Equivalent circuit model of grid-forming-converter based power grids: Visualizing control loop interactions on synchronization stability

Small-signal synchronization stability of grid-forming converter influenced by multi time-scale control interaction

With the development of renewable energy sources, power systems are gradually evolving into a system comprising both grid-forming (GFM) and grid-following (GFL) converters. However, the dynamic interactions between the two types of converters remain unclear, particularly in the presence of low-inertia GFM converters, due to the fundamental differences in their synchronization mechanisms. To address this gap, this paper develops a small-signal synchronous stability model for power systems containing GFM and GFL converters, which considers network line dynamics. Based on matrix perturbation, we reveal that GFM and GFL subsystems can be effectively decoupled when GFL converters operate near unity power factor or when GFM converters possess sufficiently large inertia or damping, and provide lower bound of control parameters ensuring decoupling. Under the decoupling condition, we propose decentralized and analytical parameter-based stability criteria which have clear physical interpretations: the positive damping of converters compensates for the negative damping of the network. In the case of coupling, we also propose decentralized stability criteria based on the small phase theorem. The effectiveness of the theoretical analysis is validated through simulations in MATLAB/Simulink.

Feasibility Analysis of Control-Loop Interaction Compensation for a Fossil-Fuel Power Plant

To reveal the influence of multiple control loops on the stability of low-frequency small disturbances in voltage source converters (VSCs) connected to weak grids, this paper proposes a state space modeling and analysis method. Taking into account the influences of factors such as external loop control and phase-locked loop, a state space model of the DC voltage control unit connected to the weak network is established. The influence laws of multiple control links such as DC voltage control, AC voltage control, and phase-locked loop control on the stability of low-frequency small disturbances in the system are studied through the variation of eigenvalues. The participation factor analysis reveals the dominant stabilityinfluencing parameters in the system. Meanwhile, the influence degree of system parameters on stability is quantitatively evaluated by using dynamic characteristic indicators such as damping coefficient and damping oscillation frequency. Finally, based on the simulation model of the DC system connected to the weak power grid built in PSCAD/EMTDC for analysis, the effectiveness of the method was verified.

Grid-forming (GFM) converters with cascaded control loops might induce small-signal instability issues over a wide frequency range, and existing damping control strategies are usually designed and verified for oscillations in specific frequency bands, exhibiting limited integrity in wideband small-signal stability assessments. In this paper, detailed control structures are considered for small-signal modeling to reveal potential wideband oscillation modes in the GFM converter systems. Then, systematic evaluation and comparison of various damping control methods are carried out regarding their positive impacts and side effects on different oscillation modes, thereby providing an integrated solution capable of mitigating instabilities over a wide frequency range. The theoretical analysis results are verified by simulations and experimental tests.

Grid-forming (GFM) converters with dc-link voltage-synchronization control (DVSC) are capable of simultaneously regulating dc-link voltage and achieving synchronization with ac grid. However, the interactions between DVSC and dc-link constant power dynamics may lead to low-frequency oscillations (LFOs) of GFM converters. This article develops a dc-link impedance-based equivalent circuit model for GFM converters with DVSC. The analysis reveals that the lack of positive resistance in dc-link impedance leads to LFO instability in GFM converters. To address this issue, a damping method, based on dc-link voltage feedforward (DVF) control is proposed, effectively suppressing oscillations and enhancing dynamic response performances. Theoretical analysis demonstrates that the proposed DVF-based damping method implements an active damper on the dc side and avoids inherent synchronous oscillations. Experimental results validate the theoretical analysis and the effectiveness of the proposed damping method.

