The University of Southampton
University of Southampton Institutional Repository

Analysis and reduction of dc-link capacitor voltage/current stress in three-level PWM converters

Analysis and reduction of dc-link capacitor voltage/current stress in three-level PWM converters
Analysis and reduction of dc-link capacitor voltage/current stress in three-level PWM converters
Power electronic converters are in the heart of modern renewable energy and motor drive systems. This Thesis focuses on the converter dc-link capacitor (bank), which is a costly component and a common source of failures. The Thesis is divided into two parts.

The first part examines the voltage and current stress induced on dc-link capacitors by the three most common converter topologies: The conventional two-level converter, the Neutral-Point-Clamped (NPC) three-level converter, and the Cascaded H-Bridge (CHB) three-level converter. The expressions derived for the rms capacitor current and its harmonics can be used as a tool for capacitor sizing. The harmonic analysis is then extended to systems that incorporate multiple converters connected to a common dc-link capacitor. The effect of introducing a phase shift to the converter carrier waveforms is examined, showing that reductions in the order of 30 to 50% in the common capacitor rms current can be achieved using appropriate phase shifts.

The second part tackles the dc-link capacitor balancing problem, also known as Neutral Point (NP) balancing problem of the three-level NPC converter. Initially, a circuit that halves the voltage stress caused by the NP voltage oscillations (ripple) on the switching devices the NPC converter is proposed. The circuit consists of low voltage rated components which offer the advantages of lower losses, volume and cost, as compared to other balancing circuits.

Subsequently, the study focuses on modulation strategies for the NPC converter. Starting with Nearest-Vector (NV) strategies, it proves that the criterion of the direction of dc-link capacitor imbalance, which is commonly adopted by NV strategies for performing the task of capacitor balancing, poses a barrier in achieving minimum NP voltage ripple. A new criterion is proposed instead, together with an algorithm that incorporates it into existing NV strategies. For the interesting case of NPC converters operating as motor drives, the resulting reduction in the amplitude of NP voltage ripple ranges from 30 to 50%.

The study finishes with an extension of the previous concept to create hybrid (combinations of NV and non NV) strategies for the NPC converter. Hybrid strategies are proposed that can eliminate NP voltage ripple, introducing lower switching losses and output voltage distortion as compared to other methods used for the same purpose. The proposed strategies perform equally well when the converter operates with non linear or imbalanced loads. All results are verified by extensive simulations using MATLAB-Simulink.
Orfanoudakis, G.I.
b0aae674-1116-4c4c-a5bd-4578b2a7db52
Orfanoudakis, G.I.
b0aae674-1116-4c4c-a5bd-4578b2a7db52
Sharkh, Suleiman
c8445516-dafe-41c2-b7e8-c21e295e56b9

(2012) Analysis and reduction of dc-link capacitor voltage/current stress in three-level PWM converters. University of Southampton, Faculty of Engineering and the Environment, Doctoral Thesis, 226pp.

Record type: Thesis (Doctoral)

Abstract

Power electronic converters are in the heart of modern renewable energy and motor drive systems. This Thesis focuses on the converter dc-link capacitor (bank), which is a costly component and a common source of failures. The Thesis is divided into two parts.

The first part examines the voltage and current stress induced on dc-link capacitors by the three most common converter topologies: The conventional two-level converter, the Neutral-Point-Clamped (NPC) three-level converter, and the Cascaded H-Bridge (CHB) three-level converter. The expressions derived for the rms capacitor current and its harmonics can be used as a tool for capacitor sizing. The harmonic analysis is then extended to systems that incorporate multiple converters connected to a common dc-link capacitor. The effect of introducing a phase shift to the converter carrier waveforms is examined, showing that reductions in the order of 30 to 50% in the common capacitor rms current can be achieved using appropriate phase shifts.

The second part tackles the dc-link capacitor balancing problem, also known as Neutral Point (NP) balancing problem of the three-level NPC converter. Initially, a circuit that halves the voltage stress caused by the NP voltage oscillations (ripple) on the switching devices the NPC converter is proposed. The circuit consists of low voltage rated components which offer the advantages of lower losses, volume and cost, as compared to other balancing circuits.

Subsequently, the study focuses on modulation strategies for the NPC converter. Starting with Nearest-Vector (NV) strategies, it proves that the criterion of the direction of dc-link capacitor imbalance, which is commonly adopted by NV strategies for performing the task of capacitor balancing, poses a barrier in achieving minimum NP voltage ripple. A new criterion is proposed instead, together with an algorithm that incorporates it into existing NV strategies. For the interesting case of NPC converters operating as motor drives, the resulting reduction in the amplitude of NP voltage ripple ranges from 30 to 50%.

The study finishes with an extension of the previous concept to create hybrid (combinations of NV and non NV) strategies for the NPC converter. Hybrid strategies are proposed that can eliminate NP voltage ripple, introducing lower switching losses and output voltage distortion as compared to other methods used for the same purpose. The proposed strategies perform equally well when the converter operates with non linear or imbalanced loads. All results are verified by extensive simulations using MATLAB-Simulink.

PDF
PhD-Orfanoudakis.pdf - Other
Download (3MB)

More information

Published date: 1 December 2012
Organisations: University of Southampton, Engineering Science Unit

Identifiers

Local EPrints ID: 352195
URI: http://eprints.soton.ac.uk/id/eprint/352195
PURE UUID: d5d4bd1c-80bc-4026-b290-a7f59d0ddf52

Catalogue record

Date deposited: 07 May 2013 14:49
Last modified: 18 Jul 2017 04:17

Export record

Contributors

Author: G.I. Orfanoudakis
Thesis advisor: Suleiman Sharkh

University divisions

Download statistics

Downloads from ePrints over the past year. Other digital versions may also be available to download e.g. from the publisher's website.

View more statistics

Atom RSS 1.0 RSS 2.0

Contact ePrints Soton: eprints@soton.ac.uk

ePrints Soton supports OAI 2.0 with a base URL of http://eprints.soton.ac.uk/cgi/oai2

This repository has been built using EPrints software, developed at the University of Southampton, but available to everyone to use.

We use cookies to ensure that we give you the best experience on our website. If you continue without changing your settings, we will assume that you are happy to receive cookies on the University of Southampton website.

×