Future power systems will source much of their electrical power from converter-based generation,be it a large scale HVDC link or a smaller system such as the back-to-back converter systems found in modern, variable-speed wind turbines. This is in stark contrast to the original AC power systems which used directly-coupled synchronous generation. The transition from the past power system to the future power system will produce power systems that have both low inertia, which compromises angular and frequency stability, and low short-circuit ratios, which compromises voltage stability.In this thesis, the modelling and control of converter-based generation in low short-circuit ratio systems are investigated. For the modelling of AC power systems and the controllers being applied to the converter(s), the unified linear state-space approach is proposed. In this approach, linear state-space models of the electrical system are combined with linear state-space models in a manner which is highly scalable and sufficiently flexible to allow multiple control algorithms acting in a system instantaneously to be considered with relative ease. Three control algorithms are considered in single converter systems: dq-axis vector current control,proportional resonant control, and power synchronization control. By adopting dq-axis vector current control, the system becomes ill-conditioned at the current level, primarily due to the dynamics of the phase-locked loop, which then causes stability issues for outer feedback loops (for example DC voltage and AC voltage controllers) which accompany the current controller. Proportional resonant control, also employing a phase-locked loop, exhibits poor dynamics in the low short-circuit ratio power system. By mimicking the basic synchronization process of a synchronous generator, power synchronization control is able to perform satisfactorily in a low short-circuit ratio system, much as a synchronous generator can. Two algorithms are considered in the multi-converter, low short-circuit ratio systems: dq-axis vector current control and power synchronization control. Performance issues observed in single converter systems when dq-axis vector current control is applied are observed in the multi-converter systems. Additional sources of undesirable coupling between control loops at the current control level are observed, potentially placing more demands on the design of the outer control loops. Power synchronization control performs satisfactorily in the multi-converter systems; however, oscillatory behaviour does arise, which requires careful tuning of the controllers. In addition, it is shown that the introduction of converters using power synchronization control enables other converters (in the same system) using dq-axis vector current control to exhibit improved performance. This is due to power synchronization control causing a converter to act as an effective voltage source/regulator,and dq-axis vector current control relying on electrical proximity to a strong voltage source. This produces systems with improved conditioning, which will reduce the complexity of the design of outer controllers for dq-axis vector current controlled converters. Keywords: control, modelling, HVDC, power systems, stability, voltage-source converter, weak ACsystems, multiple-converter systems, power system planning.

The future power grid is gradually transitioning towards a greater utilization of inverter-based resources (IBRs) to integrate renewable energy in generation portfolio. The existing synchronous generator (SG)-dominated power system is evolving into a grid, where both SGs and IBRs coexist. Since SGs are sources of mechanical inertia, their gradual replacement is resulting in a low-inertia power grid. One of the main challenges faced by such systems incorporating SGs and IBRs is the primary frequency response following a loss of generation or sudden large change in loads, which may lead to underfrequency load shedding (UFLS). Broadly, bulk power systems connected to SGs and a significant number of IBRs are the subject matter of this dissertation, with a focus on modeling, stability analysis, and control for providing frequency support from the perspective of primary frequency response. Although IBRs can be of different types depending on the control strategy, grid-forming converter (GFC) technology with a direct control over its frequency is much less understood, and is a major focus of research in this dissertation. These GFC-interfaced renewable resources in future low-inertia grids are expected to provide primary frequency support so that underfrequency load shedding is averted. The GFCs can be divided into two classes based on the control strategy: (a) class-A: droop control, dispatchable virtual oscillator control, and virtual synchronous machine, and (b) class-B: matching control. It is observed that while providing frequency support, the class-A GFCs may undergo dc-voltage collapse under current limitations during underfrequency events. On the contrary, class-B GFCs are more robust in this context. In the first part of the dissertation, we perform a stability analysis of both classes of GFCs following such events. To that end, first, the averaged phasor models of these GFC classes are developed, which can be seamlessly integrated with traditional positive sequence fundamental frequency planning models of grids. Building on this, simplified averaged models are derived to study the stability of the dc-link voltage of the GFCs under current limitations in a generic multimachine system. Using these models, the sufficiency conditions for stability for both the classes and that of instability for class-A GFCs are established. As a logical next step, a decentralized supplementary control for the droop-based class-A GFC is proposed to solve the dc-link voltage instability issue under the current limitations. This sliding mode control-based approach also aims to provide primary frequency support after the contingency. The proposed method leads to quantifiable frequency support irrespective of frequency deviation, which in turn can incentivize the plants through market participation. This approach requires the communication of frequency measurements of GFCs from adjacent buses. The proposed controller guarantees asymptotic stability of power grids with generic configurations that include multiple SGs and GFCs under dc power flow approximation and a mild assumption on the center-of-inertia based frequency dynamics model. The sliding mode controller design is challenging for a grid with multiple GFCs, as the sliding surface for each GFC requires iterative experiments for refinement. Moreover, for sliding mode control we could not establish the stability guarantee in the reduced-order system in presence of the constraints on the control input. To solve this problem, a nonlinear model predictive control (NMPC) strategy is proposed for frequency support from the GFCs, which ensures dc-link voltage stability. The NMPC approach considers a multitude of constraints including those on control input and tracks the dc-link voltage reference to indirectly regulates active power output. The controller also ensures finite-time practical stability of the close-loop system. The above-mentioned analyses and control strategies are primarily evaluated in positive sequence fundamental frequency phasor models of multiple modified IEEE benchmark systems with IBRs. Finally, the detailed electromagnetic transient (EMT) models of the IBRs are used to closely replicate the behavior of the GFCs in a real-world power grid. An EMT-TS co-simulation platform is developed for integrating the EMT models of IBRs to the phasor-based planning models of bulk power systems. This platform is used to integrate the planning model of the Western Electricity Coordinating Council (WECC) grid with an EMT-based GFC model. The proposed sliding mode control is validated in this co-simulation model to ensure the dc-link voltage stability of the GFC and provide frequency support following a contingency.

