An Improved Sliding Mode Control for Voltage Source Inverters Using Optimal Surface Selection Mechanism

Abstract

In the present era, the integration of Distributed Generation (DG) into power systems has become possible through power electronic interfaces like Voltage Source Inverters (VSIs). With the integration of effective control strategies, VSIs serves to provide reliable and high-quality power supply that meets the needs of various applications in today’s dynamic world, including Uninterruptible Power Supplies (UPS), Electric Vehicles (EVs), power quality improvement, motor drives, active power flters, grid-tied solar inverters, laboratory power supplies and lifesaving equipment in hospitals or at emergency felds. This dissertation presents a cutting-edge approach for regulating output voltage in AC microgrids (MGs), using the robust control technique i.e. Sliding Mode Control (SMC). Despite the recognized feature of robustness, SMC is prone to a phenomenon called chattering along the sliding surface, which manifests as rapid and unwanted oscillations. Chattering can result in increased power losses and decreased effciency in the VSIs, leading to higher energy consumption and reduced system effectiveness. To address this concern, an adaptive sliding surface selection mechanism is implemented here, that incorporates the advantages of the Rotating Sliding Surface (RSS) technique, along with a novel reaching law based on the magnitude of state variables, which enables the adjustment of the control gain value. The composite reaching law proposed in the study integrates three distinct functions i.e. exponential, power, and difference functions. The intelligent mix of these functions makes the law more effective in achieving both high-speed convergence rate of system states and signifcant reduction in chattering. In this approach, the sliding surface is selected using a time-varying slope based on error variables. The effectiveness of the proposed SMC method, that uses a Composite Exponential Reaching Law with a Rotating Sliding Surface (C-ERL-RSS) has been studied in comparison to two existing methods i.e. Cosine Exponential Reaching Law (Cos-ERL) SMC and Fractional Power Rate Reaching (FPRRL) SMC. The study was conducted on a single-phase VSI with varying load followed by input voltage and parametric disturbances, and the results showed that the new method outperformed the existing ones, with very low percentage of Total Harmonic Distortion %THD i.e. 0.25% and high voltage regulation of 99.9%, reduced chattering, and minimum tracking time. Moreover, the proposed C-ERL-RSS SMC, along with the Power Rate Exponential Reaching Law (PRERL) SMC, Enhanced Exponential Reaching Law (EERL) SMC, and Repetitive Reaching Law (RRL) SMC, are implemented on a two-level three-phase VSI under variable load conditions. The comparative analysis highlights the effciency and authenticity of the proposed reaching law in achieving a stable output voltage with improved robustness, reduced chattering, low %THD of 1.1% and high voltage regulation of 99.83%. On top of it, the performance of the proposed C-ERL-SMC is evaluated experimentally on Hardware In Loop(HIL) setup comprising Opal-RT and MicroLabBox-dSPACE 1202. The proposed technique responded exceptionally well in voltage regulation of 99.8% under extreme conditions with fast transient response and low %THD of 3.23%.