Abstract:In the context of China's strategic goals of achieving carbon peak and carbon neutrality, as well as the construction of a new power system and a new energy system, offshore wind power has become an important direction for the development of renewable energy. Current research focuses on offshore wind power through grid integration methods and related technologies, promoting the stable and healthy development of the offshore wind power industry. A comprehensive review of research on optimal scheduling for the grid-integrated operation of offshore wind energy systems is provided in this paper. Firstly, the spatiotemporal characteristics, aggregation strategies, grid integration technologies, and transmission control strategies of large-scale offshore wind power clusters are analyzed. Their operational mechanisms and evaluation frameworks within the context of new energy systems are explored, and typical engineering cases are further examined. Then, the current state of research on optimization scheduling for offshore wind power grid integration is comprehensively reviewed, with a focus placed on the composition of energy systems incorporating offshore wind power, scheduling issues, and optimization methods. Finally, the development trends of grid integration and operation of offshore wind power in new energy systems are forecasted, and the key technical directions requiring further study are highlighted, aiming to provide reference for subsequent research and promote the high-quality development of offshore wind power.

Abstract:The wind turbine (WT) generally provides auxiliary frequency regulation through integrated inertia control, but its frequency regulation effect is greatly affected by control parameters. In this paper, the tuning of frequency control parameters is studied by considering the adjustable ability of the WT and the system frequency response index. Firstly, a system frequency response (SFR) model is established, which takes into account the participation of wind power and thermal power in frequency regulation, and the analytic expression of the frequency response indices is derived. Secondly, after linearizing the rotor motion equation of the WT, the rotor speed model is established, and its accuracy is verified under different wind speed conditions. Then, the control parameters of the WT are set up with the aim of raising the frequency nadir and considering the rotor speed limit and the safety constraint of the system frequency. Finally, the simulation results verify that the proposed frequency control parameter tuning method can fully utilize the rotor kinetic energy of the WT under the premise of ensuring the safety of the system frequency.

Abstract:The small-signal stability problem of large-scale grid-connected wind farms consisting of permanent magnet synchronous generators (PMSGs) under wind power variations is addressed in this paper, with focus placed on the DC-link voltage timescale. A stability criterion for grid-connected PMSG-based wind farms under uncertain wind speed conditions is proposed based on the Routh-Hurwitz criterion. Firstly, a Weibull distribution model for wind speed is established, along with a power characteristic model of the PMSG and a dynamic equivalent state-space model of the large-scale wind farm, in which the DC-link voltage control loop and the phase-locked loop (PLL) are incorporated. Secondly, the stability probability of the wind farm is calculated, and the influence mechanisms of wind speed distribution, DC-link voltage outer loop control parameters, and PLL control parameters on the small-signal oscillation stability of the wind farm at the DC-link voltage timescale are thoroughly investigated. Finally, the correctness of the theoretical derivation is validated by modal analysis and time-domain simulations through a case study of a large-scale grid-connected wind farm comprising 260 wind turbines distributed across three sub-wind farms. It is found that the risk of wind farm instability is increased with wind speed, while the stability probability of the wind farm is enhanced as the system's critical stable wind speed is raised. The relevant conclusions can be provided as a basis for the planning of large-scale PMSG-based wind farms.

Abstract:At present, the related research on the internal overvoltage of the power collection system is mostly about the offshore wind farms connected to the grid through high voltage AC transmission lines, but the far-offshore wind farms that are connected to the grid through voltage source converter based high voltage direct current (VSC-HVDC) transmission are not paid enough attention. Therefore, the electromagnetic transient models of the power collection system and the VSC-HVDC transmission system are established, and the characteristics of overvoltage under three typical working conditions of grid connection, load shedding, and three-phase ground short-circuit faults in the power collection system are analyzed. It is found that the overvoltage caused by the three-phase ground short-circuit fault at the bus and its adjacent feeder port is the most serious, which poses a challenge to the insulation of the generator-terminal transformer and the VSC-HVDC transmission line, and triggers the disconnection of all the wind turbine generators from the grid. Therefore, a suppression measure that configures surge arresters at key nodes and improves the control strategy of offshore converter stations during faults is proposed, which reduces the amplitude of transient overvoltage significantly. As a result, the influence of overvoltage is controlled within the wind farm where the fault occurs, and the large-scale disconnection of wind turbine generators from the grid is avoided effectively.

