The synchronization dynamics of grid-connected power converters are known to have a tremendous impact on transient stability and fault ride-through performance under grid faults. Up till now, the modeling of converter synchronization stability, which has paved the way for numerous enhanced control methods, is developed around the assumption that the grid fault is symmetrical. However, this is rarely the case. Moreover, grid codes require dual-sequence current injection during asymmetrical faults, which implies that the previously developed models are no longer valid during unbalanced conditions. To address these issues, this article identifies the necessary stability conditions during asymmetrical conditions and presents a quasi-static large-signal reduced-order model of a grid-following converter for analyzing its synchronizing interaction with the external network during symmetrical and asymmetrical grid faults. The modeling approach is developed and tested for three different short-circuit faults: A single line-to-ground fault, a double line-to-ground fault, and a line-to-line fault. The accuracy of the proposed model is verified through detailed simulation studies and experimental tests. Thus, this model can be used to assess the transient synchronization stability of grid-following converters during any type of grid fault, and due to its low-order representation, it may be well applicable for large-scale power system studies.

This chapter starts with the various grid fault types in the power system and the evolution progress of wind turbine systems. Aligned with modern grid codes on low-voltage ride-through, Type III and Type IV wind turbine configurations are preferred nowadays. In the case of the doubly fed induction generator–based wind turbine system, due to the direct link between the generator stator and power grid, the power faults introduce the transient stator flux, which may cause the rotor overvoltage. Consequently, the control scheme of the rotor-side converter is comprehensively addressed subject to the symmetrical and asymmetrical faults. In respect to the permanent magnet synchronous generator–based wind turbine system, as the power grid is fully decoupled from the generator, the control scheme of the grid-side converter is in focus. Different controller strategies for both symmetrical and asymmetrical faults are thoroughly studied and tested. To that end, a control method that injects asymmetrical converter current in compliance with recent grid code requirements is demonstrated.

Addressing the increasing demand for high-efficiency and high-power-density converters, the flying-capacitor multilevel converter has shownitself as a promising topology. Akey advantage of this topology is the reduced voltage rating of the switches, though also makes it vulnerable to device failure during short-circuit conditions. Despite large interest in fault-tolerant operation of these converters, alongside detailed descriptions of flying-capacitor balancing, little research has focused on the converter short-circuit fault analysis, which may cause a switch failure if not properly designed for. Therefore, this article presents a comprehensive model describing the large-signal short-circuit switching behavior of a general N-level flying-capacitor multilevel converter. Highly simplified models used to predict the evolution of the switch current and voltage stress during the fault are proposed, targeted at practicing engineers for conservative design guidelines. These models are used to determine the critical time for remedial action of the converter before reaching some predefined maximum conditions. A 2-to-10-level fully configurable flying-capacitor multilevel converter and a fault circuit hardware prototype are used to experimentally perform different short-circuit tests that show a good match to the measured behavior.

This paper presents a novel phase-locked loop (PLL) structure for improving the transient stability of grid-connected converters by introducing voltage normalization control (VNC) in the conventional PLL. First, the underlying mechanism of losing synchronization during a grid fault is analyzed, and it is revealed that the key factor is the significant decrease of voltage magnitude at the point of common coupling (PCC). In order to avoid the decrease of damping ratio due to the voltage dips, a voltage normalization control method is introduced by controlling the $d$ -axis voltage magnitude to the rated value even during grid faults. To this end, the transient stability can be improved during grid faults. Phase portraits when using a conventional PLL and the PLL+VNC are both visualized for validating the effectiveness of the proposed structure. The performance of adding the additional voltage normalization control is analyzed through both small-signal and large-signal models. Finally, the experimental results are presented to verify the effectiveness of improved PLL structure for enhancing the transient stability.

