Space vector pulse-width modulation theory and topology improvement for Z-source inverters

High-performance voltage- and current- source inverters (VSIs and CSIs) are the most important components in various direct current (DC)/alternating current (AC) conversion utilities. However, with the development of the renewable energy, the traditional VSIs and CSIs have been seriously restricted due to their narrow obtainable output voltage range property. In 2002, an impedance-source power inverter (termed Z-source inverter (ZSI)) was proposed to overcome the limitations and barriers in traditional VSIs and CSIs. It has successfully integrated the function of the traditional DC/DC boost converter into the DC/AC inverter by a unique X-shape impedance network. Owing to its inherent advantages, such as wider obtainable output voltage range, higher reliability and higher efficiency, it has drawn many researchers’ attention. As a research hotspot in power electronics, the Z-source topology has been greatly explored from various aspects, including developed topologies, modulation techniques, mathematical modelings and control strategies. However, most of the carrier-based modulation strategies are found to have the drawbacks of higher switching actions than that of the traditional VSI, which would introduce more switching loss of the system. To solve this problem, the space vector pulse width modulation (SVPWM) theory of ZSI has been explored. Based on an in-depth mathematical derivation and theoretical explanation, the SVPWM theory of three-phase ZSI has been discussed in detail. Besides, novel SVPWM-based implementation schemes of two popular modulation methods (maximum boost control method and constant boost control method) have been presented. By selecting different null state and shoot-through state, three switching patterns of SVPWM technique have been explored and compared with the carrier-based strategies. In addition, the SVPWM concept has been successfully extended to the single-phase ZSI. With consideration on the circuit characteristics, the theory and implementation schemes of the SVPWM strategy for single-phase ZSI have been deduced and explained thoroughly. By properly selecting the shoot-through vectors and null vectors, switching patterns with less switching actions have been presented as well. These efforts will be helpful for understanding the SVPWM concept and promoting the industrial applications of the ZSI. As is known, the voltage boost ability is the most notable advantage of the ZSI. However, this capability of the classical ZSI is very limited, which will certainly restrict its further applications for low voltage energy sources that require strong boost inversion abilities, such as fuel cells, batteries and photovoltaic (PV) systems. Hence, an in-depth research on advanced topology with a higher voltage gain has been performed. It is noted that the voltage boost ability of the ZSI comes from its impedance network. To get further understanding of the impedance network, the small signal models of the improved ZSI have been studied by employing state-space averaging method. Through the study of the dynamic modeling, a developed impedance-type power inverter that is termed the switched-inductor (SL) ZSI is proposed based on the classical ZSI to improve the voltage adjustable ability. It employs two SL cells to replace the inductors of the impedance network. With this modification, the proposed SL ZSI increases the voltage boost inversion ability significantly compared with the classical ZSI. Only a very short shoot-through state is required to obtain high voltage conversion ratio, which is beneficial for improving the output power quality of the main circuit. In addition, the voltage buck inversion ability is also provided in the proposed inverter for those applications that need low AC voltages. Same to the classical ZSI, the proposed SL ZSI can be applied to various applications of DC/AC, AC/AC, DC/DC and AC/DC power conversion, which will be beneficial for the engineering applications using impedance-type power inverters. With the view of detailed analyzing the dynamic system and designing closed-loop controllers of the proposed SL ZSI, an accurate small signal model of the SL ZSI has also been presented. By employing the state-space averaging method, small signal models have been established and related transfer functions have been derived. Through mathematical analyses, the right-half plane (RHP) zeros are found to present in the transfer functions, which would degrade the performance of the closed-loop system. In order to keep satisfactory performance and stability, a group of root loci plots are illustrated to observe the movements of the RHP zeros. Besides, the influences of the Z-network parameters on the NMP effects caused by the RHP zeros have been investigated. Furthermore, the SVPWM theory and implementation schemes for single- and three-phase SL ZSI have also been explored. As with that of the classical ZSI, the SVPWM theories of single- and three-phase SL ZSI together with different switching patterns have been discussed in detail. In addition, implementation schemes of maximum boost control method and constant boost control method of three-phase SL ZSI based on SVPWM theory have also been presented and compared with that of the classical ZSI, which shows the advantage of the SL ZSI in boost inversion ability. All the proposed topology and control schemes have been validated by simulation and experimental investigations. These efforts will be helpful for understanding the SVWPM theory and promoting the applications of the impedance-type power inverters.DOCTOR OF PHILOSOPHY (EEE