Advanced control techniques for improving power system transient and frequency stability

The demand for electricity as a viable form of energy has been rapidly increasing due to population growth and technological advancements leading to the electrification of modern civilisation. To meet the ever-increasing demand for electricity, power networks are operated closer to their transmission capacity and stability limits to facilitate higher power transfer through existing transmission lines. This has made power networks more stressed and susceptible to disturbances which can be blamed on the lack of investment in new generation and transmission infrastructure duo to factors such as cost and environmental constraints. Lack of provision and planning under these operating conditions can lead to system failures which have encouraged system operators to develop tools and techniques to deal with these susceptibilities and weaknesses. Wide-area control systems are one of the main tools developed due to advancements in telecommunications and power systems management technologies. These systems have provided a viable framework to facilitate the implementation of centralised power systems management and control strategies which has been made possible by the availability of Phasor Measurement Unit (PMU) across power networks providing system-wide observability of system parameters and stability characteristics based on remote measurements, allowing the implementation of disturbance detection, mitigation and prevention strategies. This thesis aims to present a set of wide-area control strategies to improve the transient stability and performance of power systems subjected to severe disturbances. To achieve this objective, wide-area control methods based on Static VAR Compensators (SVC) in transmission networks and Distributed Generation (DG) systems in distribution networks, specifically Solar Photovoltaic (PV) and Battery Energy Storage Systems (BESS) are developed. The proposed wide-area control strategies are based on energy function which is used in stability assessment and design of stabilising control strategies. The proposed wide-area control strategies rely on real-time system information obtained using PMUs and a nonlinear Kalman filtering approach for estimating equivalent inter-area dynamics of the system in terms of angles and velocities based on the reduced aggregate system model. The proposed wide-area control strategy based on SVCs is designed to improve the low-frequency oscillation damping performance of power systems by enhancing the load modulation capability of SVCs in transmission networks by incorporating load dynamics in the wide-area controller design, requiring the knowledge of load dynamics characteristics. In order to obtain load dynamics, a measurement-based load model identification method based on the application of PMU and SVC is proposed, allowing the identification of load dynamics and estimation of dynamic load models, which can be incorporated into the wide-area controller design. The wide-area controller provides supplementary control for SVCs to modulate system loads in response to a disturbance based on inter-area dynamics obtained by employing the nonlinear Kalman filtering approach and PMU measurements. The proposed approach is justified by observing the effect of load dynamics on low-frequency oscillatory modes by applying spectrum analysis on generator velocity deviations while using complex loads with a diverse range of dynamics. For this purpose, a wide range of composite loads comprised of induction motor (IM) loads of different sizes and ratings are utilised. The proposed wide-area control strategy based on DGs is designed to improve the first-swing stability and low-frequency oscillation-damping performance of power systems by modulating and varying system loads using DG systems in distribution networks. This approach relies on clustering DG systems and loads to form aggregated variable generators/loads that can be controlled to regulate the total level of DG generation and loading in distribution networks, which can be used to modulate and vary the load level imposed on the transmission system. The clustering of DG systems is achieved by applying a consensus-based cooperative control algorithm to control the operation of PV and BESS systems within DG clusters, allowing them to form coherent aggregated entities that can be controlled remotely. For implementing the proposed DG-based wide-area control strategy, the capabilities of centralised wide-area monitoring and control systems and the consensus-based cooperative control approach are combined, forming a hierarchical stabilising control structure. The hierarchical stabilising control structure is used to improve the first-swing and low-frequency oscillation-damping performance of power systems. The proposed DG-based wide-area area control strategy is designed to enhance the first-swing stability of power systems by saturating power generation and absorption capacity of DG clusters, implementing a bang-bang style control to switch between maximum available generation and maximum load modes. Unlike first-swing stabilisation control, implementing a wide-area damping stabilisation control using DG clusters relies on load level and available generation capacity of DGs in distribution networks. For this purpose, an optimisation-based scheme is developed to coordinate and allocate generation between DG clusters in system areas in response to a disturbance based on inter-area dynamics obtained from the Kalman filter and PMU measurements. The performance of proposed wide-area control strategies is evaluated by performing time-domain simulation studies. The obtained results show significant improvements in system stability when proposed wide-area control strategies are applied.