High Power Electronics

The University of Tennessee 1

High Power Electronicsfor a Sustainable 21st Century

Leon M. Tolbert

The University of Tennessee

NSF Workshop for Sustainable Energy Systems

December 1, 2000

Atlanta, Georgia


Electric Power in 21st Century• Deregulation and distributed generation will result in the

need for better control of the interface between generationand transmission - hence the need for state-of-the-art highpower electronics and control systems.

• Ancillary services will also be an important resource so thatpower quality and reliability can be maintained and evenimproved in the uncertain future of distributed generation.

• Research in power engineering needs an integratedapproach for power systems and power electronics byconsidering the interactions that these two technologies haveon each other.


Power Electronics• Power electronics converts electricity from its available

form to a desired form by changing voltage and/orfrequency.

• Cross-Cutting and Enabling Technology

– Interface between distributed resources and utility

– Control flow of power in T&D system

– Increase efficiency of T&D system by eliminatingnon-active (reactive) power

– Improve power quality, stability, reliability


Power Electronic Opportunities• T&D System (require efficient, high power connections)

– Distributed Generation - connection of sources to grid

– Renewable energy - likely to make up significant portion ofgeneration capacity in future

– Ancillary Services - sag support, power factor correction,voltage regulation, power conditioning

• Transportation - hybrid electric vehicles, more electric shipsand airplanes (require efficient, lightweight electronics)

• Industrial - medium voltage variable speed motor drives(efficiency important)

In <20 years, >50% of electricity will pass through P. E. systems


Power Semiconductor Advances

102

MOSFETIGBT

GTO

SCR

BJT

107

105

104

103

106

PO

WE

R R

AT

ING

(V

·A)

10 102 103 104 105 106 107 108

FREQUENCY (Hz)

• Higher Voltage and Current Ratings• Lower ON voltage drop

• Lower Switching Times• Less Energy Required at Control Pin


Multilevel Converters• General structure

– Synthesize desired ac voltage from several levels of dc voltages

– More levels produce a staircase waveform that approaches a sinusoid

– Harmonic distortion of output waveform decreases with more levels

– No voltage sharing problems with series connected devices

– Low dV/dt reduces switching losses and EMI

• Two structures ideal for system interfaces– Cascaded H-bridges inverter with separate dc sources

– Back-to-back diode-clamped converter


Cascaded H-Bridges Inverter

S1

S3

S2

S4

Vdc

+- SDCS

S1

S3

S2

S4

Vdc

+- SDCS

S1

S3

S2

S4

Vdc

+- SDCS

S1

S3

S2

S4

Vdc

+- SDCS

a v1

v2

n

v(m-1)/2 - 1

v(m-1)/2

−5Vdc

0π/2 π

3π /2

2π

va-n

va-n

*

v5

v4

v3

v2

v1

0

0Vdc

−Vdc

5Vdc

P1

P2

P3

P4

P5

P1

P2

P3

P4

P5θ5 π−θ 5

θ4

θ1

θ2

θ3

π−θ 1

π−θ 2

π−θ 3

π−θ 4

Single phase m-level structure Line-neutral voltage for 11-level inverter


Duty Cycle Swapping

Rotation of duty cycles among the 5 H-bridges every half cycle

0 π/2π 3π /2 2π

vLa-n

vLa-n

*iLaA

v5

v4

v3

v2

v1

0

0Vdc

−Vdc

−5Vdc

5Vdc

P1

P2

P3

P4

P5

P2

P4

P3

P5

P1P2

P3

P4

P5

P1

P4

P3

P2

P1

P5

P3

P4

P5

P1

P2


Multilevel Staircase vs. PWM

5Vdc

−5Vdc

0π/2 π 2π

vac

vac*

iCa

5Vdc

-5Vdc

PWMPulse Width Modulation

(fs = 10 kHz)

MultilevelStaircase(fs = 60 Hz)

0


3-Phase Multilevel Cascaded Inverter

Three phase wye connection

H-BridgeINV.

H-BridgeINV.

H-BridgeINV.

H-BridgeINV.

H-BridgeINV.

H-BridgeINV.

H-BridgeINV.

H-BridgeINV.

H-BridgeINV.

