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The share of inverter-connected renewable energy resources (RESs) is increasing in the grid, with these resources partially displacing conventional synchronous generators. This has resulted in increased variability of active power supply, reduced overall inertia, and increased spatial heterogeneity of inertia, leading to faster system frequency dynamics along with larger and more frequent frequency control events. These effects are expected to become increasingly more important in power system control in next-generation grids, which may conceivably be made up entirely of RESs. To mitigate these challenges, a fast, area-based hierarchical control strategy is proposed. This scheme partitions the power system into small areas, estimates local power imbalances, and corrects them by utilizing local inverter-based resources (IBRs). In cases where sufficient resources are not available locally, power is preferentially sourced from electrically close neighbours using an iterative distributed optimization scheme which preserves information privacy between areas. The proposed frequency control architecture can be retrofit onto existing control systems, and allows for flexibility in the amount of model information available to the designer. The control strategy is validated on two detailed multi-area power system models. Simulation results show that the strategy provides fast and localized frequency control. [ABSTRACT FROM AUTHOR]

The increasing integration of utility-scale renewable energy sources (RESs) brings emerging challenges to the classical problems of voltage control in transmission grids, including increased potential for voltage violations. To address this challenge, we present a novel measurement-based coordinated voltage control scheme that can enable fast participation of RESs. In this approach, RESs are coordinated with the traditional voltage control devices such as synchronous generators (SGs) and static var compensators (SVCs) to maintain all bus voltages within operational limits while respecting device power limits. The control scheme allows different priorities to be assigned to different control resources, and ensures that both voltage and reactive power constraints are met in steady-state whenever it is possible to do so. The controller design requires only an approximate model of the steady-state relationships between voltage and reactive power in the system, and in online operation, processes voltage and reactive power measurements to produce set-point updates for RESs, SVCs, and SGs; this feedback provides robustness against both model uncertainty and unmeasured disturbances. The feasibility and effectiveness of the controller is demonstrated via simulation case studies on a detailed power system model. [ABSTRACT FROM AUTHOR]

Communication infrastructure (CI) in microgrids (MGs) allows for the application of different control architectures for the secondary control (SC) layer. The use of new SC architectures involving CI is motivated by the need to increase MG resilience and handle the intermittent nature of distributed generation units. The structure of SC is classified into three main categories, including centralized SC (CSC) with a CI, distributed SC (DISC) generally with a low-data-rate CI, and decentralized SC (DESC) with communication-free infrastructure. To meet the MGs’ operational constraints and optimize performance, control and communication must be utilized simultaneously in different control layers. In this survey, we review and classify all types of SC policies from CI-based methods to communication-free policies, including CSC, averaging-based DISC, consensus-based DISC methods, containment pinning consensus, event-triggered DISC, washout-filter-based DESC, and state-estimation-based DESC. Each structure is scrutinized from the viewpoint of the relevant literature. Challenges such as clock drifts, cyber-security threats, and the advantage of event-triggered approaches are presented. Fully decentralized approaches based on state-estimation and observation methods are also addressed. Although these approaches eliminate the need of any CI for the voltage and frequency restoration, during black start process or other functionalities related to the tertiary layer, a CI is required. Power hardware-in-the-loop experimental tests are carried out to compare the merits and applicability of different SC structures. [ABSTRACT FROM AUTHOR]

This brief examines the behavior of DC circuits comprised of resistively interconnected constant-power devices (CPDs), as may arise in dc microgrids containing microsources and constant-power loads. We derive a sufficient condition for all operating points of the circuit to lie in a desirable set, where the average nodal voltage level is high, and nodal voltages are tightly clustered near one another. Our condition has the elegant physical interpretation that the ratio of resistive losses to total injected power should be small compared with a measure of network heterogeneity, as quantified by a ratio of conductance matrix eigenvalues. Perhaps surprisingly, the interplay between the circuit topology, branch conductances, and CPDs implicitly defines a nominal voltage level for the circuit, despite the explicit absence of voltage-regulated nodes. [ABSTRACT FROM PUBLISHER]