A Novel Frequency Control and Flexible Dynamic Boundary Microgrids to Improve Grid Resiliency and Facilitate Renewable Generation Integration

Renewable energy generation such as Photovoltaic (PV) plays a critical role in the transition towards cleaner and carbon neutral electric power systems. In this dissertation, novel grid control strategies and architectures for renewable integration and resiliency enhancement at both the transmission level and distribution level are studied.

At the transmission level, large-scale PV farms are being interconnected and expected to replace old coal/gas/nuclear power plants. A novel adaptive frequency control strategy for PV power plants is presented based on real-time inertia estimation that minimizes the “spillage” of PV generation that has zero marginal costs. In the meantime, sufficient power reserves are maintained in the PV inverters that could respond to power system disturbances and prevent under frequency load shedding (UFLS).

At the distribution level, the flexible dynamic boundary microgrid (MG) concept is studied for improving the resiliency of the distribution grid and the integration of renewable generation and battery energy storage systems (BESSs). Firstly, the optimal operation of flexible dynamic boundary MGs is studied. Mixed-integer convex optimization techniques are utilized to model the operational objective and constraints of dynamic boundary MGs. Constraints for three-phase power unbalance and voltage unbalance are also considered for the first time. Secondly, a real-time power balance control strategy is studied for dynamic boundary MGs. Due to the smaller size and number of generation units, MGs are more susceptible to large internal disturbances such as loss of generation and load steps. The proposed control strategy is critical for the survival of the dynamic boundary MGs under internal faults. Thirdly, power flow models for grid-forming inverters are also updated to reflect the effects of different grid-forming control strategies. The proposed models significantly improve the negative sequence voltage computation accuracy in unbalanced grids. Fourthly, a novel adaptive virtual impedance control method is studied to mitigate the impact of transient inrush currents seen in dynamic boundary MGs. The adaptive virtual impedance control will not affect steady state operation nor fault ride through strategies. Lastly, a comprehensive design approach for dynamic boundary MGs is presented to help reduce engineering costs for future dynamic boundary MG projects.