Power ElectronicsPower Electronics

Chapter 4 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– AC-DC-AC frequency converter (for AC motor drive)AC-DC-AC frequency converter (for AC motor drive)

– AC-DC-AC constant-voltage constant-frequency converter (for AC-DC-AC constant-voltage constant-frequency converter (for

uninterruptable power supplies)uninterruptable power supplies)

– AC-DC-AC Converters for induction heatingAC-DC-AC Converters for induction heating

– AC-DC-AC-DC switching power suppliesAC-DC-AC-DC switching power supplies

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OutlineOutline

4.1 Commutation4.1 Commutation

4.2 Voltage source inverters4.2 Voltage source inverters

4.3 Current source inverters4.3 Current source inverters

4.4 Multiple-inverter connections and multi-level inverters4.4 Multiple-inverter connections and multi-level inverters

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4.1 Commutation types4.1 Commutation types

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

u o

io

t1 t2

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4 types of commutation4 types of commutation

Device commutation: Device commutation: Fully-controlled devices: GTO, IGBT, MOSFETFully-controlled devices: GTO, IGBT, MOSFET

Line commutationLine commutation Phase-controlled rectifierPhase-controlled rectifier Phase-controlled AC controllerPhase-controlled AC controller Thyristor cycloconverterThyristor cycloconverter

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 voltage Application: capacitive load, synchronous motorApplication: capacitive load, synchronous motor

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

Direct-CoupledDirect-Coupled With Coupling-InductorWith Coupling-Inductor

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Another classification of commutationsAnother classification of commutations

Self-commutationSelf-commutation

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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4.2 Voltage source inverter (VSI)4.2 Voltage source inverter (VSI)

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

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

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

+

-

C R LUd

V1

V2

V3

V4

VD1

VD2

VD3

VD4

uo

io

FeaturesFeatures

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Single-phase half bridge VSISingle-phase half bridge VSI

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

-

R LUd

io

uo

V1

V2

VD1

VD2

Ud

2

Ud

2

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

uo

Um

io

t1 t2

t3 t4t5 t6

V1 V2 V1 V2

VD1 VD2 VD1 VD2

UG1

UG2

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Single-phase full bridge VSISingle-phase full bridge VSI

Operation principleOperation principle+

-

CR L

Ud

V1

V2

V3

V4

VD1

VD2

VD3

VD4

uo

io

The magnitude of output square-wave voltage is UThe magnitude of output square-wave voltage is Udd..

The effective value of output voltage (or fundamental The effective value of output voltage (or fundamental

output voltage) can be changed by changing U output voltage) can be changed by changing Udd..

uoU

m

io

t1t2

t3 t4t5t6

V1 V2 V1 V2

VD1 VD2 VD1 VD2

UG1,4

UG2,3

VD4 VD3 VD4 VD3

4 3 4 3V V V V

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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 Effective value of output voltage fundamental

componentcomponent

Single-phase full bridge VSISingle-phase full bridge VSI

Quantitative analysisQuantitative analysis

ttt

Uu

5sin

5

13sin

3

1sin

4 do

dd

o1m 27.14

UU

U

dd

1o 9.022

UU

U

(4-1)(4-1)

(4-2)(4-2)

(4-3)(4-3)

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Single-phase full bridge VSISingle-phase full bridge VSI

Output voltage control by phase-shiftOutput voltage control by phase-shift

+

-

CR L

Ud

V1

V2

V3

V4

VD1

VD2

VD 3

VD 4

uo

io

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Inverter with center-tapped transformerInverter with center-tapped transformer—push-pull inverter—push-pull inverter

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Three-phase VSIThree-phase VSI

180180oo conduction conduction

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

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Three-phase VSIThree-phase VSI

Basic 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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Three-phase VSIThree-phase VSI

Fourier series extension of output line-to-line voltageFourier series extension of output line-to-line voltage

Magnitude of output voltage (line-to-line) fundamental component Magnitude of output voltage (line-to-line) fundamental component

Effective value of output voltage (line-to-line) fundamental Effective value of output voltage (line-to-line) fundamental componentcomponent

Quantitative analysisQuantitative analysis

n

k tnn

tU

tttttU

u

sin)1(1

sin32

13sin13

111sin

11

17sin

7

15sin

5

1sin

32

d

dUV

dd

UV1m 1.132

UU

U

ddUV1m

UV1 78.06

2UU

UU

(4-8)(4-8)

(4-10)(4-10)

(4-11)(4-11)

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4.3 Current source inverter (CSI)4.3 Current source inverter (CSI)

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

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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Single-phase bridge CSISingle-phase bridge CSI

Parallel Resonant Inverter Parallel Resonant Inverter

A

C

R L

Ld

Id VT

1

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 than the resonant frequency so that the 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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Three-phase self-commutated CSIThree-phase self-commutated CSI

120120oo conduction conduction

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Three-phase force-commutated CSIThree-phase force-commutated CSI

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Three-phase load-commutated CSIThree-phase load-commutated CSI

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4.4 4.4 Multiple-inverter connections Multiple-inverter connections and multi-level inverters and multi-level invertersSeries connection of 2 single-phase VSIsSeries connection of 2 single-phase VSIs

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Series connection of 2 3-phase VSIsSeries connection of 2 3-phase VSIs

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Multi-level InvertersMulti-level Inverters

Ways to deal with higher voltage and achieve better Ways to deal with higher voltage and achieve better waveformwaveform– Series connection of multiple convertersSeries connection of multiple converters– Series connection of multiple switch devicesSeries connection of multiple switch devices

Major type of multi-level invertersMajor type of multi-level inverters– Neutral point clamped multi-level inverterNeutral point clamped multi-level inverter– Flying-capacitor multi-level inverterFlying-capacitor multi-level inverter– Cascade H-bridge( series connected H-bridges)Cascade H-bridge( series connected H-bridges)

In broad sense, previously discussed series In broad sense, previously discussed series connection of multiple inverters is also called multi-connection of multiple inverters is also called multi-level inverter.level inverter.In narrow sense, only NPC and FC structures are In narrow sense, only NPC and FC structures are called multi-level inverters.called multi-level inverters.

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Neutral-Point-Clamped 3-level inverterNeutral-Point-Clamped 3-level inverter

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Neutral-Point-Clamped 5-level inverterNeutral-Point-Clamped 5-level inverter

'

'

dU

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Flying-Capacitor 3-level inverterFlying-Capacitor 3-level inverter

UdUV

W

11V

12V

41V

42V

11VD

12VD

41VD

42VD

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Series connection of 3 H-bridgesSeries connection of 3 H-bridges

U

WV

dU

dU

dU

dU

dU

dU

dU

dU

dU