Udaya D. Annakkage

The Virtual Synchronous Generator (VSG) enables inverter-based resources (IBRs) to emulate the dynamic behavior of synchronous generators, enhancing grid-forming capability and power system stability. Conventional control strategies like Multi-Loop PI (ML PI) for cascaded voltage and current control, though widely used, rely on fixed gain parameters and lack adaptability across varying grid strengths, characterized by the Short Circuit Ratio (SCR). To overcome these limitations, this paper proposes a robust Higher Order Sliding Mode Control (HOSMC) strategy for cascaded voltage and current control in VSGs. HOSMC inherently provides variable gain, strong robustness to parameter uncertainties and disturbances, and finite-time convergence, without requiring gain retuning as grid conditions change. Moreover, it mitigates chattering through continuous control laws, addressing a key limitation of traditional SMC. Unlike most nonlinear controls, HOSMC allows frequency-domain stability analysis, simplifying controller design and assessment. The proposed approach is evaluated through small-signal stability analysis and electromagnetic transient (EMT) simulations across a wide range of SCR and X/R ratios. Results show that HOSMC achieves higher and more uniform stability margins than ML PI. A scenario involving dynamic SCR variation is simulated by switching a transmission line, with the VSG operating in the IEEE 118-bus system. The system remains stable throughout, demonstrating the control’s robustness. Real-time Controller Hardware-in-the-Loop (CHIL) validation confirms the practical feasibility of HOSMC under varying grid conditions, including SCR variations, voltage and frequency disturbances, and large faults.

The power modulations carried out by a grid-forming inverter are profoundly affected by the capability of the inverter's dc-side circuit to support such modulations. Although preliminary work on dc microgrids includes the dc-side dynamics, analysis of potential ac-side interactions in the presence of grid-forming inverters is lacking. This paper presents an in-depth study of the interactions in grid-forming inverter systems considering the critical dynamics contributed by the inverter's dc-side circuitry. The study in this paper is based upon dynamic phasor modeling and eigenvalue analysis techniques. The results presented for an exemplar system of a grid-forming inverter paralleled with a synchronous machine prove the significance of including dc-side dynamics to accurately capture the full range of interactions that may occur in such systems. Detailed electromagnetic transient simulation results from PSCAD/EMTDC are included to verify the validity of the predictions of the dynamic-phasor-based model.

This paper presents a nonlinear higher‐order sliding mode control (HO‐SMC) designed for a droop control‐based grid‐forming converter. In weak grid scenarios, where the rate of change of frequency is notably high, achieving a rapid frequency response becomes imperative. The stable operation of a grid‐forming converter using droop control, coupled with classical vector control that employs cascaded voltage and current loops (multiloop) with PI controllers, faces limitations when higher droop coefficients are applied. This constraint on the application of classical vector control in weak grid conditions necessitates alternative solutions. Operating as a grid‐forming converter, the grid‐connected converter with an LCL filter represents a second‐order system. HO‐SMC mitigates the switching challenges associated with conventional SMC by integrating robust feedback linearized control. A graphical method is proposed for designing the switching gain using Lyapunov's direct method to counteract the impact of a matched disturbance. The study demonstrates that the implementation of HO‐SMC in the grid‐forming converter enhances fast frequency response by increasing the gain margin of the power frequency loop. Finally, it is illustrated that the proposed control method also improves the transient response of the converter.

This document is a summary of a report prepared by the IEEE PES Task Force (TF) on Microgrid (MG) Dynamic Modeling, IEEE Power and Energy Society, Tech. Rep. PES-TR106, 2023. In this paper, the major issues and challenges in microgrid modeling for stability analysis are discussed, and a review of state-of-the-art modeling approaches and trends is presented. In the context of the IEEE 1547 standard, the document covers issues associated with component models for MG dynamic studies and simulations, including generator and grid modeling, full and average converter models, unbalanced and balanced system conditions, dynamic and static loads, protection requirements, and detailed and simplified controls considering communications delays, packet losses, and security issues. Considering the future integration of grids and MGs to form broad integrated networks, a discussion is presented of the use of phasor vis-à-vis electromagnetic transient simulation tools for MG dynamic stability studies, as well as modeling scale-up issues and MG equivalent models. Specifically white-, grey-, and black-box models, are presented. This TF paper and companion report constitute a modeling guide for R&D groups working on developments and standards of MGs with a focus on stability issues.

