Solid State Transformer and FREEDM System Power Management Strategies

1

Prof. Alex Q. Huang, Progress Energy Distinguished ProfessorNSF FREEDM Systems Center

NC State University

Presentation Outline

1. FREEDM System: A Resilient and Smart Distribution

System

2. Solid State Transformer

• Three Generations of FREEDM Developed SST

Technology

3. Advanced Power Management Functions of the SST

4. FREEDM System Intelligent Power Management (IPM)

Control

5. Towards Energy Internet

Low Voltage Power Delivery System at the Edge of ICT: Power Electronics Impact in the last 50 Years

PowerFactor

Correction

DC/DCConverter

CPUPower

SystemPower

MemoryPower

GraphicsPower

400V (DC)

< Front end AC/DC >

< On board DC/DC >

60-264V (AC)

12V (DC)

(DC)

(DC)

(DC)

(DC)

• Plug-and-play of efficient power • Unprecedented power quality• Isolation of load dynamics by POL• (Point of Load) converters• Necessary for DC loads

3

POLs

Silicon power device’s most important contribution(BV < 600VSi BJT, Si MOSFET to GaN HFET)

BV<30VSi MOSFETGaN HFET

BJT: 1947 1970: SMPS 1980: power MOSFET 2010: GaN HFET

High Voltage and Medium Voltage Power Delivery System: Grid Edge Control Challenge

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 beyond substation• Requires a wide spectrum of products for power quality improvement (SVC,

active filter, voltage compensator, 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

Direct medium voltage power conversion

PowerFactor

Correction

DC/DCConverter

Load

Renewable Energy

Generation

Energy Storage

Others

MV DC

< Front end AC/DC >

4160-69 kV (AC)

LV DC

(400V)

Direct medium voltage power conversion [1]•power conversion without 60 Hz transformer •Smart functions for enhanced grid edge control

Inverter

LV AC

5

Solid State Transformer

Addressing climate change grand challenge: higher penetration of distributed generation/storage/renewable hence the need for better control (µs~hour)

[1] Huang, NSF ERC proposal, 2007

FREEDM System Physical Architecture

Bi-directional flow

Solid State Transformer 380 VDC Bus

6

Storage DG

Plug-and-play DC or AC Microgrid (Energy Cell)

Fault Isolation Device

120V/240V AC

StorageDG

DGISoftware

A resilient grid and an ubiquitous IT node are needed for future grid

7

SST for A Complete DC Grid

Plug-and-play DC Microgrid

Storage DG

software

software

In this case SST is absolutely needed since there is no 60Hz DC Transformer

Medium Voltage (MV) SST Technical Approach & Research Areas

8

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

"

SiC & GaN power devices, other wide bandgap

Topology/magnetics

Vision:(plug-and-play and resilient grid for high DER penetration)

WBG power device packaging

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

"

9

SST Grand Challenge: High Input Voltage

• 1968: Si bipolar transistor breakdown voltage is only ~100V• Also the topology is not directly usable due to voltage overshoot etc

????

10

Isolated

ZVS DC/DC

Converter

DC/AC Vg

Distribution grid

LV ac feeder

LVDC Link

capacitor

Medium voltage Low voltage

AC/DC

MVDC Link

capacitor

Medium Frequency Transformer for galvanic isolation

Three Stage Medium Voltage SST

11

SST Grand Challenge: High Input Voltage

+

-

+

-

3.8kV

DC

+

-

+

-

400V

DC

3.8kV

DC

SH5

SH7

SH6

SH8

S1

S3S4

SH1

SH3SH4

SH2 S2

Low Voltage

H-Bridge

+

-

+

-

400V

DC

High Frequency

Transformer

AC/DC Rectifier DC/DC Converter DC/AC Inverter

High Voltage

H-BridgeHigh voltage

H-Bridge

3.8kV

DC

7.2 kV AC

120V /

240V AC

LLs

Cs

CsLs

Port 1

Port 2

400V DC

+

-

Port 3

Gen-I SST 6.5kV Si IGBT based

Input: 7.2kVac Output: 240Vac/120Vac; 400Vdc Power rating: 20kVA

Increasing Voltage

Three stage power conversion

6.5-kV / 25-A Si IGBT

Wang et. al, “Design and hardware implementation of Gen-1 silicon based solid state transformer," APEC 2011

WBG Material Advantages

12

Low on-state losses

0

1

2

3

4

Electric Field

(MV/cm)

Energy Band Gap

(eV)

Thermal

Conductivity

(W/cm-K)

Electron

Saturation

Velocity

(10^7cm/s)

Electron Mobility

(10^3cm^2/s)

Si SiC

High voltage capability

High temperature operation

GaN

SiC Capability

13

Source: Vipin Pala et. al, ECCE2014

Theoretically about 1000X reduction over Si MOSCurrently about 100X reduction achieved for >600V rangeThe improvement is only about 10X over SJ devices!

