H. du T. Mouton

Natural Balance of Multicell Converters: The Two-Cell Case

The multicell converter topology is said to possess a natural voltage balancing property. This paper is the first of a two-part series in which multicell converters are modelled for the general case of p-cells. This paper focuses on the development of the natural balancing theory for the two-cell case. An understanding of the two-cell case is fundamental to understanding the general balancing theory. The switching functions used in switching these converters are mathematically analyzed. Equivalent circuits are derived and presented. The switching and balancing properties of these converters are mathematically analyzed. The main conclusion of the analysis is that the natural balancing of these converters are influenced by three factors namely, the harmonic content of the reference waveform, the switching frequency and the load impedance. Mathematical tools are presented that can help designers to predict if balancing problems would occur for a particular set of operating conditions. As a result of the detailed understanding of the balancing mechanism that is gained through this theory it is shown that by adding a balance booster, the load impedance can be manipulated to improve the natural balancing of the converter. Simulation results are included to verify the presented balance theory and properties

Natural balancing of three-level neutral-point-clamped PWM inverters

This paper explores the natural balancing mechanisms of the three-level neutral-point-clamped (NPC) multilevel inverter. An equivalent circuit of the three-level NPC inverter is derived that facilitates an understanding of the balancing mechanisms. This is followed by a detailed analysis of the balancing mechanisms. The theory is applied to a three-level NPC inverter under vector control. It is shown that this inverter possesses natural balancing mechanisms in which the load impedance and spectra of the switching functions play an important role. Finally, it is illustrated how the natural balancing can be enhanced by using a passive balancing circuit which provides a low impedance at the sampling frequency.

The solid-state transformer concept: A new era in power distribution

This paper considers the solid-state transformer concept as a viable alternative to the line frequency transformer in distribution circuits. The benefits of the concept are presented in terms of improvements to problem areas currently experienced on distribution networks as well as envisaged future or fringe benefits. Some barriers to universal acceptance of the concept are also identified, with the effects of these barriers analysed and discussed below.

A 1.5-MW Seven-Cell Series-Stacked Converter as an Active Power Filter and Regeneration Converter for a DC Traction Substation

A prototype 1.5 MW series-stacked converter system was built and installed at a dc traction substation. The converter inverts energy generated by slowing an electric train using regenerative braking. The regenerated energy is fed into the local ac network at the dc substation. A secondary function of the converter is to perform active power filtering (APF) while a train is drawing power from the substation. The systems control is based on the instantaneous reactive power theory. An interleaved switching scheme was used to switch the seven-cell series-stacked converter, thus exploiting the natural balancing properties of the series-stacked converter. Results obtained from tests done show that both the APF and regeneration functions of the system were successfully implemented.

An investigation of the natural balancing mechanisms of modular input-series-output-series DC-DC converters

Modular converters have many advantages, however the need to balance voltages and currents equally among the different cells requires careful consideration. Natural balancing occurs when all cells receive the same switching information and the inherent circuit operation balances the different voltages and currents. Natural balancing had been proposed for other modular topologies but it was stated that this method is not applicable to the input-series-output-series, ISOS, converter as it does not have any natural balancing abilities. In this study it is shown that the ISOS have a weak natural balancing ability similar in operation to that of the flying-capacitor converter. When the voltage dependent losses are included in the analysis an additional strong balancing mechanism is also identified.

Natural balancing of neutral-point-clamped converters under POD pulsewidth modulation

The three-level neutral-point-clamped (NPC) converter, being a widely used multilevel inverter, received a lot of attention due to problems associated with DC-link capacitor voltage balancing. There are mainly two problems associated with the neutral-point voltage of the NPC inverter: (1) at high modulation indices a low frequency ripple occurs on the neutral-point voltage; and (2) steady-state unbalance in the neutral-point voltage may arise due to a variety of factors including component imperfections, transients and other nonidealities and imbalances. In this paper we study the balancing problem with focus on the steady-state unbalance. 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 Carraras PWM strategy of phase opposition disposition (POD) and Bennets geometric model for double Fourier series adapted for use with power converter systems by Bowes. The theory is verified through simulation.

Ruston elements and fredholm theory relative to arbitrary homomorphisms

We extend some of the Fredholm theory by, among other things, developing the theory of Ruston and almost Ruston elements and spectra relative to an arbitrary homomorphism. In addition, we provide a number of applications and generalise certain well-known results.

A detailed analysis of the effect of dead time on harmonic distortion in a class D audio amplifier

This paper presents an analytical method for determining the effect of dead time on harmonic distortion in a class D audio amplifier. The analysis is based on the well established double Fourier series, providing accurate analytical solutions to the pulse width modulated spectra under natural sampling. The proposed analytical model differs from that of previous work in the sense that the precise contribution of each separate component can be determined, i.e. the fundamental component, baseband, carrier and sideband harmonics.

An AC-to-AC converter based voltage regulator

This paper presents an AC voltage regulator based on a pulse width modulated AC-to-AC converter and a step-down transformer. The regulator is aimed at electrification of small rural communities and compensates for the voltage variations ranging from +10% to 40% of the nominal. The fast dynamics associated with a pulse width modulated converter is a significant benefit when compared to more conventional techniques, for instance the use of tap changing, transformers.

Direct-coupled cascaded multilevel sag compensator

Voltage sags (or dips) are the most common power quality problem. The aim of this paper is to propose a multilevel-cascaded converter, connected in series with the supply, which acts as a sag compensator. The converter is direct-coupled and eliminates the use of costly output filters and injection transformers. The converter operation and control techniques are discussed and confirmed with simulations and practical results.