Inverter Control Methods to Support Renewable Rich Power Grids and Enhance System Stability

Abstract

Heading towards a pollution-free electricity system, renewable energy generation is expected to double by 2050 and these resources are predominantly wind and solar which are interfaced with power electronic inverters. It is evident that the future grid will rely heavily on inverter-based resources. Traditional rotating generators have large rotational inertia, slow actuation control and well-understood synchronization mechanism. In contrast, inverter-based resources are distributed, variable, and have no inherent self-synchronization. In addition, they are fast and flexible in terms of control. Thus, conventional synchronous machine control mechanisms may no longer apply to inverters and it is clear that control methodologies will play a key role to ensure stability and energy balance in the inverter-dominated grid. The present inverter control methods are mainly phase locked loop (PLL)-based current-controlled strategy with limited frequency and voltage support capability. Stability issues have been reported in weak grid scenarios using the present strategy. A fundamental question is whether or not the commonly used current-controlled inverters with proper frequency and voltage control loops can support renewable rich power grids and provide grid forming services. The answer to this question is crucial for system operators when they make service standards for system grids. The dissertation addresses this issue by evaluating frequency dynamic performance of current-controlled inverters in a low inertia system, analyzing its small signal behavior in island and weak grid operation mode. Torque analysis, which is a classic method to analyze synchronous machines, is adapted to explain the working mechanism of inverters. With gradual retirement of synchronous machines, damaging oscillation is going to be an emerging concern as power system stabilizers are also being retired. To address this issue, this project investigates a decentralized control method to improve damping for these oscillations by leveraging fast actuation and flexible control characteristics of inverter-based resources. Furthermore, an advanced nonlinear observation decoupled control strategy with local measurements and computation is being applied for inverters and examined to stabilize the system under large generation/load/network disturbances.