SJ

10X reduction

GaN

100X reduction

10X increase

25℃���

15kV SiC GTO, n-IGBT and MOSFET

SiC bipolar devices are more suitable for high temperature operation

15

Much lower switching loss compared with 6.5 kV IGBT;

Up to 8 kHz even in hard switching condition.

Turn-on loss can be eliminated in ZVSoperation

15 kV SiC MOSFET Switching Losses

16

Q1

Q2

Vin

A

Idc

V

Rd

Rs

Zi

DUT

Vs

iQ

CdcVdc

+

-

High Voltage

Half-Bridge Output Charge Measurement System

0

200

400

600

800

1000

1200

0 2000 4000 6000 8000

Qo

ss / n

C

Vdc / V

Output Charge of 15kV SiC MOSFET & JBS

fs=0.625k

fs=1.25k

(V ) 11.43 V 0.0406 (nC)oss ds dsQ

Output charge model :

∆ Simple configuration

∆ High accuracy (<1% error)

∆ No special equipment needed

∆ High voltage (up to kV)

15 kV SiC MOSFET Output Charge Measurement and Modeling

SiC MOSFETSoft

Switching

With SiC MOSFET: 15 to 300 X improvements

BreakdownVoltage (kV)

12

0.1 1 10 100

Maximum switching frequency (kHz)

4

8

SiC MOSFETHard Switching

Silicon bipolar

5 MHz-V (thermally limited)

16

90 Mhz-V 1.5 GHz-V

Figure of Merit=BV*fsw

Gen-II SST Specifications:• Input: 7.2kVac• Output: 240Vac/120Vac; 400Vdc• Power rating: 20kVA

Tested Condition:• Input: 3.6kVac• Output: 240Vac; 400Vdc• Power rating: 10kVA

18

Three stage power conversion

12 kVdc

Three Stage SST with DAB Isolated DC-DC Converter

19

95.5

96

96.5

97

97.5

98

0 5 10 15

Eff

icie

nc

y

Power(kW)

DAB DC-DC Efficiency10kHz

15kHz

20kHz

93.5%

94.0%

94.5%

95.0%

95.5%

96.0%

96.5%

97.0%

97.5%

98.0%

98.5%

99.0%

99.5%

100.0%

0 10 20 30 40

Eff

icie

nc

y

Power(kW)

AC-DC Rectifier Efficiency

6 kHz

12 kHz

98.0%

98.2%

98.4%

98.6%

98.8%

99.0%

99.2%

99.4%

0 5 10 15

Eff

icie

nc

y

Load(kW)

DC-AC Inverter Efficiency

1.2kV Si IGBT

600V Si IGBT

1.2V SiC MOSFET

Measured Efficiency of Three Stage SST with DAB Isolated DC-DC Converter

20

P2

P1

Cmv2

S1

S2

R1

R4

R3

R2

Ls

ir

vMV

Lg

Clv

Cmv1

AC-DC Rectifier Series Resonant DC-DC Converter

Medium Voltage Low Voltage

n:1 Crs2

Crs1

Lm

R5

R8

R7

R6

imvLV

DC-AC Inverter

Gen-II SST Specifications:• Input: 7.2kVac• Output: 240Vac/120Vac; 400Vdc• Power rating: 20kVA

Tested Condition:• Input: 3.6kVac• Output: 240Vac; 400Vdc• Power rating: 10kVA

Three stage power conversion

15KV/10A SiC MOSFET 1.2KV/100A SiC MOSFET

Improved Three Stage SST with SRC Isolated DC-DC Converter

21

96.0%

96.2%

96.4%

96.6%

96.8%

97.0%

97.2%

97.4%

97.6%

97.8%

98.0%

98.2%

98.4%

0 2 4 6 8 10 12 14

Eff

icie

nc

y

Power(kW)

DC-DC Stage Efficiency

10 kHz DAB

40 kHz Series Resonant Converter

Medium Voltage Isolated DAB vs SRC

• Soft Switching Over Full Load range• Improved medium frequency transformer design

