The project is to design, analyze and simulation of a single - phase 240V UPS (Uninterrupted Power Supply) system with improved power quality both at input and output. The system is simulated in MATLAB/SIMULINK environment. A detailed literature survey has been done to understand power quality issues and advantages of ‘modern UPS’ over conventional ones. Initially the ‘DC/AC inverter’ part of the UPS system is designed and simulated using ‘sinusoidal PWM inverter’ with uni-polar switching of devices to obtain pure sinusoidal output waveform. A high frequency link is used at the input of the ‘inverter’ to eliminate bulky output line frequency transformer. A PI controller is used to regulate the ‘inverter’ output against changes in load and input DC voltage magnitude. The DC bus voltage of 220V is selected where a ‘battery bank’ is connected to feed the ‘inverter’ in case of utility failure.

In the second part the project, AC/DC converter with input current shaping is designed and analyzed. This converter feeds DC power to ‘inverter’ as well as ‘battery’ charging. For ‘input current shaping’ or ‘power factor correction’ a two - stage conversion is adopted. In the first stage input ac is converted to full wave DC using uncontrolled full bridge diode rectifier without any output capacitor. In the second stage a DC/DC converter with high switching frequency is used. SEPIC DC/DC converter is chosen for this project for its inherent momentary power storage capability and positive output polarity. Both voltage and current mode controls are adopted for this DC/DC converter for output voltage regulation as will as input current shaping.

Both AC/DC converter and DC/AC inverter with HF link are concatenated to form a complete UPS system and its static and dynamic responses are studied on simulation of the system and results are presented. The simulation results have been confirmed good static and dynamic response of the system and also improved power quality at input and output of UPS.