H-BridgeINV.

H-BridgeINV.

H-BridgeINV.

Motor

ToCharger

Charge/DriveSwitch

DC AC

+

-H-Bridge Inverter

H-BridgeINV.

H-BridgeINV.

H-BridgeINV.

• Wye or delta connection possible

• Rectifier mode can charge batteries or ultracapacitors


Cascaded H-Bridges Inverter

Three phase 11-level 10 kW prototype Output voltage/current waveforms

Van

Vab

ia


Applications of Cascade Inverter

• FACTs/UPFC Applications

• Active Power Filter

• Sag Compensation

• Static Var Compensation (improve power factor)

• Interface with Distributed Generation Resources– Photovoltaics

– Fuel Cells

– Wind Turbines (rectified)

– Energy Storage


Diode-Clamped Inverter Schematic

Three phase 6-level structure Multiplexer model

V5

V4

V3

V2

V1= 0

C2

C3

C4

C5

V6

C1

VCa

VCc

VCb

Sa1

Sa2

Sa3

Sa4

D1

D2

D3

Sa5D4

Sa'1

Sa'2

Sa'3

Sa'4

D4

D3

D2

Sa'5

D1

Sb1

Sb2

Sb3

Sb4

D1

D2

D3

Sb5D4

Sb'1

Sb'2

Sb'3

Sb'4

D4

D3

D2

Sb'5

D1

Sc1

Sc2

Sc3

Sc4

D1

D2

D3

Sc5D4

Sc'1

Sc'2

Sc'3

Sc'4

D4

D3

D2

Sc'5

D1Vdc

0

iL0

iL1

iL2

iL3

iL4

iL5

ic5

ic4

ic3

ic2

ic1

iLc

iLb

iLa

vLc0

vLb0

vLa0

vc5

vc4

vc3

vc2

vc1

ha=3

hb=2

hc=0


Diode-Clamped Back-to-Back Converter

V5

V4

V3

V2

V1

C2

C3

C4

C5

V6

C1

VLa

VLc

VLb

Sa1

Sa2

Sa3

Sa4

D1

D2

D3

Sa5D4

Sa'1

Sa'2

Sa'3

Sa'4

D4

D3

D2

Sa'5

D1

Sb1

Sb2

Sb3

Sb4

D1

D2

D3

Sb5D4

Sb'1

Sb'2

Sb'3

Sb'4

D4

D3

D2

Sb'5

D1

Sc1

Sc2

Sc3

Sc4

D1

D2

D3

Sc5D4

Sc'1

Sc'2

Sc'3

Sc'4

D4

D3

D2

Sc'5

D1

VSa

VSc

VSb

Sa1

Sa2

Sa3

Sa4

D1

D2

D3

Sa5 D4

Sa'1

Sa'2

Sa'3

Sa'4

D4

D3

D2

Sa'5

D1

Sb1

Sb2

Sb3

Sb4

D1

D2

D3

Sb5 D4

Sb'1

Sb'2

Sb'3

Sb'4

D4

D3

D2

Sb'5

D1

Sc1

Sc2

Sc3

Sc4

D1

D2

D3

Sc5 D4

Sc'1

Sc'2

Sc'3

Sc'4

D4

D3

D2

Sc'5

D1

5Vdc

ac-dc converter dc-ac inverter

MotorGen

LoadSource

• DC bus voltage shared by capacitors (or batteries) in series

• Switching device voltage stress limited by clamping diodes

• All six phases can share one common dc link


Multilevel Diode-Clamped Inverter

Three phase 6-level back-to-back prototype

Output voltage and current waveforms

VL-ab

iLa


Efficiency Comparison

• Efficiency of multilevelinverter is greater than98% for loads greaterthan 40% of its ratedoutput power.

• Multilevel inverter doesnot suffer fromdisproportionate highswitching losses at lowoutput power like two-level inverters.