During severe grid faults, grid-following converters may become unstable and experience loss of synchronization when complying with requirements for low-voltage ride-through capability. This phenomenon is well described, understood, and modeled for single-converter systems but lacks a modeling framework when extended to multiconverter systems. To fill this gap, this work presents the necessary stability conditions and aggregated reduced-order models for different multiconverter configurations, which can be used to assess the transient synchronization stability of grid-following converters under symmetrical grid faults. The necessary conditions for transient stability and the aggregated models are verified through numerous simulation studies, which verify their high accuracy for large-signal synchronization stability assessment. To that end, the Anholt wind power plant is considered as a case study where the aggregated model is compared to the full operation of a wind farm string containing nine full-order grid-following converter models. High model accuracy is obtained, and the computational burden associated with the proposed model is reduced with a factor of 100 compared with a full-order representation on the tested system. Accordingly, the presented analysis and proposed modeling are attractive as a screening tool and a convenient approach for early-stage fault analysis of system design.

Modeling and design-oriented control of transient stability of grid-following converters have attained an increasing interest in recent years. Despite novel nonlinear models enabling a design-oriented enhanced transient stability controller, the focus has so far been limited to study only the synchronization dynamics of the phase-locked loop. To expand upon the knowledge of the large-signal performance and stability, this article proposes a systematic analysis procedure of a grid-following converter under weak-grid conditions and large-signal disturbances including the outer dc-link and ac-side voltage control loops. A reduced-order large-signal model is used to analyze the large-signal nonlinear behavior of the system using the area of the basin of attraction as a measure for large-signal robustness. Here, stabilizing and destabilizing trends for outer-loop controller parameters are given. Through a surrogate-model expensive black-box optimization algorithm, a computational-efficient optimal design of the outer-loop controller parameters is proposed to maximize the large-signal robustness. Finally, a recommendation and a design guideline for converter constraints and outer-loop controller parameters are given. This can be used to identify the influencing parameters for grid-following converters under large-signal disturbances, and as a tool for fast controller optimization toward large-signal robustness.

Offshore wind farms are cornerstone solutions for enabling large-scale integration of renewable energy. Due to the large number of wind turbines in a wind farm, each with their individual complex control structure, a detailed time-domain model for analysis of wind farms is impractical from a computational burden perspective. Accordingly, aggregated models have been presented to reduce the computation time. Despite the high activity in aggregated modelling of wind farms, dynamic aggregation considering heterogeneous converter controllers and filter parameters, including different phase-locked loop (PLL) parameters have not been discussed previously. To remedy this issue, this study proposes an aggregated structure-preserving model of a wind farm string where all converters have heterogeneous parameters. The proposed model is verified by simulations against a detailed numerical model of the wind farm string, showing its accuracy in preserving the wind farm string dynamics under a severe grid fault with different short-circuit ratios and PLL parameters. Through a comparison of four different PLL aggregation methods under the considered case studies, it is found that the PLL aggregation has a negligible influence on the aggregated response and, hence, the particular aggregation method employed is not of great importance.

Voltage normalization is usually adopted for a phase-locked loop (PLL) to keep a constant bandwidth unaffected from the voltage magnitude at the point of synchronization. Two conventional PLL normalization methods are analyzed from a large-signal perspective in this letter. However, different from the voltage magnitude normalization method, an unexpected stable equilibrium point emerges in the three-phase PLL based on the d-axis voltage normalization, the mechanism of which is revealed by the phase-plane analysis. Due to the unexpected equilibrium point, the d-axis voltage normalization will lose the phase tracking ability when the initial phase difference or phase jump exceeds π/2. Thus, it is suggested to use the magnitude normalization instead of the d-axis voltage normalization to avoid this kind of malfunction. Finally, the experimental results validate the theoretical analysis.

Throughout the lifetime of a dc-ac converter, abnormal ac-side short-circuit conditions may occur as a result of external component failure, grid faults, or accumulation of moisture and dust. In such conditions, converter protection is needed. Compared to a two-level converter, which may be powered down if a fault is detected, the flying-capacitor multilevel converter cannot maintain voltage balance of the flying capacitors using such an approach. This paper therefore proposes methods for fault detection, and subsequent idle-mode fault ride-through, that protects the converter from device failures while supporting flying capacitor voltage balance. This method is experimentally verified under different operating conditions and various levels on a hardware prototype.

