INVESTIGATIONS ON ENHANCED-BOOST Z-SOURCE INVERTERS

Abstract

Increased demand of energy throughout the world, shortage of fossil fuels, and environmental problems caused by conventional power generation has led to an urgent search for renewable energy sources (RES) and harvest maximum energy from them. Renewable energy is energy that comes from natural resources such as sunlight, wind, tides, and geothermal heat, which are naturally replenished at a constant rate. These comprise of wind, biomass, geothermal, thermoelectric generation (TEG), solar photovoltaic (SPV), tidal, and wave energy systems. Renewable energy sources are clean, inexhaustible, and are thought to be “free” energy sources, such as solar and wind energies. Among all these renewable energy sources, the photovoltaic energy is being widely utilized because of the ubiquity, abundance and sustainability of solar radiant energy. These photovoltaic cells or solar cells directly use the energy from the sun to generate electricity. But, the photovoltaic cell or module produces the peak (or maximum) power at a particular terminal voltage of the cell or module. Thus in order to extract this peak power from the module, a power conditioning circuit (also termed as power electronic interface) is needed. In addition, this power electronic interface is also need to feed extracted power from photovoltaic module to the grid or to the load at a required voltage level. To achieve this, generally traditional single-stage voltage source inverters (VSIs) were used as power electronic interface. But there are several limitations and disadvantages of single-stage inverters (VSIs) like: a) it is only a buck converter for DC-AC power conversion. b) The two semiconductor switches from the same arm or leg of the inverter bridge cannot be gated on simultaneously. Otherwise, a shoot-through will destroy the devices. c) Dead time must be employed, which will cause output voltage/current distortion. Therefore, usually DC-DC boost converter is cascaded in between the VSI bridge and supply terminals to boost the voltage to the required level. But, other drawbacks are remaining same. Reliability of these single-stage and two-stage power conversion topologies is less. Therefore, in order to avoid the aforementioned drawbacks and limitations of both single-stage and two-stage power conversion systems, impedance (Z)-source inverter was proposed in 2002 with increased reliability. The Z-source inverter (ZSI) provides both buck and boost DC-AC inversion in a single-stage with high electromagnetic interference (EMI). First developed Z-source inverter (ZSI) has certain drawbacks like: a) it draws discontinuous input current from the supply, b) ii more stress across the capacitors, c) huge inrush current at start-up condition, d) does not share common ground with source, and e) modulation index is limited by the shoot-through duty ratio which leads to poor utilization of the dc-link voltage and higher stress on the semiconductor switches. Except the last point, remaining all drawbacks can be eliminated by the quasi-Z-source inverters (qZSIs) with same boost factor. Many active impedance (Z)-source inverters (i.e., the impedance network consist of one active switch, two-diodes, and one capacitor) were proposed in the literature which produces the boot factor about same as that of the traditional ZSIs. But, the main drawbacks of these topologies are more stress across the capacitor and semiconductor switches. Generally, solar photovoltaic (SPV) module systems need high boost inverters to connect it to the grid/load. Therefore, to increase the boost factor, the switched-inductor Z-source inverter (SL-ZSI) was presented. But, the SL-ZSI has same drawbacks as the traditional ZSI. So, in order to avoid the drawbacks of SL-ZSI, the SL-qZSIs and Diode/Capacitor-Assisted qZSIs were proposed. To further improve the boost factor, many magnetically coupled impedance source (MCIS) network topologies were proposed in the literature with less number of components (i.e., both passive and active) in the impedance network. The main disadvantages of coupled-inductor (MCIS) based topologies are; their leakage inductance must be low or they must be tightly coupled, otherwise a high voltage spike appears across the semiconductor switches and dc-link of the inverter bridge. Moreover, the stress across the elements of the impedance network and power switch is also increased. This can lead to use of high rating devices, which in turn increase the cost of the system. Therefore, enhanced-boost Z-source inverter (EB-ZSI) with two switched-impedance networks topology was proposed recently which provides high boost at low shoot-through duty ratio without any high spikes across the dc-link voltage and power switches. Moreover, the stress across the switches and the components are less. Even though the EB-ZSI topology uses more number of components in the impedance network (i.e., four inductors, four capacitors, and five diodes) when compare to existing topologies, the stresses across those devices (capacitors, diodes, and switches) is less. Therefore, lower rating devices can be used which results in low cost. The EB-ZSI also has similar drawbacks to that of the traditional ZSI/ SL-ZSI. In order to modulate and to control the all existing Z-source inverters, iii many modulation techniques were presented in the literature. But for simplicity most of the impedance source networks were analyzed using simple boost method of control. In this thesis, improved enhanced-boost Z-source inverters are presented which provides high voltage boost in a single-stage at low shoot-through duty ratio and at high modulation index with high reliability, and shares common ground with the source and inverter bridge. Moreover, these presented topologies reduce the starting inrush current problem and capacitor stress. The expressions for inductance and capacitance design are derived. All the inductors, capacitors, switches and diodes with lower ratings can be used owing to the lower voltage stresses so that the cost is largely decreased. Throughout this thesis, the analysis