Hendrik du Toit Mouton

Variable Switching Point Predictive Torque Control of Induction Machines

This paper introduces an approach to include a variable switching time point into predictive torque control (PTC). In PTC, the switching frequency is limited by the sampling frequency; its theoretical maximum value is half the sampling frequency. However, in reality the switching frequency is lower than this value, and thus, high current and torque ripples occur compared with modulator-based control methods. In order to overcome this, an optimization problem is formulated and solved in real time. Thereby, apart from the regulation of the torque and the flux magnitude to their references, an additional control objective should be met: the minimization of the torque ripple. To do so, the time point at which the switches of the inverter should change state is calculated. Further advantages of the proposed method include the design flexibility and great performance during transients. Experimental results that verify the performance of the presented control strategy are included.

Understanding the PWM Nonlinearity: Single-Sided Modulation

This paper provides a new look at the mechanisms underlying pulse width modulation (PWM). A simple approach to analyze the behavior of a single-sided pulse width modulator is presented. By using elementary methods, the pulse-width-modulated waveform is written as the sum of two sawtooth functions and the original modulating waveform. Simply applying the well-known Fourier series expansion of the sawtooth function, an equivalent model of the pulse width modulator, which shows that it is in essence a sequence of phase modulators, is derived. This model provides a clear understanding of the nonlinearities involved in the PWM process. It is shown how the superposition of modulating waveforms in the time-domain translates into the convolution of the sidebands in the frequency domain. Finally, the interaction of the pulse width modulator and a sample-and-hold register is studied and a general expression for the Fourier transform of a regular-sampled PWM waveform is derived. The analysis applies to periodic as well as aperiodic modulating waveforms.

Design optimization of an active resonant snubber for high power IGBT converters

The design optimization procedure for a new active resonant snubber topology, specifically suited for high power insulated gate bipolar transistor (IGBT) converters, is introduced. After the basic operation principles and certain implementation consideration are discussed, the optimization strategy, based on an analytical loss evaluation, is described. Experimental results obtained on an IGBT phase-arm fitted with an optimally designed snubber are presented.

Analytical Modeling of the Effect of Nonlinear Switching Transition Curves on Harmonic Distortion in Class D Audio Amplifiers

Practical switching transition times typically occupy less than one percent of the switching period in class D audio amplifiers. These transitions, however, are a very nonlinear function of current, which give rise to significant levels of distortion. This paper presents a method for determining the effect of nonlinear switching transition curves on harmonic distortion within half-bridge and full-bridge topologies, taking into account the interaction between the various dominant pulse-timing errors. An accurate analytical model, based on the well-established double Fourier series method of analysis, for determining the effect on harmonic distortion is introduced. The analytical model is accompanied by an equivalent simulation model using an existing strategy, and the results are verified experimentally.

Predictive Control of Series Stacked Flying-Capacitor Active Rectifiers

This paper considers the use of back-to-back, full-bridge, three-level flying-capacitor converters in a series-input-parallel-output connected fashion for the implementation of a solid-state transformer. A finite-control-set model-based predictive control algorithm is developed for the control of the active rectifier front-ends. Pulse width modulation is used for the isolation back-ends. A solid-state transformer is constructed with two of the proposed back-to-back converters and experimental results are presented.

A Fixed Switching Frequency Scheme for Finite-Control-Set Model Predictive Control—Concept and Algorithm

A fixed switching frequency scheme for finite-control-set model predictive control is presented to achieve an output waveform quality that compares well to that of a pulse-width-modulator-based linear controller, while retaining the benefits of model predictive control. The controller is divided into two parts: the offline calculation and storage of the coefficients of the prediction equations and the online evaluation of the controller. The controller is experimentally evaluated on a five-level flying-capacitor converter.

One-Dimensional Spectral Analysis of Complex PWM Waveforms Using Superposition

This paper takes a new look at the mechanisms underlying the double-edge pulse-width modulation (PWM) process. It presents a novel way of deriving equations for the spectrum of double-edge PWM using basic mathematical techniques. In the process the underlying nonlinearities that generate the PWM sidebands are identified. Unlike the classical double Fourier series approach, the proposed method of deriving the PWM spectrum does not require the construction of the so-called unit cell. The interaction between this new model of the pulse-width modulator and the regular sampling process is studied, and generalized equations for the Fourier transforms of regularly sampled PWM waveforms are derived. A general solution to the important question of what happens to the PWM spectrum when the PWM reference consists of a summation of signals is presented. It is shown that the addition of reference signals in the time domain results in a double convolution of the PWM sidebands in the frequency domain. As an application of this result, it is shown how new analytic equations for the harmonics of third-harmonic injection PWM and space vector modulation can easily be derived. Finally, the new theoretical results are benchmarked against results from the well-established double Fourier series method.

Multistep Model Predictive Control for Cascaded H-Bridge Inverters: Formulation and Analysis

In this paper, a suitable long prediction horizon (multistep) model predictive control (MPC) formulation for cascaded H-bridge inverters is proposed. The MPC is formulated to include the full steady-state system information in terms of output current and output voltage references. Generally, basic single-step predictive controllers only track the current references. As a distinctive feature, the proposed MPC also tracks the control input references, which in this case is designed to minimize the common-mode voltage (CMV). This allows the controller to address both output current and CMV targets in a single optimization. To reduce the computational effort introduced by a long prediction horizon implementation, the proposed MPC formulation is transformed into an equivalent optimization problem that can be solved by a fast sphere decoding algorithm. Moreover, the benefits of including the control input references in the proposed formulation are analyzed based on this equivalent optimization problem. This analysis is key to understand how the proposed MPC formulation can handle both control targets. Experimental results show that the proposal provides an improved steady-state performance in terms of current distortion, inverter voltages symmetry, and CMV.

A series-shunt compensator with combined UPS operation

Series-shunt power quality compensators with the ability to function as an uninterruptible power supply (UPS) can solve a variety of power quality problems. This topology consists of two power electronic inverters. The shunt inverter is typically used as a voltage source during UPS operation, requiring a shunt inverter rated for the full load power. The rating of the series inverter is typically minimized to reduce cost, resulting in limited voltage compensation. This paper suggests a topology that enables the series inverter of a series-shunt compensator to assist the shunt inverter during UPS operation. The proposed topology allows the use of a shunt inverter with a power rating lower than the load rating, resulting in a compensator with reduced power rating while maintaining a high capacity for voltage compensation. This paper also suggests an arrangement of passive filters for switching ripple that does not adversely affect series voltage compensation. Practical results are presented, illustrating the operation of the proposed series-shunt UPS and series-shunt compensation for a capacitive nonlinear load.

An investigation into series power tapping options of HVDC transmission lines

The first commercial success in HVDC transmission was made in 1954 and it has since become popular as attempts have also been made to tap energy from the HVDC line itself. This paper discusses technical issues that are related to series energy tapping options for HVDC lines, with possible solutions. The performance and behavior of four different design options for a 2 MW series tap are investigated through simulations. Results are presented with comparisons, highlighting the key characteristics of each design, as proof of concept and as well as to demonstrate that series power tapping from HVDC lines is viable.