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DC-BUS capacitor rating of the back-to-back NPC converters Emilio J. Bueno, Santiago Cóbreces, Francisco J. Rodríguez, Marta Alonso, Álvar Mayor, Francisco Huerta, Carlos Girón Department of Electronics. Alcalá University [email protected] [email protected] EPE-PEMC 2006 Alcalá University Department of Electronics Researching group in Control and Power Electronics Systems

DC-BUS capacitor rating of the back-to-back NPC converters Emilio J. Bueno, Santiago Cóbreces, Francisco J. Rodríguez, Marta Alonso, Álvar Mayor, Francisco

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Page 1: DC-BUS capacitor rating of the back-to-back NPC converters Emilio J. Bueno, Santiago Cóbreces, Francisco J. Rodríguez, Marta Alonso, Álvar Mayor, Francisco

DC-BUS capacitor rating of the back-to-back NPC converters

Emilio J. Bueno, Santiago Cóbreces, Francisco J. Rodríguez, Marta Alonso, Álvar Mayor,

Francisco Huerta, Carlos GirónDepartment of Electronics. Alcalá University

[email protected] [email protected]

EPE-PEMC 2006

Alcalá University Department of Electronics

Researching group in Control and Power Electronics Systems

Page 2: DC-BUS capacitor rating of the back-to-back NPC converters Emilio J. Bueno, Santiago Cóbreces, Francisco J. Rodríguez, Marta Alonso, Álvar Mayor, Francisco

Contents

• Introduction• Relation between DC-bus capacitors in converters

of two-levels and three-levels• INP calculation• Calculation of DC-bus capacitors:

– NPC VSC– Back to back NPC VSC

• Simulation and experimental results• Conclusions

EPE-PEMC 2006

Alcalá University Department of Electronics

Researching group in Control and Power Electronics Systems

Page 3: DC-BUS capacitor rating of the back-to-back NPC converters Emilio J. Bueno, Santiago Cóbreces, Francisco J. Rodríguez, Marta Alonso, Álvar Mayor, Francisco

Introduction• Converter structures for variable-speed wind turbines

back to back VSC• Increase of wind turbine power multilevel converters as

interface between generator and the utility grid• Multilevel topologyNPC

EPE-PEMC 2006

Alcalá University Department of Electronics

Critical Design

DC-bus capacitors

Researching group in Control and Power Electronics Systems

Page 4: DC-BUS capacitor rating of the back-to-back NPC converters Emilio J. Bueno, Santiago Cóbreces, Francisco J. Rodríguez, Marta Alonso, Álvar Mayor, Francisco

Introduction

• DC-bus capacitors

• Efects of unbalance

• Aims of this presentation

EPE-PEMC 2006

Alcalá University Department of Electronics

•Same factors of two-levels topology

•Low frequency ripple due iNPunbalance

•Over-voltages in power electronics devices and DC-bus capacitors

•Decrease of quality of currents

•Even an inappropiate operation

•Unbalance in the DC-bus capacitors due to iNP on a back-to-back converter

•Relating the ripples due to iNP e iDC

•Obtaining analytical equations for each ripple of a back-to-back converter

Researching group in Control and Power Electronics Systems

Page 5: DC-BUS capacitor rating of the back-to-back NPC converters Emilio J. Bueno, Santiago Cóbreces, Francisco J. Rodríguez, Marta Alonso, Álvar Mayor, Francisco

Contents

• Introduction• Relation between DC-bus capacitors in

converters of two-levels and three-levels• INP calculation• Calculation of DC-bus capacitors:

– NPC VSC– Back to back NPC VSC

• Simulation and experimental results• Conclusions

EPE-PEMC 2006

Alcalá University Department of Electronics

Researching group in Control and Power Electronics Systems

Page 6: DC-BUS capacitor rating of the back-to-back NPC converters Emilio J. Bueno, Santiago Cóbreces, Francisco J. Rodríguez, Marta Alonso, Álvar Mayor, Francisco

Relation between DC-bus capacitors in two-levels and three-levels converters

• Two-levels converter– Working as PWM rectifier

• ΔuDC Maximum allowed ripple

• TSW IGBTs switching period

– Working as active filter

EPE-PEMC 2006

Alcalá University Department of Electronics

DCDCbase

nDC uu

SC

2

DCDC

nSW

DC

SWDCDC uu

ST

u

TiC

22

(These expressions are valid for VSC connected to the grid with passive an active load)

ΔuDC 10%uDC

Sn 100kVA

TSW 400μs

Udc 610V

CDC≥537.5μF

CDC≥4277.2μF

Very conservative!Rectifier

Filter

Researching group in Control and Power Electronics Systems

Page 7: DC-BUS capacitor rating of the back-to-back NPC converters Emilio J. Bueno, Santiago Cóbreces, Francisco J. Rodríguez, Marta Alonso, Álvar Mayor, Francisco

