Collaborative Research: Innovative Approaches for Robust and Reliable Operation of Voltage Source Converters in Critical Conditions of Emerging Grids

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Collaborative Research: Innovative Approaches for Robust and Reliable Operation of Voltage Source Converters in Critical Conditions of Emerging GridsGrid-connected voltage source converters are increasingly deployed in the energy sector, e.g. for the interconnection of distributed energy resources to the power grid. An instant of the converter's poor functionality can cause local and system level problems. For example, the cause of the 1,200 MW fault-induced solar photovoltaic resource interruption in Southern California is identified in erroneous functioning of the phase-locked loop employed in the inverter control system. The field data also show an unstable operation of photovoltaic inverters caused by grid voltage harmonics and partial or full generation loss caused by routine utility-owned capacitor switching incidents. Functions such as accurate and robust frequency estimation, voltage synchronization, and current generation during challenging conditions are crucial to maintaining the reliability of a power grid with high penetration of distributed resources. This research has identified 1) grid weakness, 2) grid voltage distortions, and 3) grid voltage and frequency disturbances, as broadly describing the major critical grid conditions. Thus, it proposes to 1) investigate the impacts of these conditions on the inverter functions, and 2) to synthesize Innovative and Effective Solutions. The proposed research invests in developing Modular, Practical and Efficient solutions that fully integrate the converter components and minimize demanding components that compromise the integrity of this multi-objective system. The project results will advance the quality and strength of inverter responses during critical conditions of future grids and will lead to the 1) improvement in the power system reliability, 2) improvement in its power quality, and 3) increase in its inverter hosting capacity. This will enhance the public power delivery services, empower the related energy industry, and develop new ties among various education and research communities. The project will stimulate and sustain the cross-disciplinary training of diversified students, particularly the underrepresented minorities enrolled in STEM programs at Mississippi State University and Georgia Southern University, and improve the broad STEM curricula. This research will establish theories to effectively formulate interactions among components of a grid-connected converter such as its phase-locked loop and its controller and between the converter and its hosting power network. Effective modeling, analysis and control processes will be developed for robust integration of the converter to the weak, polluted, and disturbed grid conditions. The phase-locked loop, optimal and robust controls, signals, and generator emulation theories are used to solidly tie the converter components together and design them in an optimal way. The project's approach is to fully integrate and optimize the converter components without adding redundant and disjoint components which may compromise other aspects of this multi-faceted and highly coupled engineering system. Particularly, recent advanced models of the phase-locked loops and recent optimal control design approaches will be deployed and joined to fully integrate the phase-locked loop into the entire control system of the converter and to design the entire control system in an optimal and robust way. High-fidelity power-hardware-in-the-loop testing will be used to examine the practicality and effectiveness of the proposed methods. Through full components integration leading to reduced oscillations and instabilities, the proposed project will increase the inverter hosting capacity of a given power grid, as well as its reliability and power quality. The project will also enhance the fundamental knowledge in those multiple fields (i.e. the phase-locked loops, optimal and robust controls, and signals) as applied to the crucial problems of integrating renewable resources to the power and energy system of our twenty-first century.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.