This paper presents an overview of the synchronization stability of converter-based resources under a wide range of grid conditions. The general grid-synchronization principles for grid-following and grid-forming modes are reviewed first. Then, the small-signal and transient stability of these two operating modes are discussed, and the design-oriented analyses are performed to illustrate the control impact. Lastly, perspectives on the prospects and challenges are shared.

Synchronous reference frame phase-locked loops (SRF-PLLs) are widely used in different technologies, such as wind turbines, electric vehicles, more electric aircraft, and motor drives, to estimate system's variables. The SRF-PLL is an adaptive notch filter that is used to estimate a sinusoidal signal's amplitude, phase angle, and frequency. However, when the input signal is subjected to a considerable variation or includes a significant noise, its stability and performance become challenging. In this paper, the stability of the SRF-PLL for substantial changes in the input signal's variables are investigated. To do so, the nonlinear time-varying (NTV) model of the system is proposed and is used for the large-signal stability assessment. Then, an adaptive tuning method, based on the proposed NTV model, is designed to improve its transient performance during and after the variation. Simulation and experimental results are used to validate the proposed method.

With an increasing capacity in the converter-based generation to the modern power system, a growing demand for such systems to be more grid-friendly has emerged. Consequently, grid-forming converters have been proposed as a promising solution as they are compatible with the conventional synchronous-machine-based power system. However, most research focuses on the grid-forming control during normal operating conditions without considering the fundamental distinction between a grid-forming converter and a synchronous machine when considering its short-circuit capability. The current limitation of grid-forming converters during fault conditions is not well described in the available literature and present solutions often aim to switch the control structure to a grid-following structure during the fault. Yet, for a future converter-based power system with no or little integration of synchronous machines, the converters need to preserve their voltage-mode characteristics and be robust toward weak-grid conditions. To address this issue, this article discusses the fundamental issue of grid-forming converter control during grid fault conditions and proposes a fault-mode controller which keeps the voltage-mode characteristics of the grid-forming structure while simultaneously limiting the converter currents to an admissible value. The proposed method is evaluated in a detailed simulation model and verified through an experimental test setup.

Paralleled grid-connected converters operated as grid-following structures are vulnerable to transient synchronization instability during grid faults. This paper mathematically describes the instability phenomenon of paralleled converters and why it is more pronounced than for single-converter operation. Based on this model, instability can be averted by modifying each converter current reference depending on the external network impedance where asymptotic stability is proven. A rapid impedance estimation algorithm is presented, which can extract the network impedance based on the disturbance of the grid fault. This estimation is used to accurately adjust the converter current references in order to guarantee stability of all paralleled converters for any severity of the grid fault. The proposed control structure is verified in a detailed simulation study and through experimental tests, which demonstrate its potential and robustness.

Transient instability is an issue for grid-following converters operating under grid faults when complying with low-voltage ride-through requirements. This has initiated much research with the aim to understand, model, and prevent loss of synchronization for synchronous-reference frame phase-locked loop (SRF-PLL)-synchronized systems. However, as the majority of grid faults are asymmetrical, a more complex synchronization unit is needed for the extraction of voltage sequences and phase tracking. This paper proposes a method for enhanced transient stability during severe grid faults for more complex synchronization structures designed to deal with asymmetrical fault conditions. This is done by freezing the frequency of a stationary-reference frame frequency-locked loop. The global asymptotic stability of the method is mathematically proven, and its performance is experimentally verified. Based on the mathematical equivalence between frequency-locked loops and phase-locked loops, it is shown that the presented method can be generalized to both stationary-reference and synchronous-reference frame structures and can, therefore, be a suitable solution in a wide range of applications.