22

Isolated

ZVS AC/AC

Converter

Vg

Distribution grid

LV ac feeder

Medium voltage Low voltage

Medium Frequency Transformer for galvanic isolation

Single Stage Medium Voltage SST

23

P2

P1

Crp2

S1

S2

R1

R4

R3

R2

Ls

ir

vMV

Lg

Co

vin

vin

Crp1

Unfolding Bridge Series Resonant Converter

Medium Voltage Low Voltage

n:1 Crs2

Crs1

Lm

Lo

R5

R8

R7

R6

imvLV

Unfolding Bridge

vo

vo

Gen-III SST Specifications:• Input: 7.2kVac• Output: 280Vac• Power rating: 20kVA

Tested Conditions:• Input: 3.6kVac• Output: 280Vac• Power rating: 10kVA

Single stage power conversion

15KV/10A SiC MOSFET 1.2KV/100A SiC MOSFET

Single Stage Medium Voltage SST

Gen-III SST Major Achievement:Vin=4.2 kV AC, Po = 8kW

24

4.2kV AC – 280V AC Experimental Waveforms @ 8kW

Crp1

Q1

Q2

Lm

Q3

n:1

Q4

Ls

Crp2

Crs1

Crs2

Lo

Co

+

-

+

-Vds2

Vin

Vo

ir

+

-

Gen-III SST Prototype(24inch*30inch*10inch)

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

"

Three Generations of SSTs

25

60Hz Transformer

Gen- 1 SST: 6.5 kV IGBT @ 3 kHz

Gen- 2 SST: 15 kV SiC@ 6 kHz & 40 kHz

Gen- 3 SST:15 kV SiC@37 kHz

36"

25

"

17

"

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

"

• Gen-1 and Gen-2 features three stage solutions, and have all the desired smart functions• Gen-3 is a single stage solution, the focus is on cost and efficiency

Transformation of the distribution power grid with smart Solid State Transformers

84

86

88

90

92

94

96

98

0 2000 4000 6000 8000 10000

Eff

icie

ncy /

%

Load / W

Efficiency vs Load

Three Generations of SST Measured Efficiency (MV AC- LV AC)

26

Gen II with DAB

Gen III

• Much higher efficiency through single stage, improved magnetics and soft switching• 2% improvement compared to Gen-II SST, 7% over Gen-I• Reached >97% for the MV AC-AC SST

Gen-I: <90%

Gen II with SRC

Vin=3.6 kVac,rms

Three Generations of SST Power Density Vs Efficiency

27

80.0%

82.0%

84.0%

86.0%

88.0%

90.0%

92.0%

94.0%

96.0%

98.0%

0.25 0.45 0.65 0.85 1.05 1.25

Eff

icie

ncy

Powerdensity (W/in3)

Power Density Vs Efficiency

36"

25

"

17

"

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

"

28

SST Winning Strategy for Smart Grid

•Fault management•Current limiting•Disconnect/reconnect

•Power Management:1. Control power factor/Var Injection2. Change/Control customer voltage 3. Low voltage ride through4. Eliminate customer side harmonics5. Provide DC power/Forming DC

Microgrid6. Bidirectional Power flow control via

Energy Cell aggregation7. Supports advanced power

managements and islanding modes

•Energy Management•Monitor energy usage (AMI)•Can control/dispatch power via microgrids (Energy Cell)•Demand side management

Microsecond

Hours

SST

Utility Distribution Feeder

LV DC 380V

LVAC 120/240VOr 3phase 480V

Software Communication(IEC61850, DNP3,Modbus)

contr

ol

29

Isolated

ZVS DC/DC

Converter

DC/AC Vg

Distribution grid

LV ac feeder

LVDC Link

capacitor

Medium voltage Low voltage

AC/DC

MVDC Link

capacitor

Three Stage Medium Voltage SST: Supports Many Power Management Smart Features

2: Load regulation

5. Forming DC Microgrid6. Power flow control3. Temporary energy

For LVRT

4. Absorb any AC side harmonic currents

1. Independent Q junction

7. Disconnect from grid to form islanding operations

30

Reactive power compensation: 3.6KV-120V,

5KW, 2KVar (Gen-I SST Result)

Inductive mode operation

Capacitive mode operation

Load PF=1

Load PF=1

1.

Grid Voltage Support Via Q injection

1: SST rectifier in diode mode (1.8kV distribution grid voltage )

– 2: SST starts with 1kW load

– 3: Grid voltage drops 10%

– 4: SST generates reactive power to restore Vpcc 31

1.