of these presented topologies is carried out using simple boost control (SBC) technique due to its simple structure. Firstly, the analysis and derivations of different voltage stresses of One switched-inductor ‘Z-source’ / ‘Improved Z’-source inverter is presented. Then, the voltage-lift type of ‘Z-source’ / ‘Improved Z’-source inverter which is derived from one switched-inductor Z-source inverter (One SL-ZSI) is proposed to get high voltage boost with same number of elements in the impedance network just by replacing the middle diode of SL-cell with voltage-lift capacitor. Then, the two configurations of enhanced-boost quasi Z-source inverters (EB-qZSIs) are proposed in this thesis which provides similar voltage boost compare to EB-ZSI using same number of passive components, and draws continuous input current from the input supply which improves the lifespan of passive components. In addition to this, the stress across the capacitors is also reduced. To reduce the capacitor stress of EB-ZSI further, improved topology of EB-ZSI is proposed in this thesis which is named as enhanced-boost series Z-source inverter (EB-SZSI). Therefore, lower rating capacitors can be used to reduce the cost of the system. Another enhanced-boost quasi Z-source inverter is proposed in this thesis which is named as four different configurations of enhanced-boost quasi Z-source inverters which reduces the capacitor stresses further. Except the discrete input current, the other advantages of EB-qZSIs are remained. All these presented topologies are compared with existing Z-network topologies. For the same input voltage and boost factor, the proposed topologies provides less stress across the capacitors, diodes, and semiconductor switches in comparison with existing topologies. Moreover, the proposed topologies require low iv shoot-through duty ratio and high modulation index to obtain the same voltage boost. Even though these proposed topologies uses more number of components in the impedance network, the stresses across those devices (capacitors, diodes, and switches) is less which results in low cost and less weight. The thesis is organized into eight chapters. In Chapter 1, a brief introduction of traditional inverters for solar photovoltaic systems such as single-stage and two-stage conversion systems has been given. The drawbacks of traditional single-stage inverter and two-stage inverter topology and the need of impedance (Z)-source inverter (ZSI) topology are discussed. The major contributions of the thesis and the organization of the thesis are also detailed. Chapter 2 starts with state-of-art of different impedance (Z)-network inverter topologies including conventional Z-source inverter are provided. All these impedance (Z)-source network topologies are broadly classified into coupled-based and non-coupled based inverter topologies and are discussed briefly. Among the non-coupled based topologies available in the literature, the enhanced-boost Z-source inverter (EB-ZSI) with two-switched impedance network topology provides very high boost factor at low shoot-though duty ratio and high modulation index and provides better quality output waveforms. The boost factor of the existing inverter topologies is provided for gain comparison. Some of the magnetically coupled impedance source (MCIS) networks are also addressed, which gives high boost factor at low shoot-through duty ratio with less number of components and the drawback of MCIS networks are also highlighted. In Chapter 3, the circuit development of the one switched-inductor Z-source inverter (one SL-ZSI) from a conventional Z-source inverter is presented. The operating principle, steady-state operation and boost factor derivation of one SL- is explained. The comparison and advantages of one switched-inductor improved Z-source inverter over one SL-ZSI is described in detail. The steady-state operation of one switched-inductor improved Z-source inverter is also validated in experiments. Chapter 4 presents the voltage-lift concept of Z-source/improved Z-source inverters which is derived from one switched-inductor Z-source inverter. Steady-state operation, derivation of boost factor and voltage gain, and performance comparison of voltage-lift ZSI/improved ZSI is described. The advantages and comparison of both the topologies is discussed in detail. The simulation and experimental results is also given to validate the theoretical analysis. v In Chapter 5, the circuit development of the enhanced-boost quasi Z-source inverters (EB-qZSIs) with two switched-impedance network is discussed in detail. The steady-state and principle of operation of EB-qZSIs is explained and the mathematical equation for capacitors and voltage gain is established. The performance comparison of the proposed and existing topologies is also described. The advantages and drawbacks of the proposed EB-qZSIs and other Z-network topologies are described. The steady-state operation of the enhanced-boost quasi Z-source inverters is validated in simulations and experiments to verify the theoretical analysis. In Chapter 6, high boost switched-impedance Z-source inverter, named enhanced-boost series Z-source inverter with two switched impedance network is studied in details. The theoretical analysis of the switched-impedance network series ZSI is validated in simulations and experiments. The detailed performance of the series switched-impedance Z-source inverter is compared in its class to uncover its operational advantages. Chapter 7 describes another enhanced-boost quasi Z-source inverter, named as four different configurations of enhanced-boost quasi Z-source inverters. In this chapter, four configurations of EB-qZSIs topologies are presented. All these topologies provide same boost factor or voltage gain but with different capacitor stresses. The detail steady-state analysis, design of impedance networks and comparison with other EB-ZSIs are presented. Simulation and experimental tests are conducted in order to validate the theoretical expressions. The general conclusions of the presented work and possible future research have been summarized