With the rapid growth of urban load density and the explosive expansion of DC loads, traditional AC distribution networks face severe challenges in capacity and stability. Medium-voltage direct current (MVDC) intertied systems, integrating grid-following (GFL) and grid-forming (GFM) converters, have emerged as a promising solution to enhance the resilience and efficiency of urban AC/DC hybrid grids. However, the coordinated site selection and planning of GFL and GFM converters, balancing control stability and economic efficiency, remains a critical unresolved issue. This paper proposes a novel site selection and planning method for embedded urban MVDC intertied systems. First, a dual-stiffness index framework is developed, incorporating voltage stiffness (<italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">K</i><sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">vtg</sub>) and current stiffness (<italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">K</i><sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ctg</sub>) to quantify the stability margins of GFL and GFM modes, respectively. These index establish critical thresholds for stability boundaries, providing a quantitative basis for coordinated converter planning. Second, an integrated planning methodology is formulated to balance control benefits and economic considerations, where <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">K</i><sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">vtg</sub> and <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">K</i><sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ctg</sub> are embedded as stability constraints, alongside investment and operational costs, to achieve technical and economic synergy. Third, to improve planning efficiency, a neural network-based decoupled stability assessment model is designed. Latin hypercube sampling generates input features, and a dual-branch ReLU neural network learns the stiffness characteristics of GFL and GFM converters separately. The model is linearized using the Big-M method for compatibility with mixed-integer linear programming (MILP) optimization. Case studies on a 36-node urban distribution system in Dongying, China, validate the proposed method. Results show that the optimal planning of intertied system with converters at nodes 9 and 17 not only reduces voltage sag domains by up to 70.4% during faults but also significantly enhances frequency stability by limiting the Rate of Change of Frequency and improving the frequency nadir, while minimizing lifecycle costs. This method effectively coordinates GFL and GFM converters, ensuring stable operation and economic feasibility in complex urban grids.

CONTROL OF THE MANIPULATOR ROTATION ELECTRIC DRIVE IN THE AUTOMATED AMMUNITION SUPPLY SYSTEM OF THE CAESAR SELF-PROPELLED GUN O. Karpovych, H. Trushkov The improvement of the automated ammunition supply system in the CAESAR 8×8 self-propelled gun through the implementation of a three-loop control system for the manipulator rotation electric drive are considered in the article. The feasibility of replacing the traditional hydraulic drive with an electric one is justified based on its higher precision, adaptability, compactness, and ease of maintenance. A motion tachogram and load diagrams are presented, forming the basis for the substantiated selection of the DC motor power. A simulation model of the three-loop cascade control system (current, speed, and position) has been developed and studied under a typical ammunition supply cycle. The interaction of control loops, which enables coordination of the system’s dynamic characteristics are reviewed. The model incorporates a control structure with a proportional (P) position controller and proportional–integral (PI) controllers for the speed and current loops. Controller tuning using technical and symmetrical optimum criteria to ensure a balance between accuracy, dynamic response, and system stability was performed. A transient analysis was carried out in all three loops, allowing the evaluation of system sensitivity to load variations and ensuring stable operation under real operating conditions. It has been established that the implementation of a position control loop in a three-loop control system, compared to the traditional two-loop structure, enables accurate positioning of the manipulator and enhances the stability of the electric drive. The research results confirm the effectiveness of the proposed control architecture under variable load conditions typical for the ammunition supply cycle in self-propelled gun. Keywords: self-propelled gun, automated ammunition supply, manipulator, control system, adjustable electric drive.

To ensure reliable power supply with good quality is an important substance of power system operation and control. To satisfy the power balance equation in order to control angle and frequency stability is more essential for system operation. It can be obtained through automatic generation control.Automatic voltage regulation aims to achieve voltage profile with acceptable tolerance. ALFC loop aims to achieve frequency within tolerable limits. The small signal stability (SSS) of the system can be enhanced with the usage of GA tuned PID Controller in both of the ALFC and AVR loops. This paper focuses on enhancement of SSS with genetic algorithm tuned PID controller. The Simulink model is comprised of AVR and ALFC control loops with PID controllers. The case study is analyzed in three cases, Case1 with combined control loops without any controller, Case2 with combined control loops with classical PID controller and Case3 with GA tuned PID controller. The results have been analyzed with response curves and bar charts and there is admirable improvement in dynamic stability with GA tuning.