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 0.2 0.4 0.6 0.8 1Fraction of Inverter's Rated Output Power

Effi

cien

cy

Commercial PWM Inverter

Back-to-Back Multilevel Converter


Applications of Diode-Clamped Inverter

• Static Var Compensation

• FACTs/UPFC Applications

• Back-to-Back Intertie of AsynchronousAC Utilities

• Interface between HVDC and AC

• Interface between distributedgeneration sources and utility

• Medium voltage motor drives


Multilevel Inverter PWM MethodsExtensions from two-level PWM to multiple levels

• Space vector

• Carrier-based

Subharmonic (SH-PWM)

Switching Frequency Optimal (SFO-PWM)

• Selective harmonic elimination

Multiple Levels offer extra degrees of freedom• State redundancies

• Device utilization

• Effective switching frequency


Level Reduction at Low Modulation Indices

-3

-2

-1

0

1

2

3

time (s)

Vol

tage

(p.

u.)

-3

-2

-1

0

1

2

3

time (s)

Vol

tage

(p.

u.)

SH-PWM SFO-PWMLevels Min Max Min Max

3 0.000 1.000 0.000 1.1554 0.333 1.000 0.385 1.1555 0.500 1.000 0.578 1.1556 0.600 1.000 0.693 1.1557 0.667 1.000 0.770 1.1558 0.714 1.000 0.825 1.1559 0.750 1.000 0.866 1.155

1

3min −

−=

m

mma

ma < 0.6

ma < 0.2


PWM Pulse Rotation

Van

Va1

Va2

Va3

Van

Vab

Ia

• Switches are allowed to “rest” for a few cycles when level is not being used- higher switching frequency possible (2X to 6X for 7-level inverter).

• Individual H-bridge inverters switch at almost constant frequency of 60 Hzover entire motor speed range.

ma = 0.3, 18 Hz (30% of rated speed), effective fsw = 300 Hz


Advantages of Multilevel Converters• Enabling technology for sustainable energy systems

• Possible connections: single-phase, multi-phase,three phase wye or delta

• Modular - lower manufacturing costs

• Compact - no transformer needed

• None or reduced output filters

• Low harmonic distortion

• Redundant levels for increased reliability

• Low switching frequency yields high efficiency


1950 60 70 80 90 2000 10

BJTs

MOSFETs

IGBTs

SiC Diodes

SiC MOSFETs

Evolution of Power Semiconductors• New semiconductor materials are expected for the next decade:

– Silicon Carbide (SiC), Galium Arsenide(GaAs), Galium Nitride(GaN), etc.


Today’sTechnology

Advantages ofSilicon Carbide Payoff

Large energy losses Reduce loss by 10x Large improvement inefficiency

Limited voltage andpower level

Increase power 103x Simplify use – no needfor multilevel inverters?

Low operatingtemperature (< 150ºC)

Increase range to500ºC

New applications;eliminate massive heatsinks

Large heat sinks &filters

Reduce size/weightby 3x

Lightweight, compactsystems

Future Power Electronics Will Be SiC

Silicon-based power semiconductor devices have reachedtheir limits - thermal and voltage blocking capabilities


VB = Blocking voltage

εs = Dielectric constant

Ec = Breakdown field

µn = Bulk mobility

Why SiC? The Physics Reason

Power dissipation in a power MOSFET iscontrolled by on-resistance:

ncs

Bon E

VR

µε 3

24=

Baliga’s Figure of Merit200× larger for SiC than for Si


New Circuit Topologies to Take Full Advantage of SiC

Ct1

Ct2

Sa1

Da1Lr

Cr2

Cr1

Sc Dc

Id Ii

Ir IoVbus

AQRT

Inverter

S1

S2 S6

S5

S4

S3

D1 D3 D5

D2 D4 D6

DC power supply

a b c

n

p

Phase Leg

Soft-switching inverter technology (Si)

12 VBattery

36 VBattery

12Vload

42Valternator

36Vload

SiC may eliminate needfor soft-switching


Future Power Electronics Issues• >50% electricity generated will pass through power

electronics systems in the near future -high efficiency and reliability are necessary.

• Need for integrated, modular units that are smaller and lessexpensive than existing power electronics circuits.

• Silicon-Carbide based power electronics have much higherthermal capabilities and voltage blocking capabilities. Futuredevices likely to be SiC instead of Si. New circuits to takeadvantage of SiC properties will be needed.

• Several growth opportunities with high power electronics inutilities, transportation, and large motor drives.

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High Power Electronics