STUDIES ON SOLAR GRID-INTERCONNECTED QUASI Z-SOURCE INVERTER

Abstract

Renewable energy sources present a trendy solution for energy shortage problem. However, these sources features variable and inconsistent power delivery over the course of the day. Power converters is, therefore, installed to stabilize the power delivery and regulate the output voltage to the utility level. Quasi Z-Source Inverter (QZSI) represents a single-stage buck-boost DC-AC converter, alternative to the traditional DC-DC converter cascaded with voltage-sourced inverter VSI topology, and is suitable for renewable energy integration systems. In this thesis, a detailed analysis and design of QZSI is presented for grid-interconnected applications. The design procedure of the output low-pass filter is addressed; taking into account the variation in harmonic performance between the traditional PWM techniques for voltage source inverters and the modified PWM techniques for ZSIs. Moreover, an analytical study of harmonic performance of six different modulation techniques for ZSIs is presented. The harmonic spectrum of each modulation technique is estimated using double Fourier series approach. The analytical approach is adopted to exclude the common errors caused by practical conditions from simulation or experimental setups, and to present a fair comparison between the different modulation techniques. A collective evaluation of the modulation techniques using weighted total harmonic distortion is presented; taking into account the impact of the applied voltage gain on the harmonic performance. This thesis, furthermore, proposes a control system design for QZSI in PV grid-tied applications. The control system is designed based on the dynamic characteristics of the converter and comprises three cascaded controllers. The first is output current controller designed in the synchronous rotating frame. The second is DC-link voltage controller. Sampling and measurement of the DC-link voltage signal is discussed using modified space-vector modulation. The last is maximum power point tracking (MPPT) in which a new strategy is proposed to provide wide operating range of DC-link voltage and shoot-through duty ratio, minimize the voltage stress on the inverter bridge, and prevent overlap between shoot-through duty ratio and modulation index. Simulation results are presented to validate the controllers design.