Power ElectronicsPower Electronics

Chapter 5 DC to AC Converters

( Inverters )

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Applications of InvertersApplications of Inverters

Conversion of electric power from DC type energy Conversion of electric power from DC type energy sources to AC type loadsources to AC type load–– BatteryBattery

–– Photovoltaic cell (Solar cell)Photovoltaic cell (Solar cell)

–– Fuel cellFuel cell

As a part of composite converterAs a part of composite converter–– ACAC--DCDC--AC frequency converter (for AC motor drive)AC frequency converter (for AC motor drive)

–– ACAC--DCDC--AC constantAC constant--voltage constantvoltage constant--frequency converter (for frequency converter (for uninterruptableuninterruptable power supplies)power supplies)

–– ACAC--DCDC--AC Converters for induction heatingAC Converters for induction heating

–– ACAC--DCDC--ACAC--DC switching power suppliesDC switching power supplies

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OutlineOutline5.1 Commutation5.1 Commutation

5.2 Voltage source inverters5.2 Voltage source inverters

5.3 Current source inverters5.3 Current source inverters

5.4 Multiple5.4 Multiple--inverter connections and multiinverter connections and multi--level inverterslevel inverters

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5.1 Commutation types5.1 Commutation types

A classification of invertersA classification of inverters–– SquareSquare--wave inverters (are discussed in this chapter)wave inverters (are discussed in this chapter)

–– PWM inverters ( will be discussed in Chapter 6)PWM inverters ( will be discussed in Chapter 6)

The concept of commutationThe concept of commutation

Basic operation principle of invertersBasic operation principle of inverters

LoadS

1

S2

S3

S4

io

uo

Ud

t

uo

iot1 t2

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4 types of commutation4 types of commutation

Device commutation: Device commutation: FullyFully--controlled devices: GTO, IGBT, MOSFETcontrolled devices: GTO, IGBT, MOSFET

Line commutationLine commutationPhasePhase--controlled rectifiercontrolled rectifierPhasePhase--controlled AC controllercontrolled AC controllerThyristorThyristor cycloconvertercycloconverter

Load commutationLoad commutation

Forced commutationForced commutation

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Load commutationLoad commutation

Condition: Load current is leading load voltageCondition: Load current is leading load voltageApplication: capacitive load, synchronous motorApplication: capacitive load, synchronous motor

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Forced commutation Forced commutation (capacitance commutation)(capacitance commutation)

DirectDirect--CoupledCoupled With CouplingWith Coupling--InductorInductor

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Another classification of commutationsAnother classification of commutations

SelfSelf--commutationcommutation

Device commutation

Forced commutation

Line commutation

Load commutation

4 types of Commutations4 types of Commutations

External External commutationcommutation

For fully-controlled devices

For thyristors

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2 classes of inverters2 classes of inverters

Voltage Source Inverter(VSI)

Current Source Inverter(CSI)

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5.2 Voltage source inverter (VSI)5.2 Voltage source inverter (VSI)

DC side is constant voltage, low impedance (voltage DC side is constant voltage, low impedance (voltage source, or bulk cap)source, or bulk cap)

AC side voltage is square wave or quasiAC side voltage is square wave or quasi--square wave. square wave. AC side current is determined by the load.AC side current is determined by the load.

AntiAnti--parallel diodes are necessary to provide energy parallel diodes are necessary to provide energy feedback path.feedback path.(freewheeling diodes , feedback diodes)(freewheeling diodes , feedback diodes)

+

-

C R LUd

V1

V2

V3

V4

VD 1

VD 2

VD3

VD4

uo

io

FeaturesFeatures

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SingleSingle--phase half bridge VSIphase half bridge VSI

The current conducting path is determined by the The current conducting path is determined by the polarity of load voltage and load current. (This is true polarity of load voltage and load current. (This is true for analysis of many power electronics circuits.)for analysis of many power electronics circuits.)

The magnitude of output squareThe magnitude of output square--wave voltage is Uwave voltage is Udd/2./2.

-

R LUd

io

uo

V1

V2

VD1

VD2

Ud2

Ud2

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SingleSingle--phase full bridge VSIphase full bridge VSIOperation principleOperation principle

+

-

CR L

Ud

V1

V2

V3

V4

VD1

VD2

VD3

VD 4

uo

io

The magnitude of output squareThe magnitude of output square--wave voltage is wave voltage is UUdd..

