HARMONIC INSTABILITY ASSESSMENT AND RESONANCE INVESTIGATION OF GRID-TIED INVERTER SYSTEM

Abstract

Energy generation around the globe is witnessing a transition from conventional fossil fuel-based generation to distributed generation based on renewable energy sources to meet climate change and other environmental uncertainties. The grid-connected inverters based on voltage source converters (VSCs) act as an interfacing medium between the renewable energy source and the grid. These VSCs are considered major technological breakthroughs responsible for energy transition success. The conventional synchronous generators in traditional power systems exhibit large time constants due to the mechanical inertia of their rotating mass. Under the events of disturbances in the power network, the combined inertia of all the generators can accommodate or absorb these oscillations. In the case of inverter-fed power systems, the network dynamics are altering due to the low inertia of the power electronic-based converters. Compared to the traditional power system network, the inverter-fed power system appears more inductive or capacitive due to the converter filters, active control loops, and distribution system components, making the network more likely to be resonant or oscillatory. In this context, stability issues of such power electronic-based power systems are of much more concern. A single distributed generation inverter connected to a grid or in islanded mode will be stable when operating alone. However, stability may not be guaranteed in multiple parallel inverter systems. The mutual interactions between the power converter impedance and passive elements of the grid will worsen the power quality, leading to harmonic instability and resonance issues. To address these problems, this thesis provides engineering insights through mathematical modeling and frequency domain analysis to understand the sources of harmonic instability and resonance in a grid-tied inverter system. Low-frequency and high-frequency harmonic stability issues are studied in detail, taking into account the effect of low bandwidth control loops, such as phase-locked loops, and high bandwidth ones, such as voltage feedforward and current control loops, on the inverter output impedance. Through bode plots and phasor diagrams, a thorough analysis is performed to investigate the impact of various control loops on the damping characteristics of inverter output impedance. The root cause in terms of negative damping offered by various control loops on inverter impedance is analyzed. Later, a solui tion for output impedance shaping through a suitable compensator design will be presented to solve the instability and resonance problems in grid-tied inverters. Lastly, two case studies are presented to explore other forms of harmonic instability, considering different current control techniques employed to regulate the output current of grid-connected inverters. Simulation and experimental results are also provided to validate the analysis. The research work in the thesis explores the importance of inverter output impedance characteristics in terms of its magnitude and phase in preserving the external stability of the grid-tied inverters with the other components present in the power system.