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The gap between electric power generation and demand increases day by day owing to the exponential growth of power consumption.
Eventually the power holidays and load shedding have become unavoidable in developing countries like ours. Power interruption leading to various devastations has been experienced and is proven to be a threat to power consumers especially in large industries. Losses in terms of life, finance and productivity are the most common aftermath of power interruption. Hence, a smart power backup and protection system has been developed. It is known as
Uninterruptible Power Supply (UPS) that provides solutions for the power supply interruptions.
UPS has become a crucial remedy for institutions like hospitals, research laboratories, offices and even in domestic circles to ensure continuous power supply. The primary role of any UPS is to provide short term power when the input power source fails. The improper output waveforms and harmonic injection on either ends are the major problems as for as any UPS system is concerned and need to be addressed immediately.
Electrical power supply interference can come in a range of forms, such as voltage dips, surges and harmonics or voltage spikes. These disruptions can cause serious harm to sensitive electrical equipment, particularly during the critical processing or production stages of an operation. To reduce the risk of power supply distortion, UPS systems are

iv often incorporated in electrical networks. UPS systems provide reliable, high quality power flow for sensitive electrical load equipment and are commonly found in industrial processing applications, medical facilities, emergency equipment, telecommunications and computerized data systems. An UPS system can be a helpful tool for ensuring proper power supply performance.
An UPS is supposed to provide quality sinusoidal signal during voltage sag and power quality problems in AC mains of the utilities. But during these periods the traditional power converters in the UPS systems are not efficient to provide quality output. The traditional PWM inverter in UPS having dead time, creates power quality problems and hence introduces harmonics, torque ripples etc. During unbalanced voltage conditions, input side and output load side create lot of troubles in power quality.
Traditionally electrical utility neutral wire is used as the fourth wire of UPS system. In case of any power failure in the AC mains, the fourth wire is isolated. In this situation four wire inverter and Four Wire Z-Source
Inverter (FWZSI) are introduced. Four wire inverter overcomes the power quality issues during unbalanced load conditions. However this is inefficient during voltage sag conditions. Unique impedance circuit (L-C circuit) couples the inverter main circuit with the power source, thus providing buck - boost voltage by controlling the boost factor that increases efficiency and reduces cost when compared with the traditional voltage source inverter and current source inverter. The proposed Z-Source inverter systems provide quality output during voltage sag and unbalanced conditions in AC mains and load

v side. Z- Source inverter operates in boost mode during both voltage sag and power failure and also it acts as a traditional inverter during normal operation.
To further improve the performance of UPS system, Z-Source
Multilevel Inverter (ZSMLI) is proposed. Multilevel inverter synthesizes a desired output voltage from several levels of input DC voltage sources. With an increasing number of DC voltage sources, the inverter voltage output waveform approaches nearly sinusoidal waveforms. As compared with traditional two level inverters, the multilevel inverters have more advantages which include lower semiconductor voltage stress, better harmonic performance, low Electromagnetic Interference (EMI) and lower switching losses. Despite these advantages output voltage of multilevel inverter amplitude is limited to DC source voltage summation. Occurrence of short circuit can destroy multilevel inverters and therefore multilevel inverters need to be operated with dead time protection. To solve these problems, Z-source multilevel inverter (ZSMLI) is proposed. The Z-Source Multilevel Inverter
(ZSMLI) utilizes Z impedance network between the DC source and inverter circuitry to achieve boost operation. The Z-Source Multilevel Inverter
(ZSMLI), unlike traditional inverters can utilize shoot through states to boost the input dc voltage of inverter switches when both switches in the same phase leg are on.
The main objective of this research is to address the above shortcomings and to develop methods to overcome them. In connection with this aim, the following works have been carried out. A Four Wire Z-Source
Inverter (FWZSI) and Z-Source Multilevel Inverter (ZSMLI) are proposed to

vi overcome the drawbacks of conventional power converter based UPS systems. The voltage gain, switching stress and efficiency of conventional and proposed UPS systems are compared. Prototype model of the both systems has been fabricated and tested.