Abstract:The offshore wind power via flexible low-frequency transmission (FLFT) system based on the modular multilevel matrix converter (M3C) faces power surplus and stability risks following faults on the power frequency grid side. To address this issue, the effects of the M3C power frequency, low-frequency decoupling control strategy on the transient process under grid side faults are first analyzed in the paper. Secondly, boundary conditions for power frequency, low-frequency side voltage and active power are deduced. Consideration into the control methods of the M3C converter, low voltage ride-through requirements, and fault types, a fault ride-through strategy based on an improved voltage-dropping method is proposed for M3C-wind turbine joint voltage-power sag control to enable wind turbine overspeed load shedding outside the low voltage ride-through range. Finally, a wind turbine via a FLFT model is built in PSCAD/EMTDC to validate the proposed strategy. It is shown that the proposed strategy can ensure the average voltage of the M3C sub-module capacitors within limits and enable safe operation during system faults.

Abstract:Wind power DC collection system can effectively solve the harmonic resonance problems of AC collection system, and the series-parallel topology can effectively reduce the system cost compared with other structures, so it is very important to study the stable operation control of series-parallel wind power DC collection system. Based on the coupling characteristics of series port voltages, the rotor and energy storage coordinated control strategy (RES-CCS) is proposed to solve the overvoltage problem caused by wind speed fluctuation during steady operation of the wind turbine. Firstly, the operating characteristics of the series-parallel wind power DC collection system and the shortcomings of the existing overvoltage control strategy are analyzed. Then, the series port voltage limit is studied. The RES-CCS method is used to clamp the series port voltage within the limit value, and the energy storage capacity of the required super capacitor is designed. Finally, a series parallel wind power DC collection system model is built in the PSCAD/EMTDC simulation platform, and RES-CCS is simulated and verified. The results show that the overvoltage control method proposed in this paper can reduce the abandonment loss and the energy storage capacity of the wind turbine, and improve the wind energy utilization rate and operation economy of the wind turbine.

Abstract:Wind energy is recognized as a renewable source due to its reliability and low cost. However, under cold and humid conditions, blade icing poses a serious hazard to the performance and durability of wind turbines. Millimeter-wave radar-based inspection techniques have gained attention for their ability to penetrate non-polarized materials and provide surface conditions and in-depth information independently of light and weather conditions. A real-time detection method for wind turbine blade icing using 77 GHz millimeter-wave radar is proposed. Mel-frequency cepstral coefficients (MFCCs) are extracted from mixed-frequency time-domain signals and fused with one-dimensional convolutional neural network (1D-CNN) for classification and identification of blade icing types. The effectiveness of the proposed method is verified through experiments with varying distances and directions. Four icing types and different thicknesses are accurately recognized, achieving a recognition rate of 94%. Thin ice coverage on wind turbine blades can be recognized and warned against.

Abstract:An improved active disturbance rejection controller (ADRC) suppression strategy is proposed to address the problem of broadband oscillation between the direct-drive wind turbine and the weak AC power grid. Firstly, the model of the direct-drive wind turbine connecting to grid is established, and the mechanism of broadband oscillation is analyzed. The ADRC design is conducted within the grid side converter. Secondly, a multi-objective optimization function is developed to tackle the difficulty of ADRC parameter tuning and improve system stability and response speed. The function includes the frequency error of the grid access and the adjustment time of the system. The parameter tuning of improved ADRC is realized by combining the method of global search and optimization to improve the rapidity, accuracy and rationality of the parameter design. Finally, MATLAB/Simulink simulations are used to compare the broadband oscillation suppression effects of controller parameters designed by the traditional bandwidth method with those from the proposed method. The overshoot, adjustment time, and harmonic content of the grid-connected current are reduced when the proposed method is applied. The results indicate that the improved ADRC strategy enables good dynamic response characteristics, noise immunity, and grid-connected current quality for the direct-drive turbine system.