The effective value of output voltage (or fundamental The effective value of output voltage (or fundamental output voltage) can be changed by changing output voltage) can be changed by changing UUdd..

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Fourier series extension of output voltageFourier series extension of output voltage

Magnitude of output voltage fundamental component Magnitude of output voltage fundamental component

Effective value of output voltage fundamental component Effective value of output voltage fundamental component

SingleSingle--phase full bridge VSIphase full bridge VSIQuantitative analysisQuantitative analysis

⎟⎠⎞

⎜⎝⎛ +++= tttUu ωωω

π5sin

513sin

31sin4 d

o

dd

o1m 27.14 UUU ==π

dd

1o 9.022 UUU ==π

(5(5--1)1)

(5(5--2)2)

(5(5--3)3)

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SingleSingle--phase full bridge VSIphase full bridge VSIOutput voltage control by phaseOutput voltage control by phase--shiftshift

tO

tO

tO

tO

tO

θ

uG1

uG2

uG3

uG4

uoio

t1 t2

t3io

uo

+

-

C R LUd

V1

V2

V3

V4

VD1

VD2

VD 3

VD 4

uo

io

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Inverter with centerInverter with center--tapped transformertapped transformer——pushpush--pull inverterpull inverter

Load

+

-

io uo

UdV1 V2

VD1VD2

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ThreeThree--phase VSIphase VSI

180180oo conductionconduction

Dead time (blanking time) to Dead time (blanking time) to avoid avoid ““shoot throughshoot through””

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ThreeThree--phase VSIphase VSIBasic equations to obtain voltage waveformsBasic equations to obtain voltage waveforms

For line voltage For phase voltage of the load

0=++ WNVNUN UUU

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ThreeThree--phase VSIphase VSI

Fourier series extension of output lineFourier series extension of output line--toto--line voltageline voltage

Magnitude of output voltage (lineMagnitude of output voltage (line--toto--line) fundamental component line) fundamental component

Effective value of output voltage (lineEffective value of output voltage (line--toto--line) fundamental line) fundamental componentcomponent

Quantitative analysisQuantitative analysis

⎥⎦

⎤⎢⎣

⎡−+=

⎟⎠⎞

⎜⎝⎛ −++−−=

∑n

k tnn

tU

tttttUu

ωωπ

ωωωωωπ

sin)1(1sin32

13sin13111sin

1117sin

715sin

51sin32

d

dUV

dd

UV1m 1.132 UUU ==π

ddUV1m

UV1 78.062

UUUU ===π

(5(5--8)8)

(5(5--10)10)

(5(5--11)11)

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5.3 Current source inverter (CSI)5.3 Current source inverter (CSI)

DC side is constant current, high impedance(current source, or largeinductor)

AC side current is quasi-square wave. AC side voltage is determined by the load.

No anti-parallel diodes are needed. sometimes series diodes are needed to block reverse voltage for other power semiconductor devices.

FeaturesFeatures

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SingleSingle--phase bridge CSIphase bridge CSIParallel Resonant Inverter Parallel Resonant Inverter

A

C

R L

Ld

Id VT1

VT2

VT3

VT4

LT1

LT2

LT3

LT4

uo

io

Switching frequency is a little higher Switching frequency is a little higher than the resonant frequency so that the than the resonant frequency so that the load becomes capacitive and load load becomes capacitive and load current is leading voltage to realize current is leading voltage to realize load commutation.load commutation.

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ThreeThree--phase selfphase self--commutated CSIcommutated CSI

120120oo conductionconduction

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ThreeThree--phase forcephase force--commutated CSIcommutated CSI

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ThreeThree--phase loadphase load--commutated CSIcommutated CSI

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5.4 5.4 MultipleMultiple--inverter connections inverter connections and multiand multi--level inverterslevel inverters

Series connection of 2 singleSeries connection of 2 single--phase phase VSIsVSIs

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Series connection of 2 3Series connection of 2 3--phase phase VSIsVSIs

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MultiMulti--level Inverterslevel Inverters33--level inverterlevel inverter