Power Electronicsee.cet.ac.in/downloads/Notes/PE/PE5-Inverters.pdf · Power Electronics INVERTERS...

Power Electronics

INVERTERS

DEPT. OF ELECTRICAL ENGINEERING, COLLEGE OF ENGINEERING TRIVANDRUM

2018 Dr. Francis M. Fernandez

Inverter Classification

Classification of inverters based on wave shape

Square wave

Quasi square wave

Sine wave

Classification of inverters based on Input

Voltage source

Current source

2DEPT. OF ELECTRICAL ENGINEERING, COLLEGE OF ENGINEERING TRIVANDRUM

Half bridge inverter

DEPT. OF ELECTRICAL ENGINEERING, COLLEGE OF ENGINEERING TRIVANDRUM 3

2DCE

2DCE

Switches S1 and S2 used are gate commutated devices such as BJT, MOSFET, IGBT, GTO etc

S1 and S2 are turned on alternately to produce an ac voltage across the load

Each switch is ON for half time period (T/2) of the desired frequency

Full bridge inverter


S1 and S2 turned on in the first half cycle (T/2)

S3 and S4 turned on in the second half cycle (T/2)

Full bridge inverter – R load


Two modes of operation:Mode 1:

S1 and S2 are ONMode 2

S3 and S4 are ON

Full bridge inverter – RL load


Four modes of operation:

Mode 1: S1 and S2 are ON, Output +ve

Mode 2: D3, D4 conducts, Output -ve

Mode 3: S3 and S4 are ON, Output –ve

Mode 4: D1, D2 conducts, Output +ve

Full Bridge Inverter – Transistor Ratings


DCCE EV )0(

R

EI DC

aveT2

)(

R

EI DC

peakT )(

Harmonics


1,3,5,..

Fourier series for symmetrical square wave4

, sin

DC

n

Ee n t

n

Original square wave

Fundamental component

Third order harmonic

Fifth order harmonic

Seventh order harmonic

Harmonics


IL

VL

tnn

Ee

n

DC

sin4

,SeriesFourier,..5,3,1

1

1

2 2Fundamental output voltage,

. 0.9

DC

DC

E E

ie E E

th 1n order output voltage, n

EE

n

2 2 2( ) 1 3 5RMS value, ...rmsE E E E

1

2 12 2 2 2

13,5,..

Harmonic voltage, 0.4352h n DCn

E E E E E

(rms)

Harmonic Parameters


12 2

23,5,..1

1Distortion Factor,

n

n

EDF

E n

12

2

3,5,..1 1

1Total Harmonic Distortion,

h

nn

ETHD E

E E

1

Harmonic Factor, nn

EHF

E

Harmonic factor for nth harmonic measures the individual harmonic contribution

Total harmonic distortion is a measures of how different is the actual waveform from its fundamental component

Harmonic factor is a measures of effectiveness in reducing unwanted harmonics using filters

Comparison of parameters in Half Bridge and Full Bridge Inverters


1

2 20.9

DC DCE E E 1

2 20.45

2 DC

DC

EE E

O DCE E2

DCO

EE

BR DCE E BR DCE E

0.4352h DCE E 0.2176h DCE E

Output voltage

Fundamentaloutput voltage

Harmonicoutput voltage

Peak breaking voltage of switches

Full Bridge Half Bridge

Example

A single phase full bridge inverter is operated from 48 V battery andsupplying power to a 24 ohm load. Determine output power THD ofoutput and transistor ratings.


2 248RMS Power, 96 W

24

EP

R

48 VCEV

( )

482A

24

DCT peak

EI

R

Solution:

1 0.9 0.9 48 43.2 VDCE E

0.4352 0.4352 48 20.89 Vh DCE E

1

20.8948.36 %

43.2hETHDE

Cross Conduction or Shoot Through Fault


Normally the switches in the bridgeswitch on in pairs – S1-S2 turn onfirst and after they are off, S3-S4turn on

Therefore the switches in the sameleg (say S1 and S4) may not turn onat the same time

Due to turn off delay, incomingdevice and outgoing device of thesame leg in the bridge conduct atthe same instant and short circuitsthe DC source.