Abstract:After the frequency support of grid-forming permanent magnet synchronous generator-based wind turbines (GFWTs) is completed, it is necessary to promptly restore the physical rotor speed to enable the GFWTs to re-operate at the maximum power point. However, in the face of actual turbulent wind speeds with random fluctuations, traditional speed recovery strategies based on a single function form may lead to the failure to recover speed in GFWTs. To address this issue, the physical mechanism behind the failure of GFWT speed recovery under turbulent wind speeds is first revealed through stability analysis. Then, combined with the alternating characteristics of gradually strengthening/weakening winds in actual turbulent wind speeds, an adaptive speed recovery strategy for GFWTs based on the state judgment of physical rotor speed is proposed. This method utilizes the measured rotor speed to distinguish between strengthening and weakening wind conditions. During strengthening winds, the reference power command is maintained, allowing the increasing aerodynamic power to accelerate the turbine's physical rotor. During weakening winds, the speed recovery process is temporarily interrupted, and the reference power command is set to a suboptimal power curve to maintain turbine stability. Finally, PSCAD/EMTDC electromagnetic transient simulation results demonstrate that the proposed improved strategy, through multiple adaptive switches between recovery and interruption processes, can achieve reliable rotor speed recovery for grid-forming wind turbines under turbulent wind conditions.

Abstract:Potential mid-frequency oscillation risks associated with offshore wind power integration via a voltage source converter based high voltage direct current (VSC-HVDC) transmission system are addressed. China's first offshore wind VSC-HVDC project, the Rudong project, is taken as the research subject. A mid-frequency oscillation suppression strategy based on virtual damping is proposed. Firstly, the mid-frequency impedance model of the offshore VSC converter is established. Through theoretical derivation and frequency scanning, it is verified that the converter exhibits inductive and positive-resistance characteristics in the mid-frequency band. Combined with an analysis of the capacitive and negative-resistance characteristics of the wind farm in the mid-frequency band, the risk mechanism of 320 Hz oscillation induced by their interaction is revealed. Based on this, the proposed strategy is implemented. Mid-frequency harmonic currents are extracted using a DC-blocking component and a narrow band-pass filter with an adjustable center frequency. These currents are then fed into a virtual damping block, where a reverse suppression voltage is generated. Consequently, the positive damping characteristic of the VSC-HVDC system is reshaped within the target frequency band. Simulation results and field tests demonstrate that oscillations can be rapidly and effectively suppressed by the proposed strategy, while the steady-state and dynamic performance of the system are maintained. Throughout the three years of project operation since commissioning, no oscillations have been reoccurred. A replicable technical solution is thereby provided for offshore wind power VSC-HVDC transmission projects.

Abstract:Due to the presence of harmonics, the noise characteristics of converter transformers differ significantly from those of ordinary transformers. Currently, the mechanism through which harmonics affect noise is not clearly understood, necessitating research on their influence patterns in converter stations. To this end, a monitoring system for converter transformer operating conditions and radiated noise is developed. Through theoretical research, simulation analysis, and noise monitoring data analysis, the characteristics of converter transformer noise under no-load and loaded conditions are summarized. Based on noise data from converter transformers under various load conditions, the noise variation patterns of converter transformers under different loads are elucidated, from which a noise prediction method for converter transformers is derived. The research results indicate that, compared to ordinary transformer noise, the noise of converter transformers under specific load conditions is concentrated mainly at higher frequencies. The high-frequency components of converter transformer no-load noise are primarily generated by the nonlinearity of the core magnetization process and have little correlation with voltage harmonics. The high-frequency components of converter transformer loaded noise are strongly related to current harmonics. Due to the human ear's greater sensitivity to high-frequency noise and the modal characteristics of converter transformers, the A-weighted sound pressure level of 400 Hz noise is the highest and varies significantly and positively with current changes. The A-weighted sound pressure levels of noise at other frequencies show insignificant changes. Based on this characteristic, the derived noise prediction formula has an average error of 0.23 dB, effectively predicting converter transformer noise. The research findings provide guidance for converter transformer noise testing and offer a method for noise prediction of operating converter transformers.

