Phase field model for electric-thermal coupled discharge breakdown of polyimide nanocomposites under high frequency electrical stress

In contrast to conventional transformers, power electronic transformers, as an integral component of new energy power system, are often subjected to high-frequency and transient electrical stresses, leading to heightened concerns regarding insulation failures. Meanwhile, the underlying mechanism behind discharge breakdown failure and nanofiller enhancement under high-frequency electrical stress remains unclear. An electric-thermal coupled discharge breakdown phase field model was constructed to study the evolution of the breakdown path in polyimide nanocomposite insulation subjected to high-frequency stress. The investigation focused on analyzing the effect of various factors, including frequency, temperature, and nanofiller shape, on the breakdown path of Polyimide(PI) composites. Additionally, it elucidated the enhancement mechanism of nano-modified composite insulation at the mesoscopic scale. The results indicated that with increasing frequency and temperature, the discharge breakdown path demonstrates accelerated development, accompanied by a gradual dominance of Joule heat energy. This enhancement is attributed to the dispersed electric field distribution and the hindering effect of the nanosheets. The research findings offer a theoretical foundation and methodological framework to inform the optimal design and performance management of new insulating materials utilized in high-frequency power equipment.

Data network traffic analysis and optimization strategy of real-time power grid dynamic monitoring system for wide-frequency measurements

The application and development of a wide-area measurement system(WAMS)has enabled many applications and led to several requirements based on dynamic measurement data.Such data are transmitted as big data information flow.To ensure effective transmission of wide-frequency electrical information by the communication protocol of a WAMS,this study performs real-time traffic monitoring and analysis of the data network of a power information system,and establishes corresponding network optimization strategies to solve existing transmission problems.This study utilizes the traffic analysis results obtained using the current real-time dynamic monitoring system to design an optimization strategy,covering the optimization in three progressive levels:the underlying communication protocol,source data,and transmission process.Optimization of the system structure and scheduling optimization of data information are validated to be feasible and practical via tests.

Typical Motion and Extinction Characteristics of the Secondary Arcs Associated with Half-Wavelength Transmission Lines

Secondary arc discharge is a complicated physical phenomenon and one of the key fundamental issues associated with ultra high voltage （UHV） half-wavelength transmission lines （HWTL）. With the establishment of a physicM simulation platform for the HWTLs, experiments were carried out regarding the motion and extinction characteristics of secondary arcs. The cathode arc root and the anode arc root were found to show an obvious polarity effect while the arc column was moving in a spirM, due to their different motion mechanisms. The extinction behavior was also recorded and experiments were designed with different compensation conditions. Results show that the arcing time can be greatly reduced if there exists an electrical compensation network. The research provides fundamentals for understanding the physics involved, especially the motion and extinction mechanisms of the secondary arcs.

Study on Valve Management of DEH for Steam Turbine

Valve management is one of the major functions of DEH for steam turbine. It has an important practical significance for the security and economy of the steam turbine. This paper starts from the valve configuration figure of the domestic-type 300 MW steam turbine, and then makes a simple comparison between the two types of valve governing modes. In order to realize the valve control, the structure of control system has been established, in which the roles of the mathematical functions are discussed. On the basis of the experiment of valve flow characteristic, this article carries out a quantitative study on the functions of the valve management and the parameter tuning method. Through a serious corrections, the sequence valve flow characteristic curve is obtained, which can provide significant guidance on the research of valve management of the similar steam turbines.

Critical Length Criterion and the Arc Chain Model for Calculating the Arcing Time of the Secondary Arc Related to AC Transmission Lines

The prompt extinction of the secondary arc is critical to the single-phase reclosing of AC transmission lines, including half-wavelength power transmission lines. In this paper, a low- voltage physical experimental platform was established and the motion process of the secondary arc was recorded by a high-speed camera. It was found that the arcing time of the secondary arc rendered a close relationship with its arc length. Through the input and output power energy analysis of the secondary arc, a new critical length criterion for the arcing time was proposed. The arc chain model was then adopted to calculate the arcing time with both the traditional and the proposed critical length criteria, and the simulation results were compared with the experimental data. The study showed that the arcing time calculated from the new critical length criterion gave more accurate results, which can provide a reliable criterion in term of arcing time for modeling and simulation of the secondary arc related with power transmission lines.

Wind turbines output power smoothing using embedded energy storage systems

The ability of an energy storage system to improve the performance of a wind turbine(WT)with a fully rated converter was evaluated,where the energy storage device is embedded in the direct current(dc)link with a bidirectional dc/dc converter.Coordinated dc voltage control design of the line-side converter and the energy storage dc/dc converters was proposed using a common dc voltage measurement for smoothing the output power.A transfer function and Bode diagram were introduced to analyze the system performance with different control parameters.MATLAB/Simulink simulations are presented to demonstrate the effectiveness of the proposed methods.It was found that the proposed methods smooth the power output from the WT to the grid and thus improve the quality of the generated power.

