Space Vector Based Hysteresis Current Control Techniques for Grid-Connected Voltage Source Inverters

Abstract

Grid-connected inverters are essential components in modern electric power systems, crucial for integrating renewable energy sources, enhancing grid stability, and enabling efficient power management. The increasing global demand for clean, sustainable energy has established the importance of grid-connected inverters due to their versatility, reliability, and ability to harness renewable resources such as solar photovoltaic (PV) systems, wind turbines, and fuel cells. These inverters are used in residential, commercial, and utility-scale applications, demonstrating their significant role in modern energy ecosystems. At the utility-scale, grid-connected inverters are vital for distributed generation, reactive power support, battery energy storage systems, and active power filtering, thereby improving power quality in the electric grid. Inverter control techniques which play a crucial role are primarily categorized into indirect current control (voltage control) and direct current control methods. Effective direct current control is essential for maintaining grid stability, optimizing power quality, and complying with stringent grid codes and regulations. The hysteresis current control method is one of the primary techniques used for direct current control. For hysteresis current control of grid-connected inverters with both the possible sinusoidal and non-sinusoidal current references, three existing techniques are studied in this thesis. It is found that the parabolic boundary-based space vector hysteresis current control (PB-SVHCC) technique performs better than the αβ stationary reference frame space vector hysteresis current control (αβ-SVHCC) technique and the dq rotating synchronous reference frame space vector hysteresis current control (dq-SVHCC) technique. However, in the literature, the PB-SVHCC technique is not studied for application to a grid-connected 3-phase 3-level neutral point clamped (NPC) inverter. This thesis addresses the enhancements necessary to adapt the existing PB-SVHCC technique for a grid-connected 3-phase 3-level NPC inverter. For a comprehensive evaluation, this inverter performs dual functions: integration of solar photovoltaic (PV) power into the distribution grid and shunt active power filtering. A single reference current, generated using instantaneous power theory, supports both active power filtering and maximum photovoltaic power integration. The introduction of a novel sliding current vii error boundary technique enhances the existing PB-SVHCC approach, effectively maintaining DC-link balance and regulating output currents simultaneously. Furthermore, leveraging the advantages of the further modified PB-SVHCC ensures a rapid response and optimal switching. Simulation and hardware results confirm the performance improvements and validate the effectiveness of the proposed control technique. However, analysis reveals that the harmonic spectrum of the inverter line-to-line voltage exhibits a spread of frequencies, limiting the applicability of the modified PB-SVHCC to L filter configurations while not being suitable for LCL filters.