PLL

+_

Phase_ FF

+

+

d= 0.5, f= 15kH z d= 0.5, f= 15kH z

I f Phase_ shift> 0, power is from H V t o LV ;

I f Phase_ shift< 0, power is from LV t o HV .

d= 0.5, f= 60H z d= 0.5, f= 60H z

Vgrid(HV)

Vgrid(LV)

13

Simulation Results (1): Power from HV Side to LV Side

Power from HV side to LV side, full power, 500kW

Control: constant frequency (15kHz) with constant phase shift (9.93o),

Vin=20kV,Vinsst=3.5kVVout=4.0kV, Voutsst=0.5kV

𝑷 ∝ 𝐏𝐡𝐚𝐬𝐞_𝐒𝐡𝐢𝐟𝐭

Power from HV side to LV side, half power, 250kW

Control: constant frequency (15kHz) with constant phase shift (0.5*9.93o),

Vin=20kV,Vinsst=3.5kVVout=4.0kV, Voutsst=0.5kV

𝑷 ∝ 𝐏𝐡𝐚𝐬𝐞_𝐒𝐡𝐢𝐟𝐭

14

Simulation Results (2): Power from HV Side to LV Side

Power from LV side to HV side, full power, 500kW

Control: constant frequency (15kHz) with constant phase shift (-9.0o),

Vin=20kV,Vinsst=3.5kVVout=4.0kV, Voutsst=0.5kV

𝑷 ∝ 𝐏𝐡𝐚𝐬𝐞_𝐒𝐡𝐢𝐟𝐭

Power from LV side to HV side, half power, 250kW Control: constant frequency (15kHz) with constant phase shift (-

0.5*9.0o),

Vin=20kV,Vinsst=3.5kVVout=4.0kV, Voutsst=0.5kV

𝑷 ∝ 𝐏𝐡𝐚𝐬𝐞_𝐒𝐡𝐢𝐟𝐭

HSST: Standardized Design for Multiple LPT Constructions

LPT Voltage 138kV/115kV 138kV/69 kV 138kV/35

kV

138kV/4

kV

Input configuration Series and parallel Series and

parallel

series series

Desirable H-SST input voltage 20 kV 20 kV 20 kV 20 kV

Number of H-SST at the input per phase 4+4+4+4=16 4+4=8 4 4

Output configuration Series Series series parallel

Desirable H-SST output voltage 4 kV 5 kV 5 kV 4 kV

Number of H-SST at the output per

phase

16 8 4 4

Minimum LPT power rating 24 MVA 12 MVA 6 MVA 6 MVA

• Modular configurations utilized to achieve various 138 kV LPTs• 6 MVA 138kV/35 kV LPT based on the 500 kVA H-SST.

16

7.2kV/60A Austin SuperMOS (1)

Features:• High blocking voltage with low on-resistance (<200 mΩ)• High speed switching (dV/dt >120kV/us) with low capacitances• Simple to drive and easy to be parallel • ZVS switching achievable • Integrated gate driver DESAT protection, UVLO protection, and Over

temperature protection• Integrated isolated power supply with 20kV insulation capability

Turn-off waveform at 4kV/80ATurn-off waveform at 5kV/40A

Picture of the 7.2kV/60A Austin SuperMOS

17

7.2kV/60A Austin SuperMOS (2)

10 20 30 40 50150

160

170

180

190

200

Rds,

on (

mo

hm

)

Drain-to-source current (A)

Conduction resistance of the 7.2kV/60A Austin SuperMOSunder different drain-to-source current @RT

-20 -15 -10 -5 0 5 10

-45

-30

-15

0

15

30

45I

DS(A)

VDS

(V)

Forward IV @Vgs=20V @RT

Reverse IV @Vgs=20V @RT

Reverse IV @Vgs=0V @RT

I-V curves of the 7.2kV/60A Austin SuperMOS @RT

18

7.2kV/60A Austin SuperMOS (3)

Parameter Value Parameter Value

Rated Voltage 7.2kV Eon @5kV/10A 15.5mJ

Rated Current60A@100oC

90A@25oCEoff @5kV/10A 1.2mJ

Rdson <200 mΩ @25oC Eoff @5kV/40A 1.8mJ

Qoss@5kV 1584nC Coss@5kV 159pF

1

10

100

100 1000 10000

Ro

n,s

p (

mΩ

-cm

2)

Breakdwan Voltage (V)

SiC Unipolar Device Rdson,sp

Cree 1.2kV Gen2 MOSFET

USCi 1.2kV 45mΩ JFET

Cree 6.5kV Gen3 MOSFET

Cree 10kV Gen3 MOSFET

SiC

1-D

Lim

it at

25°

C

Si 1

-D L

imit

at 2

5°C

Cree 15kV Gen3 MOSFET

Cree 3.3kV MOSFET

FREE

DM

Sup

er-c

asco

de 7.2kV Austin SuperMOS

Figure of Merit (FOM) of SiC unipolar devices

0 1000 2000 3000 4000 50000

200

400

600

800

1000

1200

1400

1600

1800

Qoss

Coss

Voltage (V)

