An Enhanced Exponential Reaching Law Based Sliding Mode Control Strategy for a Three Phase UPS System

Abstract

UPS is used to deliver a high quality sinusoidal waveform to consumers without any interruptions. This work proposes an Enhanced Exponential Reaching Law (EERL) based Sliding Mode Control (SMC) for a three- phase UPS 30 kVA system used in industrial Applications. The work presents the application of proposed controller on the rectifier side as well as on the inverter side feeding a linear and/or a nonlinear load. Mathematical models of the three phase rectifier (AC/DC) and three phase inverter (DC/AC) are derived. Frequency response characteristics are plotted to observe the system stability using state feedback approach. A DC/DC buck/boost converter is utilised for charging and discharging of battery which acts as a secondary power source for the UPS to feed critical loads. It also maintains power balance. The design of SMC, HOSMC and EERL-SMC are presented. The EERL based SMC is used in this work to obtain a pure sinusoidal waveform with less settling time compared to a conventional SMC. It is robust against sudden changes in load and is more efficient compared to SMC and higher order sliding mode controller (HOSMC). Using EERL, chattering phenomenon can be very much reduced with less steady state error. Chattering is observed through phase plane plots in this paper. The system is presented with both linear and nonlinear loads. A comparison is brought with respect to a classical SMC and a higher order SMC (super twisting algorithm) for a UPS system. EERL-SMC performs better in terms of pure sinusoidal waveform, good tracking, less settling time (4ms) and less steady state error (1.74%) with low THD (0.12%). It can be an alternative to a HOSMC. Simulation studies are presented in PSCAD/EMTDC version 4.6. The system stability conditions are analysed from frequency response plots obtained through MATLAB R2012b platform.