| PhD Viva

Name of the Speaker: Mr. Bishal Mondal (EE16D406)
Guide: Dr. Arun Karuppaswamy B
Online meeting link: https://meet.google.com/yhc-uxps-sqo
Date/Time: 30th January 2025 (Thursday), 2:00 PM
Title: Analysis, Design and Control of Grid-Connected Inverter with Output LCL Filter Under Non-Ideal Grid Conditions

Abstract :

Grid-connected inverters (GCIs) have gained significant attention over the past few decades due to the shift from conventional to renewable energy sources. GCIs also play a critical role in enhancing grid power quality, which is compromised by the penetration of reactive and non-linear loads. However, the design and operation of GCIs present several challenges due to poor grid voltage quality, weak grid scenarios, and requirement of adherence to grid interconnection standards. Consequently, improved design and control of GCIs are essential for reliable operation while maintaining compliance with grid codes.

This dissertation focuses on the improved design and control of GCIs. Specifically, the phase-locked loop (PLL), LCL filter design, and current control strategies have been researched. The PLL and current control has been explored under non-ideal grid voltage conditions. Additionally, the work includes a step-by-step procedure for hardware development of the GCI.

A phase-locked loop (PLL) is a vital component of the GCI, responsible for extracting the positive sequence voltage magnitude, frequency, and phase angle required for control implementation. Dual quadrature signal generators (QSGs) combined with a positive sequence calculator (PSC) are used as a prefilter in synchronous reference frame PLLs (SRF PLLs) under non-ideal grid conditions. Dual QSGs with a PSC eliminate negative sequence components and DC offsets while attenuating harmonics from the sensed voltages. Fourth-order QSGs (FO-QSGs) offer superior harmonic attenuation and complete DC-offset rejection. This work proposes a novel method of parameter selection for the existing FO-QSGs which help achieve lower settling times compared to the existing approach. Furthermore, a new FO-QSG is proposed which is capable of offering lower settling time compared to existing QSGs, with a simpler design approach.

LCL filters with passive damping are widely employed in GCIs to suppress the switching frequency components. This dissertation introduces a novel non-iterative component selection methodology for LCL filters with passive RC damping. Eight critical design constraints from the literature are addressed, ensuring optimal performance. Key innovations include introduction of grid-side inductance factor which avoids iterative method, a methodical approach to selecting the damping resistor which minimizes the quality factor while maximizing resonance damping, and the introduction of a flexible capacitor split ratio (n). An analytical method determines the optimal range of n to achieve a critically flat resonance peak, enhancing control stability under weak grids while meeting all the design constraints.

Under non-ideal grid voltage conditions, conventional dq-based current control suffers from limited harmonic rejection. To address this, this work proposes a dual-loop current control architecture with multiple resonant controllers. The disturbance suppressor gain (Kd) is optimized to enhance harmonic attenuation while maintaining stability. Resonant controllers are decoupled from the primary control loop and integrated into the disturbance suppressor loop, ensuring dynamic response preservation. This approach improves harmonic attenuation without compromising the stability of the dual-loop architecture.

Finally, the dissertation emphasizes the importance of hardware development for GCIs. A well-designed hardware platform is essential for implementing precise control algorithms under varying grid conditions. A 10 kVA Si-IGBT based two level inverter is developed for the work. The developed hardware supports the testing of advanced control strategies under diverse grid scenarios.