Abstract:Aiming at the problems of insufficient mining of load characteristic information and large scale of identification model in current non-intrusive load identification methods, a non-intrusive load identification method based on improved V-I trajectory is proposed. Firstly, the active current, instantaneous power, and V-I_f trajectory are fused into new load features by using the Gramian angular field (GAF) and color encoding techniques. Then, the convolutional neural network (CNN) model framework is optimized through the depthwise separable convolution (DSC) module and the hybrid dilated convolution (HDC) module to construct a lightweight load identification model. Finally, experiments are conducted using public datasets for analysis. The results show that the F1 score of the proposed method is 0.953, which can further improve the identification accuracy of electrical loads while reducing the occupation of software and hardware resources.

Abstract:To address the issues of poor dynamic response and insufficient robustness in virtual synchronous generator (VSG) during grid connection, closing, or system disturbances, an improved fuzzy adaptive control strategy for VSG disturbance optimization is proposed. Firstly, a small-signal model of traditional VSG control is established to analyze the influence mechanism of moment of inertia and damping coefficient on the system disturbance response. Furthermore, combining the power-angle curve and frequency characteristic curve of VSG during disturbances, adaptive adjustment rules for the moment of inertia and damping coefficient are constructed. On this basis, the membership functions and control rule base of the fuzzy controller are designed, and the parameter adjustment mechanism of the fuzzy controller is established. A universe scaling factor is further introduced to achieve dynamic adjustment of the fuzzy universe. Finally, a single VSG grid-connected model is built on the MATLAB/Simulink simulation platform. The simulation results show that the proposed control strategy can effectively suppress power oscillations and improve the adaptability of VSG during load disturbances, frequency fluctuations, and grid connection/closing stages.

Abstract:With the integration of large-scale renewable energy and the increased electrification of power systems, issues related to the weakening of power system stability due to insufficient inertia levels have become frequent in recent years. Consequently, the inertia levels evaluating of high-renewable-energy power systems is crucial for developing effective inertia enhancement strategies and ensuring the safe and stable operation of the power system. A method for power system inertia evaluating based on a recursive least squares algorithm with variable forgetting factor is proposed. Firstly, a controlled autoregressive moving average (CARMA) model, incorporating Gaussian white noise, is developed to evaluate the inertia of the power system. The Akaike Information criterion (AIC) is used to determine the appropriate model order, addressing the issue of model overfitting. Then, an improved recursive least squares algorithm with an exponentially decaying variable forgetting factor is proposed to enhance the algorithm's ability to track dynamic changes in the measured data, thereby resolving data saturation issues and improving the accuracy of inertia evaluations. Finally, the effectiveness and superiority of the proposed method are verified through the case study.