Control of a mixed culture of Lactobacillusdelbrueckii and ralstonia eutropha for the production of PHB from glucose via lactate

The potentials of grid-forming (GFM) converters in stabilizing a power system with high penetration of power electronics-based generation have been recently investigated. Due to their intrinsic behaviour of voltage source behind impedance, a crucial aspect of any GFM converter control strategy will be the handling of fault-ride through (FRT) scenarios. This paper proposes a FRT strategy for GFM converters, which respects the converter hardware limitations (i.e. current limitations) even under sever fault conditions, while maintaining GFM behavior before, during, and after the fault. The presented strategy addresses both symmetrical and asymmetrical faults, and is compliant with recently proposed draft grid codes requirements published by the British system operator NGESO. The issues related to FRT of GFM converters are first discussed in detail, and a comprehensive overview on the solutions proposed in the literature is reported. Then a proper strategy is presented, and its effectiveness is demonstrated by means of simulations and Power-Hardware-in-the-Loop (PHIL) measurements in a laboratory environment.

With the large-scale integration of renewable energy devices into the power grid, the voltage stability of the renewable energy base is becoming increasingly weak, and the problem of transient overvoltage is becoming increasingly severe. Grid-forming (GFM) converters can provide strong voltage support. When GFM converters are paralleled with grid-following (GFL) converters, they can effectively reduce transient overvoltage. However, hybrid systems involve many parameters and exhibit complex dynamics, making assessment of transient overvoltage difficult. To address this, this paper first uses Thevenin’s theorem to reduce the renewable transmission system to an equivalent model. Next, the voltage assessment of the hybrid system is analyzed across the pre-fault, mid-fault, and post-fault stages of a short-circuit fault. Then, based on the characteristics of a phase-locked loop (PLL), this paper innovatively derives an assessment method for transient overvoltage at the common coupling point (PCC) under different PLL stability conditions. Additionally, the influence of GFL converter parameters, GFM converter parameters, the GFM capacity ratio on transient overvoltage, and the external system reactance are analyzed. Finally, the proposed evaluation method and factor analysis are validated through electromechanical transient simulation using the simulation software STEPS v2.2.0.

The integration of inverter-based resources (IBRs) is reshaping power grid operation by reducing system inertia, which impacts small-signal rotor angle stability and increases low-frequency oscillations (LFOs). While power-electronics-based flexible AC transmission systems (FACTSs) have been the primary solution, the shift of IBR control toward grid forming (GFM) is changing this approach. GFM control inherently provides inertia and affects small-signal stability, but implementing power oscillation damping (POD) algorithms in these inverters presents challenges, particularly regarding active-power-based ones (POD-P). Although various POD-P solutions are emerging for GFM inverters, few studies have evaluated their impact on the GFM device itself and their inherent capabilities, such as inertia and damping. This paper proposes that any design methodology should consider, besides the impact of POD controls on the grid, their effect on the properties of GFM devices. It introduces a theoretical framework using the network frequency perturbation (NFP) approach to assess this impact. Additionally, a simple POD-P control method is proposed for GFM controllers, with simplicity as its key advantage. The desired damping effect, along with the absence of impact on other frequency components, is verified through NFP analysis. The theoretical findings are experimentally validated with test bench results.

In the last decade, the concept of grid-forming (GFM) converters has been introduced for microgrids and islanded power systems. Recently, the concept has been proposed for applications in wider and interconnected transmission networks, and several control structures have thus been developed, giving rise to discussions about the characteristics and the functionalities of such converters. In this paper, an overview of control schemes for GFM converters is provided. By identifying the main subsystems in respect to their functionalities, a generalized control structure is derived and different solutions for each of the main subsystems composing the controller are analyzed and compared. Subsequently, several selected open issues and challenges regarding GFM converters, i. e., angle stability, fault ride-through (FRT) capabilities, and transition between islanded to grid-connected modes are discussed. Perspectives on challenges and future trends are lastly shared.

In the last decade, the concept of grid-forming (GFM) converters has been introduced for microgrids and islanded power systems. Recently, the concept has been proposed for use in wider interconnected transmission networks, and several control structures have thus been developed, giving rise to discussions about the expected behaviour of such converters. In this paper, an overview of control schemes for GFM converters is provided. By identifying the main subsystems in respect to their functionalities, a generalized control structure is derived and different solutions for each of the main subsystems composing the controller are analyzed and compared. Subsequently, several selected open issues and challenges regarding GFM converters, i. e. angle stability, fault ride-through (FRT) capabilities, and transition from islanded to grid connected mode are discussed. Perspectives on challenges and future trends are lastly shared.