Yunjie Gu

The increasing penetration of inverter-based resources in Great Britain's power system has resulted in a decline in system strength, which has given rise to sub-synchronous oscillations. Traditional methods of assessing system strength are inadequate for system operators to mitigate these oscillations, and therefore, new system strength assessment methods are needed. This paper presents the design of an end-to-end small-signal system strength monitoring process using online impedance measurements, developed through collaborative efforts of a consortium of industry and research partners. The process is based on the evaluation of frequency-dependent impedances from grid measurement data, followed by the computation of a new small-signal system strength metric, Impedance Margin Ratio. Considerations on the feasibility of a practical implementation of the process in a real-world system subject to measurement uncertainty, along with technical aspects such as the injection configuration, are investigated. The process is validated in electromagnetic transient simulations in PSCAD, on a power system model that resembles a high-inverter-dominated region in Great Britain. The results demonstrate that system strength is highly dependent on operating conditions highlighting the importance of evaluating it across multiple system operating scenarios.

Grid-following (GFL) inverters have very different large-signal stability characteristics to synchronous generators. Existing studies mainly focus on the synchronization stability of an individual GFL inverter, while interactions between multiple inverters are less often addressed. This paper elucidates interaction mechanisms between heterogeneous inverters, covering GFL, grid-forming (GFM), and grid-supporting (GSP) types, to determine stability boundaries of systems with mixed inverter compositions. The generalized large-signal model for two-inverter systems is derived for various inverter combinations. This establishes that systems containing GFL inverters do not admit a global energy function, fundamentally limiting the applicability of traditional direct methods. To overcome this barrier, a manifold method is employed to determine the region of attraction (ROA). To address the computational complexity of the manifold method, reduced-order models of inverter are used. The large-signal stability margin is assessed by the shortest distance from a stable equilibrium point (SEP) to the boundary of the ROA, termed the stability radius (SR). Using the proposed framework, results of two-inverter system show that both GFM and GSP inverters significantly enhance large-signal stability where the other inverter is GFL, with GFM providing slightly superior performance. All findings are validated through both EMT simulations and power hardware-in-the-loop (PHIL) experiments.

Identifying the whole-system admittance model of an interconnected power system with multiple inverter-based resources (IBRs) typically requires injecting small voltage perturbations at each bus connected with an IBR, which is impractical in both simulations and real-world systems. To overcome this challenge, this paper presents a hybrid data/model-driven approach to identifying the whole-system admittance model through combinations of the measured data and the theoretical network model. By introducing voltage perturbations at a single bus in the system and measuring current responses at all buses, the measured data can be integrated with the known network nodal admittance matrix to reconstruct the whole-system admittance model. The whole-system admittance model, which is a closed-loop model, encompasses the dynamic characteristics of the entire system and facilitates oscillation analysis, root cause tracing, as well as system strength assessment. The proposed new approach is validated through simulations and power hardware-in-the-loop experiments, demonstrating a practical solution for implementing a limited number of injection sources to identify potential oscillations and their root causes for the entire power system.

As the growing penetration of inverter-based resources (IBRs) in modern power systems, the system strength is decreasing. Due to the inherent difference in short-circuit capacity contributions of synchronous generators and inverters, the short-circuit ratio is not a one-size-fit-all metric to assess the system strength. Following the distinct dynamic behavior of the IBR in small- and large-signal disturbance, the system strength is separated accordingly. To address the large-signal system strength assessment, a control type-dependent metric, Power Margin Ratio (PMR), is proposed in this paper. PMR is defined as the ratio between the maximum power that can be injected to the system without causing any instability and the nominal power of the IBR. It can be obtained via power flow calculation with a modified algorithm. The theoretical foundation of PMR is established from the viewpoint of dynamical systems. PMR is identical to SCR for the single-plant-infinite-bus system, while presents advancement for multi-infeed power systems. Comprehensive case studies and discussions have validated that PMR reveals the large-signal system strength from a static perspective.

The global transition toward a decarbonized electricity sector is fundamentally reshaping power systems. Central to this transformation is the increasing deployment of variable renewable energy sources, primarily wind and solar photovoltaics, supported by grid-scale battery energy storage systems and high-voltage direct current interconnections. These inverter-based resources (IBRs) are interfaced with the main power grid through power electronic converters whose dynamic characteristics differ markedly from those of conventional power plants with synchronous generators (SGs).

Grid-forming (GFM) converters have shown great potential for voltage and frequency support in power electronics dominated power systems. However, their power-based voltage regulation is greatly affected by the power injection of paralleled grid-following (GFL) converters. As a result, complex and dynamic interactions arise between converters, which makes the transient stability of paralleled voltage source converter (VSC) systems with both GFL and GFM converters difficult to analyze. To address this issue, this letter develops a transient angle dynamic model for paralleled-VSC systems and reveals that instability typically originates between the GFM converter and the grid. Based on this, the impact of the interactive power between converters on the GFM angle motion is quantified via relative angle range estimation, and then a simplified GFM angle model is further proposed for transient stability evaluation. Moreover, a transient stability criterion for paralleled-VSC systems is established and well extended to multi-VSC systems, and its effectiveness is validated with experimental results. The proposed method enables quantitative transient stability analysis for paralleled-VSC systems without any unexpected iteration, which may offer some good insights for transient stability assessment of systems with multiple converters.

In grid-forming (GFM) inverters, the control loops for power, voltage, current, and virtual impedance are inherently coupled due to limited switching frequency and bandwidth constraints, complicating controller design and potentially compromising system stability. To address this challenge, a complexvector-based impedance circuit model is developed to characterize the interactions among these control loops. Building upon this model, a co-design methodology is proposed to coordinate the swing mode, flux mode, and resonance mode of GFM inverters. The robust stability of GFM inverters is further analyzed under varying grid strengths and control delays, with explicit consideration of digital discretization effects. Theoretical analysis and the proposed design are validated through comprehensive experimental results.