32

Load regulation: : 3.6KV-120V, 5KW-3KW (Gen-I)

Output DC voltage, AC voltage and current

2.

Load step down

AC Voltage

DC Voltage

33

Smart Feature: Low Voltage Ride Through (LVRT)

Input voltage, current, PWM voltage, high voltage DC link 3Output DC voltage, AC voltage and current

Gen-I SST

120V AC

3.6 kV AC

5 kW LOAD

3.

Smart Feature: Load Harmonic Mitigation

34

Grid voltage

Grid current

LV ac voltage

LV ac current

Test setup and condition

Grid voltage: 1.8kV

HV dc link voltage: 3kV

Low voltage dc output: 200V

Low voltage ac output 120V

APF

4.

Grid-Connected Operation:3.6kV steady state/DC Microgrid

• Grid voltage: 3.6kV

• MVDC link voltage: 6kV

• Low voltage dc output: regulated at 400V

35

3.6kV grid voltage

Grid current

High voltage dc link (half)

400V low voltage dc output

5.

PF=1

Grid-Connected Operation:Regenerative mode/bidirectional power flow

• Grid voltage: 900V

• MVDC link voltage: 1.5kV

• Low voltage dc: 100V

36

Filter REC DC/DC INVLV AC

MV AC

SST

LV DC DCcurrentsource

Load

LV dc current (from positive to negative)

LV dc voltage

MV grid voltage MV grid current

6.

37

Soft Start

SST Power Flow Control: Control DC Bus Voltage As a Aggregation Signal

Energy cell

SST“Energy Router”

6.

SST and DC Microgrid Testbed

38

Gen-I SST

switch

Load bank

120 W PV panel20Ah battery

500 w DC/DC Converter for battery

200 w DC/DC converter for PV

Key hardware parameters

FREEDM Control Classification

SST1

SST2 SST3

69 KV 12.45 KV

FID 1 FID 2

120 V AC

380 V DC

Residential

AC system

Residential

DC systemDC microgrid

AC microgrid380 V DC

Residential

DC systemDC microgrid

120 V ACResidential

AC system

AC microgrid

Communication line

Power line

Control center

Grid

FID 3

Key features of the innovative FREEDM system:

1. Plug-and-play AC and DC Microgrid that integrate distributed renewable

energy resources (DRER) and distributed energy storage devices (DESD).

2. Intelligent power management (IPM) through high bandwidth SST.

3. Intelligent fault management (IFM) with ultra-fast and intelligent FID.

4. Intelligent energy management (IEM) via coordinated optimization and

dispatch of distributed resources. Slower communication can be used

Intelligent Power Management (IPM) Control of FREEDM System

Operation modes and transitions

IPM Challenge: operate each component/subsystem in a distributed fashion

while maintaining system stability under all operation conditions

Mode I:

FREEDM system

in grid-connected

Mode II:

FREEDM system

in islanding

Mode III:

SST in

islanding

Case 1: Substation

SST detects grid faults and

disconnects

Case 3: MVAC bus

under frequency

Case 4:Substation SST synchronizes MVAC bus to

grid and connects

Case 5:Substation SST restores MVAC bus and SSTs

reconnects

Case 2

IPM Objective:

Voltage & Frequency Regulation

Autonomous and Distributed IPM Control Strategy

• IPM control has higher priority than IEM

• Encompasses L1 to L3 level of control

• L1 (energy cell) devices have it own control

• L2 (SST) has its own control

• Require no communication

• Benefit from the inherent control bandwidth of the underlining power electronics

L1

L2

L3

Dual Droop Control Strategy for DC Microgrid

Dual droop control:

First droop for SST: f-P droop (AC frequency to SST active power) and P-

Vldc droop (active power to DC voltage) combine to form the first droop: f-

Vldc (AC side frequency to DC side voltage); SST acts like a real

transformer, only lets power flow through it.

Second droop for DC microgrid:

DESD: Vldc-P droop; affording/absorbing power to/from SST according

to the Vldc (upper level AC frequency f indeed);

DRER: normally MPPT control; Vldc-P droop when Vldc is too high;

Load: shedding at low Vldc point;

DESD, DRER are the only energy sources in system; the working point is

determined by loads and available sources.

