Development of quasi z source multilevel inverter for photovoltaic systems

Abstract

Photovoltaic (PV) power generation systems have emerged as an attractive area of research recently due to its rich persistence, universality and harmless nature. As PV is susceptible to variation in temperature and irradiation, the inverter should perform the essential tasks such as adjusting the output voltage, generating sinusoidal output voltage and current with reduced harmonics. The conventionally employed double-stage inverters uses a DC-DC boost converter to boost the widely varying input voltage to a constant DC output voltage. The additional components used in it increases the system cost and reduces the efficiency. Therefore, single-stage inverter that boost/buck the input voltage using shoot-through state is an attractive solution due to its high reliability, low cost and compactness. In this thesis, the quasi impedance source inverter (qZSI) is taken as the preeminent topology. Nowadays, the multilevel inverter topologies have gained momentum in renewable energy systems due to its various advantages such as reduced voltage stress on power devices, common mode voltage and harmonics in the output voltage and current. The desired output voltage can be synthesized by series connection of several single-stage qZSI s fed from separate PV sources. In this work, a single-phase photovoltaic quasi Z-source multilevel inverter (PV-qZMLI) is proposed and its overall performance is analysed in detail. The quasi impedance network components designed using traditional method employs the linearized voltage waveforms of the operating states and results in bulky impedance network size. Hence, this work focusses on designing the impedance network by double frequency ripple analysis to minimize the size of the impedance network elements. The size reduction of the components in qZMLI is validated by comparing with conventional method and three more topologies such as Z, Embedded Z and Trans quasi Z networks by applying the same analysis. The functionality of the inverter is assessed by the modulation techniques. In this work, different modulation strategies for the proposed qZMLI consisting of phase shift techniques such as simple boost control, maximum boost control and maximum constant boost control for sinusoidal pulse width modulation, pulse width amplitude modulation and hybrid pulse width modulation are investigated. A modified pulse width amplitude modulation is proposed for the qZMLI. Among the carrier phase shift techniques, the proposed modified PSPWAM produces output voltage with required boost factor, reduced harmonics, reduced switching loss and improved efficiency. A comprehensive mathematical analysis is carried out for the proposed modulation strategy. newline