An investigation into the output spectral characteristics of PWM power inverters

Abstract

Pulse-Width-Modulated inverters become very important in power applications in recent decades. Many different types of topology and control strategy have been developed to improve performance to cater for the increasing needs. One of the major concerns of the control strategy is to reduce the undesirable harmonics in order to improve the performance and efficiency. Facing the need of high power applications, multilevel inverters have been widely studied. To exploit the high efficiency, PWM inverter has also been used in digital audio power amplifier application, which will operate the inverter at a relatively high switching frequency. For all these applications, the quality of PWM output spectral is of particular concern since it can degrade the system performance considerably. This thesis presents an extensive study on the spectral characteristics of a number of PWM inverter topologies. The analysis adopts a generalized approach and the solution is applicable to most sinusoidal PWM inverter topologies. It has the advantages of easy understanding of the derivation and only simple numerical integration technique is required to compute the analytical solution, and in particular closed-form solutions are also available under certain operating conditions. The general solutions provide a very convenient tool for researchers who would like to have an in-depth insight of the PWM spectrum under various control strategies. In addition, non-ideal operating characteristics such as dead-time, transient response and pulse dropping have also been included in the study for understanding their spectral distortion effects. By decomposing the multilevel PWM into a series of sub PWM and using the double Fourier analysis, compact analytical solutions for arbitrary level with arbitrary carrier phase composition have been obtained. An elegant 3-D function is obtained to describe the spectral characteristics through the inter-modulation combinations between the carrier and modulating signal frequency. Both the solutions for naturally and uniformly-sampled multi-carrier based PWM have been derived. There has been considerable research emphasis on multilevel power converters, driven substantially by their ability to operate at higher power levels and the potential for a reduction in the harmonic content. In this thesis, we present a theoretical study and experimental results of using phase-shift technique to dilate the output spectrum of a five-level neutral-point-clamped (NPC) PWM H-bridge inverter. It is shown that the output spectrum can be made to only consist of the multiple of the fourth order inter-modulated harmonics with appropriate phase-shift combination. The spectrum dilation offers an advantage of reducing both the number and magnitudes of the low-order signal harmonics without increasing the carrier frequency which will inevitably increase the switching loss. Pulse dropping is one of the key parameters which cause poor performance of PWM output. A novel design, called Transient Dynamic Boost, has been studied, which makes full use of the DC link voltage and at the same time avoids pulse dropping. The pulse dropping effects and the compensation due to the transient dynamic boost have been theoretically analyzed to justify the new topology. Various considerations for determining the control parameters are also studied in detail. An experimental PWM inverter based prototype digital power amplifier has been implemented to demonstrate the effectiveness of spectral compensation of the transient dynamic boost. Transient ringing at the switching edges of the PWM pulse commonly exists due to various reasons. However, it has been claimed as one of the causes for the harmonic distortion simply judging from experimental observations. Theoretical analysis on the spectral distortion caused by transient ringing has never been provided due to its complexity involved. An advanced 3D model has been developed with the consideration of the transient effects on both rising and falling edges. The ringing is modeled using the step response of a second-order circuit. With this theoretical solution, the role of the transient ringing in harmonic distortion is clearly verified for the first time and it is found that its effects on harmonics distortion are insignificant. This solution is an important supplement to the PWM research area. Finally, a novel dead-time generation and compensation method for 2-level PWM inverters is presented. Inserting dead-time to the PWM driving signal is essential in hardware implementation despite of causing spectral distortion. By varying the slope of the tri-angular carrier signal, the dead-time produced is auto-adaptive and the duration of the dead-time is self-adjustable to avoid pulse dropping. In addition, it can also help increase the dynamic range of the input signal. The spectral distortion introduced is suppressed using a simple compensation approach and its effectiveness has been confirmed via simulations with the consideration of the output filter and load.

Date of Award	16 Jul 2007
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Wing Hong Ricky LAU (Supervisor)