PERFORMANCE ANALYSIS OF UPS INVERTERS SYSTEM

Abstract

Ups Inverters have been continuously growing during the past years due to power deficits, frequent power cuts, unreliable grid, digital dependency of day to day life, and most important, the demand of clean energy. Developments in power electronic devices, fast processors and advancements in utilisation techniques of renewable sources further catalyse the upsurge. Information Technology (IT) or Information Technology enabled Services (ITeS), such as, Banking, Financial Services and Insurance (BFSI), telecom, healthcare, education and manufacturing sectors heavily rely on UPS. Due to their criticality, at some instances, even a brief interruption is too hazardous and cannot be ignored. Therefore, under such situations, an added backup is utmost essential. Further, predicting future power requirements is almost next to impossible for anyone at the initial stages of planning. Therefore, reliability and power expansion of UPSs are among the major concerns for power supply designers. In addition, high performance, energy efficiency and robust design with low cost have been amongst prime pre-requisite s from manufacturer as well as customer end. This thesis investigates various control strategies for single-phase Pulse Width Modulated (PWM) voltage source inverters used in Uninterruptible Power Supplies (UPS) first for a unit inverter module and then for a parallel bi-module UPS inverter system. For the control of single inverter module, both single-loop and multi-loop control schemes have been examined. It has been observed that multi-loop approaches are better in terms of voltage regulation performance and robustness. However, voltage performance also depends on the compensator or controller utilised in realising the control strategy. Three commonly used integral controllers, namely; Proportional Integral (PI), Proportional Resonant (PR) and Synchronous Reference Frame Proportional Integral (SRF-PI) have been applied to obtain desired the voltage output. Conventionally, PI control suffers from large steady-state error and PR has fixed-point DSP implementation issues. On the other hand, SRF-PI can achieve excellent performance but the design and implementation complexities are high, particularly for the single-phase VSIs. These limitations have been moderated by implementing voltage-loop control in synchronous frame and current-loop in stationary frame of reference. The presented research investigation further explores different control strategies on parallel inverter modules for enhancement in power rating of the UPS system. The focus has been on different Instantaneous Average Current Sharing (IACS) control schemes due to their better current sharing and expansion flexibilities. At first, using the structure of multi-loop U ii inductor current feedback control for unit inverters, an Instantaneous Average Current Feed Forward (IACFF) based current sharing scheme has been proposed for multi-inverter UPS system. In this scheme, the instantaneous average current has been feed-forwarded to increase the impedance for the inter-modular circulating current. Along with the conventional PI and PR based controller based Instantaneous Average Current Sharing (IACS) control schemes, a non-linear and a periodic controller based current sharing control schemes have been attempted for multi-module UPS. An IACS scheme using non-linear controller based Hierarchical Fuzzy Logic (HFL) has been suggested for parallel connection of multi-module UPS inverters. Being a fuzzy based control scheme, the UPS system modelling needs not be precise and robustness of the control is high over wide variations in system parameters. In another IACS control scheme, a periodic controller based Hybrid Iterative Learning (HIL) has also been proposed for the multi-inverter system. Since signals are periodic in UPS application, the HIL has been realised by combining two controllers, i.e. Iterative Learning (IL) and PI control. The steady-state error of the stationary frame PI control can be successfully overcome by employing the IL based controller, whereas the poor transient dynamic of the later can be improved by the PI controller. Therefore, two controllers complement each other in overall performance efficacy of the HIL control. Further, the presented control uses an inductor current feedback to provide both damping to the inverter plant and current sharing control of the multi-inverter UPS system. Therefore, the HIL control reduces the requirement of one sensor per module in the multi-module UPS inverters system to achieve a good voltage regulation and proper current sharing, simultaneously. A systematic design procedure and control analysis has been presented in due course for the respective control schemes. Simulation investigations and experimental implementations using two single-phase PWM VSIs prove the effectiveness of the proposed theoretical conceptions.