## Natural balancing of the neutral-point-clamped converter

Thesis (MScEng)--University of Stellenbosch, 2003. / ENGLISH ABSTRACT: The three-level neutral-point-clamped(NPC) converter, being a widely used multilevel
inverter, received a lot of attention recently due to problems associated with de-link
capacitor voltage balancing. There are mainly two problems associated with the neutralpoint
voltage of the NPC inverter:
1. At high modulation indices a low frequency ripple occurs on the neutral-point voltage.
2. Steady-state unbalance in the neutral-point voltage may arise due to a variety of
factors including component imperfections, transients and other non-idealities and
imbalances.
In this thesis we study the balancing problem with focus on the steady-state imbalance.
This is achieved by a systematic and mathematically rigorous study of the natural balancing
mechanisms of the three-level three-phase NPC inverter. Orthogonality of two sets of
switching spectra in the frequency domain would imply that the DC-bus voltages balance
in the steady state. This is done through mathematical analysis using Carrara's PWM
strategy of alternative phase opposition disposition(APOD), phase opposition disposition(
POD) and phase disposition(PD); and Bennet's geometric model for double Fourier
series adapted for use with power converter systems by Bowes. The theory is verified
through simulation. / AFRIKAANSE OPSOMMING: Aangesien die drie-vlak, geklemde, neutrale-punt omsetter(NPC) 'n algemene omsetter
konfigurasie is, is daar onlangs baie aandag gegee aan die probleme wat geassosieer word
met die balansering van die omsetter se gelykstroombuskapasitorspanning. Die twee hoof
probleme wat gepaart gaan met die neutraalpuntspanning van die NPC omsetter is:
1. Met 'n ho modulasie-indeks ontstaan daar 'n lae frekwensie rippel op die neutralepuntspanning.
2. 'n Bestendige toestand wanbalans van die neutrale-puntspanning kan ontstaan as
gevolg van 'n verskeidenheid faktore, onder andere komponent nie-idealiteite,
oorgangs- en ander wanbalanse.
In hierdie tesis word op die bestendige-toestandwanbalans gefokus. Dit word gedoen
deur middel van die neutraalbalanseeringsmeganisme van die drie-vlak, drie-fase NPC
omsetter, sistematies en gedetaileerd wiskundig te bestudeer. In die bestendige toestand
sal orgonaliteit van twee stelle skakel spektras in die frekwensie gebied, GS-bus spanning
balans impliseer. Dit word wiskundig geanaliseer deur gebruik te maak van Carrara se
alternatiewe fase opposiesie disposisie (APOD), fase oposisie disposisie (POD) en fase
disposisie(PD), puls-wydte modulasie strategie, asook Bennet se geometriese modelle vir
die dubbel Fourier reeks wat aangepas is vir drywingsomsetters deur Bowes. Ten slotte
is die teorie geverifieer deur simulasies.

 Identifer oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/53514 Date 12 1900 Creators Salagae, Isaac Mahijoko Contributors Mouton, H. du T., Stellenbosch University. Faculty of Engineering. Dept. of Electrical and Electronic Engineering. Publisher Stellenbosch : Stellenbosch University Source Sets South African National ETD Portal Language en_ZA Detected Language English Type Thesis Format 177 p. : ill. Rights Stellenbosch University

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