Modulation technique and control of DC/AC inverters with low harmonic distortion

Abstract

This thesis presents a research on the modulation technique and control of DC/AC inverters with low harmonic distortion. The strategies are applied to the main parts in an inverter, including the power stage and the controller. A new transient boost technique for the inverter has been proposed for the power stage. This technique can solve the problem of pulse dropping and widen the modulation range, in order to reduce the harmonic distortion at the output to give better input/output reproduction and amplification. The use of boundary control with second-order switching surface for the control of DC/AC inverters has been proposed for the controller section. The control method is simple and does not require any complicated computation of the system transfer function or control loop compensation. The output voltage can obtain near-optimal response when it is subject to large-signal disturbances or input voltage variation. With further enhancement in the boundary control scheme, the high-order switching surface of the boundary control for DC/AC inverters has been proposed. The proposed switching surface takes the state variation into consideration, this gives better performance of replication and amplification of the reference signal. The contents of this thesis are as follows. In Chapter 1, the operational characteristics and limitations of existing DC/AC inverters will be discussed. Then, a survey on various methods for dealing with harmonic distortion will be presented. The methods under investigation can be classified into two main streams, including power stage modulation technique and control of the switching profile. In Chapter 2, the concept of transient dc-link boost technique will be derived. It is based on increasing the dc link voltage and reducing the modulation index momentarily, when the modulating signal is large. Apart from effectively reducing harmonic distortion at the output, the proposed method can also widen the dynamic modulation range and thus reduce the required supply voltage. A 10 W prototype with a full-bridge inverter has been built. Some pulses are artificially dropped in the gate signals to simulate the pulse-dropping effect. The inverter output harmonic spectra with and without the transient boost have been compared. In Chapter 3, the concept of using a second-order switching surface in the boundary control of inverters will be derived. It is based on estimating the state trajectory movement after a switching action, resulting in a high state trajectory velocity along the switching surface. This phenomenon accelerates the trajectory moving toward the target operating point. A 100 W full-bridge inverter has been built. Dynamic responses of the inverter supplying to a resistive load, an inductive load and a diode rectifying circuit have been studied. In Chapter 4, a generalized high-order switching surface for the boundary control of inverters will be derived. It gives better dynamic control of inverters than the first-order or second-order switching surfaces discussed in chapter 3. As the proposed switching surface is much more close to the ideal switching surface, the trajectory velocity along the switching surface is very high. This makes the trajectory move toward the target operating point in two switching actions during any level of signal disturbance. The proposed control method has been successfully applied to a 300 W full-bridge inverter. The system can revert to the steady state in two switching actions during the any level of signal variations in the input voltage, load disturbances, and output reference. In Chapter 5, an overall conclusion of the research topics and some suggestions for further research will be given.

Date of Award	15 Feb 2007
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Shu Hung Henry CHUNG (Supervisor)