Relation between DC-bus capacitors in two-levels and three-levels converters

EPE-PEMC 2006

Alcalá University Department of Electronics

• Three-levels converter– CDC dependes on the low frequency ripple due to the NP current – fiNP = 3·fm

– Dynamic circuit of NP connection:

• Assumptions:– Each converter independently controlled– CP=CN=2CDC

– Only fundamental harmonics are considered 2P

NP

NPDCequiv

C

CC

CCC

DCNPNPequiv CCCC 2

Researching group in Control and Power Electronics Systems

Page 8: DC-BUS capacitor rating of the back-to-back NPC converters Emilio J. Bueno, Santiago Cóbreces, Francisco J. Rodríguez, Marta Alonso, Álvar Mayor, Francisco

Contents

• Introduction• Relation between DC-bus capacitors in converters

of two-levels and three-levels• INP calculation• Calculation of DC-bus capacitors:

– NPC VSC– Back to back NPC VSC

• Simulation and experimental results• Conclusions

EPE-PEMC 2006

Alcalá University Department of Electronics

Researching group in Control and Power Electronics Systems

Page 9: DC-BUS capacitor rating of the back-to-back NPC converters Emilio J. Bueno, Santiago Cóbreces, Francisco J. Rodríguez, Marta Alonso, Álvar Mayor, Francisco

INP Calculation• SVPWM: ma=1 and DPF=0

• SPWM: ma=1 and DPF=0

EPE-PEMC 2006

Alcalá University Department of Electronics

0 50 100 150 200 250 300 350-150

-100

-50

0

50

100

150

inp

1(A

)

0 50 100 150 200 250 300 350-150

-100

-50

0

50

100

150

inp

2(A

)

0 50 100 150 200 250 300 350-150

-100

-50

0

50

100

150

inp

3(A

)

ang(deg)0 50 100 150 200 250 300 350

-150

-100

-50

0

50

100

150

inp

4(A

)

ang(deg)

titi basephaseNPbase

3cosˆ823

phasei74,0 • THSPWM

– Under the same condition the third order component is relatively smaller

Researching group in Control and Power Electronics Systems

Page 10: DC-BUS capacitor rating of the back-to-back NPC converters Emilio J. Bueno, Santiago Cóbreces, Francisco J. Rodríguez, Marta Alonso, Álvar Mayor, Francisco

Contents

• Introduction• Relation between DC-bus capacitors in converters

of two-levels and three-levels• INP calculation• Calculation of DC-bus capacitors:

– NPC VSC– Back to back NPC VSC

• Simulation and experimental results• Conclusions

EPE-PEMC 2006

Alcalá University Department of Electronics

Researching group in Control and Power Electronics Systems

Page 11: DC-BUS capacitor rating of the back-to-back NPC converters Emilio J. Bueno, Santiago Cóbreces, Francisco J. Rodríguez, Marta Alonso, Álvar Mayor, Francisco

Calculation of DC-bus capacitors

• Harmonics components of the ripple of the DC-bus capacitors:

EPE-PEMC 2006

Alcalá University Department of Electronics

Zero frequency

Conmutation frequency or high frequency ripple

Unbalance grid voltages or harmonics

(2 ωbase)

Two-levels converter

3 ω mod or low frequency ripple due to

iNP

The most restrictive!

Researching group in Control and Power Electronics Systems

Due to unbalances in the voltage of the two banks. (Depends on modulation

technique.)

If modulation signal has no offset

Equalization resistances

Three-levels converter

Page 12: DC-BUS capacitor rating of the back-to-back NPC converters Emilio J. Bueno, Santiago Cóbreces, Francisco J. Rodríguez, Marta Alonso, Álvar Mayor, Francisco

Calculation of DC-bus capacitors: NPC VSC

• Analysing the VSC working as active rectifier

EPE-PEMC 2006

Alcalá University Department of Electronics

iDC ripple are exactly the same in CP and CNNO unbalance

iNP ripple is absorbed by CP and CN unbalance

ΔvP= ΔvN

10 10.002 10.004 10.006 10.008 10.01 10.012 10.014 10.016 10.018 10.02

-100

-50

0

50

100

I(A

)

10 10.002 10.004 10.006 10.008 10.01 10.012 10.014 10.016 10.018 10.02

450

500

550

V(V

)

10 10.002 10.004 10.006 10.008 10.01 10.012 10.014 10.016 10.018 10.02

-100

-50

0

50

100

time(s)

V(V

)

iNP

iP iN

vP vN

vNP

Variation of the voltages in the two banks of capacitors of a NPC in function of iNP

