Grid-forming (GFM) inverters with an overcurrent limiting strategy called cross-forming control published in recent literature are endowed with both voltage angle-forming and current magnitude-forming capabilities, which ensure inertia support and precise overcurrent limiting during faults. However, the risk of losing stability caused by positive feedback in the cross-forming control during grid fault recovery is found in this paper. To understand and address this problem, this paper first analytically derives the operating ranges of constant voltage control (CVC) and current limiting control (CLC) in terms of power angle. Then, an unstable region induced by the loss of operating points is identified within the operating range of CLC. Subsequently, a saturated cross-forming control method is proposed to completely eliminate the unstable region. The proposed method guarantees global fault recovery capability of GFM inverters without compromising the original operating region of cross-forming control.

The switching and saturation nonlinearities introduced by current limiting strategies not only reshape the power-angle characteristics but also change the damping properties of the virtual synchronous generator (VSG)-controlled grid-forming (GFM) inverters. However, the damping effect on the transient stability of GFM inverters with current limiting has not been intuitively and analytically investigated and quantified yet. This paper first analyzes the piecewise characteristic of the power-angle curve for the GFM inverter when current limiting is activated. Then concavity and convexity of the phase portrait under different operating modes of the GFM inverter are analytically investigated. An important proposition on the curvature of the phase portrait of GFM inverter with different current-limiting strategies is proposed and proven. Based on this, the dissipated energy of the GFM inverter is graphically illustrated and precisely estimated. Consequently, the critical clearing angle (CCA) and energy difference indicator of GFM inverters with current limiting are analytically determined with low conservativeness. The effectiveness of theoretical analyses is validated by simulation and experimental tests.

Ice accretion on overhead transmission ground wires threatens power grid stability, causing galloping and tower collapse. Existing de-icing robots face challenges in adapting to steep, low-friction terrains and enduring extreme low temperatures. To address these issues, this paper proposes a novel de-icing robot system. Mechanically, a dual-arm trapezoidal wheeled walking mechanism is designed. A differential clamping force distribution strategy based on the centre of gravity (COG) is proposed to enhance climbing capability (>30°) and anti-skid performance. Control-wise, an autonomous battery heating system and intelligent return logic are designed for low-temperature environments. Simulation and experimental results verify that the proposed system significantly improves climbing stability and ensures reliable operation at −20°C with an endurance exceeding 8 km.

The icing process is simulated by using CFD methods, incorporating the influence of liquid water content, median volume diameter, temperature, duration, wind speed, etc.. By comparing results with wind tunnel test, the numerical technique used by present study is validated. The key parameters for icing thickness would be the wind speed, liquid water content, and time duration. Relative to baseline conditions, these parameters accentuate maximum ice thickness by 62%, 98%, and 22%, respectively. Then the aerodynamics characteristics are calculated and fitted by using nine-order polynomials, which can be used for further galloping analysis.

This paper investigates the synchronization stability of one multiple virtual synchronous generators (VSGs) embedded power system with the current saturation and limits of frequency. The dynamic model of the multiple VSGs integrated system with controller limits is built. Accordingly, the transient energy function (TEF) for the autonomous system without touching controller limits is established, and the stability region is determined via the method of the closet unstable equilibrium point (UEP). Further, it is found that the system leaves away from the bounds of frequency at several leaving bound points . Among these, there exists two special ones with the minimal transient energy, and they are fixed irrespective of the switching times. Finally, one stabilizing condition is proposed that only if the energy of two special leaving bound points is less than the system critical energy, the global stability is ensured. The system critical energy is determined by the minimum of two values, namely, the method by closet UEP, and the maximum potential energy to ensure the system does not enter the mode of current limiting control (CLC). The numerical simulations of 9-buses and 39-buses power systems have verified the correctness of the proposed analysis.

This paper provides a new insight for the transient stability of the grid-connected virtual synchronous generator (VSG) with multiple controller limits. The dynamic model of the grid-connected VSG with the current saturation and limits of frequency is firstly established. Accordingly, one conservative stability region of the system without limits of frequency is derived via the Lyapunov method, and the critical energy is determined by the potential energy of the intersection point of the right switching line and x-axis. Furthermore, it is found that the trajectories of the system with limits of frequency leave from the bounds of the frequency at two fixed switching points (FSPs). As a result, the global stabilizing condition is derived that the Lyapunov function values of the two FSPs are less than the defined critical energy of system with no limits of frequency based on the common Lyapunov function theory.