Surface-modification effect of MgO nanoparticles on the electrical properties of polypropylene nanocomposite

Polypropylene(PP)-based nanocomposite is a promising insulation material for recyclable high-voltage direct current(HVDC)cables,where the coupling agent plays an important role.In this study,four silane coupling agents with different alkyl chain groups(methyl,propyl,octyl,and octadecyl)were used to surface-modify magnesium oxide(MgO)nanoparticles.The surface-modification effect on the electrical properties of PP/MgO nanocomposites was investigated.The results show that surface-modified nanoparticles introduce quantities of deep traps,whose quantity increases as the alkyl chain length increases.The similar tendency also occurs on DC volume resistivity.All these nanocomposites show remarkable space charge suppression ability and improved DC breakdown strength.Among them,the nanocomposites with octyl-modified MgO show the best electrical properties,which could be attributed to the introduction of a large quantity of deep traps.The work may give a reference for the selection of coupling agents in PP-based nanocomposite insulation material for HVDC cable.

Subsynchronous oscillation monitoring and alarm method based on phasor measurements

Owing to the large-scale grid connection of new energy sources, several installed power electronic devices introduce sub-/supersynchronous inter-harmonics into power signals, resulting in the frequent occurrence of subsynchronous oscillations(SSOs). The SSOs may cause significant harm to generator sets and power systems;thus, online monitoring and accurate alarms for power systems are crucial for their safe and stable operation. Phasor measurement units(PMUs) can realize the dynamic real-time monitoring of power systems. Based on PMU phasor measurements, this study proposes a method for SSO online monitoring and alarm implementation for the main station of a PMU. First, fast Fourier transform frequency spectrum analysis is performed on PMU current phasor amplitude data to obtain subsynchronous frequency components. Second, the support vector machine learning algorithm is trained to obtain the amplitude threshold and subsequently filter out safe components and retain harmful ones. Finally, the adaptive duration threshold is determined according to frequency susceptibility, amplitude attenuation, and energy accumulation to decide whether to transmit an alarm signal. Experiments based on field data verify the effectiveness of the proposed method.

Field PMU Test and Calibration Method——PartⅡ:Test Signal Identification Methods and Field Test Applications

Synchrophasor measurement units(PMUs)provide synchronized measurement data for wide-area applications.To improve the effectiveness of synchrophasor-based applications,field PMUs must be tested to ensure their performance and data quality.In the companion paper(Part I),we proposed a field PMU test and calibration framework consisting of a PMU calibrator and analysis center.Part I presents the development and test of the PMU calibrator.This paper focuses on the analysis center and field test applications.First,the critical component of the analysis center is the signal identification module,for which the step and oscillation signal identification methods are proposed.Here,the performance evaluation criteria of PMU in these two cases are different from others.The methods include a step signal detection method based on singular value decomposition(SVD),which has the capability of weak step detection to account for energy leakage of the signal during the step process,and an oscillation signal identification method based on SVD and fast Fourier transform,which can accurately extract oscillation components that benefit from the adaptive threshold setting method.Second,the analysis center software is implemented based on identification results.By integrating the PMU calibrator in Part I with the analysis center in Part II,we can examine in depth the field PMU test applications in three test scenarios,including standard,playback,and field signal test.Results demonstrate the effectiveness and applicability of the proposed field PMU test methods from both Parts I and II.

High-accuracy and Low-complexity Phasor Estimation Method for PMU Calibration

Due to the increasing development of renewables in power systems,the requirements for phasor measurement units(PMUs)becomes higher.A PMU calibrator is an important tool to test and calibrate PMUs to ensure their measurement performance.This device can provide accurate reference values for error analysis of PMUs.In this paper,a phasor algorithm with low computational complexity and high accuracy is proposed for the PMU calibrator.This method reduces the processor requirements and development costs of the calibrator,thereby facilitating its popularization.At first,an enhanced discrete Fourier transform(DFT)method is put forward:1)the frequency response of the windowed DFT method is analyzed to reveal its large measurement errors under dynamic conditions;2)the parameter requirements of the DFT window that is regarded as a lowpass filter are analyzed,and thus a lowpass filter with better filtering performance is designed as the window coefficients to improve the estimation accuracy.Then,based on the enhanced DFT algorithm,a calibrator algorithm framework consisting of two-stage filters and a signal recognition module is established.This algorithm can consider the anti-interference ability and dynamic measurement accuracy at a low reporting rate.Simulation and experimental test results show that the proposed calibrator algorithm provides high-accuracy measurements of the static and dynamic signals with low computational complexity.

Field PMU Test and Calibration Method–Part I:General Framework and Algorithms for PMU Calibrator

Laboratory testing of phasor measurement units(PMUs)guarantees their performance under laboratory conditions.However,many factors may cause PMU measurement problems in actual power systems,resulting in the malfunction of PMU-based applications.Therefore,field PMUs need to be tested and calibrated to ensure their performance and data quality.In this paper(Part I),a general framework for the field PMU test and calibration in different scenarios is proposed.This framework consists of a PMU calibrator and an analysis center,where the PMU calibrator provides the reference values for PMU error analysis.Two steps are implemented to ensure the calibrator accuracy for complex field signals:①by analyzing the frequency-domain probability distribution of random noise,a Fourier-transform-based signal denoising method is proposed to improve the anti-interference capability of the PMU calibrator;and②a general synchrophasor estimation method based on complex bandpass filters is presented for accurate synchrophasor estimations in multiple scenarios.Simulation and experimental test results demonstrate that the PMU calibrator has a higher accuracy than that of other calibrator algorithms and is suitable for field PMU test.The analysis center for evaluating the performance of field PMUs and the applications of the proposed field PMU test system are provided in detail in Part II of the next-step research.