This fault damages both the devices

Cross Conduction or Shoot Through Fault


A solution for cross conductionfault is to introduce a dead bandor delay between the trailingedge of the gate input ofoutgoing device and the leadingedge of the gate input ofincoming device

Dead band should be longer than the turn off time of the inverter

3-Phase Inverter


A

C

B

RR

R

Load configurations

AB

C

S1 D1

S4 D4 S6

S3 D3

D6

EDC

S2

S5 D5

D2

ThreePhaseLoad

3-Phase Inverter180° conduction


ABC

N

Connection during first interval

S1 D1

S4 D4 S6

S3 D3

D6

EDC

S2

S5 D5

D2

DCE3

1 dcE3

1

dcE3

2



ABC

S1 D1

S4 D4 S6

S3 D3

D6

EDC

S2

S5 D5

D2

dcE3

1

dcE3

2

N

Connection during second intervalDCE

3

2



ABC

S1 D1

S4 D4 S6

S3 D3

D6

EDC

S2

S5 D5

D2

dcE3

1

dcE3

2

IntervalIncoming

DeviceConducting

Devices

1 S1 5,6,1

2 S1 6,1,2

3 S3 1,2,3

4 S4 2,3,4

5 S5 3,4,5

6 S6 4,5,6

Features of 180 degree conduction

Conduction period for each switch is 180°

Three switches conduct at a time

There is possibility of cross conduction if a dead band delay is not deliberately introduced.

Phase voltages are six step waves

Line voltages are quasi square waves




ABC

S1 D1

S4 D4 S6

S3 D3

D6

EDC

S2

S5 D5

D2

N

Connection during first interval

2dcE

dcE

2dcE



ABC

S1 D1

S4 D4 S6

S3 D3

D6

EDC

S2

S5 D5

D2

2dcE

dcE

2dcEN

Connection during second interval



ABC

S1 D1

S4 D4 S6

S3 D3

D6

EDC

S2

S5 D5

D2

2dcE

dcE

2dcEN

Connection during third interval



ABC

S1 D1

S4 D4 S6

S3 D3

D6

EDC

S2

S5 D5

D2

2dcE

dcE

2dcE

IntervalIncoming

DeviceConducting

Devices

1 S1 6,1

2 S1 1,2

3 S3 2,3

4 S4 3,4

5 S5 4,5

6 S6 5,6

Features of 120 degree conduction

Conduction period for each switch is only 120°

Only two switches conduct at a time

Two switches in the same leg of bridge have inherent dead band of 60° and there is no possibility of cross conduction

Phase voltages are quasi square waves

Line voltages are six step waves


dcE

Parallel Inverter

T1

T2

C

L

EDC


SCRs can be used as switch, simpleforced commutation is possible

When T1 is turned on, the transformeris energised in one direction and thecapacitor C is charged with a voltageof 2EDC

When T2 is turned on capacitorvoltage is applied to T1 in reversedirection and commutates it; thecapacitor C is charged with a voltageof -2EDC

T1 is turned on again forcing T2 toturn off and the cycle repeats

Higher output voltage is possible bysuitable transformer turns ratio

Voltage Control with Pulse Width Modulation

Single Pulse width Modulation Consists of a pulse with

variable width in each half cycle

width varies from 0 to π

Multiple Pulse Width Modulation Is an extension of single PWM and

uses several equidistant pulses in each half cycle

Sinusoidal Pulse Width Modulation Pulse width is a sinusoidal function

of angular position of the pulse in a cycle


Single Pulse PWM


Comparator

Carrier Wave

Square Wave

Single Pulse PWM


2

d2

2

3

2

1

2 tage,output vol of valueRMS

dVV DC

ndn

VV DConm sin

4 harmonic,nth of Peak value

This implies, when the pulse width is 120°, third harmonics will be eliminated

60d ,120 is pulsewidthWhen

3

4Then peak value of 3rd harmonic, sin 3 60 0

3 DC

o m

VV

Multiple Pulse PWM


1

2

RMS value of output voltage,

p

DC

N PV V

Comparator

Carrier Wave

Square Wave

Np = number of pulses in a half cycle

P = pulse width

Sine PWM


c

r

V

VMI Index, Modulation

where Vr is the peak of reference wave and Vc is the peak of carrier wave

There will be low order harmonics when MI is greater than 1

Typical MOSFETS


Typical IGBTs


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