Abstract:The source-grid-load collaborative interaction and the precision regulation of adjustable resources with high sensitivity provide important support for improving the regulation ability and efficiency of new power system, and promoting the efficient and reliable operation of new power system. Therefore, precision regulation method for source-grid-load collaborative interaction considering power node and resource sensitivity is proposed in this paper. Firstly, from the two perspectives of power grid application requirements and node adjustable resources, the overall idea of double-layer sensitivity analysis method is described, and the layered logic architecture of the precision regulation platform for regional source-grid-load collaborative interaction is constructed, in order to realize and apply the proposed double-layer sensitivity model and precision regulation method. Secondly, the first layer power branch sensitivity analysis model is proposed for the power grid side to obtain the power branch sensitivity matrix. A fine-grained second layer adjustable resources sensitivity analysis model is further established for the diversified adjustable resources aggregated under the power node to complete the quantitative ranking of adjustable resources regulation sensitivity. Then, the application process of double-layer sensitivity analysis results is designed in detail. The regulation sequence and regulation quantity of source and load are defined. Finally, taking a power system as an example, the effectiveness and rationality of the proposed model and method are verified. The results indicate that the proposed precision regulation method can accurately select high sensitivity power nodes and effectively sort high sensitivity adjustable resources when solving the multiple scenarios application needs of power grids.

Abstract:Gas to liquid (GTL) synthetic oil, produced from natural gas, exhibits excellent environmental and electrical properties, making it suitable for use as a new type of transformer insulating oil. In order to better understand the gas generation characteristics of GTL insulating oil, this study utilizes molecular dynamics simulation to construct a microscopic system of GTL insulating oil and simulate its decomposition process under electro-thermal combined fault conditions. Through analysis of the simulation results, changes in the types and quantities of decomposition products of GTL insulating oil are examined, and the gas generation pathways and mechanisms are determined using isotope labeling. The simulation results indicate that the final products of GTL insulating oil decomposition include small-molecular gases such as C₂H₄, C₂H₂, CH₄, H₂, C₂H₆, and free radicals. With increasing fault temperatures, the decomposition of GTL insulating oil becomes more complete. Under the electro-thermal faults, temperature is identified as the dominant factor influencing decomposition, while the presence of a strong electric field further accelerates the process. Under the same fault conditions, the proportion of H₂ and CH₄ generated from GTL oil is about 5% higher than that from conventional mineral insulating oil. This study's findings can provide theoretical support and the reference for the fault diagnosis and condition assessment of transformers using GTL insulating oil.

Abstract:To address the issue of leakage in traditional pattern recognition classifiers when identifying transformer oil-paper insulation defects within partial discharge ultrasonic mixed pulses, a multi-source partial discharge diagnostic model based on an improved version of YOLOv8 is proposed. Firstly, three typical defective ultrasonic pulse signals from transformers are collected. Each single-source pulse undergoes transformation into a two-dimensional time-frequency spectrogram using continuous wavelet transform (CWT). The signals are converted into grayscale to enhance contrast while preserving relative intensity information for time-frequency features. Subsequently, a data augmentation method based on the Wasserstein generative adversarial network with gradient penalty (WGAN-GP) is employed to expand the sample library. This addresses the imbalance issue among classes in samples used for training ultrasound pulse defect recognition. Additionally, the t-distributed stochastic neighbor embedding (t-SNE) algorithm is utilized to perform dimensionality reduction on generated samples. This filters out low-quality samples and ensures the overall quality for the augmented dataset. Finally, the global attention mechanism (GAM) is introduced to enhance the target detection algorithm YOLOv8. The proposed multi-source localized impulse diagnostic model for transformer ultrasonic time-frequency mapping is applied to identify the multi-source localized impulse test set. The average identification accuracy of each type of localized impulse reaches 95.67%, validating the effectiveness of the proposed methodology.