Series compensation of transmission lines increase the risk of Sub-Synchronous Resonance (SSR) problems. Use of Thyristor Controlled Series Compensators (TCSC) in place of Fixed Series Capacitors (FSC) has benefited in terms of damping SSR. SSR Damping with TCSC can be achieved with supplementary controls or through its inherent damping capability. Use of discrete time models for small signal stability analysis of TCSC is well established. This paper uses a simplified Dynamic Phasor (DP) model of the TCSC for an in-depth evaluation of the inherent SSR damping capability through small signal stability analysis. Source of inherent damping in open loop operation and the representation of it in the DP model is elaborated and the small signal model is validated against Electro Magnetic Transient (EMT) simulations. Effect of inductive-resistive nature of the TCSC on Induction Generator Effect (IGE) and Torsional Interactions (TI) is shown through small signal stability assessment. It is shown that the network resonant frequency exists only in a narrow range of firing angles of the TCSC and thus IGE and TI are avoided in most part of its operating range. IEEE first benchmark system for SSR studies is used to demonstrate the inherent damping capability of a TCSC.

This paper proposes a novel three-stage near real-time phase-driven procedure to locate the generator-type source of forced oscillations, inspired by the concept of Transient Energy Function (TEF), independent of the amplitude of any signal. In the commencement of the process, each generation bus is assigned with its active power and angular velocity angles reached from the reduced power system graph. Next, the difference in the synchronous generators’ active power angles and the difference in their angular velocity angles is exploited as the graph branches’ weight coefficients. Then, three exclusive and first-proposed decision functions are applied sequentially. First, the weighting coefficients of branches and nodes are fed to the first decision function. Regardless of the grid size, the result of this step is limited to up to four generators. The next step focuses exclusively on the first step outcomes, which results in two generators as recommended items. The last step, relying on the output of the second one, reveals the target generator accurately. The methodological procedure has been validated on the New England 10-machine 39-bus benchmark power system modelled in the Real-Time Digital Simulator (RTDS/RSCAD) and then scrutinised in the MATLAB environment. In each study scenario, the results are compared with the conventional transient energy method. The simulation results revealed that the presented approach reliably releases all sources, including limit cycle and turbine governing reference signal modulation, under the most intense parametric resonance.

Leading to the enormous growth in renewable and power electronics technologies and the global drive towards environmental friendliness and sustainability, a significant number of renewable energy sources are being connected to the power system via inverter-based systems. The inverter-based generations (IBG) have no stored energy and less fault current injection capability compared to the conventional synchronous machines. Consequently, a large penetration of IBG creates challenges to maintaining the stability of the power system, especially the transient stability. The weaker the power system, the higher the significance of instability. Few solutions exist in the literature to improve the fault recovery of IBG connected to weak power systems. This paper considers the method of storing energy in sub-module capacitors of the Modular-Multi-level Converter (MMC) along with temporarily boosting the inverter’s current limit. Conversely, increasing the ratings of the inverter will result in high manufacturing costs. Hence an optimization strategy is proposed in this paper, for obtaining a robust set of inverter control parameters that enhances fault recovery without excessively increasing the manufacturing cost of MMC. A frequency scanning technique supplemented with Generalized Nyquist criteria is incorporated into the optimization methodology to constrain the search space for the optimization algorithm. This enables the optimization algorithm to converge to an acceptable solution with a reasonable computing time. Furthermore, validation of the resultant set of parameters for different system conditions is presented. Finally, IBG with optimized fault recovery controllers is integrated into a simplified real-world power system, and the applicability of the proposed optimized controllers is illustrated.