Traditional power systems are transitioning toward more sustainable electricity generation and supply systems. One of the major contributors toward this transition is the increased penetration of renewable energy resources which help to promote clean energy production, diversify the energy mix, reduce carbon emissions, and so on. However, the trend of increasing renewable energy resources has started to disrupt the conventional paradigm of power system operations. Therefore, modern electric utilities are concerned and looking for solutions to integrate these resources without disturbing the security and reliability of their existing systems.Along with the rise of renewable energy resources and the retirement of conventional generation, distributed energy resources (DERs) are becoming more prevalent in modern electric grids. DERs are small-scale resources connected at the medium and low voltage distribution networks, which include, but are not limited to, photovoltaics (PV), wind, battery energy storage, and microturbines. With the evident expectation of heavy penetration of DERs in the near future, it has become more important than ever before to enable DERs to provide ancillary grid services. In this regard, DERs can be used independently or through aggregation to provide ancillary services to the grid. Though the contribution of a single DER or a distribution system consisting of multiple DERs to grid services may not be significant, stacked and coordinated contributions from several active distribution systems or aggregators can provide frequency regulation and other grid services at scale. However, the large-scale integration of DERs poses challenges in the planning, operation, and management of an existing power grid. These challenges call for developing a framework that provides avenues for their large-scale integration and assists in employing them for ancillary grid services. In this context, FERC Order $2222$ has also established standards to enable and promote the participation of behind-the-meter DERs for several grid services. Whereas the regulations have been formulated, the practical challenges associated with their integration and adoption for ancillary grid services are still a concern for electric power utilities.This dissertation addresses these critical challenges by developing a comprehensive framework and real-time control strategy to coordinate and optimally dispatch DERs and utility-scale resources for one of the important ancillary grid services, which is secondary frequency regulation. The study is conducted from the perspective of designing a novel mathematical model for implementing secondary frequency regulation at both distribution and transmission levels. A deep reinforcement learning-based strategy is proposed that effectively manages the diverse portfolios of resources, effectively handles the complexities associated with diverse characteristics, and accurately dispatches the resources for Automatic Generation Control (AGC). Furthermore, serverless cloud computing architecture and grid response time analysis are conducted for practical deployment of the proposed secondary frequency control algorithm in a real field. Moreover, a comprehensive framework is developed to build an electromagnetic transient (EMT) model of a large-scale power grid that can be used to validate the proposed secondary frequency control on accurate power system models in real-time. In addition, the proposed serverless cloud computing architecture along with the integration of simulation in real-time digital simulator (RTDS) provides a high-fidelity prototype for the practical deployment of secondary frequency regulation and is also flexible to implement for other power system control problems.The results, mathematical models, and large-scale power system models proposed in this study provide major advances and important insights to enable active distribution network consisting of DERs and utility-scale resources for secondary frequency regulation.
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