Modulation techniques comparison for three levels VSI converters

November 2002 MODULATION TECHNIQUES COMPARISON FOR THREE LEVELS VSI CONVERTERS

1

MODULATION TECHNIQUES COMPARISON FOR THREE LEVELS VSI

CONVERTERS

Emilio J. Bueno, Roberto García, Marta Marrón,Felipe Espinosa, Jesús Ureña

Departamento de Electrónica. Universidad de Alcalá[email protected]

IECONIECON’’20022002


2

IndexIndex

1. Introduction.

2. PWM modulation techniques for NPC converters.

3. NP unbalancing in NPC.

4. Simulation results.

5. Conclusions.


3

RED ELÉCTRICA MÁQUINA ACASÍNCRONA

INVERSOR 2

Vw

Vu

Vv

Iw

Iv

Iu

VN

VNP

VP

Vc

Vb

Ic

Va

Ib

Ia

D2

D1

NP

C

C

Sa2

Sa1

Sa2

Sa1

Sb2

Sb1

Sb2

Sb1

Sc2

Sc1

Sc2

Sc1

N

P

IM

A/DMedida de variables

Control inversor 1.Generación señales SVPWM



Control inversor 2.Generación señales SVPWM

CPU principal

INVERSOR 1

NPC double converter block diagram

1. 1. Introduction Introduction (1/3)(1/3)


4

Comparison between a NPC VSI and a two-level VSI


2 levels 3 levels

UDC (V) 484,62 483,14IDCBUS (A) 17,395 17,241PDC (W) 8430 8330Pout activa (W) 8132,4 7992,3Pinv = PDC – Pout activa (W) 297,6 337,7

PIGBT (W) 49,6 28,14

2-level VSI line voltage

NPC VSI line voltage

• Load: R =1Ω y L = 10mH

• fS = 1Khz

Harmonics to the conmutation frequency


5

Diode rectifier/NPC inverter system: System used to simulate

L R

D 2

D 1 NP

C

C

Sa2

Sa1

Sa2

Sa1

Sb2

Sb1

Sb2

Sb1

Sc2

Sc1

Sc2

Sc1

N

P

VP

VN

INP

The simulations have been developed with the Matlab Power System Blockset

• Line voltage 400V/50Hz

• C = 4700μF

• IGBT’s are simulated by ideal- switches.

• R = 0.01Ω and L = 0.01H



6

Cosinusoidal modulation function (1/3)

( )αcos=OF

Sa1’scarrier

Sa2’scarrier

Reference signal

2. PWM2. PWM modulation techniques formodulation techniques for NPC NPC convertersconverters (1/8).(1/8).

Harmonic components of vab.


7

Cosinusoidal modulation function (3/3)

DC-bus voltage and variation of NP voltage INP current

NPNP VVV −=



8

Vectorial modulation function (1/3)Switching-state vectors of the three-level VSI

6 sectors of 60º

24 subsectors (each one equilateral triang. )

201

α

β

220

210

200

202102002

012

022

021

020 120

211100

101212

112001

122011

121010

221110

VREF

21

1

2

Vectores cero:222111000

¿How does it obtain the PWM signals?

Placing VREF inside in once of the 6 sectors of 60º

Placing VREF in the triangles1, 2, 3, or 4 of the once of

the sectors of 60º

Computing of the three dutycycles

Choosing of the conmutation-sequence

VREF: m y θ



9

200100

d11 d03d13

211

221110

VREF

220

210

4

213

θ

d01

d21

d04 d14

d24

d23

d02

d12

( ) ( )( )( ) ( )( )

( )θθθ

θθ

sin2sincos3

sincos31

21

11

01

⋅⋅=−⋅⋅=

+⋅⋅−=

mdmd

md

( )( ) ( )( )

( ) ( )( )θθ

θθ

θ

cos3sin1

cos3sin1

sin21

22

12

02

⋅−⋅+=

⋅+⋅+−=

⋅⋅−=

md

md

md

( ) ( )( )( ) ( )( )

( )θθθ

θθ

sin2sincos31

sincos32

23

13

03

⋅⋅=−⋅⋅+−=

−⋅⋅−=

mdmd

md

( ) ( )( )( ) ( )( )( )θ

θθ

θθ

sin21sincos32

sincos3

24

14

04

⋅⋅+−=+⋅⋅−=

−⋅⋅=

mdmd

md

1.-

2.-

3.-

4.-

( ) ( )θθ sincos31+⋅

<m

( ) ( )θθ sincos31+⋅

>m

( ) ( )θθ sincos31−⋅

>m

( )θsin21

⋅<m

¿In which subsector is the reference vector?

