Final Technical Report Stabilizing The Power System In 2035 And Beyond Evolving From Grid Following To Grid Forming Distributed Inverter Controllers

Final Technical Report: Stabilizing the Power System in 2035 and Beyond: Evolving from Grid-Following to Grid-Forming Distributed Inverter Controllers

Under existing grid operations, large synchronous generators provide sufficient rotational inertia to form a rigid backbone for the bulk power system. With photovoltaics (PV) forecasted to provide more than 600 GW of generation by 2050 under the U.S. Department of Energy's SunShot Initiative objectives, however, it is clear that power electronic inverters will play a dominant role in future systems, and low-inertia stability must be ensured to maintain system reliability. Today, the risks to system stability can be observed on geographically small islands, such as Hawaii, which contain a relatively large amount of installed PV. These risks stem from a fundamental shortcoming of contemporary control strategies-existing inverter controllers cannot guarantee grid stability. Given that future power systems driven by sustainable resources will be characterized by low inertia, locations such as Hawaii provide a glimpse into the obstacles facing future power systems. Considering these challenges, the aim of this project wasto develop and demonstrate distributed inverter controllers that enable the reliable control of low-inertia power systems with hundreds of gigawatts of integrated PV.

Final Technical Report

From Grid Following to Grid Forming

Electrical power generation is drastically shifting from centralized power generation to decentralized distributed power generation as a result of the rising integration of renewable energy sources into the electrical grid. The primary challenge in this transition is replacing synchronous generators (SGs) with inverter-interfaced renewable generations. When there are one or more synchronous generators in the system, grid-connected inverters follow the voltage and frequency reference generated by the synchronous generator and act as a controlled current source to supply necessary quantity of active and reactive power. In the presence of one or more stiff voltage sources, such inverter operation has recently been labeled as ‘Grid-Following’ (GFL) mode of operation. If all synchronous machines are taken out of service, there will not be any voltage reference, rendering grid-following inverter operation infeasible. Hence, the way that the GFL inverters are controlled today results in the inability of the grid to operate 100% inverter-based resources (IBR). Therefore, in the absence of a synchronous generation as a stiff voltage source, the frequency and voltage of the grid must be controlled by some of the inverters. These inverters, referred to as "Grid-Forming" (GFM) inverters, are tasked with supporting a stable voltage and frequency in a variety of situations, including the connection or disconnection of a load or a generator, or the occurrence of a power system fault. Grid-forming inverters (GFMIs) will have a crucial role with the increase in renewable penetration during the coming years. This thesis aims to study the modeling approach and control technique of a GFM inverter in an islanded grid. The droop-based control of a GFL inverter is also studied and compared to that of a GFM inverter to understand the fundamental difference in their operation. As GFM inverters will gradually replace synchronous generators, GFM inverters are expected to behave very similarly to synchronous generators in a grid without a utility connection. Hence, the voltage balancing and short circuit behavior of GFM inverters are further compared to that of synchronous generators. Additional controller modifications are also proposed for the enhanced performance of the GFM inverter. Finally, GFM inverter-based virtually islanded Hybrid AC-DC microgrid architecture is proposed for the power distribution of future residential buildings