If iNP triangular baseNPTiQ ˆ24

1

If iNP sinusoidalbase

NPbaseNP

iTiQ

3

ˆˆ

6

1

Worst case

P

basephaseDC v

TiC

ˆ

3

23

CDC due to iNP

DC

baseNP

i

NPiNiP C

Tivvv

NP

NPNP 2

ˆ

6

1

2

phaseNP ii

base

ˆ8ˆ23

Researching group in Control and Power Electronics Systems

Page 13: DC-BUS capacitor rating of the back-to-back NPC converters Emilio J. Bueno, Santiago Cóbreces, Francisco J. Rodríguez, Marta Alonso, Álvar Mayor, Francisco

Calculation of DC-bus capacitors: NPC VSC

EPE-PEMC 2006

Alcalá University Department of Electronics

CDC due to iDC

DCDC

SWnDC uu

TSC

2

Total ripple ripple iNP + ripple iDC Negligible

DCDC

SWnSW

DC

DCiDC

iNiP Cu

TST

C

iuvv DC

DCDC 422

1

2

DCDC

SWn

DC

basen

Cu

TS

C

Ti

42

ˆ8

6

1 2

0 10 20 30 40 50 60 70 80 90 1000

100

200

300

400

500

600

700

800

900

1000

uD

C (

V)

mf

Main ripple due to iNP

Main ripple due to iDC

Researching group in Control and Power Electronics Systems

f

baseDC m

Uu

28

33 3

Page 14: DC-BUS capacitor rating of the back-to-back NPC converters Emilio J. Bueno, Santiago Cóbreces, Francisco J. Rodríguez, Marta Alonso, Álvar Mayor, Francisco

Calculation of DC-bus capacitors: back-to-back NPC VSC

Alcalá University Department of Electronics

Researching group in Control and Power Electronics Systems

tititi NPINPRNP

iNPR and iNPI have different frequency capacitors for each frequency

iNPR = iNPI have different frequency iNP = 0

0DCDC iNiP

vv

0DCDC iNiP

vv

The worst situation iNP=2iNPI NPINPR iiP

basephaseDC v

TiC

ˆ

3

43

EPE-PEMC 2006

Page 15: DC-BUS capacitor rating of the back-to-back NPC converters Emilio J. Bueno, Santiago Cóbreces, Francisco J. Rodríguez, Marta Alonso, Álvar Mayor, Francisco

Contents

• Introduction• Relation between DC-bus capacitors in converters

of two-levels and three-levels• INP calculation• Calculation of DC-bus capacitors:

– NPC VSC– Back to back NPC VSC

• Simulation and experimental results• Conclusions

EPE-PEMC 2006

Alcalá University Department of Electronics

Researching group in Control and Power Electronics Systems

Page 16: DC-BUS capacitor rating of the back-to-back NPC converters Emilio J. Bueno, Santiago Cóbreces, Francisco J. Rodríguez, Marta Alonso, Álvar Mayor, Francisco

Simulation and experimental results

Fsw 2.5kHz

Cp =CN 1000μF

Ri 0.075Ω

LI 0.75mH

fbase 25 Hz

Ubase 400V

Ri 0.075Ω

LI 0.75mH

fbase 50 Hz

Ubase 400V

EPE-PEMC 2006

Alcalá University Department of Electronics

Researching group in Control and Power Electronics Systems

Inverter VSC2

Rectifier VSC1

ModulationTHSPWM

VvP 30

Simulation

23THSPWMam

VvP 30

Page 17: DC-BUS capacitor rating of the back-to-back NPC converters Emilio J. Bueno, Santiago Cóbreces, Francisco J. Rodríguez, Marta Alonso, Álvar Mayor, Francisco

Simulation and experimental results

EPE-PEMC 2006

Alcalá University Department of Electronics

Researching group in Control and Power Electronics Systems

Experimental

VvP 30

Page 18: DC-BUS capacitor rating of the back-to-back NPC converters Emilio J. Bueno, Santiago Cóbreces, Francisco J. Rodríguez, Marta Alonso, Álvar Mayor, Francisco

Contents

• Introduction• Relation between DC-bus capacitors in converters

of two-levels and three-levels• INP calculation• Calculation of DC-bus capacitors:

– NPC VSC– Back to back NPC VSC

• Simulation and experimental results• Conclusions

EPE-PEMC 2006

Alcalá University Department of Electronics

Researching group in Control and Power Electronics Systems

Page 19: DC-BUS capacitor rating of the back-to-back NPC converters Emilio J. Bueno, Santiago Cóbreces, Francisco J. Rodríguez, Marta Alonso, Álvar Mayor, Francisco

Conclusions

• Conclusions

EPE-PEMC 2006

Alcalá University Department of Electronics

Researching group in Control and Power Electronics Systems

• Acknowledgment