Qoss

(nC

)

10

100

1000

Coss (p

F)

Output charge and output capacitance of the 7.2kV Austin SuperMOS

19

Primary Side Full Bridge Setup

• 3kV film capacitors are used in series to construct the 5kV dc link.• DC+, DC- and the midpoint cable entry points are located on the

bottom layer of the PCB• Dimensions are 262mm x 240mm x 168mm

Q1

Q4

Q3

Q2

AC1

AC2

20

Gate Driver Power Supplies with High Isolation Voltage

• 10kV isolation• Series LLC resonant circuit• Secondary side circuit included inside integrated module• Primary winding is a HV wire that loops around 4 toroidal cores

included inside 4 SuperMosfet modules of the primary full bridge• Transfer capacitance = 2pF• Input = 15Vdc; Output = 24V dc• High insulation capability realized by 3D printed bobbin which

separates the primary hv wire and secondary winding.• Switching frequency = 235kHz

21

Low Voltage Side Power Stage

Rohm SiC modulesInfineon IGBT modules

• Secondary Side dimensions : 400 x 270 x 130mm• Includes Interface Board, which allows for optical control

from the controller , thus achieving high level of isolation• Interface Board also responsible for

• Output Contactor Control• Sensor signal conditioning• Driver Fault detection and shutdown

1200V, 8mohm

1200V,200A, 1.2V600A, 2V

22

High-power Medium-frequency Transformer

Version 1 transformer

Core Material:FINEMET®FT-3TL

150 * 125 * 170mm

Turn ratio: 49:7

Magnetic inductance_HV/LV: 276.5mH/5.88mh

Leakage inductance_HV/LV: 78.3uH/1.86uH

0

2

4

6

8

10

12

14

0.1k 1k 10k 20k 30k 40k 50k 60k 70k 80k 90k 100k

Low voltage side AC resistance

0

100

200

300

400

500

600

700

0.1k 1k 10k 20k 30k 40k 50k 60k 70k 80k 90k 100k

High voltage side AC resistancemΩ mΩ

f f

23

Transformer insulation design and Partial discharge test

Dielectric Breakdown 20,292 volts

Dielectric Strength -1194 volts/mil

Heat Shrink Thin Wall Tubing

Dielectric Strength>20kV / mm

Insulation sheet

AC RMS 2.5kV 60HZ

PDIV is 2.5kV, Improved design for higher insulation voltage is needed.

Version 2

Inner-layer bobbin

Outer-layer bobbin

Version 2 design is planned Using inner bobbin to enhance

the insulation

24

100 kVA DABSST Converter 3D drawings (Alpha Design)

• Tentative Arrangement yields an SST of dimensions : 1250 x 270 x 170 mm• However, since the LFT is considerably bigger (1575 x 1524 x 2032 mm ), the SST should be able to fit in

the LFT enclosure itself (drawing on next slide)

Preliminary MV Test Result (Vin=3.6 kV/Vout=103V)

DABSST operation with 3.6kV input (blue 2kV/div.) and 103V output (green 50V/div.) and grid current (red 1A/div.).

Transformer primary voltage (blue), primary winding current (red) and secondary voltage (green).

26

Estimated Loss Breakdown and Efficiency

HV rectifier

HV Conduction

HV Switching

LV unfolding

LV Conduction

LV switching

Transformer

Inductor

TOTAL 1509W

DAB SST total loss @0.5kV, 62.5kW output

0.96

0.965

0.97

0.975

0.98

0.985

0.99

0 10000 20000 30000 40000 50000 60000 70000

Efficiency

0.96

0.965

0.97

0.975

0.98

0.985

0.99

0.995

0 100000 200000 300000 400000 500000 600000

Efficiency

HSST

DAB SST

• LFT efficiency estimated based on 500 kVA prototype from Control Transformer

27

500kVA HSST drawing • 500 kVA LFT order in place• 100 kVA DABSST is 80% complete• DABSST modeling and control finished• Next step

• Contract in place for all subs (task 1)• Line frequency model of the DABSST (Task 5)• System level modeling of LPT (Task 5)• HSST monitoring strategy (Task 4)• Improved isolation capability of DABSST transformer (Task 2)

DELIVERABLES (*assume 3/18/2019 start date)

Deliverable

Planned

Completion

Date

Status (8/2019)

Alpha 100 kVA SiC DABSST 9/30/2019 80%

Beta 100 kVA SiC DABSST 3/31/2020 0%

500 kVA Hybrid Solid State Transformer 9/30/2020 50%

Monitoring and failure/fault detection

platform

12/31/2019Started, 10%

Control, modeling and simulation analysis of

LPT based on H-SST

12/31/2020Started, 10%