Abstract:With the rapid development of ubiquitous power internet of things, a large number of sensing and monitoring nodes are deployed across all aspects of the power system to enable real-time monitoring of power equipment operating status and surrounding environmental parameters. However, the traditional power supply mode has gradually failed to meet the power supply demand of the sensor nodes in the power system, especially for transmission systems in the complex outdoor environment. To address this, a self-powered multi-wind driven energy acquisition is proposed, based on the triboelectric nanogenerator (TENG) that combines a rotary TENG and a flapping TENG. The collector of two modules:wind energy acquisition module based on the combination of rotating TENG, and wind vector sensing module based on flapping TENG. Firstly, the output characteristics of the wind energy acquisition module are tested and analyzed, and the hybrid power supply strategy of the backup power supply is proposed to realize the efficient power supply of commercial temperature and humidity sensors under the multi-stage wind speed from low to high. Secondly, experiments demonstrate that the wind vector sensing module can respond sensitively to eight direction wind vectors in the range of 2.6~13.5 m/s. The logical relationship between wind vector and the aeolian vibration is established to evaluate abnormal wire vibration. Finally, on this basis, a self-driven sensing system consisting of an energy acquisition, an eight-channel signal acquisition circuit, and the ambient temperature, humidity and wind vector monitoring of the LabVIEW host computer is developed. The real-time monitoring of temperature, humidity, wind speed and wind direction is fully realized, and the aeolian vibration warning of the transmission line is realized through the multi-directional wind vector information in response to aeolian vibration.

Abstract:Aiming at the safety threat posed by metal particle contamination inside DC gas-insulated transmission line (GIL), where metal particles tend to adhere to the surface of basin insulators, a simulation model reflecting the actual operating conditions in DC GIL is established, and the impact mechanism of metal particles on the insulator's surface electric field is analyzed. In this paper, the millimeter-sized particle is selected, and the effect of various factors, such as particle size, shape, attachment location, and the aggregation of multiple particles, is researched on the electric field distribution of basin insulators. The findings indicate that the safety threshold of the insulator surface electric field strength is exceeded with a single metal particle adhesion defect. There is a boundary-diminishing effect of particle size on electric field distortion. The potential and curvature of particle attachment location are positively correlated with the degree of electric field distortion on the insulator surface. Furthermore, when particles are irregularly shaped, such as conical or linear, or when multiple particles aggregate longitudinally, there is a significant increase in the degree of electric field distortion on the insulator surface. The electric field distortion near the particles becomes more severe under the condition of voltage polarity reversal.

Abstract:To enhance the economy and low-carbon of virtual power plant scheduling considering exergy efficiency, a cascade optimization low-carbon scheduling strategy of virtual power plant considering organic Rankine cycle is put forward. Firstly, with the goal of increasing exergy efficiency of heat energy, the mechanism of organic Rankine cycle power generation is studied and a mathematical model of utilization of low quality waste heat output is established. Secondly, the coupling mechanism of organic Rankine cycle and power-to-gas device in virtual power plant is studied, and a carbon reduction guidance model considering cascade carbon trading is established to realize the coupling of cascade utilization of thermal energy and low-carbon scheduling in virtual power plant. Then, the carbon emission right model including cogeneration unit, gas boiler and gas load is established to clarify the carbon emission responsibility. Finally, the multi-load comprehensive demand response model including electricity, heat and gas is established on the load side, and the low-carbon scheduling model of cascade optimization of virtual power plant energy is established with the minimum total cost of virtual power plant as the optimization goal. The simulation results show that the proposed scheduling strategy can improve the energy efficiency and low carbon efficiency of the virtual power plant.

Abstract:There is a great difference between the transient response characteristics and the power frequency response characteristics of the grounding device under lightning strike. Its grounding impedance decreases under the action of soil impulse characteristics. In this paper, a test platform for the frequency characteristics of soil electrical parameters is built, and the frequency characteristics of soil electrical parameters are tested based on the quadrupole method. The real part of the relative dielectric constant of the soil and the change rule of conductivity are obtained. Meanwhile, the accuracy of the test results is verified in this paper by using the frequency domain relationship between the conductivity and the real part of the dielectric constant. Combined with the AC steady state response and impulse transient response tests of the soil, the calculated voltage waveforms are in good agreement with the measured voltage waveforms. It also verifies the validity of the AC frequency characteristics of the soil's electrical parameters in the calculation of the impulse response. The results of the impact response tests and calculations of the sample soils show that the consideration of the frequency characteristics of the soil electrical parameters is crucial to the accuracy of the impact response calculation, which provides a reference for the calculation of the impact grounding impedance of the conductor.