Simulated FREEDM System

Battery1

Substation

SST

Battery2 Load1

380VDC

GRID

DC Microgrid DC Microgrid

12.47kV MVAC bus

SST2SST1

PV1 PV2

380VDC

Battery3 Battery4 Load2

VMV, fMV

Vldc1 Vldc2

Component Rated Power Component Rated Power

Substation SST 6000VA Battery1, Battery3 600W

SST1 1200VA Battery2, Battery4 400W

SST2 1000VA Load1 120ΩPV1, PV2 600W Load2 240Ω

DC Microgrid 1 DC Microgrid 1

L1

L2

L3

Simulation Case 1: Mode I to II

AC frequency and active

power of SST1, SST2

Vldc1 and power of PV1,

battery1, battery2

Vldc2 and power of PV2,

battery3, battery4

• 1.7 kW grid power is lost at 2.5s• Battery automatically change from charging to discharging following the

double droop strategy• Battery power sharing automatically achieved • Power balance is reached within 0.1s

Simulation Case 4: Mode II to I

AC frequency and active

power of SST1, SST2

Vldc1 and power of PV1,

battery1, battery2

Vldc2 and power of PV2,

battery3, battery4

• When MV grid recovers, dual droop re-balance the power flow . Battery in charging mode

Simulation Case 3: Mode II to III

AC frequency and active

power of SST1, SST2

Vldc1 and power of PV1,

battery1, battery2

Vldc2 and power of PV2,

battery3, battery4

• DC microgrid 1 battery and PV production stops• Available power not capable to regulate medium voltage frequency. SST1 and

SST2 disconnect from medium voltage feeder • DC microgrid 2 is capable to keep DC microgrid 2 operation

DC Microgrid 1 DC Microgrid 2

Simulation Case 5: Mode III to I

AC frequency and active

power of SST1, SST2

Vldc1 and power of PV1,

battery1, battery2

Vldc2 and power of PV2,

battery3, battery4

• When MV grid recovers, SST reconnect to the MV feeder• dual droop re-balance the power flow . • DC microgrid 1 restarts, battery in charging mode• DC microgrid 2 battery moves into charging mode

48

SST Winning Strategy for Smart Grid

•Fault management•Current limiting•Disconnect/reconnect

•Power Management:1. Control power factor/Var Injection2. Change/Control customer voltage 3. Low voltage ride through4. Eliminate customer side harmonics5. Provide DC power/Forming DC

Microgrid6. Bidirectional Power flow control via

Energy Cell aggregation7. Supports advanced power

managements and islanding modes

•Energy Management•Monitor energy usage (AMI)•Can control/dispatch power via microgrids (Energy Cell)•Demand side management

Microsecond

Hours

SST

Utility Distribution Feeder

LV DC 380V

LVAC 120/240VOr 3phase 480V

Software Communication(IEC61850, DNP3,Modbus)

contr

ol

Electric Market Reality & Transformation

49

Where is the customer: You?USA Residential Customer: 128m $114/Per Month/Per Customer ~15B/per monthSource: EIA

Issue: Top-down business architecture

SSTStep 1:

Towards a resilient grid

SSCB

SST

Energy Cells are to

operate and manage local generators,energy storage, and dispatchable load

compete with each other to maximizetheir own profits

Utilities are expected to make more profiton providing ancillary services to ensurethe residential distribution systemsecurity and reliability, in addition toelectricity transaction.

Who owns data rules

Step 2: Towards Energy Internet

[1] Huang, A.Q.; Crow, M.L.; Heydt, G.T.; Zheng, J.P.; Dale, S.J.; , "The Future Renewable Electric Energy Delivery and Management (FREEDM) System: The Energy Internet," Proceedings of the IEEE , vol.99, no.1, pp.133-148, Jan. 2011

[2] W. Su, and A.Q. Huang, “A Game Theoretic Framework for a Next-generation Retail Electricity Market with High Penetration of Distributed Residential Electricity Suppliers” Applied Energy, vol.119, pp.341-350, April 2014.

[3] W. Su, “The Role of Customers in the U.S. Electricity Market: Past, Present, and Future”, The Electricity Journal, 2014. (invited)

Analogous to internet consumer-to-consumer commerce (eBay) or stock market

The Third Industrial Revolution

using Internet technology to transform the power grid of every

continent into an Energy Internet that acts just like the Internet

(when millions of buildings are generating a small amount of renewable energy locally, on-site, they can sell surplus green electricity back to the grid and share it with their continental neighbors); and

Picture from ”Economist”51

THANK YOU

52