1. The reference vector is placed inside una de las 6 regiones de 60º.

2. How m is equal to:

se obtiene el subsector en el que está. 3

35.13

LL

REF

DC

REF

VV

VV

m⋅

==

Space-vectors for 0 θ60º (phase volltages)

200211100

221110

VREF

220

210

4

21 3

DCV⋅31

DCV⋅3

1

DCV⋅32

θ

Vectorial modulation function (2/3): VREFsynthesis3. Duty cycles calculation



10

Vectorial modulation function (4/5): Conmutation sequence

0

1

2

0

1

2

0

1

2

c

b

a

100 200 210 211d0 d1 d2 d0

20d

20d

1d 2d

0

1

2

0

1

2

0

1

2

c

b

a

100 100 200 210d0 d0 d1 d2

2d0d 1d

ia

ib

ic

INP

VNP

P

N

Sa

Sb

Sc

O

C

C

a

b

c0

12

0

12

0

12

0

1

2

0

1

2

0

1

2

c

b

a

100 200 210 211d0 d1 d2 d0

40d

20d

21d

22d

20d

22d

21d

211d0

210d2

200d1

100d0

Example: VREF is in the subsector 3 of the first sector of 60º. Inside of this subsector there is one conmutation sequence:

100-200-210-211

After choose the conmutation sequence, the PWM signals are obtained.

Chosen option: Symmetrical PWM

Asymmetrical modulation Asymmetrical modulation withour 1 con.

Symmetrical

Modulation



11

Vectorial modulation function (3/3): Results

Line voltage signal and its harmonic components

VREF (phase voltage)

Out line voltage



12

Cosinusoidal modulation function with zero-sequence component (1/2)

Cosinusoidal modulation function is very easy to implementbut has two disadvantages:the maximum linear modulation index is 0.79, and

1. the current distortion is not minimized.

Both factors can be improved adding a zero-sequence ofthe reference signals containing only third order harmonics, and adding it to the original cosinusoidal reference signals. In this work the following continuous zero-sequence has been chosen:

{ } { }2

,,,,max cbacbaZ

VVVmínVVVV +−=

The zero-sequences can to generatemodulation functions:

1. continuous, and

2. discontinuous



13

Line voltage harmonic components PWM generator circuit

Cosinusoidal modulation function with zero-sequence component (2/2): Results

Zero-sequence generator module

Offset



14

INP current

NP Voltage: NPNP VVV −=

3. NP 3. NP unbalacingunbalacing in NPC (1/3).in NPC (1/3).

ia

ib

ic

INP

P

Sa

Sb

Sc

O

C

C

a

b

c0

12

0

12

0

12

VNP

N

Unbalance between the voltages of the two capacitor banks of the DC-bus (low frequency ripple).

Third order component harmonic.


15

It’s depend on ... (1/2)

VNP with para R=1Ω and L=10mH, ma=0.45 VNP with R=1Ω and L=10mH, ma=0.9



16

VNP with R=10mΩ and L=10mH, ma=0.9

CONCLUSIONS:• VNP is a function of the amplitude modulation index.• VNP is a function of the load, that is to say, of the load power factor.


Same conditions, except C = 10mF

It’s depend on ... (2/2)


17

NPC Inverter.- Regenerative circuit

UDC L

Lua(t)

CDC ub(t)

uc(t)

eb(t)

ec(t)

Lea(t)

NPC VSI

FILTER GRID

4. 4. Simulation results (1/3)Simulation results (1/3)

Vab

Ia

• Load: R = 0.1Ω y L = 0.01mH• Phase = 5º

• UDC = 750V

• SVM with zero-sequence.

α

º5=δua

ea

ia


18


NPC Rectifier.- Controlled rectifier (active filter)

L

L

Lua(t)

CDCeb(t)

ec(t)

ub(t)

uc(t)

ea(t)

LOAD UDC

NPC VSI

iaea

ua

• Load: R = 0.1Ω y L = 0.01mH

• Phase = 5º

• SVM with zero-sequence.

º5=δ ua

eaia


19


NPC double converter

L

L

L ua(t)

CDCub(t)

uc(t)

ea(t)eb(t)

ec(t)UDC

Inverter 1.- RECTIFIERL R

Inverter 2.- INVERTER

• Load: R = 0.1Ω y L = 0.01mH

• Grid : R = 0.1Ω y L = 0.01mH


20

Modulation tecniques comparison

• Loads with a low power factor generate high ripple currents in NP.• Cosinusoidal modulation functions: Modify the offset of the reference signals.• Vectorial modulation functions: Choose of other redundant vectors.

Cosinusoidal Vectorial Cosinusoidalwith zero-seq.

Maximum ma 0.79 0.91 0.92

VNP variation δ Conmutation sequence δ

Implementation Easy Difficult Easy

NP unbalancing

5. 5. ConclusionsConclusions

Modulation techniques comparison for three levels VSI converters

Documents

Transcript of Modulation techniques comparison for three levels VSI converters