Prosumers, such as energy storage, smart home, and microgrids, are the consumers who also produce and share surplus energy with other users. With capabilities of flexibly managing the generation, storage and consumption of energy in a simultaneous manner, prosumers can help improve the operation efficiency of smart grid. Due to the rapid expansion of prosumer clusters, the planning and operation issues of prosumer energy systems have been increasingly raised. Aspects including energy infrastructure design, energy management, system stability, etc., are urgently required to be addressed while taking full advantage of prosumers' capabilities. However, up to date, the research on prosumers has not drawn sufficient attention. This proposal presents the need to introduce a Research Topic on prosumer energy systems in Frontiers in Energy Research. We believe this Research Topic can promote the research on advanced planning and operation technologies of prosumer energy systems and contribute to the carbon neutrality for a sustainable society.

Grid Connected Converters: Modeling, Stability and Control discusses the foundations and core applications of this diverse field, from structure, modeling and dynamic equivalencing through power and microgrids dynamics and stability, before moving on to controller synthesis methodologies for a powerful range of applications. The work opens with physical constraints and engineering aspects of advanced control schemes. Robust and adaptive control strategies are evaluated using real-time simulation and experimental studies. Once foundations have been established, the work goes on to address new technical challenges such as virtual synchronous generators and synergic inertia emulation in response to low inertia challenges in modern power grids.The book also addresses advanced systematic control synthesis methodologies to enhance system stability and dynamic performance in the presence of uncertainties, practical constraints and cyberattacks. Addresses new approaches for modeling, stability analysis and control design of GCCs Proposes robust and flexible GCC control frameworks for supporting grid regulation Emphasizes the application of GCCs in inertia emulation, oscillation damping control, and dynamic shaping Addresses systematic control synthesis methodologies for system security and dynamic performance

The environmental and economic impact of fossil fuel generating plants has resulted in the need to find more efficient and clean sources of electricity power generation. As an alternative, promising power generation options such as renewable energy sources, especially wind energy, are receiving increased interest. This dissertation addresses some important problems in power system operations (stability and power quality) when renewable energy resources (generation and storage) are integrated into the conventional power system. In particular, the use of energy storage (STATCOM/Battery) to enhance the small-signal stability, transient stability, and voltage regulation of an electrical power system with renewable power generation is investigated as follows. 1) A linearized model of the nonlinear power system in the region of a steady-state operating point is used in conjunction with an LMI-based control design method, including D-stability, to achieve small-signal angle and frequency stability, voltage regulation, when the system is perturbed by changes in mechanical power inputs to the synchronous generators in the system. 2) A nonlinear model of the power systems with energy storage is used with an Interconnection and Damping Assignment-Passivity-Based Control design (IDA-PBC) framework to achieve transient power angle stability along with frequency and voltage regulation after the occurrence of a large disturbance (a symmetrical three-phase short circuit transmission line fault). 3) Using the combination of the IDA-PBC control law and a reduced-order nonlinear observer, voltage regulation and system stability enhancement are simultaneously accomplished following both temporary and permanent faults. To illustrate the effectiveness of the STATCOM/Battery to enhance small-signal and transient stability, the LMI-based controller design with D-stability and IDA-PBC controller design are validated using the following simulation studies: (1) a single machine infinite bus (SMIB) (2) a two-machine (synchronous generator and doubly-fed induction generator) connected to an infinite bus. For the small-signal case, simulation results show that the proposed controller can simultaneously achieve frequency and voltage regulation along with improved transient performance. For the large-signal case, the results show that the proposed controller provides improved critical clearing times (CCTs) and an enlarged domain of attraction (DOA) along with improved frequency and voltage regulation.