To meet both the requirements of current limiting and ancillary services during fault ride-through (FRT) of the GFM inverters, various current limiting strategies are developed. Recently, their post-fault behaviors and the switching characteristics have been found to have a significant impact on fault recovery capability. However, the correspondence between the switching conditions and the fault recovery capability has not been fully revealed. The commonalities and uniqueness of various current limiting strategies during fault recovery have not been thoroughly investigated and summarized. To fill these gaps, this paper first graphically illustrates the switching conditions of three typical current limiting strategies. Then, on the basis of the graphical illustration, the initial current-based conditions are converted to be based on the power angle with a fixed range. In addition, the fault recovery capability of three current limiting strategies is investigated and summarized based on the proposed power angle-based switching model. The correspondence between the fault recovery capability and switching conditions is revealed. Consequently, sufficient conditions are obtained for a successful fault recovery. The correctness of the theoretical analyses is verified and validated by numerical simulations and experimental tests.

The existing priority-based current limiting control (CLC) for grid-forming (GFM) inverters may lead to failures in fault recovery, including being locked in CLC and mode oscillation between CLC and constant voltage control (CVC). To resolve these problems, this letter first reveals the fault recovery mechanism of droop controlled GFM inverters by illustrating the conditions of three fault recovery states in the phasor diagram. Then, two methods including determination of the optimal range of current angle and upper limit of saturation block are presented to guarantee successful fault recovery of GFM inverters. The correctness of theoretical analyses is verified and validated by multiple experimental tests.

Transmission lines are subject to various faults due to extreme weather conditions. Monitoring can provide multiple sources of data on the transmission lines. By leveraging the advantages of Bayesian estimation and neural networks, a comprehensive analysis method for transmission line faults based on multi-source data fusion is proposed. This method uses Bayesian estimation to analyze and obtain data strongly related to fault risks. and applies neural networks to construct a fault analysis model. Taking wind-induced faults as an example, the method analyzes a case study of typical transmission line data. The results demonstrate that the proposed method can effectively analyze the occurrence of wind-induced faults on the lines, providing technical support for intelligent operation and maintenance of the transmission lines.

Grid-forming (GFM) inverters are DC-AC converters that regulate their AC terminal frequency and voltage in response to real-time measurements. Virtual oscillator control (VOC) is an emerging strategy of grid-forming controllers. Unlike conventional phasor-based control methods such as droop control and virtual synchronous machine control, VOC is a time-domain controller that global convergence can be almost guaranteed by adding a nonlinear element. In this article, the state space model of a VOC-based inverter system connected to an infinite bus is established to access the steady-state values. In order to study the effect of nonlinear terms on the system stability, a detailed impedance model of the VOC controller is proposed. Numerical simulations validate the impedance model. Through the impedance-based method, how the nonlinear term can influence system stability is analyzed.

The ice thickness plays a key role for design and maintenance of overhead transmission lines. The direction and diameter of the conductors have an impact on the ice thickness. The design specifications propose different types of conductor diameter correction formulas. In order to further accumulate basic icing data, in this paper, we introduces the conductor icing field observation conducted in the winter of 2020–2021 in Nanyue Mountain, Hunan province, China, as well as the related results. The analyzing results show that in the case of fog and mixed glaze, the smaller the diameter of the conductor, the larger ice thickness, and the ice thickness of conductors in most east-west directions is greater than that in the north-south direction.

Existing research on high-frequency oscillation indicates that the high-frequency oscillation during no-load charging of modular multilevel converter (MMC) station is primarily linked to time delay and the ground capacitance of AC lines. Nevertheless, a high-frequency oscillation around 2000 Hz has been observed in Rudong high voltage direct current project even when the AC lines are not connected. Except time delay, there still exists another key factor of the high-frequency oscillation, i.e., the transformer stray capacitance. This paper delves into the interaction among the dynamics of MMC station and the transformer. And the mechanism of high-frequency oscillation under no-load charging condition is determined. Firstly, the harmonic state space (HSS) model of MMC station considering the effect of stray capacitance is established. By plotting the eigenvalue trajectory, the essentiality of taking into account stray capacitance is elucidated. Subsequently, a simplified model is obtained by neglecting the dynamics of elements with low participation in the oscillation modes. Next, the mechanism of high-frequency oscillation is clarified according to impedance characteristics. Finally, the impact of proportional gain and shear frequency on small signal stability is revealed. The correctness and accuracy are identified by electromagnetic transient simulations.