Voltage source converters (VSCs) are one of the most adopted power converter topologies in power electronic-based power systems. Over the years, various control strategies are introduced for grid-connected VSCs, aiming to provide high quality sinusoidal current generation, bidirectional power flow, robustness to uncertainties and mismatches, acceptable steady-state and dynamic performances as well as simplicity and low computational burden. To meet these goals, this paper proposes a direct adaptive current control strategy for three-phase PWM-VSC in grid-connected applications. In the proposed control method, a simple adaption law for updating the feedback control gains based on the Lyapunov stability theory is proposed, to compensate system uncertainties. The performance of the proposed control method is evaluated under various conditions by extensive simulation and experimental tests and show that the method is powerful and robust.

Increased penetration of converter-based power generation has enforced system operators to require ancillary services from distributed generation in order to support the grid and improve the power system stability and reliability. Recent and next-generation grid codes require asymmetrical current provision during unbalanced faults for optimal voltage support. To address this, based on the highly used flexible positive- and negative-sequence control method for current reference generation, this paper presents a general current reference strategy for asymmetrical fault control, where a direct and explicit method is proposed to calculate power references and controller gains while simultaneously complying with converter current limitation and fulfilling the next-generation grid code requirements. The proposed method is tested for three distinct asymmetrical grid faults considering the requirements for dynamic voltage support of the recently revised German grid code as well as the next-generation grid codes. It is shown that the proposed method can improve the fault ride-through performance during asymmetrical faults compared with conventional solutions and comply with modern grid code requirements in a general and flexible manner.

In this article, a novel passivity-based control strategy is proposed for the exponentially stable tracking controller design of static synchronous compensator (STATCOM) system, which is a single input and single output. The STATCOM is not an input-affine system but a special port-controlled Hamiltonian system form. Hence, it is regularized by using a dynamic extension algorithm so that the proposed tracking control strategy is designed in an input-output linearization framework with a bounded solution to the driven zero dynamics equation. The proposed control strategy is proposed with consideration of the performance and stability of the input-output linearized dynamics. Simulation results show that the proposed control strategy improves the transient performance of the system compared to the previous results even in the lightly damped operating range.

As grid-connected converters are at risk of losing synchronism with the grid when exposed to extreme voltage sags, this might jeopardize the stability during a fault and a converter's ability to comply with fault ride-through requirements. This article investigates the synchronization stability of grid-tied converters during severe symmetrical faults with phase jumps. To achieve zero-voltage ride-through capability, a frozen phase-locked loop (PLL) structure can be employed to guarantee stability during faults. However, as the frozen PLL approach is unaware of frequency drifts and phase-angle jumps in the grid voltage, its performance during nonconstant frequency and phase is unknown. Therefore, this article investigates and provides new insight into how the frozen PLL performs during phase jumps and reveals whether phase compensation should be utilized to improve the converter response during a severe symmetrical fault. It is disclosed, that even though phase compensation can improve the injected currents during a fault situation including large phase jumps, a noncompensated frozen PLL can inherently ensure stability and allow for zero-voltage ride-through capability at an acceptable current injection. Furthermore, the robustness of the frozen PLL has been analyzed through a comprehensive simulation study where three test cases have been experimentally verified, which confirms the presented findings.

Renewable energy sources interfaced with the grid through power-electronic converters may lose stability and capability to perform as desired when exposed to severe grid faults. As a result of this, transient stability analysis and assessment are particularly important for power system studies. Usually, synchronization stability and transient stability analysis are performed by simulation studies containing a large amount of details, which makes this process highly time-consuming for large-scale systems. To circumvent this issue, a nonlinear second-order model is developed to capture the essential effects of the synchronization process of grid-tied converters during faults. Due to this low-order model, the stability assessment can be approached using phase-plane analysis with a low computational burden - more than 4000 times faster than the full-order switching model. The simplified model is verified against a detailed switching model and laboratory setup of the entire converter system indicating a high accuracy (> 96%). Accordingly, the simplified reduced-order model can be used for accurate transient stability studies when a low availability of computational power is present, if large-scale systems are considered, or for detailed uncertainty and sensitivity analysis.