The modern power system exhibits a double high characteristic, where the physical structure and control strategies of power electronic devices have altered the stability characteristics of the modern power system. This paper investigates the transient instability issue faced by virtual synchronous generators (VSG) equipped with a current limiter during grid faults. Firstly, the transient stability model of the VSG is established. Then, the equal-area criterion (EAC) is employed to reveal the mechanism of transient instability in VSG and elucidate the influence of the current limiter on the transient stability of VSG. Based on this analysis, current angle configuration methods are further proposed to enhance the transient stability of VSG. Finally, the correctness of the mechanism analysis and proposed methods is validated through simulation.

In winter, freezing rain may lead to eccentric icing on conductors. Due to the effects of steady wind on eccentric icing, the conductor may feature a low frequency, high amplitude vibration, which is conductor galloping. The conductor galloping can lead to electric and mechanical damage to overhead transmission lines. In this paper, based on the study on more than 1000 conductor galloping events of overhead transmission lines in China, the thresholds of meteorological parameters which are likely to fulfill the conditions necessary for conductor galloping have been defined. Via the meteorological data from 1999 to 2023 of 122 weather stations, the galloping days for each station can be achieved. Then the total number of galloping days for each winter can be computed. The type I extreme value distribution model is employed as the probability distribution model to calculate the galloping days with specific return period for each station. Finally, the galloping distribution map for Shandong province, China can be depicted by ordinary kriging interpolation method integrated in the mapping software. It is suggested that, the 30a, 50a, and 100a galloping distribution map can be used to the planning and design of OHLs for 35kV~330kV, 500kV~750kV, ±800kV~1000kV, respectively. By utilizing the above-mentioned maps, differentiated design and maintenance can be issued for OHLs with different voltages.

This paper focuses on the development of an online transmission line de-icing device that integrates energy harvesting, monitoring, and de-icing technologies. The device comprises a vibration generation module, an environmental monitoring module, and an energy harvesting module. The vibration generation module produces specific frequency vibrations to stimulate the vibrations of ice-covered conductors, while the environmental monitoring module adjusts the mode and frequency of vibration to optimize the de-icing effect. The device's energy supply comes from the static electric field generated by the transmission line, known as electrostatic energy harvesting. The main advantage of this electrostatic energy harvesting method is its self-sufficiency, independence from external power sources, reduced maintenance costs, and environmentally friendly energy collection. The vibrational de-icing method involves installing vibration devices directly on the conductors to disrupt the adhesion between ice and the conductor surface, create internal cracks, and enhance the de-icing effect through resonance. The technology and device proposed in this study show significant advantages in terms of safety, efficiency, and environmental adaptability, and are expected to see broader application in the future.

In recent years, with the development of the economy, the scale of China's transmission lines has continued to grow, the environment in which transmission lines are located has become more and more complex, and the status assessment and maintenance of transmission equipment are facing greater challenges. The requirements for power supply are also getting higher and higher. The research on the safety assessment method of transmission lines is an important link in improving the safety of transmission lines and ensuring the power consumption of life and production. This paper will try to establish a Monte Carlo model of the transmission line, which includes factors that may affect the safety of the line, such as natural disasters and the state of line components. And based on the model, the whole operation process of the transmission line is simulated by the Monte Carlo method, and various factors that may affect the safety state of the transmission line such as natural disasters and line component anomalies are comprehensively considered. The impact of these factors on line safety is evaluated, and the relationship between transmission line safety and monitoring maintenance modes such as regular maintenance and condition maintenance is also explored through the established Monte Carlo model.

Previous studies regards that the grid-forming (GFM) converter switches from the constant voltage control (CVC) to the current limiting control (CLC) at the point of intersection of the unsaturated and saturated power-angle curves, which might not be held in most situations. This article firstly derives the switching rule of GFM converter between two controls from the current limiting inequality. Accordingly, the conventional algebraic-variable switched model is transferred to the proposed state-variable switched model. Based on Lyapunov Theory, the stability of equilibrium points (EPs) is studied and the stable regions under two controls are further derived. In addition, one necessary stabilizing condition for the power synchronization loop based GFM (PSL-GFM) converter being locked in CLC is derived. As a result, one global stabilization on CVC is proposed by regulating the angle between the saturated current vector and d-axis . Finally, the maximum system boundary of the first swing can be obtained by placing stable equilibrium points (SEPs) under CLC on the right switching line.